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Susitna-Watana Hydroelectric Project Document
ARLIS Uniform Cover Page
Title:
Fluvial geomorphology modeling approach : technical memorandum
SuWa 200
Author(s) – Personal:
Author(s) – Corporate:
Tetra Tech
AEA-identified category, if specified:
Final study plan
AEA-identified series, if specified:
Series (ARLIS-assigned report number):
Susitna-Watana Hydroelectric Project document number 200
Existing numbers on document:
Published by:
[Anchorage : Alaska Energy Authority, 2013]
Date published:
June 30, 2013
Published for:
Alaska Energy Authority
Date or date range of report:
Volume and/or Part numbers:
Study plan Section 6.6A
Final or Draft status, as indicated:
Document type:
Pagination:
3, iv, 86, 25 p.
Related work(s):
A memorandum to: Fluvial geomorphology modeling below
Watana Dam study, Study plan Section 6.6 : Final study plan
Pages added/changed by ARLIS:
Notes:
Contents: Letter of transmittal -- Attachment A. Fluvial geomorphology modeling approach
technical memorandum (June 2013) -- Attachment B. Summary of consultation on geomorphology
study plans.
All reports in the Susitna-Watana Hydroelectric Project Document series include an ARLIS-
produced cover page and an ARLIS-assigned number for uniformity and citability. All reports
are posted online at http://www.arlis.org/resources/susitna-watana/
July 1 , 2013
Ms. Kimberly D. Bose
Secretary
Federal Energy Regulatory Commission
888 First Street, NE
Washington, DC 20426
Re: Susitna-Watana Hydroelectric Project, FERC Project No. 14241-000;
Fluvial Geomorphology Modeling Approach Technical Memorandum
Dear Secretary Bose:
On April 1, 2013, the Federal Energy Regulatory Commission (Commission or FERC)
issued its Study Plan Determination (April 1 SPD) for 14 of the 58 proposed individual
studies in the Alaska Energy Authority’s (AEA) Revised Study Plan (RSP) for the
Susitna-Watana Hydroelectric Project, FERC Project No. 14241 (Project).
When approving the Study of Fluvial Geomorphology Modeling Below Watana Dam
Study (RSP Section 6.6), the Commission recommended that AEA file a technical
memorandum that provides additional information on the models and methods for
addressing several aspects of the study plan. The recommendations were:
1. Modeling in Focus Areas
We recommend that AEA file by June 30, 2013, the proposed technical
memorandum related to the selection and application of the one- and two-
dimensional models (proposed for development in the second quarter of
2013). We also recommend that the technical memorandum include the
following information:
1. Specification of the one- and two-dimensional models to be used in the fluvial
geomorphology modeling pursuant to this study as well as the aquatic habitat
models pursuant to Study 8.5 (fish and aquatics instream flow);
2. Location and extent of one- and two-dimensional geomorphology and aquatic
habitat modeling in project reaches, focus areas, and other study sites;
3. Rationale and criteria for model selection including an overview of model
development;
4. For fluvial geomorphology modeling only, a detailed description of the
processes and methods by which ice and large woody debris (LWD) would be
incorporated into the modeling approach (as described in our
2
recommendations for incorporating large woody debris and ice processes into
fluvial geomorphic modeling); and
5. Documentation of consultation with the Technical Work Group (TWG),
including how the TWG’s comments were addressed.
2. Interaction of Geomorphic Processes in the Mainstem and Tributaries
We recommend the study plan be modified to include a defined approach
to evaluating geomorphic changes at the confluence of the Chulitna,
Talkeetna, and Susitna rivers. The evaluation should extend from the
mouth of both the Chulitna and Talkeetna rivers to the potentially affected
upstream reaches of these tributaries. We recommend that AEA prepare a
technical memorandum detailing a proposed approach for evaluating
geomorphic changes in the three rivers confluence area, including
explicitly stated objectives for evaluating geomorphic changes, an
overview of the technical approach, additional data collection required,
models and model components to be used, and additional analyses that
would be conducted to address the stated objectives. We recommend that
AEA file by June 30, 2013, this technical memorandum to include
documentation and consultation with the TWG, including how the TWG’s
comments were addressed.
3. Incorporating Large Woody Debris and Ice Processes into Fluvial
Geomorphic Modeling
As noted above in our analysis and recommendations for Modeling in
Focus Areas, we are recommending that AEA file a technical
memorandum with additional information on AEA’s proposed model
selection process. We recommend that an additional provision be added
to the technical memorandum requiring that AEA describe in detail how
ice and LWD would be incorporated into both one- and two-dimensional
modeling approaches. The technical memorandum should explicitly state
where and how each of the five scenarios for incorporating ice processes
into one-dimensional and/or two-dimensional fluvial geomorphology
modeling would be implemented, as well as details regarding where and
how LWD pieces and/or accumulations would be incorporated into two-
dimensional modeling.
Consistent with the Commission’s recommendations within the April 1 SPD, AEA is
filing the attached Fluvial Geomorphology Modeling Approach Technical Memorandum
(attached as Attachment A).
The draft version of this document was made available for review on May 3, 2013 and
the material was presented at a Technical Workgroup Meeting on May 21, 2013.
Attached as Attachment B is a comment response table that includes a description of how
comments are incorporated into the final plan; and an explanation for why certain
comments were not incorporated into the final plan.
3
As always, AEA appreciates the participation and commitment to this licensing process
demonstrated by Commission Staff, federal and state resource agencies, and other
licensing participants. AEA looks forward to working with licensing participants and
Commission Staff in implementing the approved studies, which AEA believes will
comprehensively investigate and evaluate the full range of resource issues associated
with the proposed Project and support AEA’s license application, scheduled to be filed
with the Commission in 2015.
If you have questions concerning this submission please contact me at wdyok@aidea.org
or (907) 771-3955.
Sincerely,
Wayne Dyok
Project Manager
Alaska Energy Authority
Attachments
cc: Distribution List (w/o Attachments)
Attachment A
Fluvial Geomorphology Modeling Approach Technical Memorandum (June 2013)
Susitna-Watana Hydroelectric Project
(FERC No. 14241)
Fluvial Geomorphology Modeling Approach
Technical Memorandum
Prepared for
Alaska Energy Authority
Prepared by
Tetra Tech
June 30, 2013
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 June 2013
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TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page i June 2013
TABLE OF CONTENTS
1. INTRODUCTION ................................................................................................................. 1
1.1 Background ...................................................................................................................... 3
1.1.1 Reach-Scale Issues .................................................................................................... 5
1.1.2 Local-Scale Issues ..................................................................................................... 6
1.2 Objectives ......................................................................................................................... 6
2. OVERALL MODELING APPROACH .............................................................................. 9
2.1 Background ...................................................................................................................... 9
2.2 Comprehensive Modeling Approach ............................................................................. 10
3. SELECTION OF HYDRAULIC AND BED EVOLUTION MODELS ......................... 13
3.1 1-D Models ..................................................................................................................... 13
3.1.1 Overview of 1-D Model Development ................................................................... 14
3.1.2 Selection Criteria for 1-D Models........................................................................... 15
3.1.3 Potential 1-D Models .............................................................................................. 16
3.1.3.1 HEC-RAS ............................................................................................................ 16
3.1.3.2 SRH-1D ............................................................................................................... 17
3.1.3.3 MIKE 11 .............................................................................................................. 17
3.1.3.4 HEC-6T ............................................................................................................... 17
3.1.4 Selection of 1-D Model ........................................................................................... 18
3.2 2-D Models ..................................................................................................................... 18
3.2.1 Overview of 2-D Model Development ................................................................... 19
3.2.2 Selection Criteria of 2-D Models ............................................................................ 21
3.2.3 Potential 2-D Models .............................................................................................. 22
3.2.3.1 SRH-2D ............................................................................................................... 22
3.2.3.2 ADH .................................................................................................................... 23
3.2.3.3 MD_SWMS/SToRM ........................................................................................... 23
3.2.3.4 MIKE 21 .............................................................................................................. 24
3.2.3.5 River2D Modeling Suite ..................................................................................... 24
3.2.3.6 RiverFLO-2D ...................................................................................................... 25
3.2.4 Selection of 2-D Model ........................................................................................... 26
4. MODEL APPLICATION ................................................................................................... 29
4.1 Models and Survey Extent ............................................................................................. 29
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page ii June 2013
4.1.1 Reach-Scale Model ................................................................................................. 29
4.1.2 Local-Scale Focus Area Models ............................................................................. 30
4.1.3 Other Tributary Models .......................................................................................... 32
4.2 Reach-Scale 1-D Modeling ............................................................................................ 32
4.2.1 Bed Evolution and Hydraulic Modeling ................................................................. 32
4.2.2 Large Woody Debris Effects .................................................................................. 34
4.2.2.1 Large Woody Debris Mapping ............................................................................ 34
4.2.2.2 Large Woody Debris Modeling........................................................................... 35
4.2.3 Ice Effects ............................................................................................................... 36
4.2.4 Summary of Reach-Scale Model Results ............................................................... 36
4.3 Local-Scale 2-D Modeling ............................................................................................. 37
4.3.1 Bed Evolution and Hydraulic Modeling ................................................................. 38
4.3.1.1 Morphology Modeling of Focus Areas ............................................................... 38
4.3.1.2 Hydraulic Modeling for Habitat Analysis ........................................................... 39
4.3.2 Large Woody Debris Effects .................................................................................. 40
4.3.3 Ice Effects ............................................................................................................... 40
4.3.4 Summary of Local-Scale Model Results ................................................................ 42
4.4 Other Tributary Modeling .............................................................................................. 43
5. CONSULTATION DOCUMENTATION ......................................................................... 45
6. REFERENCES .................................................................................................................... 47
7. TABLES ............................................................................................................................... 51
8. FIGURES ............................................................................................................................. 57
LIST OF TABLES
Table 2-1. 1-D versus 2-D model capabilities ................................................................................ 51
Table 3-1. Evaluation of 1-D models ............................................................................................ 53
Table 3-2. Evaluation of 2-D models ............................................................................................ 54
Table 4-1 Tributary modeling ....................................................................................................... 55
Table 4-2. Large woody debris digitizing within geomorphic features ....................................... 56
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page i June 2013
LIST OF FIGURES
Figure 4-1. Survey and Model Cross Sections from PRM 30 to PRM 47. ................................... 59
Figure 4-2. Survey and Model Cross Sections from PRM 45 to PRM 66. ................................... 60
Figure 4-3. Survey and Model Cross Sections from PRM 63 to PRM 81. ................................... 61
Figure 4-4. Survey and Model Cross Sections from PRM 79 to PRM 99. ................................... 62
Figure 4-5. Survey and Model Cross Sections from PRM 98 to PRM 116. ................................. 63
Figure 4-6. Survey and Model Cross Sections from PRM 114 to PRM 131. ............................... 64
Figure 4-7. Survey and Model Cross Sections from PRM 131 to PRM 148. ............................... 65
Figure 4-8. Survey and Model Cross Sections from PRM 142 to PRM 154. ............................... 66
Figure 4-9. Survey and Model Cross Sections from PRM 166 to PRM 179. ............................... 67
Figure 4-10. Survey and Model Cross Sections from PRM 176 to PRM 188. ............................. 68
Figure 4-11. Proposed Cross Section Layout for Chulitna and Talkeetna Rivers, 1-D Model
Tributary Reaches. ................................................................................................................ 69
Figure 4-12. FA104 (From R2 Resource Consultants Inc. 2013b). .............................................. 70
Figure 4-13. FA113 (From R2 Resource Consultants Inc. 2013b). .............................................. 70
Figure 4-14. FA115 (From R2 Resource Consultants Inc. 2013b). .............................................. 71
Figure 4-15. FA128 (From R2 Resource Consultants Inc. 2013b). .............................................. 71
Figure 4-16. FA138 (From R2 Resource Consultants Inc. 2013b). .............................................. 72
Figure 4-17. FA141 (From R2 Resource Consultants Inc. 2013b). .............................................. 72
Figure 4-18. FA144 (From R2 Resource Consultants Inc. 2013b). .............................................. 73
Figure 4-19. FA151 (From R2 Resource Consultants Inc. 2013b). .............................................. 73
Figure 4-20. FA173 (From R2 Resource Consultants Inc. 2013b). .............................................. 74
Figure 4-21. FA184 (From R2 Resource Consultants Inc. 2013b). .............................................. 74
Figure 4-22. Middle River Tributary Locations Relative to Geomorphic Reach and
Focus Areas. .......................................................................................................................... 75
Figure 4-23. Lower River Tributary Locations Relative to Geomorphic Reach. ........................ 76
Figure 4-24. Example of Fine Mesh Applied in Whiskers Slough Focus Area. .......................... 77
Figure 4-25. Example of Coarse Mesh Applied in Whiskers Slough Focus Area. ...................... 78
Figure 4-26. Examples of mesh detail for habitat analysis requirements. .................................... 79
Figure 4-27. Example of Ice Blockage Altering Flow Distribution in Multiple
Channel Reaches (Zabilansky et al. 2003). ........................................................................... 79
Figure 4-28. Ice Jam Locations at FA104. .................................................................................... 80
Figure 4-29. Ice Jam Locations at FA113. .................................................................................... 81
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page ii June 2013
Figure 4-30. Ice Jam Locations at FA115. .................................................................................... 82
Figure 4-31. Ice Jam Locations at FA128. .................................................................................... 83
Figure 4-32. Ice Jam Locations at FA138. .................................................................................... 84
Figure 4-33. Ice Jam Locations at FA141. .................................................................................... 85
Figure 4-34. Ice Jam Locations at FA144. .................................................................................... 86
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page iii June 2013
ACRONYMS AND ABBREVIATIONS
1-D One dimensional
2-D Two dimensional
ADCP Acoustic Doppler Current Profiler
AEA Alaska Energy Authority
AOW additional open water
ASPRS American Society of Photogrammetry and Remote Sensing
BEI Bank Energy Index
BG Background
cfs cubic feet per second
D Depth
DHI Danish Hydraulic Institute
D/S Downstream
FA(s) Focus Area(s)
FaSTMECH Flow and Sediment Transport with Morphologic Evolution of Channels
FERC Federal Energy Regulatory Commission
FHWA Federal Highway Administration
GIS Geographic Information System
HEC-RAS Hydrologic Engineering Centers River Analysis System
HSC Habitat Suitability Criteria
iRIC International River Interface Cooperative
LiDAR Light detecting and ranging
LWD Large woody debris
MBH Mobile Boundary Hydraulic
MD_SWMS Multi-Dimensional Surface-Water Modeling System
OS Operational Scenario
PDO Pacific Decadal Oscillation
PRM Project River Mile
RoR Run of River
RSP Revised Study Plan
SPD Study Plan Determination
SRH-1D (2D) Sedimentation and River Hydraulics-One Dimension (Two Dimensions)
SToRM System for Transport and River Modeling
TIN Triangulated Irregular Network
TWG Technical Work Group
U/S Upstream
USACE U.S. Army Corps of Engineers
USGS U.S. Geological Survey
V Velocity
WSE Water-surface elevation
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page iv June 2013
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TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 1 June 2013
1. INTRODUCTION
After submittal of the Fluvial Geomorphology Modeling Technical Memorandum (Tetra Tech
2012) and the Revised Study Plan (Alaska Energy Authority [AEA] 2012), the Federal Energy
Regulatory Commission (FERC) issued a Study Plan Determination (SPD) on April 1, 2013, that
included three recommendations to provide additional information on the models and methods
for addressing several aspects of the study plan. The recommendations were:
1. Modeling in Focus Areas
We recommend that AEA file by June 30, 2013, the proposed technical
memorandum related to the selection and application of the one- and two-
dimensional models (proposed for development in the second quarter of 2013).
We also recommend that the technical memorandum include the following
information:
1. Specification of the one- and two-dimensional models to be used in the fluvial
geomorphology modeling pursuant to this study as well as the aquatic habitat models
pursuant to Study 8.5 (fish and aquatics instream flow);
2. Location and extent of one- and two-dimensional geomorphology and aquatic habitat
modeling in project reaches, focus areas, and other study sites;
3. Rationale and criteria for model selection including an overview of model
development;
4. For fluvial geomorphology modeling only, a detailed description of the processes and
methods by which ice and large woody debris (LWD) would be incorporated into the
modeling approach (as described in our recommendations for incorporating large
woody debris and ice processes into fluvial geomorphic modeling); and
5. Documentation of consultation with the Technical Work Group (TWG), including
how the TWG’s comments were addressed.
The items in this recommendation are addressed in the following sections of this
Technical Memorandum:
• Selection of 1- and 2-D models is included in Section 3. “Selection of
Hydraulic and Bed Evolution Models.”
• Application of 1- and 2-D models is included in Section 4. “Model
Application.”
• Specification of the 1-D model is Section 3.1.4 “Selection of 1-D Model.”
• Specification of the 2-D model is Section 3.2.4 “Selection of 2-D Model.”
• The selected 1- and 2-D models will be used for hydraulic input to the aquatic
habitat analyses and modeling as described in Section 4.3.1.2 “Hydraulic
Modeling for Habitat Analysis.”
• Location and extent of 1- and 2-D models is included in Section 4.1 “Models
and Survey Extent.” Project reaches are described in Section 4.1.1 “Reach-
Scale Model,” Focus Areas are described in Section 4.1.2 “Local-Scale Focus
Area Models,” and other study sites (tributaries) are described in Section 4.1.3
“Other Tributary Models.”
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 2 June 2013
• Rationale and criteria for model selection are included in Sections 3.1.2
“Selection Criteria for 1-D models” and 3.2.2 “Selection Criteria 2-D
Models.”
• Overviews of model development are Sections 3.1.1 “Overview of 1-D Model
Development” and 3.2.1 “Overview of 2-D Model Development.”
• See recommendation 3, below, for items related to LWD and Ice modeling.
• Documentation of consultation is Section 5. “Consultation Documentation.”
2. Interaction of Geomorphic Processes in the Mainstem and Tributaries
We recommend the study plan be modified to include a defined approach to
evaluating geomorphic changes at the confluence of the Chulitna, Talkeetna, and
Susitna rivers. The evaluation should extend from the mouth of both the Chulitna
and Talkeetna rivers to the potentially affected upstream reaches of these
tributaries. We recommend that AEA prepare a technical memorandum detailing
a proposed approach for evaluating geomorphic changes in the three rivers
confluence area, including explicitly stated objectives for evaluating geomorphic
changes, an overview of the technical approach, additional data collection
required, models and model components to be used, and additional analyses that
would be conducted to address the stated objectives. We recommend that AEA
file by June 30, 2013, this technical memorandum to include documentation and
consultation with the TWG, including how the TWG’s comments were addressed.
The items in this recommendation are addressed in the following sections of this
Technical Memorandum:
• The defined modeling approach is included as Section 2.2
“Comprehensive Modeling Approach.”
• The evaluation of the Chulitna and Talkeetna Rivers, including the
approach and objectives is part of Section 1.2 “Objectives.”
3. Incorporating Large Woody Debris and Ice Processes into Fluvial Geomorphic
Modeling
As noted above in our analysis and recommendations for Modeling in Focus
Areas, we are recommending that AEA file a technical memorandum with
additional information on AEA’s proposed model selection process. We
recommend that an additional provision be added to the technical memorandum
requiring that AEA describe in detail how ice and LWD would be incorporated
into both one- and two-dimensional modeling approaches. The technical
memorandum should explicitly state where and how each of the five scenarios for
incorporating ice processes into one-dimensional and/or two-dimensional fluvial
geomorphology modeling would be implemented, as well as details regarding
where and how LWD pieces and/or accumulations would be incorporated into
two-dimensional modeling.
The items in this recommendation are addressed in the following sections of this
Technical Memorandum:
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 3 June 2013
• LWD processes and modeling methods are described in Sections 4.2.2.2 “Large
Woody Debris Modeling” for 1-D modeling and 4.3.2 “Large Woody Debris
Effects” for 2-D modeling.
• The five scenarios for ice modeling are described for 1-D modeling in
Section 4.2.3 “Ice Effects” and for 2-D modeling in Section 4.3.3 “Ice
Effects.”
This technical memorandum was developed to provide responses to the SPD recommendations.
The intent is to identify the models and methods for addressing the specific comments and
recommendations, recognizing that adjustments to the approaches may occur as additional
information is acquired. The draft technical memorandum was included as a topic at the May 21,
2013, TWG meeting. This document will be updated in Q1 2014 as methods are refined based
on field data collection, further coordination between this and other study components, and
initial model development.
The selection of the one-dimensional (1-D) and two-dimensional (2-D) models, as well as the
modeling approaches, has been coordinated with the other pertinent studies and the licensing
participants. As part of the coordination process, an early draft of this technical memorandum,
titled Fluvial Geomorphology Modeling (Tetra Tech 2012), was posted on the AEA website in
May 2012.
The fluvial geomorphology modeling team will continue to develop the modeling approaches
and coordinate with other studies on modeling needs. Site reconnaissance, data collection, and
field observations in the summer of 2013 will also result in additional detail and possible
adjustments to the modeling approaches. The length of modeling for the tributaries will be
identified in the field, and hydraulic and sediment-transport modeling domains for the Focus
Areas may be extended slightly, on the order of a channel width or less, to improve boundary
conditions, but no adjustments will be made to the actual Focus Area limits. A revised modeling
approach technical memorandum will be submitted in the first quarter of 2014. The revised
memo will incorporate further understanding of the system gained from the summer 2013 field
data collection and observations as well as the experience of performing the initial 1-D and 2-D
model runs.
1.1 Background
The purpose of the fluvial geomorphology studies is to assess the potential effects of the Susitna-
Watana Hydroelectric Project on the dynamic behavior of the river downstream of the proposed
dam, with particular focus on potential changes in instream and riparian habitat. The Project will
alter flow rates and sediment supply downstream of the dam, and the channel form is expected to
respond to the changes. Whether the existing channel morphology will remain the same or at
least be in “dynamic equilibrium” under post-project conditions is a significant question in any
instream flow study. In other words, is the channel morphology in a state of dynamic equilibrium
such that the distribution of habitat conditions will be reflected by existing channel morphology
or will changes in morphology occur that will influence the relative distribution or characteristics
of aquatic habitat over the term of the license (Bovee 1982)? This key issue prompts four overall
questions that must be addressed by the geomorphology study:
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 4 June 2013
• Is the system currently in a state of dynamic equilibrium?
• If the system is not currently in a state of dynamic equilibrium, what is the expected
evolution over the term of the license?
• Will the Project affect the morphologic evolution of the Susitna River compared to pre-
project conditions?
• If the Project will alter the morphology of the river, what are the expected changes over
the term of the license?
The methods and results from the geomorphology study and the fluvial geomorphology
modeling study will address these questions. These studies will be coordinated to:
• determine how the river system functions under existing conditions;
• determine how the current conditions form and maintain a range of aquatic and channel
margin habitats; and
• identify the magnitudes of changes in the controlling variables and how these will affect
existing channel morphology;
The fluvial geomorphology modeling will include a range of analyses for Existing and with-
Project scenarios. These analyses will be used to:
• develop calibrated models to predict the magnitudes and trends of geomorphic response
to the Project;
• apply the models to estimate potential for channel change for with-Project operations
compared to existing conditions;
• support coordination with the geomorphic and other studies to integrate model results
with understanding of geomorphic processes and controls to identify potential Project
effects; and
• support the evaluation of potential Project effects by other studies that relate to channel
and hydraulic results throughout the river corridor over the license period
To develop the modeling approach, specific issues that need to be addressed have been
identified. These issues have been further differentiated into reach-scale and local-scale issues
since the scale influences the proposed approach. The reach-scale modeling of the Susitna River
will be performed using 1-D models, as they are well suited for long term simulations over long
river reaches. Reach-scale results will address channel morphologic change on the order of 101
to 100 x the Susitna River width and subdivide the flow between channel, left floodplain, and
right floodplain. The 1-D models will be used to assess reach-scale sediment-transport
conditions, potential changes in bed and water-surface elevations, changes in channel profile,
and potential changes in bed material gradation. The 1-D models will also provide boundary
conditions for the local-scale modeling (i.e., the Focus Areas) that will be performed for
relatively short reaches of the Middle Susitna River Segment using 2-D models.
The detailed results of the 2-D models will provide more localized information on changes in
hydraulic and bed conditions over a range of flows for existing and with-project conditions. The
local-scale results within Focus Areas will provide results over a range of scales. For the main
channel the element size will be on the order of 10-1 x the Susitna River width, but results can be
integrated to obtain channel average results (100 x width). Model refinement must be greater in
areas of appreciable geometric change or where velocity magnitude and direction change rapidly.
In these areas model refinement will be between 10-1 and 10-2 x Susitna River width. A high
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 5 June 2013
level of refinement is required for detailed habitat analysis in the lateral features. These areas
will be analyzed at the 10-2 x Susitna River width. Floodplain and island areas tend to have
higher flow resistance and the shallow, low velocity conditions do not require as detailed of a
resolution in the models. These areas will be modeled at scales ranging from 100 to 10-1 x
Susitna River width.
• The fluvial geomorphology modeling will include evaluation of existing conditions
compared to four operational scenarios over the 50-year term of the license. The four
operational scenarios include maximum load following, base load, intermediate load
following, and Run of River (RoR). These scenarios are described as: Max Load
Following OS-1: The Maximum Load Following OS-1 scenario is based on the
assumption that the entire load fluctuation of the Railbelt would be provided by the
Susitna-Watana Project, and that all other sources of electrical power in the Railbelt
would be running at base load. This assumed condition is not realistic for an entire year,
and the results of this condition should be conservative with respect to assessing
downstream impacts of load following.
• Base Load: This scenario assumes that the Project is operated to support the Railbelt base
load and does not operate in a load following mode.
• Intermediate Load Following: This scenario represents an intermediate condition between
the Maximum Load Following OS-1 and the Base Load scenarios in which the Susitna –
Watana Project provides a portion of the load fluctuation of the Railbelt (the portion of
the load fluctuation that the Project would supply has not been determined).
• Run of River: In this scenario the Project is operated to match outflow from the reservoir
with inflow to the reservoir. The exception would be during the initial filling of the
reservoir when inflow would exceed outflow. The major difference between this scenario
and the pre-Project condition would be the trapping of the vast majority of the sediment
load in the reservoir under the Run of River scenario.
1.1.1 Reach-Scale Issues
Reach-scale issues refer to aspects of the system that involve the overall behavior and general
characteristics of the Susitna River over many miles. Each reach represents a spatial extent of the
Susitna River that has a consistent set of fluvial geomorphic characteristics. Reach-scale issues
include:
• Historical changes in the system and the existing status with respect to dynamic
equilibrium (i.e., the overall sediment-transport balance).
• Changes in both the bed material (sand and coarser sizes) and wash (fine sediment) load
sediment supply to the system due to trapping in Watana Reservoir.
• Long-term balance between sediment supply and transport capacity and the resulting
aggradation/degradation response of the system for pre- and post-Project conditions.
• Changes in bed material mobility in terms of size and frequency of substrate mobilized
due to alteration of the magnitude and duration of peak flows and sediment supply by the
project.
• Project-induced changes in supply and transport of finer sediments that influence
turbidity.
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 6 June 2013
• Potential for changes in channel dimensions (i.e., width and depth) and channel pattern
(i.e., braiding versus single-thread or multiple-thread with static islands) due to the
Project and the magnitude of the potential change.
• Project-induced changes in river stage due to reach-scale changes in hydrology, bed
profile, channel dimensions, and potentially hydraulic roughness.
• Characterization of the types, amounts, and features of LWD both in terms of supply and
the overall effects of LWD on sediment transport.
• Changes in ice cover effects on sediment mobilization.
1.1.2 Local-Scale Issues
Local-scale issues refer to aspects of the system that involve the specific behavior and
characteristics of the Susitna River at a scale associated with site-specific geomorphic and
habitat features. Local-scale issues are addressed using a more detailed assessment over a smaller
spatial area; however, these analyses must draw from and build upon the understanding and
characterization of the system behavior determined at the reach scale. Local-scale issues
include:
Processes responsible for formation and maintenance of the individual geomorphic features and
associated habitat types. Potential changes in geomorphic features and associated aquatic habitat
types that may result from effects of Project operation on riparian vegetation and ice processes.
Effects of changes in flow regime and sediment supply on substrate characteristics in lateral
habitat units. Changes in upstream connectivity (breaching) of lateral habitats due to alteration of
flow regime and possibly channel aggradation/degradation. These changes may induce further
changes in the morphology of lateral habitats, including:
• potential for accumulation of sediments at the mouth;
• potential for accumulation of fine sediment supplied during backwater
connection with the mainstem;
• potential for changes in riparian vegetation that could alter the width of lateral
habitat units.
Project effects at representative sites on the magnitude, frequency, and spatial distribution of
hydraulic conditions that control bed mobilization, sediment transport, sediment deposition, and
bank erosion.
Potential for change in patterns of bedload deposits at tributary mouths that may alter tributary
access or tributary confluence habitat.
Potential for changes in accumulation of LWD and related effects on hydraulics, erosion, scour,
and sediment transport.
Relating potential changes in ice cover and ice jams and the related potential effects on
hydraulics; flow distribution between the main channel, lateral features, and floodplains;
sediment transport; and erosion.
1.2 Objectives
The objective of this technical memorandum is to document the procedures for modeling the
fluvial geomorphology of the Susitna River below Watana Dam. The overall goal is to model
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 7 June 2013
and evaluate the potential Project effects of the proposed Susitna-Watana Hydroelectric Project
on the fluvial geomorphology of the Susitna River and tributaries, and provide input to other
team members for evaluating potential Project effects on habitat. The results of this and other
geomorphology studies will be used in combination with geomorphic principles and
criteria/thresholds defining probable channel forms to predict the potential for alterations of
channel morphology. The purpose of this technical memorandum is to explain the proposed
approach, including which models will be used, for the Susitna River fluvial geomorphology
modeling..
Specific objectives include:
• Identify the 1-D sediment-transport model that will be used for reach-scale modeling and
2-D sediment-transport model that will be used for local-scale modeling, including
o Providing the rationale and criteria for model selection;
o Specifying the selected models; and
o Providing an overview of model development of the selected models.
• Identify the location and extent of the 1-D and 2-D models that will be performed.
• Provide a description of the processes and methods for incorporating ice and LWD into
the 1-D and 2-D geomorphic modeling.
• Describe the modifications to the study plan for evaluating geomorphic changes at the
confluence of the Susitna, Chulitna, and Talkeetna Rivers.
As with the reach-scale 1-D sediment-transport analysis of the Susitna River, the objectives for
evaluating geomorphic change at the Three Rivers Confluence are to compare existing
conditions to with-Project scenarios related to 1) hydraulic interactions; 2) sediment-transport
interactions; 3) channel form (aggradation, degradation, and width), and to relate these outcomes
to potential Project effects.
There are no additional analyses for the Three Rivers Confluence anticipated beyond what is
currently planned for the 1-D modeling of the Susitna River because the Chulitna and Talkeetna
Rivers will be included as tributary reaches in the 1-D modeling. The reach-scale 1-D modeling
of the Susitna, Chulitna, and Talkeetna Rivers will provide information on potential Project
effects on hydraulics, sediment transport, and channel form through the analysis of:
• Velocity
• Depth
• Water-surface elevation
• Sediment loads
• Effective discharge
• Coincident flows and stage
• Aerial photo analysis of channel change
• Bed material gradation
• Aggradation and degradation
• Channel profiles
• Channel width
• Channel plan form
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 8 June 2013
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 9 June 2013
2. OVERALL MODELING APPROACH
2.1 Background
The proposed modeling approach considers the need to address both reach-scale and local-scale
conditions and the practicality of developing and applying various models based on data
collection needs, computational time, analysis effort, and model limitations. Based on these
considerations, an approach that uses 1-D models to address reach-scale issues and 2-D models
to address local-scale issues will be used. A comparison of the capabilities of 1-D and 2-D
models is provided in Table 2-1. Based on these capabilities and the need to evaluate potential
Project effects over the majority of the system and at small habitat scales, a combination of 1-D
and 2-D modeling approaches is required. Considering the broad physical expanse of the Susitna
River system, the general hydraulic and sediment-transport characteristics of the various
geomorphic reaches that make up the overall study area will be evaluated using 1-D computer
models. The 2-D models will be used to evaluate the detailed hydraulic and sediment-transport
characteristics at locations where it is necessary to consider the more complex flow patterns to
understand and quantify flow distribution, habitat, breaching, and erosion/deposition issues
related to changing hydrology, sediment supply, ice, and LWD conditions.
The 2-D models will be applied to the 10 Focus Areas that are representative of important habitat
conditions and the various geomorphic reaches and associated channel classification types. These
sites were chosen in coordination with the Fish and Aquatics Instream Flow, Riparian Instream
Flow, Ice Processes, and Fish and Aquatic Resources studies to facilitate maximum integration
of available information between the studies (see Sections 6.6.4.1.2.4, 8.5.4.2.1.2, and 8.6.3.2 of
the Revised Study Plan (RSP), AEA 2012; R2 Resource Consultants 2013a, 2013b).
In addition to the reach-scale 1-D models for existing and with-project conditions, 1-D models
will be developed for a selected subset of tributaries to provide sediment inputs to both the reach-
scale model and the 2-D Focus Area models. These tributaries will be evaluated using models
developed with cross section and bed material data collected near the mouth. Temporary gages
and stage-discharge relationships at selected tributaries from other studies will provide a basis
for estimating the flow record, and the models will be used to evaluate sediment loads. Because
it is not practical to develop these models for all tributaries, a subset of tributaries will be
modeled, and the resulting information will be used to develop flow and sediment supplies for
other ungaged tributaries. The reach-scale model will be used to develop boundary conditions
for the 2-D models that include water-surface elevation versus discharge rating curves for the
downstream boundary, and the sediment supply at the upstream boundary.
Section 4, Model Application, provides locations and extents of the reach-scale 1-D model,
local-scale 2-D models, and tributary models. Integration of the 1-D reach-scale modeling with
the local-scale 2-D Focus Area modeling will provide the following advantages:
• The 1-D model will allow for efficient assessment of the hydraulic conditions and
sediment-transport balance over the length of the study reach between Watana Dam and
Susitna Station.
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 10 June 2013
• The 1-D model reaches will extend up the Chulitna and Talkeetna Rivers to more fully
represent these sediment sink/sources and to evaluate potential Project effects on the
morphology and flooding potential of these tributary channels.
• The 1-D model uses cross-sectional data that are being obtained as part of the Open-
Water Flow Routing portion of the Instream Flow studies plus additional cross sections to
represent stream-wise variation in planform and profile.
• The 1-D model will provide the boundary conditions for the 2-D model in the Focus
Areas, including starting water-surface elevations and upstream sediment supply.
• The 1-D model will provide reach-scale evaluation of potential sediment-transport effects
due to changes in LWD amounts.
• The 2-D model applied at the Focus Areas, which are also being evaluated for the ice
processes and riparian instream flow studies, will allow for the fullest level of integration
of these efforts, particularly as they relate to assessments of potential changes in channel
width and pattern.
• The 2-D model will provide additional information on erosion and sedimentation
processes related to ice jam surge, channel and lateral features blockage by ice, and flows
diverted onto floodplain areas by ice jams.
• The 2-D model at the Focus Areas will provide an understanding of the hydraulic
conditions and sediment-transport processes that contribute to formation of individual
habitat types.
• The 2-D model at the Focus Areas will be used to evaluate flow conditions and bed
mobilization around LWD obstructions.
• The 2-D model provides a much more detailed and accurate representation of the
complex hydraulic interaction between the main channel and the lateral habitats than is
possible with a 1-D model.
2.2 Comprehensive Modeling Approach
As described above, 1-D modeling will be used to evaluate reach-scale channel morphology and
2-D modeling will be used at the focus areas for channel morphology and habitat analyses.
These models require input (boundary conditions) on inflowing water and sediment, and
downstream water surface (stage-discharge relationships). Sediment sampling will provide bed
material gradations for the 1- and 2-D morphology modeling and field observations and
calibration will be used to establish roughness values for all the morphology and hydraulic
models. Therefore, the various types of models will need to be conducted in a sequence where
certain models or analyses provide input to other models. For example, the dam operations and
flow routing models will be used to provide flow hydrographs to the 1- and 2-D morphology
models and the 2-D hydraulic models will provide hydraulic results for the habitat analysis
models.
Table 2.2 illustrates the series of four types of models that will comprise the majority of the
fluvial geomorphology modeling component of the study. For each of these model types the
hydrology, sediment, hydraulic, and geometric (channel and floodplain) input and results are
summarized. The source of the input information is identified when it is provided by another
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 11 June 2013
study component. The type of information that will be used by other study components is
identified for the fluvial geomorphic modeling results.
A prerequisite for the 1-D reach-scale morphology models is to determine the sediment supplied
by each of the tributaries. Table 2.2 shows 1-D Tributary Sediment Modeling is the first
modeling task. This modeling will be conducted for a range of flows to develop sediment rating
curves at all tributaries located at Focus Areas, selected tributaries in the Lower Susitna River
Segment for sediment supply and limited habitat analyses, and other selected tributaries in the
Middle Susitna River Segment for sediment supply only. The range of tributaries will be used to
develop sediment inflow for other tributaries throughout the model domain. Some of these
tributaries will also be analyzed to provide information for the aquatic habitat and barrier studies.
The 1-D reach-scale morphology modeling will be run for a 50-year continuous flow record for
Existing conditions and with-Project operational scenarios. The inflows to the model are the
outflows from the dam for these conditions plus tributary inflows. Similarly, sediment passing
the dam site will be included at the upstream limit of the model for these four conditions.
Tributary sediment inflow will also be included. The only hydraulic boundary condition for this
model is the stage-discharge relationship at the Susitna Station gage. The existing (2012 and
2013) channel and floodplain geometry will be the starting condition and the model will simulate
potential channel change throughout the 50-year license period.
The reach-scale modeling will provide information to the 2-D local-scale morphology modeling
efforts and to other study components. Local-scale models will be developed at the Focus Areas
representing conditions at years-0, -25, and -50. If bed elevations or channel widths change over
the 50 year period, the reach-scale model results will not only be used to alter the future (years-
25 and -50) geometry, but will provide future downstream stage-discharge and upstream
sediment supply rating curves to the local-scale models. The geometry and rating curve
information must all be changed to maintain consistency between the models and to maintain
internal consistency of the specific local-scale model. Because the local-scale models will be run
for approximate 6-month open-water hydrographs, AEA does not anticipate that the rating
curves will change appreciably. This assumption will be tested by evaluating the reach-scale
model results and a shifting rating curve could be used if necessary.
The reach-scale models will also provide information to other studies. For example, aquatic and
riparian habitat studies will use stage-discharge information at specific locations over the 50-year
license period. There may also be the need to incorporate future channel change into the
River1D ice model or flow routing models. This would only be necessary if the magnitude of
geometric change would significantly affect the results of these studies.
The 2-D local-scale morphology models of the focus areas will not be run for the full 50-year
period, but will be run for initial conditions and at years 25 and 50. These short duration runs
(~6 months) will be performed for a range of hydrologic conditions including wet, average, and
dry years with warm and cool Pacific Decadal Oscillation. This range of hydrologic conditions
will be used to interpret the local-scale morphology models and compare Existing and with-
Project conditions in the main channel, secondary channels, other lateral features, islands, and
floodplain areas. Just as the channel changes in the reach-scale models are used to develop the
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 12 June 2013
future geometry of the local-scale morphology models, the local-scale morphology model results
will be used to modify lateral feature geometry in the 2-D local-scale hydraulic models.
The local-scale hydraulic models are necessary because they have much greater mesh refinement
than can be achieved in the morphology models and they are steady-state models run for a range
of flows rather than dynamic models run for seasonal hydrographs. The habitat analyses require
a sequence of steady flows that can be applied to the range of flow magnitudes, durations, and
timings of the analysis scenarios. The 2-D hydraulic modeling provides depth, velocity, water-
surface elevation and other parameters for the range of flows throughout the local-scale model
domains. These data will be used by the aquatic habitat, riparian habitat, and barrier studies to
evaluate potential Project effects.
Though not specifically included in Table 2.2, additional 2-D morphology modeling will be
conducted for a range of ice blockage and breakup conditions to evaluate erosion and deposition
potential. The specific conditions for these simulations will be coordinated with input from
agencies and other study components. Also not included in the table are changes in LWD that
may occur over time. Descriptions of ice and LWD effects on sediment transport and model
simulations are described in Section 4.2 for reach-scale modeling and 4.3 for local scale
modeling.
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 13 June 2013
3. SELECTION OF HYDRAULIC AND BED EVOLUTION MODELS
Many computer programs are available for performing movable boundary sediment-transport
simulations. The choice of an appropriate model for this study depends on a number of factors,
including: (1) the level of detail required to meet the overall study objective, (2) the class, type,
and regime of flows that must be modeled, (3) characteristics of the bed material and wash load
and, and (4) data necessary for model development and calibration. In addition, because of the
wide range of sediment sizes present in the Susitna River, both the 1-D and 2-D models must be
capable of routing sediment by size fractions, and ideally be capable of addressing deposition of
fine sediments (wash load).
A variety of candidate models were evaluated for application on the Susitna River. The models
fall into three categories of availability: (1) public domain, (2) commercial, and (3) proprietary.
Public domain models are often developed by federal agencies or at universities and are available
without cost. While they typically include a user interface, a commercial interface may also be
available for these models. Commercial models are also available, though there are costs
associated with acquiring the initial license, annual renewals, and support. Proprietary models
are available only by contracting with the developer to perform an analysis. Proprietary models
were not included as candidates. The candidate models for the 1-D and 2-D portions of the study
are discussed below.
3.1 1-D Models
Most 1-D movable-boundary, sediment-transport models are designed to simulate changes in the
cross-sectional geometry and river profile due to scour and deposition over relatively long
periods of time. In general, the flow record of interest is discretized into a quasi-unsteady
sequence of steady flows of variable discharge and duration. For each model time-step and
corresponding discharge, the water-surface profile is calculated using the step-backwater method
to compute the energy slope, velocity, depth, and other hydraulic variables at each cross section
in the network. The sediment-transport capacity is then calculated at each cross section based on
input bed material information and the computed hydraulics, and the aggradation or degradation
volume is computed by comparing the transport capacity with the upstream sediment supply (i.e.,
the supply from the next upstream cross section for locations not identified as an upstream
boundary condition). The resulting aggradation/degradation volume is then applied over the
cross-sectional control volume (i.e., the sub-channel concept), and the shape of the cross section
is adjusted accordingly. The computations proceed from time-step to time-step, using the
updated cross-sectional and bed material gradations from the previous time-step.
The 1-D sediment-transport models should not be applied to situations where 2-D and 3-D flow
conditions control the sediment-transport characteristics because they do not consider secondary
currents, transverse movement and variation, turbulence, and lateral diffusion; thus, the 1-D
models cannot simulate such phenomena as point bar formation, pool-riffle formation, and
planform changes such as river meandering or local bank erosion. The 1-D models typically
distribute the volume of aggradation or degradation across the entire wetted portion of the
channel cross section after each time-step; thus, the effects of channel braiding are not directly
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 14 June 2013
considered. The 1-D models are, however, useful in evaluating the general sediment-transport
characteristics and overall event or long term sediment balance of a given reach, and they are
useful in providing boundary conditions for local-scale 2-D models.
3.1.1 Overview of 1-D Model Development
The following steps will be followed to develop the 1-D sediment-transport model. With few
exceptions (as noted) the model development will be very similar regardless of the selected
model. An overview of calibration and validation is included below. Additional information on
model parameterization, calibration, validation, and sensitivity analysis will be provided in the
study reports. Review and quality control procedures will be implemented throughout the model
development process and are not indicated as individual steps. The steps are:
1. Determine the overall model layout.
• Downstream boundary selected at a location of known stage-flow conditions.
• Upstream boundary location(s) of known discharge and sediment supply information.
• Tributaries that will be modeled geometrically with sediment routing.
• Flow change locations of tributaries that are modeled as flow and sediment inputs.
• Identification of split flow reaches around islands or in multiple-channel locations.
2. Develop cross-sectional data.
• Determine cross section locations to represent the channel network.
• Obtain channel cross-sectional geometry from land and bathymetric survey data.
• Extend surveyed channel cross sections over islands and into floodplains using land-
based survey and light detecting and ranging (LiDAR) data.
• Determine channel and floodplain flow distances between cross sections.
3. Develop flow resistance (roughness) data for cross sections.
• Channel base roughness based on bed material size.
• Adjust base roughness to account for other sources of flow resistance such as channel
irregularities, obstructions (including LWD), bed forms, and channel sinuosity. Note:
project-related changes in amounts of LWD and sediment size can be related to flow
resistance values.
• Channel bank and floodplain (overbank) roughness based on land use, vegetative
ground cover, and obstructions using field observations and aerial photography.
4. Develop bed and bank material gradation and layer information.
• Surface sampling,
• Subsurface sampling, and
• Bank material samples.
5. Develop inflow hydrographs and sediment inflows for existing and with-project
conditions.
• For quasi-unsteady models, develop step hydrographs for the main channel and
tributary inputs.
• For fully unsteady models, use complete flow hydrographs.
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 15 June 2013
• Develop sediment inflow rating curves based on tributary models or gaging station
records that include sediment measurements.
6. Other considerations.
• Bridge constrictions and geometries.
• Ineffective flow areas around bridges and other rapid expansion and contraction
areas.
• Use of depth- or flow-variable roughness input.
7. Test the hydraulic model over a range of flow conditions.
• Evaluate cross-sectional spacing to determine the need for interpolated cross sections.
• Review for potential geometric input errors in reach lengths or station-elevation data
in areas of appreciable change or instability in hydraulic results.
8. Calibrate and validate the hydraulic model.
• Adjust flow resistance input values (within reasonable limits) to calibrate the
hydraulic results using available data including:
o Water-surface elevations at the time of cross-sectional survey,
o Water-surface elevations collected at other flows,
o Gaging station records,
o Water level loggers at Focus Areas and other locations,
o Discharge and velocity measurements including main channel and lateral features,
and
o High water marks reported from extreme flood events.
9. Test the sediment-transport model.
• Conduct a sediment-transport time-step sensitivity analysis to evaluate
appropriate computational time steps for different flow magnitudes.
10. Calibrate and validate the sediment-transport model.
• Adjust sediment input values, bed layer properties, sediment-transport time
step (within reasonable limits) to calibrate the hydraulic results using available
data including: 1) Gage station measurements sediment loads, specific gage
plots, flow area, width, depth, and velocity measurements, 2) Comparison
cross sections, and 3) Longitudinal profiles.
11. Run and evaluate the results of the sediment-transport simulations.
3.1.2 Selection Criteria for 1-D Models
The criteria for selecting a 1-D model for this project are primarily based on required
functionality given the specific conditions of the Susitna River and its tributaries. There are
several desirable characteristics as well, which may influence the decision if models are
otherwise similar in their capabilities and performance. The desirable characteristics include:
public domain, high level of experience with the model, and advanced graphical user interface
for model input and review of results.
The required characteristics include:
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 16 June 2013
• The model must accommodate sufficiently large number of cross sections to model over
100 miles of river including split flow reaches.
• The model must be capable of storing sufficiently large number of hydrograph ordinates
to model flows over the 50-year license period.
• The model must be capable of simulating sufficient number and range of sediment sizes
to represent the range of materials.
• Sediment-transport calculations must be performed by size fraction, especially to
simulate bed material sorting and armoring processes in coarse bed channels.
• The model must include either (or both) the Parker (1990) or Wilcock and Crowe (2003)
bedload sediment-transport equations, as these are the most applicable to the range of
coarse bed conditions in the Susitna River and tributaries.
• Closed loop sediment-transport capability must be included to model sediment
transported around islands and in multiple channel reaches. This is especially common in
the Lower Susitna River Segment but is also important in the Middle River.
3.1.3 Potential 1-D Models
The 1-D models that are being considered for this study are:
• U.S. Army Corps of Engineers’ (USACE) Hydrologic Engineering Centers-River
Analysis System (HEC-RAS), version 4.1 (USACE 2010),
• U.S. Bureau of Reclamation’s Sedimentation and River Hydraulics-One Dimension
(SRH-1D), version 2.8 (Huang and Greimann 2011),
• Danish Hydraulic Institute’s (DHI’s) MIKE 11, version 2011 (DHI 2011a), and
• Mobile Boundary Hydraulics’ (MBH’s) HEC-6T, version 5.13.22_08 (MBH 2010).
3.1.3.1 HEC-RAS
HEC-RAS, version 4.1.0 (USACE 2010) is a publicly available software package developed by
the USACE to perform steady flow water-surface profile computations, unsteady flow
simulations, movable boundary sediment-transport computations, and water quality analysis.
HEC-RAS includes a Windows-based graphical user interface that provides functionality for file
management, data entry and editing, river analyses, tabulation and graphical displays of
input/output data, and reporting facilities. The sediment-transport module is capable of
performing sediment-transport and movable boundary calculations resulting from scour and
deposition over moderate time periods, and uses the same general computational procedures that
were the basis of HEC-6 and HEC-6T (USACE 1993; MBH 2010). In HEC-RAS, the sediment
transport potential is estimated by grain-size fraction, which allows for simulation of hydraulic
sorting and armoring. This model is designed to simulate long term trends of scour and
deposition in streams and river channels that could result from modifying the frequency and
duration of the water discharge and stage, sediment supply, or direct modifications to channel
geometry. Benefits of the HEC-RAS software include widespread industry acceptance, public
availability, and ease of use. Potential limitations of the program include excessive computer
run-times, file size output limitations, and the inherent problems associated with 1-D modeling
of aggradation and degradation by equal adjustment of the wetted portion of the bed that can
result in unrealistic channel geometries. Another significant limitation of using HEC-RAS for
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 17 June 2013
this project is that it does not currently incorporate “looped” networks (split flows around
islands), which are common in the Middle and Lower Susitna River segments.
3.1.3.2 SRH-1D
SRH-1D (Huang and Greimann 2011) is a publicly available, mobile-boundary hydraulic and
sediment-transport computer model for open channels that is capable of simulating steady or
unsteady flow conditions, internal boundary conditions, looped river networks, cohesive and
non-cohesive sediment transport (Ruark et al. 2011), and lateral inflows. The hydraulic and
sediment-transport algorithms in SRH-1D are similar to those in HEC-RAS 4.1 and HEC-6T
except that it also includes the capability to perform fully unsteady sediment-transport
simulations. Advantages of SRH-1D include robust algorithms for hydraulic conditions and
sediment routing, including sediment sorting. Potential disadvantages include limited testing for
a broad range of conditions outside the U.S. Bureau of Reclamation and the lack of graphical
user interface, which complicates data input and display of output.
3.1.3.3 MIKE 11
DHI’s MIKE 11 is a commercial software package developed for 1-D dynamic modeling of
rivers, watersheds, morphology, and water quality. The model has the ability to solve the
complete non-linear St. Venant equations (in only the streamwise direction) for open channel
flow, so the model can be applied to any flow regime. MIKE 11 provides the choice of diffusive
and kinematic wave approximation and performs simplified channel routing using either the
Muskingum or Muskingum-Cunge methods. The program includes a module for simulating
erosion and deposition of non-cohesive sediments. Advantages of MIKE 11 include its robust
hydrodynamic capabilities (though not necessarily better than HEC-RAS), the user-friendly
graphical interface, and good reporting and presentation capabilities. Disadvantages primarily
stem from the commercial nature of this model and associated high cost of the software license.
The MIKE 11 model does not include either the Parker (1990) or Wilcock and Crowe (2003)
sediment-transport equations, which are favored for simulating bed material transport and sorting
processes in coarse bed channels.
3.1.3.4 HEC-6T
HEC-6T is a commercially available program that was developed by William A. Thomas, former
Chief of the Research Branch at the USACE Hydrologic Engineering Center. Mr. Thomas
planned, designed, wrote, and applied the publically available version of HEC-6; HEC-6T is a
commercial enhancement of the original version. HEC-6T is a DOS-based program that includes
a Windows-based graphical user interface for input data manipulation and post-processing of
simulation results. Limitations of this program include reduced capabilities for modeling
numerous ineffective flow areas as compared to HEC-RAS 4.1 and limited capabilities of the
graphical user interface. The model uses a quasi-unsteady flow representation. Advantages of
HEC-6T are its wide application experience, looped channel capability, and large number of
sediment-transport equations (including both Parker and Wilcock and Crowe), sediment sizes,
and hydrograph ordinates. This model includes algorithms to limit the potential for unrealistic
cross-sectional geometry. Model input is limited to 5,000 cross sections, which should be more
than sufficient for the analysis of the Susitna River and tributaries as an average 250-foot cross-
sectional spacing, which is less than half the channel width for much of the river, would be able
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FERC Project No. 14241 Page 18 June 2013
to cover nearly 240 miles of channel. This software is relatively inexpensive; the fact that it is
commercial is not a significant limitation. The fluvial geomorphology modeling team has
extensive experience with this program.
3.1.4 Selection of 1-D Model
Specific model characteristics and selection criteria are summarized in Table 3-1 along with an
evaluation of each candidate model relative to the criteria. Based on the information provided
above and experience with these models, the geomorphology study team will use HEC-6T for
the reach-scale sediment-transport analysis. HEC-6T is capable of modeling looped networks,
which eliminated HEC-RAS from consideration. It also includes both the Parker (1990) and
Wilcock and Crowe (2003) sediment-transport relationships, which eliminated MIKE 11. The
advantages of HEC-6T over SRH-1D include a high level of team experience with the model and
its broader range of project applications. An advantage of SRH-1D is full unsteady flow analysis
that can directly simulate flow attenuation. This is not an overriding consideration as quasi-
unsteady analyses have been used successfully for many large rivers. The selection is supported
by the modeling team’s confidence that HEC-6T is capable of effectively and efficiently
modeling the processes that are important for this scale of geomorphic analysis.
3.2 2-D Models
The 2-D models provide a much more detailed and accurate representation of the flow field than
1-D models because they predict both the magnitude and direction (in the horizontal plane) of
the velocity, whereas 1-D models only predict magnitude of velocity in the downstream
direction. Because the 2-D models input includes the complete bed topography at the resolution
of the mesh, they also provide a more accurate representation of velocity, flow depth, and water-
surface elevation throughout the model domain. In contrast, 1-D models assign a single water-
surface elevation across each cross section. The velocity distribution can be estimated based on
the distribution of conveyance across the cross section. The 2-D models vary water-surface
elevation and distribute velocity based on the equations of motion (continuity and Newton’s
second law) and, therefore, account for flow conditions up- and downstream of the location of
interest. As a result, 2-D models are superior in defining detailed hydraulic conditions in areas
of special interest such as key habitat units.
The 2-D models are often categorized based on the solution technique and grid structure. Finite
difference models use a regular grid, which simplifies the solution but limits the level of detail
that can be achieved. Finite element and finite volume models use an irregular mesh that allows
for more detail in areas of interest or in areas where there is appreciable variability. A subset of
finite element models uses a curvilinear grid, which shares advantages and disadvantages of both
regular grid and irregular mesh. For the requirements of this project, only models that use an
irregular mesh are considered because of the highly variable channel and floodplain
configurations (main channel, secondary channels, side sloughs, upland sloughs, tributaries,
islands, and floodplains) and the need to provide accurate and detailed results for habitat
evaluation.
The 2-D sediment-transport models are much more in their infancy. Publicly available 2-D
sediment-transport models had very limited capability. One of the earliest available models,
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FERC Project No. 14241 Page 19 June 2013
STUDH (McAnally 1989) could only be used to simulate a single grain-size of fine sediment for
evaluating sand transport. Models with capability to simulate coarse beds and multiple grain-
size analyses are much more recent.
The 2-D hydraulic models of a specific location should be developed to accurately represent the
geometry (bathymetry and topography) and variability of flow resistance, with appropriate
boundary conditions. The mesh should include greater detail in areas with appreciable variability
in geometry, velocity magnitude, velocity direction, depth, and roughness. The required
boundary conditions include downstream water-surface elevation and upstream discharge
(mainstem and tributary sources). The model boundaries should be located where flow is
generally one-dimensional, although this requirement is not absolute and the effects can be
reduced by extending the model limits up- or downstream from the areas of interest. 2-D
sediment-transport models must include from good quality hydraulic modeling capability, and
they must accurately represent surface and subsurface sediments, sediment depths, erodibility,
and appropriate starting and boundary conditions. 2-D models are fully dynamic, which is a
requirement for sediment routing, though many can be operated in a steady state. A sediment-
transport simulation routes the sediment through the network and adjusts the elevation of the grid
points (nodes) due to erosion and deposition. Modeled changes in node elevations provide a
feedback on the hydraulic simulation due to changes in flow depth and conveyance. Unlike 1-D
models, which aggrade or degrade the wetted portion of each cross section in concert, 2-D
models adjust nodes individually based on the spatial variability of velocity, depth, sediment
supply, and sediment-transport capacity.
3.2.1 Overview of 2-D Model Development
The following steps will be followed to develop the 2-D hydraulic and sediment-transport
models of the Focus Areas. The model development and application will be similar regardless of
the selected model. An overview of model calibration and validation is included below.
Additional information on model parameterization, calibration, validation, and sensitivity
analysis will be provided in the study reports. Review and quality control procedures will be
implemented throughout the model development process and are not indicated as individual
steps. The steps are:
1. Determine the overall model layout.
• Downstream boundary stage-flow conditions developed from 1-D model,
• Upstream (i.e., inflowing) discharge and sediment supply from 1-D model, and
• Tributary flow and sediment input from tributary models.
2. Develop geometric base data.
• Data from TIN (Triangulated Irregular Network) surface representation from land and
bathymetric survey including necessary breaklines, and
• Data from LiDAR bare earth data set for unsurveyed island and floodplain areas.
3. Develop model network.
Determine node and element locations and configurations to accurately represent
geometry (bathymetry and topography) and changes in roughness. This may be either a
network of triangular or a combination of triangular and quadrilateral elements,
depending on the selected model.
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FERC Project No. 14241 Page 20 June 2013
Refine the network in areas of appreciable change or areas of significant habitat interest.
Determine the node elevations from the geometric data.
Review mesh quality to assure that element size transitions and other modeling
requirements are reasonably met. These include increased mesh refinement where there
is appreciable geometric change or where velocity magnitude or directions changes
occur. Identifying where additional model refinement is refinement is needed is
somewhat based on experience and judgment. Large element sizes may miss large-scale
flow separation (circulation) or may have numerical instabilities (oscillating or greatly
changing velocities). If the instabilities are too large the model will terminate. Areas of
instability are easily identified in the model results and these areas will be refined. The
model results will also be reviewed to determine if there are currents that are not
“reasonably” depicted based on our experience and these areas will be refined.
4. Develop flow resistance (roughness) and turbulence stress data.
• Channel base roughness based on bed material size.
• Adjust base roughness to account for other sources of flow resistance such as
obstructions (including LWD) and bed forms. Note: project-related changes in
amounts of LWD and sediment size can be related to flow resistance values. Also
note that LWD will be simulated by including large debris areas as part of the
geometry.
• Channel bank and floodplain (overbank) roughness based on land use, vegetative
ground cover, and obstructions using field observations and aerial photography.
• Turbulence stress data, such as eddy viscosity coefficients, are used to incorporate
internal flow stresses. Reasonable values depend on each model’s numerical
representation of these stresses. ADCP data will be used to calibrate these
coefficients.
5. Develop bed and bank material gradation and layer information.
• Surface sampling,
• Subsurface sampling, and
• Bank material samples.
6. Develop water and sediment inflows for existing and with-project conditions.
• For fully unsteady models, use complete flow hydrographs.
• Steady flow simulations will be performed for habitat analysis based on the range of
flows in the simulation record.
• Develop sediment inflow rating curves based on tributary models and from the 1-D
reach-scale model.
7. Other considerations.
• Ice jam breakup hydrographs.
• Ice jam blockage of main channel or lateral features causing redistribution of flow.
• LWD as obstructions or changes in roughness.
• Erodibility of floodplain areas.
8. Test the hydraulic model over a range of flow conditions.
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FERC Project No. 14241 Page 21 June 2013
• Further evaluate mesh quality and the need for additional mesh refinement for areas
with appreciable changes in velocity magnitude or direction to adequately capture
flow transitions.
9. Calibrate and validate the hydraulic model.
• Adjust flow resistance input values (within reasonable limits) to calibrate the
hydraulic results. Calibration and validation will be performed using available data
including:
o Measured water-surface elevations throughout the focus areas during site
survey and water-surface elevations measured at other times.
o Measured velocities collected using acoustic doppler current profiler along
selected cross sections and longitudinal profiles. Note that flow resistance
values in 2-D models are often lower than comparable 1-D models
because 2-D models directly account for processes that 1-D models must
treat as lumped flow resistance parameters.
o Water level loggers.
o Discharge distribution between main channel and secondary channels.
o High water mark information if available.
10. Test the sediment-transport model.
• Conduct a sediment-transport time-step sensitivity analysis to evaluate appropriate
computational time steps for different flow magnitudes. These tests identify the
longest stable time-step for model applications.
11. Calibrate and validate the sediment-transport model.
• Adjust sediment inflow rates and sizes, bed layer properties, sediment-transport time
step (within reasonable limits) to calibrate the hydraulic results using available data
including:
o Main channel bed level changes observed in the 1-D modeling,
o Comparisons of cross sections using 1980s and current data and between the
1-D and 2-D models, and
o Longitudinal profiles.
12. Run and evaluate the results of the sediment-transport simulations.
3.2.2 Selection Criteria of 2-D Models
The criteria for selecting a 2-D model for this project are primarily based on required
functionality based on the specific conditions of the Middle Susitna River Segment. As with 1-D
models, there are also several desirable characteristics that may influence the decision if models
are otherwise similar in their capabilities and performance. The desirable characteristics include:
public domain, high level of experience with the model, moderate to fast execution speed, and
advanced graphical user interface for model input and reviewing results. The required
characteristics include:
• Capability for sufficiently large number of elements to model the Focus Areas at the
required spatial resolution.
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FERC Project No. 14241 Page 22 June 2013
• Flexible mesh (irregular mesh) to accurately depict geometric and hydraulic
variability.
• Capability to simulate a sufficiently number and range of sediment sizes to represent
the range of materials in each Focus Area.
• Sediment-transport calculations must be performed by size fraction, especially to
simulate bed material sorting and armoring processes in coarse bed channels.
• The model must include either (or both) the Parker (1990) or Wilcock and Crowe
(2003) bed-load sediment-transport equations, as these are the most applicable to the
range of coarse bed conditions in the Susitna River and tributaries The model must be
numerically stable under a wide range of flow conditions, especially as portions of the
network wet and dry.
3.2.3 Potential 2-D Models
Potential 2-D models that are being considered for this study are:
• U.S. Bureau of Reclamation’s SRH-2D, version 3 (Lai 2008; Greimann and Lai 2008),
• USACE’s Adaptive Hydraulics ADH, version 4.3 (USACE 2013),
• U.S. Geological Survey’s (USGS) Multi-Dimensional Surface-Water Modeling System
(MD_SWMS) suite, which includes SToRM or System for Transport and River Modeling
and FaSTMECH or Flow and Sediment Transport with Morphologic Evolution of
Channels models (McDonald et al. 2005; Nelson et al. 2010),
• DHI’s MIKE 21, version 2011 (DHI 2011b),
• River2D modeling suite (Steffler and Blackburn 2002; Kwan 2009), and
• RiverFLO-2D model (Hydronia 2012).
3.2.3.1 SRH-2D
The U.S. Bureau of Reclamation’s SRH-2D (Lai 2008) is a finite-volume, hydrodynamic model
that computes water-surface elevations and horizontal velocity components by solving the depth-
averaged St. Venant equations for free-surface flows in 2-D flow fields. SRH-2D is a well-tested
2-D model that can effectively simulate steady or unsteady flows and is capable of modeling
subcritical, transcritical, and supercritical flow conditions. The model uses an irregular mesh
composed of a combination of triangular and quadrilateral elements. SRH-2D incorporates very
robust and stable numerical schemes with a seamless wetting-drying algorithm that results in
minimal requirements by the user to adjust input parameters during the solution process. A
potential limitation of this software is that the mobile bed sediment-transport module is currently
not publically available; however, Tetra Tech has gained permission to use the sediment-
transport module on a number of other projects. Contact with the model developers indicates
that permission would be granted for use in this study. The public download version of the
model (Greimann and Lai 2008) includes a morphology module that calculates bedload transport
capacities at each model node based on user-defined bed material sediment gradations, but does
not simulate routing of that sediment and related adjustments to the channel bed. The advanced
version of SRH-2D also includes a second module that uses the capacities from the morphology
module to perform sediment-routing calculations and associated bed adjustments. Based on
guidance from the model developers and confirmed by Tetra Tech’s use of the model for other
studies, the maximum practical model size is about 16,000 elements, which could be a potential
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FERC Project No. 14241 Page 23 June 2013
limitation in applying the model to larger-scale areas. This size limitation only applies to
sediment routing simulations so much more detailed networks can be developed to support
habitat simulations. Another potential limitation of the model is that sediment gradations are
limited to eight size fractions, though the sizes are user specified so the size-class intervals can
be tailored to the site conditions. SRH-2D uses the Manning equation for flow resistance, and
does not provide a mechanism to vary the Manning coefficient with depth and discharge. In
some cases, such as relatively shallow flow over a coarse bed, the hydraulic roughness can vary
appreciably with depth. In these cases, other flow resistance equations that incorporate the
roughness height relative to the flow depth may be preferable. The program performs wetting
and drying by turning on and off elements. This approach is stable for the finite-volume method.
The model is publicly available, with no associated licensing cost.
3.2.3.2 ADH
The USACE ADH program was developed by the Coastal and Hydraulics Laboratory (Engineer
Research Development Center) to model saturated and unsaturated groundwater, overland flow,
3-D Navier-Stokes flow, and 2-D or 3-D shallow-water, open-channel flow conditions. ADH is a
depth-averaged, finite-element hydrodynamic model that has the ability to compute water-
surface elevations, horizontal velocity components and sediment-transport characteristics
(including simulations to predict aggradation and degradation) for subcritical and supercritical
free-surface flows in 2-D flow fields. The ADH mesh is composed of triangular elements with
corner nodes that represent the geometry of the modeled reach with the channel topography
represented by bed elevations assigned to each node in the mesh. A particular advantage of the
ADH mesh is the ability to increase the resolution of the mesh—and thereby the model
accuracy—by decreasing the size of the elements during a simulation in order to better predict
the hydraulic conditions in areas of high hydraulic variability. However, use of the adaptive
mesh option often results in excessively long simulation run times (several days per run) that
could be impractical for this study. The model uses either the Manning or roughness height flow
resistance equations. Additionally, the wetting and drying algorithm in this model has
significant numerical stability limitations when applied to shallow, near-shore flows that occur in
rivers like the Susitna River. The ADH model does not include either the Parker (1990) or
Wilcock and Crowe (2003) sediment-transport equations, which are favored for coarse bed
channels and simulating armoring processes. The model is publically available.
3.2.3.3 MD_SWMS/SToRM
The USGS’s MD_SWMS model (McDonald et al. 2005) is a pre- and post-processing
application for computational models of surface-water hydraulics. This system has recently been
incorporated into a public-domain software interface for river modeling distributed by the
International River Interface Cooperative (iRIC) (Nelson et al. 2010). iRIC is an informal
organization made up of academic faculty and government scientists whose goal is to develop,
distribute, and provide education for the software. iRIC consists of a graphical user interface
that allows the modeler to build and edit data sets, and provides a framework that links the
interface with a range of modeling applications. The graphical user interface is an interactive
1-D, 2-D, and 3-D tool that can be used to build and visualize all aspects of computational
surface-water applications, including grid building, development of boundary conditions,
simulation execution, and post-processing of the simulation results. The models that are
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FERC Project No. 14241 Page 24 June 2013
currently included in iRIC are FaSTMECH and SToRM, which are part of the MD-SWMS
package, as well as NAYS, MORPHO2D, and a Habitat Calculator for assessing fish habitat
under 2-D conditions. Of these models, SToRM is the most relevant for modeling the Susitna
River for purposes of this project, primarily because it uses an unstructured triangular mesh (in
contrast to the curvilinear mesh required for FaSTMECH) and provides both steady-flow and
unsteady-flow capability. NAYS is a fully unsteady, 2-D model designed for a general, non-
orthogonal coordinate system with sophisticated turbulence methods that can evaluate the
unsteady aspects of the turbulence, and MORPHO2D is 2-D model capable of analyzing the
interactions between sediment transport and vegetation and between surface water and
groundwater. Both NAYS and MORPHO2D were developed in Japan, and have not been widely
used or tested in the U.S. The SToRM model blends some of the features of finite volumes and
finite elements, and uses multi-dimensional streamline upwinding methods and a dynamic
wetting and drying algorithm that allows for the computation of flooding. Subcritical,
supercritical, and transcritical flow regimes (including hydraulic jumps) can be simulated. The
program includes advanced turbulence models and an automatic mesh refinement tool to better
predict the hydraulic conditions in areas of high hydraulic variability. The most recent version of
the SToRM model does not include the capability to model sediment-transport, but the program
authors are currently working on implementing sediment-transport algorithms that may be
available for use in this study (J. Nelson, pers. comm., 2012). MD_SWMS has been successfully
applied to a number of rivers in Alaska, including the Tanana River near Tok (Conaway and
Moran 2004) and the Copper River near Cordova (Brabets 1997); some of the modules are
currently being validated using high-resolution scour data from the Knik River near Palmer. This
modeling package is publicly available, with no associated licensing cost.
3.2.3.4 MIKE 21
Developed by DHI, MIKE 21 is a commercial modeling system for 2-D free-surface flows that
can be applied in rivers, lakes, coastal, and ocean environments. It has the ability to simulate
sediment transport and associated erosion and deposition patterns. The software includes a
Windows-based graphical user interface as well as pre- and post-processing modules for use in
data preparation and analysis of simulation results, and reporting modules that have graphical
presentation capabilities. MIKE 21 has the ability to model a range of 2-D mesh types that
include Single Grid, Multiple Grid, Flexible Mesh, and Curvilinear Grid. The MIKE-21 model
uses either Manning number (numerically similar to Manning n), or Chezy flow resistance
equations, but does not include roughness height. Wetting and drying are element-based with a
transitional condition where a layer of water is maintained on dry nodes until the element is fully
dry. The model does not include either the Parker (1990) or Wilcock and Crowe (2003)
sediment-transport equations, which are favored for coarse bed channels and simulating
armoring processes. MIKE-21 is commercially available with a relatively expensive licensing
cost compared to other available models.
3.2.3.5 River2D Modeling Suite
River2D is a 2-D, depth-averaged finite-element hydrodynamic model developed at the
University of Alberta and is publically available from the university with no associated licensing
cost. The River2D suite consists of four programs: R2D_Mesh, R2D_Bed, River2D, and
R2D_Ice, each of which contains a graphical user interface. The R2D_Mesh program is a pre-
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FERC Project No. 14241 Page 25 June 2013
processor that is used to develop the unstructured triangular mesh. R2D_Bed is used for editing
the bed topography data and R2D_Ice is used to develop the ice thickness topography at each
node for simulating ice-covered rivers. Following mesh development, the hydrodynamic
simulations are run using the River2D program, which also includes a post-processor for
visualizing the model output. River2D is a very robust model capable of simulating complex,
transcritical flow conditions using algorithms originally developed in the aerospace industry to
analyze the transitions between subsonic and supersonic conditions (transonic flow).
Many 2-D models become numerically unstable due to wetting and drying of elements; however,
River2D uniquely handles these conditions by changing the surface flow equations to
groundwater-like flow equations in these areas. The model computes a continuous free surface
with positive (above ground) and negative (below ground) water depths, which allows the
simulation to continue without changing or updating the boundary conditions, increasing model
stability. The transmissivity of the subsurface flow is essential for the wetting and drying
algorithm but can create surface-flow continuity issues. The model uses only roughness height
for flow resistance. For some conditions, such as in vegetated banklines or floodplains, the
Manning roughness equation is often preferable.
River2D also has the capability to assess fish habitat using the PHABSIM weighted-usable area
approach (Bovee 1982). Habitat suitability indices are input to the model and integrated with the
hydraulic output to compute a weighted useable area at each node in the model domain.
River2D Morphology (R2DM) is a depth-averaged, two-dimensional hydrodynamic-
morphological and gravel transport model developed at the University of British Columbia. The
model was developed based on the River2D program, and is capable of simulating flow
hydraulics and computing sediment transport for uni-size and mixed-size sediment using the
Wilcock-Crowe (2003) equation over the duration of a hydrograph. R2DM can be used to
evaluate the changes in grain-size distributions, including fractions of sand in sediment deposits
and on the bed surface. The number of size classes is set at 12 and the sediment sizes are preset
in the software ranging from 0.125 mm to 256 mm. The sediment-transport module has been
verified using experimental data, and was successfully applied to the Seymour River in North
Vancouver, British Columbia (Smiarowski 2010). River2D is available in the most recent
version of iRIC (version 2.0).
3.2.3.6 RiverFLO-2D
RiverFLO-2D is a commercial two-dimensional, depth-averaged finite-element hydrodynamic
and mobile-bed model developed by Hydronia LLC. The model uses triangulated mesh
(irregular grid) and efficient wetting and drying methods. The wetting and drying algorithm
includes partially wet elements by assigning nodes with positive depth as zero velocity. The
model used Manning equation to represent surface roughness. RiverFLO-2D is commercial at
moderate cost (approximately $5,000) including the SMS interface. The model includes eight
sediment-transport equations but does not include Parker (1990) or Wilcock and Crowe (2003).
The sediment is represented by a single (median) particle size except for the Van Rijn equation
which also includes D90. Without a particle size gradation or multiple layers, armoring processes
cannot be simulated. This is a significant shortcoming for evaluating potential Project effects.
This model is commercially available.
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FERC Project No. 14241 Page 26 June 2013
3.2.4 Selection of 2-D Model
Table 3-2 provides a summary of the 2-D models, their characteristics, and limitations. Four of
the six models can be eliminated based on the model selection criteria (Section 3.2.2). SToRM
was eliminated as it does not currently include sediment transport. ADH, MIKE 21, and
RiverFLO-2D were eliminated because they do not include either the Parker (1990) or Wilcock
and Crowe (2003) sediment-transport equations and RiverFLO-2D does not include sediment
gradations. Based on river conditions and project requirements, two of the models (SRH-2D and
River2D) are good candidates for sediment-transport anal yses related to the project. The SRH-
2D model includes both of the desired sediment-transport relationships and the River2D model
includes Wilcock and Crowe (2003). Other differences between the models include the method
for specifying flow resistance, approach for wetting and drying of elements, and limits on model
size (number of elements). SRH-2D only includes the Manning equation and River2D only
includes roughness height for estimating hydraulic roughness. Because there are situations
where either approach for flow resistance has advantages, this difference was not a deciding
factor.
Wetting and drying is a significant issue for 2-D modeling because model instability can be
significant when areas of the model are added or eliminated from the network as the water-level
changes. Experience with SHR-2D indicates that it performs very well for wetting and drying in
shallow areas along the margins of the channel. When the centroid of the element is dry, the
element is eliminated and it is reintroduced into the network when the centroid is rewetted.
River2D does not eliminate elements from the model, but converts nodes to a subsurface flow
controlled by the transmissivity and storativity. Without including transmissivity, the entire
mesh needs to be submerged. These parameters should be set such that the amount of flow
traveling below the surface is negligible, but can be adjusted to improve transient analysis. If
transmissivity is too high, surface-flow continuity could be a problem, especially for simulating
low flow conditions or when large portions of the network are “dry.”
Neither model has significant size limitations for hydraulic simulations; both can accommodate
more than 100,000 elements. SRH-2D is limited to approximately 16,000 elements for sediment
routing simulations, which may be a limitation for the Focus Areas. Significantly larger numbers
of elements will be included for habitat simulations, as needed. Although the number of
sediment size classes in SRH-2D is limited to eight, they are user specified so they can be
adjusted based on site conditions. River2D has 12 size classes, but are preset in the model and
sizes greater than 256 mm or less than 0.125 mm are not included. The fluvial geomorphology
team has considerable experience using the SRH-2D model for both hydrodynamic and mobile
boundary simulations. Other team members have experience using the ice and habitat
functionality of River2D. The SRH-2D model does not compute habitat suitability indices
directly, but the output can be readily used for that type of analysis with spreadsheet and GIS
tools, a procedure that has been used by the modeling team for many projects. Although
River2D’s groundwater approach for element wetting and drying is a concern given the potential
range of flows needed for sediment-transport analysis, it is not known if this will create
continuity problems for the specific simulations that are required for this study. River2D will be
one of the tools used for the ice processes modeling. This factor and the habitat functionality
that has been incorporated directly into the model represent advantages of using River2D for the
project.
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Because of the uncertainty in how the models will perform, the geomorphology modeling team
members recommend testing the SRH-2D and River2D models for sediment transport and
habitat analysis at one Focus Area to assess their capabilities and limitations with respect to the
characteristics of the Susitna River and the specific questions that must be answered by the
modeling. The primary criteria for making the final model selection will center on the ability of
the model to produce representative flow and sediment-transport results for existing conditions,
including flow continuity, comparisons to observed velocities and depths, overall flow
distribution, sediment-transport capacity, bed evolution, and armoring. Other criteria will
include ease of model development, limitations on model size and spatial resolution, execution
speed, and convenience performing post-run analyses. Since these models use essentially the
same basic types of data, the outcome from the proposed test will not affect the data collection
plan.
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4. MODEL APPLICATION
The selected models will be used to address hydraulics, sediment transport and morphology at
reach and local scales. Additional 1-D models will be used to provide input to these models.
This section describes the application of the individual models to address specific aspects of the
study and the interaction between the models.
The overall river will be simulated using a 1-D reach-scale model applied to existing and with-
project operation scenarios. The reach-scale model will extend from the Watana Dam site down
to Susitna Station and will include portions of the Talkeetna and Chulitna rivers as tributary
reaches. Local-scale modeling will be performed with 2-D models at each of the 10 Focus
Areas. Some Lower Susitna River tributaries will be modeled with 1-D models to support
habitat evaluation, but also to develop sediment input for the reach-scale model. In the Middle
Susitna River Segment, sediment input will be evaluated for tributaries at the Focus Areas to
evaluate delta formation, fish barrier analysis, and habitat analysis. The results from tributary
analyses at Focus Areas will be supplemented by analyzing sediment supply from other
tributaries. Not all tributaries will be modeled, but a sufficient range of tributary conditions will
be evaluated to develop sediment and flow input for the tributaries throughout the Middle and
Lower Susitna River Segments.
4.1 Models and Survey Extent
4.1.1 Reach-Scale Model
Figures 4-1 through 4-10 show the cross sections and channel network for the Susitna River 1-D
model that, as previously discussed, will extend from Susitna Station (Project River Mile [PRM]
29.9) to the Watana Dam site (PRM 187.1). The figures show four types of cross sections: (1)
cross sections shown in green were surveyed in 2012, (2) cross sections shown in yellow will be
surveyed in 2013, (3) lower priority cross sections in red that may be surveyed in 2014, and (4)
cross section locations on small secondary channels are contained in red circles. The yellow
lines depict the extent of the model cross sections that approximately encompasses the 100-year
floodplain. Only the below-water, in-channel areas (similar to the 2012 cross sections) will be
surveyed directly. The remaining extent will be developed using detailed LiDAR data. The
lower priority sections are not believed to be needed for model development because they are in
locations where cross section interpolation should be adequate based on having relatively smooth
transitions between up- and downstream surveyed sections. These 2013 sections were prioritized
to have sufficient information to develop a complete model recognizing the potential for a
shortened survey field season. Additional survey (2014) may be conducted if it is apparent that
more cross sections are required in the modeling.
The Terrascan (Tetra Tech customized software) ground classification algorithm will be used to
create the bare-earth model from the LiDAR data. The bare-earth class is developed from the set
of returns classified as ‘Only’ and ‘Last of Many’ by an iterative method. First, a rectangular
filter is passed over the “Unclassified” points and a set of local low points is selected to seed the
bare-earth class. Then all the unclassified points are compared to the triangulated surface defined
by the set of bare-earth points and those that are found be close enough to fall within a certain
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 30 June 2013
angle and distance of the surface are added to the bare-earth class (ASPRS Class 2). The process
is repeated with the expanded bare-earth class until the number of points being added to the bare-
earth class declines.
The figures also show the reaches (blue lines) and junctions (red dots). When flow splits around
islands or there are significant and distinct flow paths, the model will include separate reaches.
Junctions are required where flow and sediment splits or re-connects. The most complex area of
reaches occurs between the Yentna River confluence and PRM 44.5 (Figure 4-1). This area is
quite complex from a 1-D modeling perspective; thus, some simplification was required,
especially between PRMs 34 through 38.
Tributaries that are modeled only as point-source flow and sediment input do not require
junctions, but tributaries that are modeled as reaches do. Tributaries that will be modeled as
reaches are the Talkeetna and Chulitna rivers. The Yentna River will be modeled as a
flow/sediment input based on flow data from the upstream gage and sediment rating curves.
Based on the discussion at the June 14, 2012, Water Resources TWG meeting and review
comments and recommendations in the April 1, 2013, Study Plan Determination, the Three
Rivers Confluence area (Susitna, Talkeetna, and Chulitna confluence) can be adequately
modeled for purposes of this study with the 1-D sediment-transport model (Figure 4-11). Figure
4-11 shows the cross sections that are prioritized for survey in 2013. More cross sections will be
developed for the Chulitna River based on LiDAR data and some additional cross sections may
be surveyed in 2014 on the Talkeetna River, if necessary. The existing gage on the Talkeetna
River reach is at the upstream limit of the survey reach, and this will be the upstream extent of
the model.
The modeled reach of the Chulitna River will extend approximately 10 miles to a location of
channel narrowing. Model topography that will be developed from the LiDAR data will be
collected during low flow. The surveyed bathymetry from the lower Chulitna River will be used
to guide adjustments to the low flow channel to account for the missing topography below the
water surface in the LiDAR data. The LiDAR data will be used to estimate water-surface slope
(elevation drop over channel distance). Using slope, roughness, discharge, and flow top-width,
AEA will be able to estimate the flow area and average depth at each cross section. The channel
form from the surveyed cross sections will used to guide the development of the below water
channel shape. This approach is feasible because the LiDAR will be collected at low flow when
flow depths are shallow on this wide and braided section of the Chulitna River.
4.1.2 Local-Scale Focus Area Models
In addition to the large-scale 1-D model of the Middle and Lower Susitna River segments, local-
scale 2-D models will also be developed in the 10 Focus Areas located in the Middle River. The
10 focus areas are shown in Figures 4-5 through 4-10 and are designated by the downstream
PRM as FA104, FA113, FA115, FA128, FA138, FA141, FA144, FA151, FA173, and FA184.
Each of the Focus Areas is shown in detail in Figures 4-12 through 4-21. At each of the Focus
Areas, detailed survey (land and bathymetric) will be performed including the areas along the
water’s edge and below water. Some floodplain survey will also be performed, but the primary
topographic data source for the floodplain areas will be the LiDAR data (see Section 4.1.1 for
discussion of development of bare-earth model). The survey and LiDAR data will be used to
generate a detailed TIN surface-representation of the area that will be the basis for assigning
elevations to the nodes of the 2-D model networks.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 31 June 2013
The Riparian Instream Focus Areas (FAs) that are co-located with the geomorphology FAs 104,
115, 128, 138, and 173 extend into the vegetated islands and floodplain areas adjacent to the
channel (Figures 4-12 through 4-21). The 2-D hydraulic and sediment-transport model will
include main channel, secondary channels, sloughs, tributaries, islands and floodplains. The 2-D
model limits will extend up to one channel width up- and downstream from the Focus Area
limits to reduce boundary condition effects within the primary area of interest, though in many
cases the model limits will coincide with the FA limits. In most cases, surveyed cross sections
are located relatively close to the Focus Area limits and can be used to extend the models. At
FA104 (Figure 4-12), the 2-D model will be extend approximately one channel width up- and
downstream of the FA. At FA113 (Figure 4-13), the model downstream boundary will be
located at PRM 113.6. The upstream boundary of FA113 is coincident with the downstream
boundary of FA115 (Figure 4-14) and these FAs will probably be included in a single 2-D
model. If the combined model is too large, then two overlapping models will be developed to
provide coverage of the areas. The upstream boundary of FA115 is suitable as the upstream
model boundary.
Neither the upstream nor downstream boundary of FA128 (Figure 4-15) is ideal, but neither can
be improved significantly within a reasonable distance of the FA boundary. As a result, the
model boundaries will be located approximately one channel width up- and downstream to
reduce boundary condition effects.
The upstream boundary of FA138 (Figure 4-16) is suitable for 2-D modeling, but the
downstream boundary is not. The model boundary will, therefore, be moved downstream
approximately one channel width and rotated perpendicular to flow.
The downstream model boundary at FA142 (Figure 4-17) will be located approximately one
channel width downstream to move the boundary condition away from the mouth of Indian
River. The upstream boundary is suitable as a 2-D model boundary. Both the up- and
downstream model boundaries will be located approximately one channel width from the
boundaries at FA144 (Figure 4-18). The upstream boundary at FA151 (Figure 4-19) is close to
the mouth of Portage Creek, so extending the model boundary upstream will be considered,
though the variable river width at this location may also present additional challenges. The
downstream model boundary at FA151 will be located approximately one channel width from
the FA boundary. The downstream boundary of FA173 (Figure 4-20) is suitable for 2-D
modeling and the upstream model boundary will be moved approximately one-half channel
width upstream. Both of the boundaries at FA184 (Figure 4-21) are adequate for 2-D modeling,
and moving either could create modeling challenges because moving the downstream boundary
would put it into the Tsusena Creek delta and moving the upstream boundary would put it into a
widened area with channel bars.
Eight of the Focus Areas encompass 11 tributaries including: Portage Creek, Indian River, Skull
Creek, Gash Creek, Slash Creek, Whiskers Creek, and 5 unnamed tributaries (Figure 4-22, Table
4-1). Each of the tributary mouths and a nominal reach length will be included in the Focus Area
models to simulate delta processes and tributary bed response. The length of the reaches will be
determined in the field, but are anticipated to be between 0.2 and 0.7 miles. Approximately 5
cross sections will be surveyed in each tributary to develop sediment input rating curves to the
Focus Area models.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 32 June 2013
4.1.3 Other Tributary Models
In addition to the Focus Areas, HEC-RAS sediment-transport models will be developed for four
tributaries outside the Focus Areas in the Middle River and five additional tributaries in the
Lower River (three in 2013 and 2 in 2014). The Middle River tributaries will include short
reaches in the downstream portions of Tsusena Creek, Fog Creek, Gold Creek, and Lane Creek
to determine sediment inputs to the reach-scale model (see Figure 4-22). In the Lower River,
local-scale sediment-transport models will be developed for the mouth and approximately 1-mile
reaches of Trapper Creek, Birch Creek, Sheep Creek, Caswell Creek and Deshka River to
determine potential morphologic effects of with-Project scenarios and to provide sediment inputs
to the reach-scale model (Figure 4-23, Table 4-1).
4.2 Reach-Scale 1-D Modeling
4.2.1 Bed Evolution and Hydraulic Modeling
The 1-D model will be used to simulate bed evolution throughout the model domain for existing
conditions and with-Project operational scenarios. Inclusion of the Chulitna and Talkeetna rivers
as modeled reaches will allow direct evaluation of potential Project effects on hydraulic and
sediment-transport conditions in the lower portions of these tributaries. This approach provides a
more complete evaluation of potential Project effects on these tributaries than would be achieved
by treating them only as point-source water and sediment inputs. For example, morphological
characteristics of the Chulitna River indicate that this tributary reach is a potential sediment
source or sink; treating it as a modeled reach allows for simulation of sediment accumulation or
bed lowering. Similarly, the Talkeetna River as a modeled reach will allow simulation of
changes in both water-surface elevations and channel geometry. Flow and sediment input will
be based on the gaging station records and measurements for the Chulitna and Talkeetna rivers.
The 1-D modeling will compute bed and water-surface elevations, flow depths and velocities
throughout the modeled reaches of the mainstem Susitna River and tributaries under each
scenario. These data will provide input for evaluating habitat conditions, changes in lateral
habitat connectivity, and the potential for developing barriers at tributaries. The model will also
provide information to assess changes in bed material composition and effects on fine material
transport (wash load) and turbidity. The model will include the transition from an extremely low
sediment supply at the dam to the longitudinally increasing sediment loads through tributary
inputs and entrainment of existing bed material. The results of the 1-D modeling will be also be
used to evaluate changes in effective discharge, which, along with the flow-frequency analyses
and potential for changes in vegetation from the riparian instream flow study, will be used to
evaluate potential changes in channel width. The Riparian Instream Flow Study will provide
data on area and elevation for zones of woody vegetation recruitment. These elevations will be
correlated to flow recurrence, which in turn will be used to evaluate morphologic changes along
the margins of the main channel, secondary channels, side sloughs, upland sloughs, and
tributaries. Data from the Riparian Instream Flow Study, the Fish and Aquatics Instream Flow
Study analysis of changes in hydrology operational scenarios, and 1-D sediment-transport
analysis provide a basis for adjusting the channel width in the 1-D model throughout the
simulation.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 33 June 2013
As noted in Section 3.1.1 (Overview of 1-D Model Development) the sediment-transport model
will be calibrated based on available information. Ideally, the calibration would involve
simulating an extended time period where the channel morphologic change has been
documented. This possibility has been considered in this case because a survey was conducted
in the 1980s. The model would be run from the 1980s to present as a calibration step. However,
AEA is not taking this approach because the survey data from the 1980s is not deemed sufficient
to develop a 1980s model. AEA will, however, compare 1980s and current cross sections to
determine the amount of channel change for that approximate 30-year period.
Another useful tool for assessing variability and trends in channel morphology is specific gage
analyses at the long-term USGS gages. These plots are based on measurements of discharge and
water-surface elevation through time. At low flow the water surface will closely track bed
elevation change and the specific gage plot will identify trends and variability in bed elevation
through time. Although limited by the number of available cross sections, channel bed profiles
will also be used to assess the channel morphologic change since the 1980s. Although the
available information may not be sufficient for a precise calibration of the sediment-transport
model, it will be adequate to evaluate potential Project effects as these will be relative
comparisons of Existing conditions to the simulated operational scenarios.
It is anticipated that adjustments will be made to the channel width in the with-Project scenario
runs at up to five times during each simulation. Long-term width change (narrowing) is expected
to occur along the Susitna River based on reductions in ~2-year recurrence interval flows for the
with-Project scenarios. Using typical downstream hydraulic geometry relationships, bankfull
channel width is proportional to bankfull discharge to the 0.5 power (Leopold and Maddock
1953), though other values of the exponent have been proposed. For example Parker (2010)
indicates a power of 0.461 for gravel-bed rivers. A consistent flow frequency (Q2) or effective
discharge is used to predict the eventual equilibrium width for the new hydrologic regime.
With a target channel width determined for the new hydrologic regime, we will need to estimate
the rate of width change over the 50-year license period. The rate of width adjustment may be
greatest in the initial years after closure, so the time interval for simulating width change may be
shorter during the initial periods of the simulation and increase with time during the simulation.
The rate of width adjustment may also be limited by the supply of sediment available for
deposition in the channel margins. This potential limiting factor will be checked and if
necessary, the rate of width change modified. One approach for developing the width versus
time relationship is the application of rate law, which is an exponential decay function (Graf
1977, Wu et al. 2012). Rate law is not a process-based approach, but is a useful relationship for
describing geomorphic response due to a disturbance. We will coordinate with the other study
team members and agencies to agree on an amount of channel width change expected to occur
over the 50-year license period. For example, the Middle Susitna River Segment may be deemed
to reach 80 percent of the ultimate width change and the Lower Susitna River Segment may
reach 60 percent. The rate law equation would be used to determine the channel width for
intermediate time periods. These channel widths will be imposed on the reach-scale models
during the simulation and ultimately on the local-scale 2-D models for future (years-25 and -50)
conditions.
The reach-scale models will be run for a 50-year period, corresponding to the length of the
FERC license. This 50-year period will be selected from the 61-year record in coordination with
the Technical Team. AEA will incorporate at least one large, relatively rare (i.e., 50- to 100-year
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 34 June 2013
recurrence interval) event if one does not already occur in the selected model period. Although it
is likely (64% chance) that at flood exceeding the 50-year recurrence flow is contained within
any selected 50-year period, if that event is not present it could be included to account for
operational effects on this flood. Similarly, there is a 40% chance of a 100-year event occurring
in a 50-year period, so this event could be included for similar reasons. The final decision to
include such event(s) will be coordinated with the Technical Team.
The simulation time-steps in the 1-D sediment-transport model will vary with discharge to
optimize the balance between model stability and total simulation time. The length of the
discharge-dependent time-steps will be determined in the early phases of the modeling through a
series of tests with the initial, baseline model that will identify the maximum stable time-step for
each range of discharges using procedures similar to those spelled out in USACE (1992).
In addition to simulating a long term, continuous period of flows, it will also be possible to
include rare flood events associated with unusual climatic conditions or ice-jam breakup to
understand conditions that form or maintain the existing habitats and how those conditions may
be altered by the project. For these conditions, an appropriate time step will be determined on a
case-by-case basis. For example, a time step ranging from several hours to 1 day may be
appropriate for a flood event; however, the time step necessary to model breaking of an ice jam
may need to be on the order of minutes.
4.2.2 Large Woody Debris Effects
4.2.2.1 Large Woody Debris Mapping
The effects of LWD on sediment transport and bed evolution will be evaluated using data from
2012 aerial photography to define baseline conditions, and the amounts of LWD will be reduced
proportionally within each reach based on the changes in supply as a result of the Project.
The 2012 aerial photographs (1-foot pixel resolution) will be used as a base to digitize existing
LWD within the Geomorphic Feature classifications (Table 4-2). Portions of LWD features that
extend into the listed GeomFeat polygons in the Middle and Upper River (RM 99 to RM 260)
will be digitized, but LWD that is contained wholly within vegetated islands (VI), additional
open water (AOW) or background (BG) areas will not be digitized because these features will
have little or no effect on hydraulic conditions in the channel. All wood in the Middle and Upper
River (RM 99 to RM 260) will be digitized, but only a sub-sample of the wood in the Bar Island
Complex features in the Lower River will be digitized to obtain representative wood densities on
these mobile features.
Individual Pieces: LWD at least 25 feet long that are wholly within, or extend into, the
designated geomorphic features will be digitized as single segment line features from the root
wad or thickest end (start of line) to the thinnest end of the LWD (end of line). Digitizing will
take place at a 1:1000 scale within ArcMap. In log jams (see below), individual pieces that are
over 25 feet in length and are discernible will be digitized.
The following attributes will be assigned to each individual LWD feature:
RootWad (Y or N) – Is there a visible root wad, defined as visible thickened end, on the piece of
LWD? (This is a judgment call because the resolution of the photos is not always sufficient to be
definitive.)
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 35 June 2013
Jam (Y or N) – Is the LWD contained within a log jam, defined as three or more touching pieces
of visible/digitized LWD?
Local_Scr – Is the LWD definitively from a local (adjacent bank) source, generally determined
to be a local source if the LWD extends perpendicular to, or at an oblique angle from, the
vegetated bank into the flow (e.g., not parallel to the bank), or if the piece of large wood has the
majority of the branches intact, indicating that it was not transported very far from its source.
Channel Position – The channel position of the wood will be identified in the following
categories:
BJ – Bank Adjacent – adjacent to vegetated bank at the side of a channel
AB – Apex of Bar – at the apex of a bar
DB – Downstream end of Bar – at the downstream end of an unvegetated bar
SB – Side of a Bar – along the side or in the middle of an unvegetated bar
MDC – Middle of the Channel – within the wetted channel
HSC – Head of a Side Channel – spanning the head of a side channel
SPC – Span Channel – spanning a small channel at a location other than the head
of the channel
BG – Biogeomorphic, e.g., contained in beaver dams
Image Date – the date of the aerial photograph image that was used for digitizing
Length (ft) – length of the piece of LWD as calculated within ArcMap from length of line feature
Log Jams: In addition to single pieces, if there are large log jams that contain small, un-
differentiated pieces of wood, the area of these log jams will also be digitized as a polygon
feature. Single, distinguishable pieces of LWD within these polygons will also be digitized as
line features as described above. The following attributes will be recorded for log jam features:
RM_ID – Identifier coded as RM-XXX with XXX being sequential number in an upstream
direction
Channel Position – same as used for individual pieces of wood, described above
Image Date – the date of the aerial photograph image that was used for digitizing
Area (sq-ft) of the polygon that will be calculated with ArcMap
4.2.2.2 Large Woody Debris Modeling
One of the objectives of the geomorphology study (AEA 2012, Section 6.5.1.1) is to “Assess
Large Woody Debris Transport and recruitment, their influence on geomorphic forms and, in
conjunction with the Fluvial Geomorphology Modeling Study, effects related to the Project.”
The geomorphology study will evaluate large woody debris sources, loading, and transport in the
Susitna River. Loading from upstream and major tributaries will be evaluated for pre- and with-
Project scenarios. The fluvial geomorphology modeling study will also provide input on the
potential Project effects on large woody debris input. Bank erosion rates under the existing and
with-Project scenarios will be evaluated using the Bank Energy Index (BEI) (Mussetter et al.
1995; Mussetter and Harvey 1996), a semi-quantitative index of the total energy applied to the
channel banks. One- and 2-D modeling results will be used to compute the BEI values. The BEI
values will be correlated to existing bank erosion rates at specific locations. With-Project bank
erosion rates will be estimated using this correlation to estimate LWD recruitment.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 36 June 2013
At the reach-scale, large woody debris increases overall flow resistance, reduces velocity, and
reduces sediment transport (Smith et al. 1993, Shields and Grippel 1995; Assani and Petit 1995;
Buffington and Montgomery 1999). The cumulative drag force of debris in a particular reach
will be distributed over the reach by equating area-distributed drag force to the equivalent shear
stress to compute an incremental increase in flow resistance associated with the LWD (Hygelund
and Manga 2003). For existing conditions, the amount of debris, type of obstruction, size, and
other attributes will be used to evaluate the contribution of debris to total flow resistance. The
input flow resistance coefficients will then be modified in the Project-conditions models to
reflect changes in LWD due to the Project by proportioning the amounts of debris and the
resulting total flow resistance based on the altered LWD supply. Depending on the relative
LWD supply, effects on reach-average hydraulics may be negligible in some areas, but could be
significant in others. In general, LWD supply from upstream of the dam will be eliminated by
the Project, but LWD supplied from tributaries downstream from the dam will be unchanged. If
bank erosion rates decrease based on the BEI analysis, then this supply will also be reduced.
Existing debris characteristics, including size, height and frequency, will be evaluated both in the
field and by analyzing aerial photography, as described above. For large clusters and large
individual pieces, height and size can also be evaluated using the LiDAR data.
4.2.3 Ice Effects
As part of the ice processes study for the Susitna River, “predictive ice, hydrodynamic and
thermal modeling using River1D is planned for the Middle River between the proposed dam and
the Three River Confluence near Talkeetna” (Section 7.6.3.2, AEA 2012). Additional ice-
related, reach-scale modeling will be performed as part of the fluvial geomorphology modeling
study. It is tentatively assumed that the existing bed material is stable (i.e., below incipient
motion conditions) under ice conditions, due to reduced velocities and shear stresses associated
with low river flows and the ice cover. The validity of this assumption under both existing and
with-Project conditions will be tested by performing an incipient motion analysis using shear
stress results from the River1D modeling. Should the results indicate that substantial sediment
transport should occur at the reach scale, the 1-D model will be adjusted to incorporate
appropriate rates of sediment transport for ice covered conditions. AEA anticipates this could be
accomplished by extending the simulation period, and adjusting flow magnitudes and durations
of the 1-D modeling to account for sediment transported under these conditions.
1-D dynamic modeling will also be performed of ice jam breakup surges to develop inflow
hydrographs for 2-D dynamic models. The 1-D modeling will be performed using HEC-RAS
and will be similar to dam break simulations of the rapidly released water stored above the ice
jam. The 2-D simulations of ice jam surges are discussed in Section 4.3.3.
4.2.4 Summary of Reach-Scale Model Results
The reach-scale, 1-D modeling will provide a basis for assessing Project effects on the following
issues:
Existing status and potential Project effects with respect to dynamic equilibrium with respect to
sediment transport.
Changes in both the bed material (sand and coarser sizes) and wash (fine sediment) load under
with-project conditions.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 37 June 2013
Long-term balance between sediment supply and transport capacity, and the resulting
aggradation/degradation response of the system.
Changes in bed material mobility.
Changes in effective discharge.
Changes in flow distribution within multiple channel reaches.
Project-induced changes in supply and transport of finer sediments that influence turbidity.
Potential for changes in channel dimensions (i.e., width and depth) and channel pattern (i.e.,
braiding versus single-thread or multiple-thread with static islands).
Project-induced changes in river stage due to reach-scale changes in hydrology, bed profile,
channel dimensions, and hydraulic roughness.
Characterization of the types, amounts, and features of LWD both in terms of supply of LWD
and the overall effects of LWD on sediment transport and channel morphology.
Changes in ice cover effects on sediment mobilization.
Boundary conditions for the 2-D local-scale modeling.
Hydraulic parameters to calculate Bank Energy Index and estimate bank erosion rates.
4.3 Local-Scale 2-D Modeling
The 2-D hydraulic modeling will be performed at the Focus Areas to support habitat analysis
using steady flow simulation performed over the range of discharges that occur in the study
reach. Sediment-transport modeling is inherently unsteady because the sediment is routed
through the system and the bed deforms in response to changes in flow and sediment supply.
Boundary conditions for all of the 2-D model simulations, including downstream water-surface
elevations, and upstream flow and sediment supply, will be obtained from 1-D modeling results.
For open-water conditions the downstream flow-stage and upstream flow-sediment supply rating
curves will be developed from the 1-D sediment-transport models and upstream flow rates will
be from the 1-D open-water flow routing models. For ice cover conditions AEA will develop
upstream flow and downstream flow-stage boundary conditions from the River1D models
developed by the Ice Processes Study.
The flexible mesh formulation that is available in the SRH-2D and River2D models is ideal for
obtaining detailed results in areas of significant change or interest. Large channels require less
detail than small channels, and floodplains generally require the lowest resolution because the
topographic and hydraulic variability is often the lowest for floodplain areas. In developing the
models, the mesh will be refined, as necessary, to capture the effects of appreciable topographic
or flow resistance variability. Figure 4-24 shows an example of a fine mesh, though the level of
detail is varied throughout the model domain to provide greater mesh refinement in areas of
anticipated geometric or hydraulic change. An example of a coarse mesh is shown in Figure 4-
25. This level of mesh resolution may be required for 2-D morphology modeling to reduce
computer simulation time. For the 2-D sediment-transport models, the element sizes will be as
large as practicable, but with sufficient detail to represent variability in bathymetry, topography,
roughness, and bed composition.
Areas with significant habitat value will be identified by the aquatic habitat team members, and
these areas will be modeled at a level of mesh resolution sufficient to describe the variability in
hydraulic conditions that is necessary for the habitat analysis. Figure 4-26 is an example of the
mesh requirements of the 2-D hydraulic models used for aquatic habitat analysis. Based on input
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 38 June 2013
from the aquatic and riparian habitat analysis teams, it is anticipated that the mesh resolution of
approximately 2 m will be used in key areas of the habitat analysis. Element sizes of up to 10
m will be used for the main channel, and up to 30 m elements in floodplain areas. The element
sizes will transition smoothly between these ranges to maintain good mesh quality. These
general guidelines represent starting values for the spatial resolution that will be refined and
adjusted as necessary throughout the model development phase. The level of mesh refinement
will be determined first for hydraulic requirements and then the fine 2 m mesh will be used in
areas of detailed habitat analysis. The mesh examples in Figures 4-24 and 4-25 do not fully meet
the element size criteria outlined above because greater detail will be incorporated in the lateral
features and habitat areas.
4.3.1 Bed Evolution and Hydraulic Modeling
4.3.1.1 Morphology Modeling of Focus Areas
Due to the intensive computational requirements of 2-D sediment-transport modeling and the
potentially long execution times, it is not practical to run the 2-D Focus Area sediment-transport
models over a multi-year time-frame. These models will be run over a select set of three
seasonal hydrographs to assess river behavior during typical wet, average, and dry annual runoff
seasons, and warm or cool Pacific Decadal Oscillation (PDO). The specific seasonal
hydrographs will be selected by categorizing each year in the 61-year extended record and
selecting a representative year from each subset. The criteria for identifying the runoff
categories and the specific hydrographs from each category will be determined in coordination
with the other study teams and agencies. Because of the nature of the 2-D model formulation,
the time increment for 2-D mobile-boundary simulations is typically on the order of seconds to
insure model stability; however, results are reported at longer time intervals to limit output file
size.
Riparian vegetation plays a key role in the development of islands and lateral habitats, primarily
by protecting surfaces from erosion and promoting sediment deposition. Vegetation can also
contribute to channel narrowing by encroaching onto bars and islands, causing riverward growth
of banks through trapping of sediments. Conversely, changes in the flow regime and/or ice
processes can alter riparian vegetation patterns, including the extent, species composition and
age-classes, providing a feedback mechanism between the processes. As a result, the influence of
riparian vegetation on the morphology of the Susitna River is an important consideration in these
studies. The riparian instream flow and geomorphology studies will be closely coordinated
because of the interactions described above. The teams will develop an understanding of the
interactions between the processes that are responsible for creation and maintenance of the
islands and lateral habitats by coordinating their respective study approaches and integrating the
study results. Estimates of the ages of island and floodplain surfaces from the Riparian Instream
Flow Study, based on dendrochronology combined with the inundation results from the 2-D
modeling, will greatly facilitate this effort by helping to identify rates of sediment deposition and
reworking of these surfaces. The turnover analysis based on overlay of aerial photos from the
1950s, 1980s and current conditions will also provide quantification of existing lateral floodplain
accretion and erosion rates.
The 2-D morphology modeling will include the analyses of the existing channel geometry with
existing hydrology as a baseline for comparison. The existing channel geometry will then be
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FERC Project No. 14241 Page 39 June 2013
combined with the hydrology for the range of operational scenarios. These results will be used
to evaluate potential, initial Project effects on sediment transport and bed morphology, including
changes in bed composition and flow distribution to lateral channels. Runs will also be
performed using the projected 25- and 50-year channel geometry and with-Project hydrology.
The future channel geometry will include main channel bed level changes from the 1-D model.
Width change in the main channel that was incorporated in the 1-D modeling results will also be
included in the 2-D models. Changes in sediment supply and downstream boundary conditions
will be incorporated from the 1-D modeling. Vegetation encroachment and changes in lateral
features will also be incorporated into the 2-D morphology models. These models will be run for
the three seasonal hydrographs to be used in the development of the detailed hydraulic models.
4.3.1.2 Hydraulic Modeling for Habitat Analysis
Output from the 2-D hydraulic models will be provided to the habitat analysis teams in tabular
(either ASCII or spreadsheets, as appropriate) format for each flow condition. The output values
for the required hydraulic variables that include depth, velocity, and water-surface elevation, will
be provided at each node along with the associated geo-referenced horizontal coordinates and
elevations. Additional hydraulic parameters (such as Froude number and shear stress) can be
computed from the output variables. These will be selected in consultation with the Instream
Flow Habitat study and agencies. The range of flows will be determined in coordination with the
instream flow habitat study and can be adjusted, as necessary, during the modeling phase.
Habitat will be calculated at each node by combining the Habitat Suitability Criteria (HSC) for
each species and life stage with the hydraulic data. The method of calculation will depend on the
hydraulic model used for the simulations. If River 2D is the model selected for hydraulic
modeling, the habitat may be calculated directly, though GIS may still be used. If the SHR-2D
model is selected, a Geographic Information System or GIS-based approach will be used to
calculate habitat. There will be several steps required to convert the model output from
hydraulic data to habitat data.
The 2-D modeling will include steady-state analyses of either the existing channel or projected
future topography. Results for the existing channel topography will provide the baseline for
comparison of potential Project effects. Where appropriate, the existing-conditions topography
will be adjusted to represent the projected channel form at year-25 and at the end of the 50-year
license period, and the models will be re-run to provide a basis for assessing Project effects. The
future channel geometry in the 2-D models will include main channel bed level changes from the
1-D model. Predicted width changes in the main channel will also be incorporated, based on the
1-D model results and additional, off-line analyses that consider Project-related changes in the
effective discharge analysis, riparian vegetation, and sediment supply. Projected changes in
secondary channels and other lateral features will also be incorporated into the models where
appropriate. Through this process, model output will be provided for the habitat analysis for
existing conditions, initial, with-Project conditions associated with the change in hydrology, and
the projected longer-term (50-year) changes in channel morphology combined with operational
hydrology.
In summary, the geomorphic modeling will provide the following information:
Existing geometry, vegetation, and bed composition:
Steady-state simulations over a range of discharges, and
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 40 June 2013
Hydraulic variables including depth, velocity, and water-surface elevation tied to
geo-referenced horizontal coordinates (x, y) as input to the habitat analysis.
The same output as the previous bullet for with-project operational scenarios to represent interim
conditions after Project implementation, but before long-term channel adjustments.
The same output as the previous bullet based on Project future-conditions geometry, vegetation,
and bed composition for with-project operational scenarios to represent conditions at year-25 and
at the end of the 50-year Project license.
The future conditions 2-D models will be developed through a combination of the 1-D model
results, 2-D morphology modeling results, and coordination with the other study teams and the
agencies.
4.3.2 Large Woody Debris Effects
The data described in Section 4.2.2.1 will also be used to develop 2-D modeling scenarios to
assess the effects of changes in the amount and distribution of LWD on local hydraulic and
sediment-transport conditions. Projected changes in the size and location of large woody debris
will be simulated by adjusting mesh resolution, bed elevations, and the relative erodibility of the
affected area. If a large jam is likely to be removed or become smaller with time, the roughness
would be reduced and modeled bed elevation will be lowered. The hydraulic effects of large
woody debris can also be simulated by locally adjusting flow resistance without changing
elevation. In either case, erosion due to the acceleration of flow around the obstruction can be
simulated. This erosion is not a complete representation of the scour that can occur at large
woody debris accumulations because scour is also related to vertical flow, vortices, and
turbulence that are not simulated by the 2-D models. Where appropriate, the additional local
scour will be estimated using scour equations developed for other applications. For example, a
recently developed equation for abutment scour would be useful for evaluating scour around
large log jams. This equation, described in the Federal Highway Administration (FHWA) HEC-
18 bridge scour manual (Arneson et al. 2012), relates obstruction shape, bed material mobility,
unit discharge upstream, and unit discharge adjacent to the obstruction to potential scour depth.
The equation is conservative, as it is intended for design of bridge foundations, but the
magnitude of the conservatism can also be determined (Lagasse et al. in press).
4.3.3 Ice Effects
Ice processes influence both the channel morphology and riparian vegetation. For example, ice
can prevent vegetation from establishing on bars by annually shearing off or uprooting young
vegetation. Similarly, ice can scour vegetation from the banks, increasing their susceptibility to
erosion. Both of these influences can affect channel morphology. Ice jams can also directly
influence the channel morphology by diverting flows onto the floodplain where new channels
can form, particularly when the downstream water-surface elevations are low, allowing the
return flows to headcut back into the floodplain. Ice can also move bed material that would not
be mobilized under open-water conditions by rafting large cobbles and boulders.
The Geomorphology and Ice Processes studies will work together to identify the key physical
processes that interact between the two. A significant portion of the influences of ice processes
on morphology are directly related to their effects on riparian vegetation. The Geomorphology
and Ice Process studies will also coordinate with the Riparian Instream Flow study to identify
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FERC Project No. 14241 Page 41 June 2013
and interpret evidence of ice conditions such as ice scarring locations and elevations on trees.
Additional influences of ice processes beyond the riparian vegetation issues that will be
incorporated directly into the fluvial geomorphology modeling include:
• Simulation of the effects of surges from ice jam breakup on hydraulics, sediment
transport and erosive forces using unsteady-flow 2-D modeling with estimates of breach
hydrographs.
• Simulation of the effects of channel blockage by ice on the hydraulic and erosion
conditions resulting from diversion of flow onto islands and the floodplain.
• Use of the detailed 2-D model output to assess shear stress magnitudes and patterns in
vegetated areas, and the likelihood of removal or scouring.
• Use of the detailed 2-D model output to assess shear stress magnitudes and patterns in
unvegetated areas, and the likelihood of direct scour of the boundary materials.
• Application of the 2-D model to investigate whether ice jams are a significant contributor
to floodplain and island deposition as a result of ice jams inundating these features and
causing sedimentation.
The analyses of ice-affected morphologic change will rely on observations and information from
the Ice Processes Study, the Riparian Instream Flow Study, and geomorphology field work. The
results of 1-D and 2-D simulations, performed by the Ice Processes Study, will also be used. The
information to be developed for both existing and with-Project scenarios will include: (1) size,
location, and frequency of ice jams, (2) location, extent, and duration of bank attached ice, (3)
location, extent, and duration of ice blockage in main versus secondary channels, (4) model
output from 1-D and 2-D ice model simulations, (5) estimates of fine-sediment concentrations
during ice cover conditions, and (6) field observations of the impacts of ice movement or flow
diversion on floodplain areas. The types of analyses and specific conditions to be evaluated will
be coordinated with the other study teams and agencies as information from the 2013 field
season is evaluated.
Although sediment transport in ice-affected conditions is not fully understood, Ettema (2010)
indicates that bed-load and suspended load equations are able to represent these conditions and
that for a given discharge sediment transport potential is less than for open water conditions due
to reduced velocity and shear stress. He indicates that bed-load equations compared well with
measurements in ice-covered conditions and that expected increases in suspended bed-material
load transport, due to temperature effects on water density, viscosity, and particle fall velocity,
are supported by lab and field studies. Suspended load is often supply limited, so an increase in
this sediment-transport component may depend more on supply than on transport capacity. Ice
jams and breakup may exert significant impact on unregulated rivers (Ettema 2010) and where
water discharge fluctuates appreciably, such as during winter operation of hydropower dams, ice
cover formation and presence may also have significant effects (Ettema 2010; Zabilansky et al.
2002).
The 2-D modeling will be used to evaluate the effects of altered hydrology and ice conditions on
local erosion, mobilization, and sedimentation processes. The hydraulic results from the
River1D and River2D modeling performed as part of the ice processes study will be reviewed to
evaluate whether general mobilization of the bed is possible during ice-cover conditions.
Using ice jam breakup hydrographs, unsteady 2-D models will be used to evaluate the hydraulics
and sediment transport for these dynamic flow conditions. The hydrographs will be developed
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FERC Project No. 14241 Page 42 June 2013
with unsteady hydraulic routing using HEC-RAS modeling similar to dam-break modeling.
Blackburn and Hicks (2003) provide information on simulating ice jam breakup surges and
indicate that unsteady-flow, hydraulic routing is applicable to this type of simulation. The
location and height of blockages created by ice dams will be determined through coordination
with the Ice Processes and Riparian Instream Flow study teams. The unsteady dam break
capabilities of the HEC-RAS model will be used to simulate the release of ponded water
upstream of the ice dam to generate a flow hydrograph that will be input to the unsteady 2-D
models when the flood wave reaches the downstream Focus Area.
Ice cover typically increases active flow area and decreases velocity and bed-load transport.
Bank-attached ice can redistribute flow within a channel. As shown in Figure 4-27, in multiple
channel areas, partially or completely blocked channels can redistribute flow between channels.
Channel blockage can also divert flow onto islands and floodplains. Figures 4-28 through 4-34
show the locations and descriptions of ice jams in 2012 and in the 1980s. This type of
information will provide a basis for selecting the FAs where additional 2-D erosion modeling
will be performed to evaluate ice effects. We anticipate that three or four FAs will be selected.
Full blockage will be simulated using altered geometry and partial blockage will be modeled
with a combination of altered geometry and high flow resistance. These methods will be used to
evaluate erosion potential of vegetated and unvegetated areas with ice conditions. The specific
locations and characteristics of ice jams will be coordinated with the other study teams and
agencies.
Diversion of flow onto vegetated floodplain areas may also contribute to floodplain
sedimentation. Data from winter operation of gages and other field measurements or
observations will be used to estimate fine-material concentrations. Depending on the material
size and type of channel blockage, the fine-material diverted onto vegetated areas may be
trapped by vegetation and accumulate on the floodplain surface. The 2-D models will not
simulate the accumulation directly, but the results can be post-processed to estimate rates of
accumulation and floodplain accretion. This will be done by computing the unit discharge of
fine sediment (unit discharge of flow times the fine sediment concentration) delivered to
floodplain areas based on the proportion of Rouse-type suspended sediment profiles extending
above the top-of-bank. As vegetation will trap some of this sediment, rates of accretion can be
estimated. We will work with data collected by the Riparian Instream Flow Study team to
evaluate historical rates of sediment accretion to validate the above analytical approach. With-
Project accretion rates will be proportioned based on comparison of the expected overbank flows
and sediment concentrations with existing conditions flow and sediment concentrations.
4.3.4 Summary of Local-Scale Model Results
The local-scale, 2-D modeling will provide information on:
• Existing status and potential Project effects with respect to changing morphology in
lateral features.
• Detailed hydraulic input to habitat modeling for existing and with-Project conditions.
• Changes in bed material composition.
• Changes in bed material mobility.
• Changes in flow distribution.
• Project-induced changes in supply of finer sediments into floodplains.
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FERC Project No. 14241 Page 43 June 2013
• Project-induced changes in river stage on delta formation at tributary mouths.
• Effects of LWD on sediment transport and channel morphology.
• Changes in ice effects on sediment mobilization, lateral features, and floodplains.
• Hydraulic parameters to estimate Bank Energy Index and bank erosion rates.
4.4 Other Tributary Modeling
In the Lower River, short-reach sediment-transport models will be developed for the mouth area
and downstream approximately 1 mile of Trapper Creek (2013), Birch Creek (2013), Sheep
Creek (2014), Caswell Creek (2014) and Deshka River (2013) to determine potential
morphologic effects of with-Project scenarios and to provide sediment inputs to the reach-scale
models. The models will include portions of the main channel or side channels below the
tributary mouth to also evaluate change in bed elevation and water surface. The geometry of
confluence areas of these models will be developed by performing a sediment budget (sediment
continuity analysis) where excess sediment will be represented by expanding the height and/or
extent of the tributary delta into the receiving channel. The size of the tributary delta will be
adjusted until the long-term sediment-transport capacity of the receiving channel achieves an
equilibrium with the sediment supplied by the tributary.
One final set of tributaries models will be developed. These models will be for short reaches of
Tsusena Creek, Fog Creek, Gold Creek, and Lane Creek upstream of backwater influence from
the Susitna River to estimate sediment input for the 1-D bed evolution models.
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5. CONSULTATION DOCUMENTATION
The draft version of this document was made available for review on May 3, 2013 and the
material was presented at a Technical Team Meeting on May 21, 2013. The minutes from the
meeting were posted on June 4, 2013. The draft document, presentation, minutes, and comments
(Agency and public comments with responses) are all available on the Susitna-Watana public
website (www.susitna-watanahydro.org). The following additions, clarifications, and corrections
to the draft document have been made in this version based on the discussions at the May 21,
2013 meeting and comments received:
Section 1.1 Background
• Expanded discussion of dynamic equilibrium of the Susitna River
• Expanded discussion of connections and objectives of the geomorphology and
fluvial geomorphology modeling study components
• Introduction to simulation scales for reach- and local-scale models for main
channel, side channels, other lateral features, islands, floodplains, and aquatic
habitats
Section 1.2 Objectives
• Inclusion of objectives for the Three Rivers Confluence reach-scale modeling
Section 2.2 Comprehensive Modeling Approach
• Section added to discuss connections between the types of modeling that will
be conducted as part of the fluvial geomorphology modeling study
• New table added (Table 2.2) that illustrates model connections, input
requirements, and how results will be used by subsequent models
• Discussion of how morphologic change may need to be incorporated by other
study components
Section 3.1.3.2 SRH-1D
• Corrected the number of sediment size classes
Section 3.2.1 Overview of 2-D Model Development
• Clarification of how model refinement will be related to appreciable change in
velocity
Sections 3.2.3.1 SHR-1D, 3.2.3.5 River2D, and 3.2.4 Selection of 2-D Model
• Correction of number of sediment sizes in SRH-2D, River2D and related
discussion
Section 4.1.1 Reach Scale Model (survey)
• Identification of cross sections that are prioritized for survey in 2013
• Discussion of how LiDAR will be used to develop channel cross sections in
the Chulitna River
• Figures 4-1 through 4-11 updated to show 2013 high priority sections and low
priority sections
Section 4.2.1 Bed Morphology and Hydraulic Modeling (1-D)
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FERC Project No. 14241 Page 46 June 2013
• Discussion of how comparison cross sections, channel profiles, and specific
gage analyses will be used for model calibration
• Discussion of suitability of the 1980s survey data for morphology modeling
• Discussion of how changes in channel width will be simulated over the 50-
year license period
• Expanded discussion of potentially including 50- and/or 100-year events into
the simulation alternatives
Section 4.2.3 Ice Effects
• Expanded discussion of how ice effects could be included in 1-D morphology
model simulations
Section 4.3 Local-Scale 2-D Modeling
• Discussion of using 1-D model results to develop 2-D Focus Area boundary
conditions and channel geometries
• Expanded discussion of mesh refinement for aquatic habitat requirements and
inclusion of new figure (Figure 4-26) illustrating the habitat areas
Section 4.3.1 Bed Morphology and Hydraulic Modeling (2-D)
• Reordered sub-sections so the order matches the order of modeling tasks
• Included Pacific Decadal Oscillation as a hydrologic condition that will be
addressed
• Included year-25 as a time period for Focus Area morphology and hydraulic
modeling
Section 4.4 Other Tributary Modeling
• Expanded discussion of how some tributary deltas will be simulated with site-
specific 1-D models
Section 6 References
• Added references related to channel width change
Section 7 Tables
• Table 2.2; added to illustrate connections between types of morphology
models and results used by other study components
• Tables 3.1 and 3.2; corrected for number of sediment size classes in SRH-1D,
SRH-2D, and River2D
• Table 4.1; added note about which tributaries will have data collection in 2013
Section 8 Figures
• Updated Figures 4-1 through 4-11 to show prioritized cross section surveys in
2013
• Updated Figure 4-11 showing cross sections on the Chulitna River that will be
developed with LiDAR
• Updated Figures 4-12 through 4-21 to show habitat types at the Focus Areas
• Updated Figure 4-22 to show riparian focus areas based on current
information
• Included new figure (Figure 4-26) added to illustrate mesh detail requirements
for aquatic habitat analyses
• Figure numbers incremented for subsequent figures
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 47 June 2013
6. REFERENCES
AEA. 2012. Revised Study Plan. Susitna-Watana Hydroelectric Project FERC Project No.
14241. Alaska Energy Authority, Anchorage, Alaska.
Arneson, L.A., Zevenbergen, L.W., Lagasse, P.F., and Clopper, P.E. 2012. Evaluating Scour at
Bridges, Fifth Edition, Federal Highway Administration Report No. FHWA-HIF-12-003
HEC-18. 340 p.
Assani. A.A., Petit, F., 1995. Log-jam effects and bed-load mobility from experiments
conducted in a small gravel-bed forest ditch. Catena 25, 117-126.
Blackburn, J., Hicks, F., 2003. Suitability of Dynamic Modeling for Flood Forecasting during Ice
Jam Release Surge Events. Journal of Cold Regions Engineering, 17(1), March 2003.
ASCE.
Bovee, K.B., 1982. A guide to stream habitat analysis using the instream flow incremental
methodology. Instream Flow Information Paper No. 12. FWS/OBS-82/26. U.S. Fish and
Wildlife Service, Office of Biological Services, Fort Collins, Colorado.
Brabets, T.P, 1997, Geomorphology of the Lower Copper River, Alaska: U.S. Geological Survey
Professional Paper 1581, 89 p.
Buffington, J. M., and D. R. Montgomery. 1999. Effects of hydraulic roughness on surface
textures of gravel-bed rivers. Water Resources Research 35, 3507-3521.
Conaway, J.S., and Moran, E.H., 2004, Development and calibration of two-dimensional
hydrodynamic model of the Tanana River near Tok, Alaska: U.S. Geological Survey
Open-File Report 2004-1225, 22 p.
DHI. 2011a. Danish Hydraulic Institute, Water and Environment. MIKE 11, A Modeling
System for Rivers and Channels – User Guide.
DHI. 2011b. Danish Hydraulic Institute, Water and Environment. MIKE 21 Flow Model,
Hydrodynamic Module User Guide, 90p.
Ettema, R. 2010. Ice Effects on Sediment Transport in Rivers. Chapter 10 of Sedimentation
Engineering, 614-648, M.H. Garcia, Ed., American Society of Civil Engineers,
Graf, W.L. 1977. The rate law in fluvial geomorphology. American Journal of Science, 277 p
178-191.
Greimann, B. and Y. Lai, 2008. Two-Dimensional Total Sediment Load Model Equations,
ASCE Journal of the Hydraulics Division, 134(8): 1142–1146.
Huang, J.V. and Greimann, B.P., 2011. SRH-1D 2.8 User’s Manual, Sedimentation and River
Hydraulics – One Dimension, Version 2.8, U.S. Department of Interior, Bureau of
Reclamation, Technical Service Center, Sedimentation and River Hydraulics Group.
227p.
Hydronia. 2012. RiverFLO-2D V3, Two-Dimensional Finite-Element River Dynamics Model,
User’s Guide Release V3.0, Hydronia, LLC, Pembroke Pines, Florida, 39 p.
Hygelund, B. and Manga, M., 2003. Field measurements of drag coefficients for large woody
debris. Geomorphology, 51 (2003) 175-185
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 48 June 2013
Kwan, S. 2009. A Two Dimensional Hydrodynamic River Morphology and Gravel Transport
Model. University of British Columbia, Vancouver, CA, 113p.
Lagasse, P.F., Ghosn, M., Johnson,. P.A., Zevenbergen, L.W., and Clopper. P.E. 2013. Risk-
Based Approach for Bridge Scour Prediction, Draft Final Report, Transportation
Research Board, National Academy of Science. Washington D.C.
Lai, Y.G., 2008. SRH-2D version 2: Theory and User’s Manual, Sedimentation and River
Hydraulics – Two-Dimensional River Flow Modeling, U.S. Department of Interior,
Bureau of Reclamation, November, 113 p.
Leopold, L.B. and Maddock, T. Jr., 1953. Hydraulic Geometry of Stream Channels and some
Physiographic Implications. Professional Paper No. 252, USGS, 57 p.
McAnally, W.H. 1989. STUDH: A Two-Dimensional Numerical Model for Sediment Transport,
in Sediment Transport Modeling, Sam S. Y. Wang, Ed., ASCE, p. 659-664.
McDonald, R.R., Nelson, J.M., and Bennett, J.P., 2005, Multi-dimensional surface-water
modeling system user’s guide: U.S. Geological Survey Techniques and Methods, 6-B2,
136 p.
Mobile Boundary Hydraulics, 2010. Sedimentation in Stream Networks (HEC-6T) User Manual.
388 p.
Mussetter, R.A., Harvey, M.D. and Sing, E.F., 1995. Assessment of dam impacts on
downstream channel morphology. In Lecture Series, U.S. Committee on Large Dams,
San Francisco, California, May 13-18, pp. 283-298.
Mussetter, R.A. and Harvey, M.D., 1996. Geomorphic and hydraulic characteristics of the
Colorado River, Moab, Utah: Potential impacts on a uranium tailings disposal site. Proc.
Conference on Tailings and Mine Waste, '96, Colorado State University, January 16-19,
1996, Balkema, Rotterdam, pp. 405-414.
Nelson, Jonathon, 2012. Personal Communication. U.S. Geological Survey. June 18.
Nelson, J.M., Shimizu, Y., Takebayashi, H., and McDonald, R.R. 2010. The International River
Interface Cooperative: Public Domain Software for River Modeling. In the 2nd Joint
Federal Interagency Conference, Las Vegas, NV, June 27-July1, 2010.
Parker, G. 1990. Surface-based bedload transport relation for gravel rivers. Journal of Hydraulic
Research, 28(4), 417-436.
Parker, G. 2010. Transport of Gravel and Sediment Mixtures. Chapter 3 of Sedimentation
Engineering, 165-251, M.H. Garcia, Ed., American Society of Civil Engineers.
R2 Resource Consultants, Inc. 2013a. Selection of Focus Areas and Study Sites in the Middle
and Lower Susitna River for Instream Flow and Joint Resource Studies – 2013 and 2014,
Technical Memorandum for Alaska Energy Authority Susitna-Watana Hydroelectric
Project, FERC No. 14241.
R2 Resource Consultants, Inc. 2013b. Adjustments to Middle River Focus Areas, Technical
Memorandum for Alaska Energy Authority Susitna-Watana Hydroelectric Project, FERC
No. 14241.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
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FERC Project No. 14241 Page 49 June 2013
Ruark, M., Niemann, J., Greimann, B., and Arabi, 2011. Method for Assessing Impacts of
Parameter Uncertainty in Sediment Transport Modeling Applications, Journal of
Hydraulic Engineering, 137(6): 623–636.
Shields, F.D. and Gippel, C.J., 1995. Prediction of effects of woody debris removal on flow
resistance. Journal of Hydraulic Engineering 121, 341-354.
Smiarowski, A., 2010. The evaluation of a two-dimensional sediment transport and bed
morphology model based on the Seymour River. University of British Columbia,
Vancouver, CA. 111 p.
Smith, R.D., Sidle, R.C., Porter, P.E., Noel, F.R., 1993. Effects of experimental removal of
woody debris on the channel morphology of a forest, gravel-bed stream. Journal of
Hydrology 151, 153-178.
Steffler, P. and Blackburn, J. 2002. Two-Dimensional Depth Averaged Model for River
Hydrodynamics and Fish Habitat – Introduction to Depth Averaged Modeling and User’s
Manual, University of Alberta, CA, 120 p.
Tetra Tech Inc. 2012. Fluvial Geomorphology Modeling. Technical Memorandum for Alaska
Energy Authority Susitna-Watana Hydroelectric Project, FERC No. 14241.
U.S. Army Corps of Engineers (USACE), 1992. Guidelines for the Calibration and Application
of Computer Program HEC-6. Training Document No. 13, Hydrologic Engineering
Center, Davis, California
U.S. Army Corps of Engineers (USACE), 1993. HEC-6, Scour and Deposition in Rivers and
Reservoirs, User’s Manual, Hydrologic Engineering Center, Davis, California.
USACE, 2010. HEC-RAS, River Analysis System, User’s Manual, Version 4.1.0, Hydrologic
Engineering Center, Davis, California.
USACE, 2013. Adaptive Hydraulics User Manual Version 4.3. U.S. Army Corps of Engineers,
Engineering Research and Development Center.
Wilcock, P.R., and Crowe, J.C. 2003. Surface-based transport model for mixed-size sediment.
Journal of Hydraulic Engineering. 129(2), 120-128.
Wu, B., Zheng, S. and Thorne, C.R. 2012, A general framework for using the rate law to
simulate morphological response to disturbance in the fluvial system. Progress in
Physical Geography, 36(5), p. 575-597.
Zabilansky, L.J., Ettema, R, Wuebben, J., and Yankielun, N., 2002. Survey of River Ice
Influences on Channel Bathymetry Along the Fort Peck Reach of the Missouri River,
Winter 1998-1999. ERDC/CRREL TR-02-14, Cold Regions Research and Engineering
Laboratory, USACE, 151 p.
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7. TABLES
Table 2-1. 1-D versus 2-D model capabilities
Consideration 1-D Models 2-D Models
Sediment balance Reach-scale Local-scale
Aggradation/degradation response Reach-scale Local-scale
Changes in bed material gradation Reach-scale Local-scale
Sediment accumulation at slough mouths / localized deposition X
Bed material mobilization X X
Effective discharge X
Flushing of fines from side slough habitats X
Complex flows in floodplain and potential erosion X
Frequency and duration of overbank flooding Reach-scale Local-scale
Distribution of flow and flow patterns between channel features Larger side channels X
Distribution of flow between channel(s) and floodplains X X
Transverse hydraulic gradients X
Bed deformation (distributed aggradation and degradation) X
Ice effects on sediment mobilization and erosion Reach-scale Local-scale
Large woody debris effects Reach-scale Local-scale
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 52 June 2013
Table 2-2 Model input and results for various study aspects.
Modeling Task Input and Results Hydrology Sediment Hydraulics Channel & Floodplain Geometry
1-D Tributary
Sediment
Modeling
Input Range of steady flows Bed material from site samples Site-specific D/S stage-discharge Existing at T = 0 (yr-0)3
Results for: Results for range of steady flows to develop sediment rating curves at mouth of each tributary
1-/2-D Morph. trib. sediment rating curves
Aquatic Habitat V, D, WSE some trib. mouths barrier/delta change some tribs.
Other studies
1-D Reach-Scale
Morphology
Modeling
Input 50-yrs Existing & OS1 Existing & OS2 stage-discharge at Susitna Sta. Existing at T = 0 (yr-0)3
Results for: Results for continuous 50-year simulations throughout 1-D modeling domain
2-D Morphology U/S sed. rating curves at FAs D/S stage-discharge at FAs main channel change
1-D Ice stage-discharge at 3-Rivers main channel change4
Flow Routing main channel change4
Aquatic Habitat substrate change4 stage-discharge relationships main channel change4
Riparian Habitat sediment supply to overbanks stage-discharge relationships bar/island/floodplain change
2-D Local-Scale
Morphology
Modeling
Input <1-yr wet, avg., dry with
PDO, Existing & OS1
U/S sed. rating curves at FAs for
yrs-0,25,50 for Existing & OS5
D/S stage-discharge at FAs for
yrs-0,25,50 for Existing 3 OS5
Existing (yr-0)3, yrs-25,505
in main channel
Results for: Results for range of <1-yr simulations throughout FA modeling domain
2-D Hydraulic bed material gradation change4 lateral feature trends
2-D Ice lateral feature trends4
Flow Routing
Aquatic Habitat substrate change4 barrier/delta change
Riparian Habitat sediment supply to overbanks bar/island/floodplain change
2-D Local-Scale
Hydraulic
Modeling
Input Range of steady flows6 Bed material gradation change7 D/S stage-discharge at FAs for
yrs-0,25,50 for Existing & OS5
Existing (yr-0)3, yrs-25,50 main
channel5 and lateral features7
Results for: Results for range of steady flows throughout FA modeling domain
Ice, Flow Routing
Aquatic Habitat V, D, WSE, etc. throughout FAs
Riparian Habitat V, D, WSE, etc. throughout FAs
Notes: 1 From flow routing study. 2 From gage data, sediment transport study, and reservoir sedimentation study. 3 From hydrographic survey, land-based survey, and LiDAR. (Survey by Tetra Tech for tributaries.) 4 Only if magnitude of change is sufficiently large to warrant inclusion in other study aspects. 5 From 1-D Reach-Scale morphology models. 6 From habitat study requirements. 7 From 2-D Local-Scale morphology modeling trends.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 53 June 2013
Table 3-1. Evaluation of 1-D models
Model Characteristics and Evaluation Criteria Models
HEC-RAS SRH-1D MIKE 11 HEC-6T
General
Commercial/cost (if applicable) ○ ○ ● / $8,000 ● / $3,000
Full or quasi unsteady for sediment transport simulation Quasi Both Full Quasi
Ice for fixed bed ● ○ ○ ○
Ice for moveable bed ● ○ ○ ○
# of transport equations supported 7 13 10 18
Supports user defined transport equation ○ ○ ○ ●
Closed loop capability ○1 ● ● ●
Experience with model: High (H); Moderate (M); Low (L) H L M H
Model Size Limitations
# of cross sections NL NL NL 5,000
# of hydrograph ordinates 40,000 NL NL NL
# of sediment sizes 20 NL NL 20
Sediment Sizes Supported
Wash load (silts, clays) ● ● ● ●
Considers settling and resuspension ● ● ● ●
Sand ● ● ● ●
Gravel and cobble ●W ●P,W ● ●P,W
Notes: ● = Yes; ○ = No; NL = No Limit
P = Parker (1990), W = Wilcock & Crowe (2003) sediment transport relations 1 Not currently available, but in development.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 54 June 2013
Table 3-2. Evaluation of 2-D models
Model Characteristics and Evaluation Criteria
Model
SRH-2D ADH SToRM MIKE
21
River2D
(R2DM)
River
FLO-2D
General
Commercial/cost (if applicable) ○ ○ ○ ● / $20k ○ ● / $5k
Unsteady flow capability ● ● ● ● ● ●
Ice for fixed bed ○ ● ○ ● ● ○
Ice for moveable bed ○ ● ○ ● ● ○
Number of transport equations supported 4 2 ○1 10 2 8
Supports user defined transport equation ○ ● ○1 ● ○ ●
Relative execution speed: Fast (F), Moderate (M), Slow (S) M S F F M F
Model stability: High (H), Moderate (M), Low (L) H M M H H H
Experience with model: High (H), Moderate (M), Low (L) H M L L M L
Mesh Wetting and Drying Approach:
Element (E), Node/Partial (N), Sub-Surface (S) E N N E S N
Roughness Equation: Manning n (n), Chezy (C),
roughness height (Ks), drag coefficient (Cd, Cd α 1/C2) n n, Ks Cd, Ks n, C Ks n
Moveable boundary simulation ● ● ○1 ● ● ●
Grid Structure/Model Formulation
Finite element (FE)/Finite Volume (FV) FV FE FV/FE FV/FE FE FE
Grid structure: Flexible Mesh (FM) FM FM FM FM FM FM
Model Size Limitations
# of grid elements for sediment routing, (U = unlimited) 16k 2 U ○1,3 U >100k >100k
# of sediment sizes (NS = not specified in documentation) 8 NS ○1 NS 12 1
Sediment Sizes Supported
Wash load (silts, clays) ○ ● ○1 ● ○ ○
Considers settling ○ ● ○1 ● ○ ○
Sand ● ● ○1 ● ● ●
Gravel and cobble (P = Parker, W = Wilcock & Crowe) ● P,W ● ○1 ● ● W ●
Sediment Gradation/Multiple Layers/Armoring ● ● ○1 ● ● ○
Notes: ● = Yes; ○ = No; 1 Not currently available, but in development.
2 >100k elements for hydraulic modeling, 3 Unlimited for hydraulic modeling.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 55 June 2013
Table 4-1 Tributary modeling
Tributary Name PRM Entering Bank Geomorphic Reach Focus Area Sediment Input only 1-D or 2-D
Tsusena Creek 184.6 RB MR-2 X 1-D
Fog Creek 179.3 LB MR-2 X 1-D
Unnamed 174.3 LB MR-2 FA173 2-D
Unnamed 173.8 RB MR-2 FA173 2-D
Portage Creek 152.3 RB MR-5 FA151 2-D
Unnamed* 144.6 LB MR-6 FA144 2-D
Indian River* 142.1 RB MR-6 FA141 2-D
Gold Creek* 140.1 LB MR-6 X 1-D
Skull Creek* 128.1 LB MR-6 FA128 2-D
Lane Creek* 117.2 LB MR-7 X 1-D
Unnamed* 115.4 RB MR-7 FA115 2-D
Gash Creek* 115.0 LB MR-7 FA113 2-D
Slash Creek* 114.9 LB MR-7 FA113 2-D
Unnamed* 113.7 LB MR-7 FA113 2-D
Whiskers Creek* 105.1 RB MR-8 FA104 2-D
Trapper Creek* 94.5 RB LR-1 1-D
Birch Creek* 92.5 LB LR-1 1-D
Sheep Creek 69.5 LB LR-2 1-D
Caswell Creek 67.0 LB LR-2 1-D
Deshka River* 45.0 RB LR-3 1-D
*Tributaries that will be analyzed in 2013.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 56 June 2013
Table 4-2. Large woody debris digitizing within geomorphic features
Geomorphic Feature Code Description Lower River Middle River LWD Digitized?
MC Main Channel X X Yes
EXP MC Exposed Substrate Main Channel X Yes
SC Side Channel X X Yes
EXP SC Exposed Substrate Side Channel X Yes
SCC Side Channel Complex X Yes
BIC Bar Island Complex X Sub-sample1
BAB Bar/Attached Bar X
SS Side Slough X X Yes
EXP SS Exposed Substrate Side Slough X Yes
US Upland Slough X X Yes
EXP US Exposed Substrate Upland Slough X Yes
TR Tributary X X Yes
EXP TR Exposed Substrate Tributary X Yes
TD Tributary Delta X Yes
TM, MCTM,
SCTM, TRTM
Tributary Mouth (Main Channel, Side Channel,
Tributary)
X Yes
VI Vegetated Island X X No
AOW Additional Open Water X X No
BG Background X X No
1 Due to the high number of pieces of large wood on the Bar Island Complex features in the lower river, the large area of Bar
Island Complex, and the likely transient nature of the wood here, these areas will be sub-sampled to obtain a density of large
wood and log jams. The density of wood features will be apportioned over the total area of Bar Island Complex to estimate
total wood loading.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 57 June 2013
8. FIGURES
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 58 June 2013
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TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 59 June 2013
Figure 4-1. Survey and Model Cross Sections from PRM 30 to PRM 47.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 60 June 2013
Figure 4-2. Survey and Model Cross Sections from PRM 45 to PRM 66.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 61 June 2013
Figure 4-3. Survey and Model Cross Sections from PRM 63 to PRM 81.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 62 June 2013
Figure 4-4. Survey and Model Cross Sections from PRM 79 to PRM 99.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 63 June 2013
Figure 4-5. Survey and Model Cross Sections from PRM 98 to PRM 116.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 64 June 2013
Figure 4-6. Survey and Model Cross Sections from PRM 114 to PRM 131.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 65 June 2013
Figure 4-7. Survey and Model Cross Sections from PRM 131 to PRM 148.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 66 June 2013
Figure 4-8. Survey and Model Cross Sections from PRM 142 to PRM 154.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 67 June 2013
Figure 4-9. Survey and Model Cross Sections from PRM 166 to PRM 179.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 68 June 2013
Figure 4-10. Survey and Model Cross Sections from PRM 176 to PRM 188.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 69 June 2013
Figure 4-11. Proposed Cross Section Layout for Chulitna and Talkeetna Rivers, 1-D Model Tributary Reaches.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 70 June 2013
Figure 4-12. FA104 (From R2 Resource Consultants Inc. 2013b).
Figure 4-13. FA113 (From R2 Resource Consultants Inc. 2013b).
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 71 June 2013
Figure 4-14. FA115 (From R2 Resource Consultants Inc. 2013b).
Figure 4-15. FA128 (From R2 Resource Consultants Inc. 2013b).
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 72 June 2013
Figure 4-16. FA138 (From R2 Resource Consultants Inc. 2013b).
Figure 4-17. FA141 (From R2 Resource Consultants Inc. 2013b).
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 73 June 2013
Figure 4-18. FA144 (From R2 Resource Consultants Inc. 2013b).
Figure 4-19. FA151 (From R2 Resource Consultants Inc. 2013b).
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 74 June 2013
Figure 4-20. FA173 (From R2 Resource Consultants Inc. 2013b).
Figure 4-21. FA184 (From R2 Resource Consultants Inc. 2013b).
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 75 June 2013
Figure 4-22. Middle River Tributary Locations Relative to Geomorphic Reach and Focus Areas.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 76 June 2013
Figure 4-23. Lower River Tributary Locations Relative to Geomorphic Reach.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 77 June 2013
Figure 4-24. Example of Fine Mesh Applied in Whiskers Slough Focus Area.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 78 June 2013
Figure 4-25. Example of Coarse Mesh Applied in Whiskers Slough Focus Area.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 79 June 2013
Figure 4-26. Examples of mesh detail for habitat analysis requirements.
Figure 4-27. Example of Ice Blockage Altering Flow Distribution in Multiple Channel Reaches (Zabilansky et al. 2003).
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 80 June 2013
Figure 4-28. Ice Jam Locations at FA104.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 81 June 2013
Figure 4-29. Ice Jam Locations at FA113.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 82 June 2013
Figure 4-30. Ice Jam Locations at FA115.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 83 June 2013
Figure 4-31. Ice Jam Locations at FA128.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 84 June 2013
Figure 4-32. Ice Jam Locations at FA138.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 85 June 2013
Figure 4-33. Ice Jam Locations at FA141.
TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 86 June 2013
Figure 4-34. Ice Jam Locations at FA144.
Attachment B
Summary of Consultation on Geomorphology Study Plans
1
Table 6.4-1. Summary of Consultation on Geomorphology Study Plans
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NMFS General Comments
1 Transmittal
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5/18/2013 Eric
Rothwell
NMFS Although these comments are directed at fluvial
geomorphology, there is a tremendous amount of
overlap with what is really a common technical
platform representing the riverine physical
environment – how patterns of water and sediment
movement over time play out to produce spatial and
temporal combinations of hydraulic, topographic, and
biotic conditions.
The model selection portion is well considered and
documented for this stage in the modeling process.
However, it is unclear if mobile bed models will be
used independently for geomorphic analysis or if they
are going to be integrated in the instream flow habitat
analysis too?
If the mobile bed models will be used in calculating
WUA, how will differing project operation scenarios be
assessed? Mobile bed modeling requires every
hydrologic iteration to be modeled since every flow
has the potential to alter the channel compared to
fixed bed models where a single bed geometry in used
for all flow scenarios.
Mobile-bed (morphology) modeling will not be
used to provide hydraulic input to habitat models,
which will be steady-state models run for a range
of flows. The mobile-bed results will be used in
the development of the hydraulic (habitat)
models. See Fluvial Geomorphology Modeling
Approach Technical Memorandum (TM) at
Sections 2.2 and 4.3.
2 Transmittal
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(attachment)
5/18/2013 Eric
Rothwell
NMFS By utilizing a fixed bed model (for a given channel
geometry at a given point in time) over a range of flow
conditions (ie. low flow to max flow), any number of
hydrologic scenarios can be analyzed without having
to re-run the model to calculate associated hydraulic
conditions and resulting habitat suitability.
AEA will use the approach as outlined in the TM
at Section 4.3.1.2.
2
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3 Transmittal
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5/18/2013 Eric
Rothwell
NMFS There are certainly some complex and uncertain steps
in the overall modeling approach – e.g., integrating
variation and simulation scales ranging from sub-
meter to 100s of kms and potentially seconds and
hours to decades; readjusting and analyzing channel
dimensions, vegetated zones, representing altered
topography in 1-d and 2-d; mechanical ice breakups;
and ice behavior with intra-daily load-following.
AEA agrees that there are many complex
technical and coordination aspects to the Project.
Ice behavior will be evaluated by the Ice
Processes Study and this study will coordinate
with that study on modeling of morphologic
changes related to a range of ice conditions and
potential for feedback between the ice and fluvial
geomorphology modeling studies. See TM at
Section 4.3.3. AEA will also coordinate with the
Instream Flow Riparian Study on adjusting
vegetation zones related to morphologic change
See TM at Section 4.3.3.
4 Transmittal
Email
(attachment)
5/18/2013 Eric
Rothwell
NMFS Nonetheless, it is a very professional, rational, and
well-considered approach. Given our comments
provided below, we don’t see a better way to proceed
without having results from the current and ongoing
field work and analysis that are being conducted.
Pure validation with truly independent and replicated
data sets is elusive if not impossible in this type of
situation. However, there are a couple of activities
that might provide some corroborating evidence.
Thank you for the observations. AEA agrees
that the field work and early stages of analysis
will provide useful insights that will necessitate
making adjustments to the approach. AEA also
agrees that true validation will be difficult to
achieve. For further information regarding
calibration and validation, see Revised Study
Plan (RSP) Section 6.6.4..
4.1 Transmittal
Email
(attachment)
5/18/2013 Eric
Rothwell
NMFS One is to use the historical depiction of channel
locations, dimensions, and meta-habitat distribution
described in the “Mapping of Geomorphic Features…”,
FERC 14241, T2012 Tech. Memo to edit the current
topography. Then, re-run historical flows with that
estimated historical topography to see if it matches the
pattern of changes producing current topography.
As part of the Fluvial Geomorphology Study,
AEA will use available data from the 1950s,
1980s, and present to evaluate changes in
channel form, width, vegetation using cross
sections, profiles, aerial photos (dating back to
the 1950s), and gage data. AEA does not
anticipate running models that represent 1980s
to present based on concerns of data adequacy.
See TM at Section 3 and Section 4.2.1.
3
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4.2 Transmittal
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5/18/2013 Eric
Rothwell
NMFS Another is to compare simulated stages of mechanical
ice-break surges with maps of ice scar elevations.
Determining elevation (+/- ~ 15 cm) and year of ice
scars is a relatively accurate and inexpensive
dendrochronology task (e.g., compared to year and
elevation of establishment).
AEA will coordinate with the Riparian Instream
Flow Study, Study 8.6, to identify ice scarring in
relation to ice-breakup surges and incorporate
this information into the ice-surge modeling that
will be part of the Fluvial Geomorphic Modeling
Study. See TM at Section 4.3.3.
NMFS Specific Comments
5 Transmittal
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5/18/2013 Eric
Rothwell
NMFS Page 8: “The 1-D model will provide the boundary
conditions for the 2-D model in the Focus Areas,
including starting water surface elevations and
upstream sediment supply.”
How will potential changes in channel geometry due to
aggradation/degradation and associated WSE from
the 1D model output be accounted for in providing
boundary conditions to the focus area when the 2D
model is using pre-project topography?
The 1-D model will be run for a range of flows
that can be used to develop rating curves at the
downstream boundaries of the Focus Area 2-D
models. Since the Focus Area models will be
run for less than one year, we anticipate that the
rating curve will be relatively stable for the <1-yr
periods, but this hypothesis will be tested with
the 1-D model results. If the rating curve shifts
over the time span of the 2-D analysis other
adjustment will be made, such as segmenting
the time span of the 2-D into shorter durations.
AEA anticipates using different rating curves for
the year-0, -25, and -50 2-D models. Similarly,
AEA will use the sediment transport results from
the 1-D modeling to provide sediment supply
input for the 2-D models. As with the
downstream rating curves, the sediment supply
rates are also likely to be different for years 0, 25
and 50. TM at Section 3 and Table 2-2.
4
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6 Transmittal
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5/18/2013 Eric
Rothwell
NMFS Page 8: “The 2-D model at the Focus Areas will
provide an understanding of the hydraulic conditions
and sediment transport processes that contribute to
formation of individual habitat types.”
Will the formation of the individual habitat types (new
bed geometry) be used in the habitat modeling
analysis? i.e. will output from mobile bed 2D model be
used to generate “future” channel geometry that would
then be used to conduct fixed bed 2D hydraulic/habitat
modeling.
Yes. The detailed 2-D hydraulic models that
provide input to the habitat models will be run for
year-0, -25, and -50 conditions that include new
bed geometry and potentially new substrate.
See TM at Section 4.3.1.2.
5
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7 Transmittal
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5/18/2013 Eric
Rothwell
NMFS Page 11 - “Calibrate and validate the sediment
transport model.
Adjust sediment input values, bed layer properties,
sediment transport time step (within reasonable limits)
to calibrate the hydraulic results using available data
including:
Gage station measurements sediment loads, specific
gage plots, flow area, width, depth, and velocity
measurements,
Comparison cross sections, and Longitudinal profiles.”
While the input values can be adjusted to match
current gage station measurements as listed in the
first bullet, it seems like the calibration of the 1D
mobile bed model for the second and third bullets
should be conducted utilizing the 1980s cross-
sectional data. This type of comparison is needed in
order to determine if the model is able to properly
simulate the change in channel
aggradation/degradation or total volume of sediment
transport over the past 30 year time period. Then the
model could be used to estimate future channel
change and sediment volumes throughout the project
extent.
AEA did not incorporate the requested change
because AEA is concerned that the 1980s cross-
section data will not be sufficient to achieve
reliable results. AEA’s hypothesis is that the
Susitna River and its tributaries are largely
unaffected by development, water usage,
sediment control, or flood control, and that
reaches will be very close to dynamic equilibrium
or somewhat constrained and non-alluvial. AEA
will test this hypothesis by comparing 1980s and
present cross sections, specific gage plots, and
with other factors. See TM Section 3 and Section
4.2.1.
6
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8 Transmittal
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5/18/2013 Eric
Rothwell
NMFS Page 16 – “Refine the network in areas of significant
change or areas of significant habitat interest.”
Identify how ‘significant change’ and ‘areas of
significant habitat interest’ will be defined.
By significant change, AEA means significant
geometric or hydraulic change. Where the
geometry changes rapidly there must be
additional refinement of the network. Even in the
absence of geometric change, there can be
significant change in velocity magnitude or
direction. This can occur due to varying flow
resistance from vegetation. In addition to the
hydraulic requirements, AEA will include a
refined network in areas of significant habitat
interest identified by the Instream Flow Habitat
Study. Other areas that could affect downstream
habitat, such as the heads of side sloughs, will
also be modeled with additional network
refinement. AEA will not simply develop the
model and run it for final results. AEA will run
the model and review the output to determine if
additional refinement is needed. See TM at
Section 3.2.1 (Develop Model Network bullet –
Review Mesh Quality sub-bullet) for a description
of the Model Approach and refinement.
7
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9 Transmittal
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5/18/2013 Eric
Rothwell
NMFS Page 16: How will LWD be represented in River 2D?
How will flow both under and through LWD piles be
represented and measured?
The 2-D modeling does not include vertical
velocity (3-D) so flow under Large Wood Debris
(LWD) cannot be directly represented, nor would
it be safe to attempt to measure it. Although flow
under individual logs is likely due to local scour,
flow under or through LWD jams is anticipated to
be relatively minor compared to the redirection of
flow around the jam because the open spaces
are filled with smaller limbs and sediment.
Therefore, AEA anticipates including debris jams
in the model geometry. If AEA finds a situation
where flow through a debris jam needs to be
analyzed, AEA can represent the physical
obstruction with an extremely high Manning n
value that diverts flow around the low-
conveyance “obstruction,” but would still allow
some flow through the area. AEA would adjust
the Manning n value to achieve the desired
through-flow, but determining the desired amount
would be difficult. See TM at Section 3.2.3.5 for
description of the processes of the River2D
Modeling Suite. See TM at Section 4.3.2 and
RSP Section 6.6.4.1.2.7 for a description of
modeling Large Woody Debris.
10 Transmittal
Email
(attachment)
5/18/2013 Eric
Rothwell
NMFS Page 16: “Develop inflow hydrographs and sediment
inflows for existing and with-project conditions”
How many and what range of flows will used to
develop inflow hydrographs and sediment transport
models?
The range of flows will include the complete 50-
year flow record as analyzed in the 1-D models
and will provide input to the selected seasonal
flow hydrographs and sediment inputs for the 2-
D sediment transport models. See TM at Table
2-2.
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11 Transmittal
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(attachment)
5/18/2013 Eric
Rothwell
NMFS Page 17: “Calibrate and validate the sediment
transport model.
Adjust sediment input values, bed layer properties,
sediment transport time step (within reasonable limits)
to calibrate the hydraulic results using available data
including:
Main channel bed level changes observed in the 1-D
modeling,
Comparison cross sections, and Longitudinal profiles.”
It doesn’t seem clear what data will be used to
calibrate the 2D mobile bed model. Where will the
cross section comparisons and longitudinal profile
data come from? Will this be generated from 1980s
data or how will it be determine if the model is able to
properly simulate the change in channel geometry
over time?
The cross sections and longitudinal profiles will
come from the 1980s and current data and will
primarily be used to calibrate the 1-D sediment
transport model. The 2-D model results will need
to be consistent with the 1-D model, at least in
the main channel and major secondary channels.
1980s cross sections within Focus Areas will
also be used directly to compare with 2-D model
results. Flow resistance input values, measured
water-surface elevations and measured
velocities will be used to calibrate the 2-D mobile
bed model. See TM at Section 3.2.1 and RSP
Section 6.6.4.1.2.5 (description of model
calibration and validation).
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11.1 Transmittal
Email
(attachment)
5/18/2013 Eric
Rothwell
NMFS What flows will be used to test the models and what
amount of deviation from field measured conditions
will be deemed suitable?
AEA will target deviations in bed elevation in the
1- and 2-D models that are commensurate with
the amounts of deviation seen in the comparison
cross sections from the 1980s and present and
in the specific gage analysis. AEA anticipates
that the variability will also change from one
Focus Area to another due to tributary
interactions and other constraints. Until the
comparisons are made AEA cannot specify exact
amounts. See RSP Section 6.6.4.1.2.5
(guidelines for calibration is reasonable) and TM
Table 2-2 (list of model input parameters,
including flows).
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12 Transmittal
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(attachment)
5/18/2013 Eric
Rothwell
NMFS Page 26: “The modeled reach of the Chulitna River
will extend approximately 10 miles to a location of
channel narrowing. Model topography for the
upstream 6 miles of this reach will be developed from
the LiDAR data that will be collected during low flow.
The surveyed bathymetry from the lower 4 miles of the
Chulitna River will be used to guide adjustments to the
low flow channel to account for the missing
topography below the water surface.”
What algorithms will be used to create the bare-earth
model from the LiDAR data?
AEA will use a customized Terrascan ground
classification algorithm to create the bare-earth
model from the LiDAR data. The bare-earth class
is developed from the set of returns classified as
“Only” and “Last of Many” by an iterative method.
First, a rectangular filter is passed over the
“Unclassified” points and a set of local low points
is selected to seed the bare-earth class. Then all
the unclassified points are compared to the
triangulated surface defined by the set of bare-
earth points and those that are found be close
enough to fall within a certain angle and distance
of the surface are added to the bare-earth class
(ASPRS Class 2). The process is repeated with
the expanded bare-earth class until the number
of points being added to the bare-earth class
declines. In response to this comment, additional
detail has been added to the TM at Section
4.1.1.
12.1 Transmittal
Email
(attachment)
5/18/2013 Eric
Rothwell
NMFS How will the topography be determined under water,
conifers and dense trees from the LiDAR data?
Hydrographic survey will be used for the
underwater portions of surveyed cross sections.
See TM at Table 2.2 for model inputs that will be
developed from hydrographic surveys. LiDAR
will not be used directly for under water
topography, but as described in the response to
comment 12.2, below water portions of some
cross sections will be developed for the Chulitna
River. For the comment on vegetation, see
AEA’s response to Comment 12.
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12.2 Transmittal
Email
(attachment)
5/18/2013 Eric
Rothwell
NMFS What portion (percentage) of the channel is under
water during low flow and how will the underwater
portion of the channel be constructed? i.e. dropping
the edge of water points by a pre-determined amount
(normal depth for the flow in the river at the time of the
LiDAR data collect) and then connecting them with a
slightly lower point in the middle of the low flow
channel.
AEA will target a low flow to have the least
amount of wetted area. As described in the TM
at Section 4.1.1, AEA will develop additional
cross sections on the Chulitna River outside the
area of hydrographic survey using the surveyed
cross sections as a guide and infer the flow area
based on normal depth calculations and the
observed water surface slope. The channel bed
will be developed for the below-water areas
using a shape that is consistent with the
surveyed cross sections.
13 Transmittal
Email
(attachment)
5/18/2013 Eric
Rothwell
NMFS Page 27: “Each of the tributary mouths and a nominal
reach length will be included in the Focus Area models
to simulate delta processes and tributary bed
response. The length of the reaches will be
determined in the field, but are anticipated to be
between 0.2 and 0.7 miles. Approximately 5 cross
sections will be surveyed in each tributary to develop
sediment input rating curves to the Focus Area
models.”
It is difficult to say if 5 cross sections would provide
enough detail in these tributaries based on various
lengths. It would better to identify the number of cross
sections as a function of channel/trib width (ie. XSs
will be spaced at a minimum of X-channel widths apart
and no more than X-channel widths).
There may be more or fewer cross sections
depending on local conditions (channel slope,
geometric variability, and controls). The number
and locations will be determined in the field to
best represent sediment transport and supply.
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14 Transmittal
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(attachment)
5/18/2013 Eric
Rothwell
NMFS Page 28: “Data from the Riparian Instream Flow
Study, the Fish and Aquatics Instream Flow Study
analysis of changes in hydrology operational
scenarios, and 1-D sediment transport analysis
provide a basis for adjusting the channel width in the
1-D model throughout the simulation. It is anticipated
that adjustments will be made to the width in the with-
Project scenario runs at up to five times during each
simulation.”
What criteria will be used to determine when/how/why
channel width adjustments are needed and how will
that ultimately change the results of the modeling?
Channel width is anticipated to change due to
changes in hydrologic regime (~2-yr recurrence
interval flows, effective discharge) and will be
influenced by sediment supply and riparian
vegetation. AEA will review hydraulic geometry
for current conditions as a first step in relating
hydrologic regime to channel width, though
constrained reaches are not expected to have
consistent hydraulic geometry relationships. We
anticipate that we will need to differentiate
between Middle and Lower Susitna River as a
minimum and probably between specific
geomorphic reaches. The Middle River,
especially the area above Devils Canyon, will
have an immediate hydrologic and sediment
change and throughout the 50 years of
operation. The Lower River will experience the
hydrologic change immediately but will have a
delayed sediment supply change. We anticipate
using Rate Law (exponential decay) with rapid
initial change approaching an asymptotic long-
term value. The difficulties in predicting the rate
of width change will require AEA with the input of
Technical Workgroup (TWG) participants to
arrive on an acceptable target. Width change
will affect hydraulics and sediment transport just
as bed elevation change will. See TM at Section
4.2.1 for further discussion on width-change.
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15 Transmittal
Email
(attachment)
5/18/2013 Eric
Rothwell
NMFS Page 30 – Large Woody Debris Modeling
How will LWD be represented in the 2D models and
how will flow in, under and through be accounted for?
See AEA’s response to Comment 9 and TM at
Section 4.3.2 for a description of modeling Large
Woody Debris and RSP Section 6.6.4.1.2.7.
16 Transmittal
Email
(attachment)
5/18/2013 Eric
Rothwell
NMFS Page 32 – “Boundary conditions for all of the 2-D
model simulations, including downstream water
surface elevations, and upstream flow and sediment
supply, will be obtained from the 1-D modeling
results.”
Is this the case for both the fixed bed and mobile bed
models?
Yes, with the exception that the fixed-bed models
will not have a sediment supply.
16.1 Transmittal
Email
(attachment)
5/18/2013 Eric
Rothwell
NMFS For the fixed bed case (current conditions), why not
use field collected discharge and wse for inputs to the
2D models, vs, 1D model outputs for boundary
conditions?
AEA has not included these inputs because the
1-D models will have a more complete and
greater range of flows than the field
measurements. The 1-D model will be calibrated
using discharge and water surface elevation data
collected in the field.
16.2 Transmittal
Email
(attachment)
5/18/2013 Eric
Rothwell
NMFS For the mobile bed modeling, it is unclear how
changes in channel geometry and associated WSE in
the 1D model output could be used to provide
boundary condition input for the focus area 2D
modeling when the channel geometries are different?
AEA will adjust channel geometry of the year-25
and year-50 2-D morphology models based on
changes in geometry at those dates in the 1-D
modeling. The boundary conditions will also
correspond to year-25 and year-50 conditions.
Therefore, it is intended that the geometry and
boundary condition information will be consistent.
See TM at Sections 2.2 and 4.3.
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17 Transmittal
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(attachment)
5/18/2013 Eric
Rothwell
NMFS Page 32: “The output values for the required hydraulic
variables that include depth, velocity, and water-
surface elevation, will be provided at each node along
with the associated geo-referenced horizontal
coordinates and elevations.”
Consider Froude number as a model output, as it is
may be an important habitat metric for some species
(benthic fish and macroinvertebrates).
AEA will coordinate the selection of habitat
metrics with other studies and seek the input of
the TWG participants. See TM at Section
4.3.1.2 and RSP Section 6.6.4.1.2.1, and RSP
Section 8.5.4.6.1.4. The Froude number can be
calculated as a post-processing step based on
depth and velocity. Similarly, shear stress can
be computed using velocity, depth and bed
roughness as a post processing step or as direct
model output. See TM at Sections 4.2.3 and
4.3.3.
17.1 Transmittal
Email
(attachment)
5/18/2013 Eric
Rothwell
NMFS Pages 56-58 – Figures 4-5, 4-6, 4-7: Focus Areas
104, 138, 141, 144, 128 boundaries should be
adjusted to incorporate geomorphology cross
sections, in order to maximize the usability of collected
field data in modeling Focus Areas.
Geomorphology cross sections were placed
outside, but near the Focus Areas (FA) to define
the model boundary conditions somewhat away
from the areas of interest. AEA’s goal is to
reasonably represent flow conditions within the
FA limits for habitat analysis. AEA would
strongly discourage including the area between
the FA limit and the model boundary in any
habitat analysis due to boundary condition
effects.
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NMFS COMMENTS cont.
18 Transmittal
Email (body
of email)
5/18/2013 Eric
Rothwell
NMFS During the conversations there were several fluvial
geomorph related comments that came up. It may be
useful to bring these up again next Tuesday at the
geomorph meeting.
Study integration - how will the mobile bed model be
incorporated into assessment of changes in habitats
(will they be reclassified - Greg Auble asked about
this) at FA's?
The 1-D mobile bed modeling will address
channel change (aggradation, degradation,
depth, and width) and potential changes in
substrate for the main channel and major
secondary channels on a reach-scale over the
50-year license period. The 2-D morphology
modeling will be conducted for <1-year periods at
years 0, 25 and 50, and will incorporate bed
change consistent with the 1-D results. The 2-D
morphology modeling will provide additional
information to the 2-D hydraulic modeling used to
develop input to the habitat analysis. Specifically,
changes in the connectivity or breaching flows
will be determined and if reclassification of the
lateral habitats results from changes in the
breaching flows, these changes will be identified.
See TM at Section 2 for a description of the
overall modeling approach.
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19 Transmittal
Email (body
of email)
5/18/2013 Eric
Rothwell
NMFS Study integration - a discussion of fish access to off-
channel habitats, how will the FA bed models be used
to assess future changes to fish access of tributaries,
sloughs, side-channels, etc? Also a review of how the
1D model would be used to address this same
question outside of the FA's. Bill called in today and
addressed this, but it would be beneficial to me to
revisit.
Sediment supply will be estimated for the
tributaries. This supply will be used directly in
the FA 2-D models to evaluate delta formation
and fish access. In the 1-D models the supply
will be used to manually adjust delta geometry
and encroachment into the main channel. See
TM at Section 2.2 and RSP Section 6.6.4.1.2.6
for further discussion on tributary modeling. For
other types of side channels, the FA models will
include detailed representation of side-channel
entrances and exits to evaluate potential for
morphologic change affecting fish access.
Individual Comments
20 Email 6/4/13 Becky Long Individual Under Comprehensive Modeling Approach, the draft
TM has shown 4 operation scenarios for modeling
purposes: existing, load following, base load and
intermediate. This was for the 1 D Reach-Scale
Morphology Models for sediment input, geometry and
d’s rating curves; 2 D Morphology for Unsteady
Models at Focus Areas for sediment inflow, substrate
and lateral feature geometry and d’s rating curve; 2 D
Hydraulic (habitat) Steady Models at Focus Areas for
hydraulic data to habitat models for range of flows.
There needs to be a RoR operation scenario for these
models.
AEA will include a run of river scenario in the
Fluvial Geomorphology Modeling effort. See TM
at Section 1.1.
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21 Email 6/4/13 Becky Long Individual For ungaged tributary modeling, 3 operation scenarios
will be run: no dam, load following and baseload. A
RoR operation scenario needs to be added to these
scenarios. The RoR scenario will pinpoint dam effects
on the tributaries and represents a project operation
scenario that could be run during sensitive natural
ecosystem processes such as salmon spawning, redd
incubation periods, post- hatching and pre-emergence
periods and salmon migration.
AEA will include a run of river scenario in the
Fluvial Geomorphology Modeling effort. See TM
at Section 1.1.
22 Email 6/4/13 Becky Long Individual The draft TM mentioned “In addition to simulating long
term continuous period of flows, will be possible to
include rare flood events associated with unusual
climactic conditions or ice jam break up.”
I support this. I also support that reach scale models
should include at least 1 large relatively rare (i.e. 50-
100 year recurrence interval) event that does not
occur in the selected model 50 year period. I would
like to request that these models could be run to
simulate cumulative impacts from future climate
change conditions and post project impacts.
AEA anticipates including an extreme event in
the continuous 50-year flow record if one does
not occur within that record. There is a
reasonable chance (64%) that a 50-year or
greater event would occur during a 50 year
period. See TM at Section 4.2.1. Climate change
is not included in the fluvial geomorphic modeling
because using the historic record provides a
better comparison of Existing conditions to the
operational scenarios without introducing
hypothetical and uncertain modifications to the
flow record.
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23 Email 6/4/13 Becky Long Individual I am concerned about the 1 year snapshots of short
term looks at patterns of degradation and aggradation.
Judgment and interpretation of results will have to be
used due to the short time frame. Therefore, I am
concerned about the scientific validity.
The 50-year continuous simulation using the 1-D
model will provide useful information on system-
side channel response including aggradation and
degradation. The 2-D models will be run for
year-0, -25, and -50 for a range of hydrologic
conditions (dry, average, and wet seasonal
conditions). The 2-D models will include channel
changes consistent with the 1-D models for the
three time periods. Therefore, AEA anticipates
that the modeling approach will provide the
information needed to evaluate potential Project
effects on geomorphology and habitat. See TM
at Section 2.2.
24 Email 6/4/13 Becky Long Individual The AEA team will have to pick a 50 year record out of
the 61 year USGS record, a significant portion of
which is based on modeling itself. I heard a National
Weather Service person state in a radio report that
there is not much confidence in the Sunshine Gage. I
have not been able to find out what is meant by that.
But this concerns me.
AEA will evaluate the data from each of the
gages. It may be that the gage has variable
rating curves (stage-discharge relationships).
AEA will perform specific-gage analyses using
historic flow measurements at the gages.
Variability of the rating curves will be useful
information during model development. See TM
at Section 4.2.1.
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25 Email 6/4/13 Becky Long Individual I support looking closely at the Talkeetna and Chulitna
Rivers for post- project geomorphologic impacts.
Specifically it is important to answer the question will
the elevation of the main tributary beds change due to
the mainstream changes of lower water surface
elevations? What does this mean for those tributaries?
The aggradation simulation is important. As a long
time resident of the Talkeetna River watershed
dependent on the Talkeetna River salmon resources, I
appreciate this effort.
Thank you for the comment. The modeling
approach was amended to include the Chulitna
and Talkeetna Rivers as tributary reaches rather
than sediment/water sources only. See TM at
Section 4.1.1. This will allow AEA to evaluate
potential channel and water surface change on
these tributaries.
26 Email 6/4/13 Becky Long Individual 4.3.3 Ice Effects - the “tentative assumption” that the
river bed will be “stable due to low flows and ice cover”
seems incorrect. Proposed 10,000 cfs winter flows
are far from low. The ice transport capacity of the
river this fall left 6-8’ ice sheets far into lateral brush
habitat and close to the end of Main Street,
Talkeetna. The potential for winter ice jamming
should be seriously addressed, as should general
winter sediment transport.
As stated by the Ice Processes Study (RSP
Section 7.6.4.7), “for the Middle River, the
calibrated River1D model will be used to model
the proposed Project operational scenarios. The
model will predict water temperature, frazil ice
production, ice cover formation, elevation and
extent of ice cover, and flow hydrograph (winter
flow routing and water levels) between the
proposed dam site and Talkeetna. The model will
also predict ice cover stability, including potential
for jamming, under load-following fluctuations.”
Although flows will be higher for winter operation
conditions, the discharges may not be high
enough to cause general sediment transport and
the potential for general sediment transport will
be assessed from the model results. Ice
jamming will also be evaluated related to
formation and localized sediment transport from
ice jam breakup. See TM at Section 4.3.3.
TCCI
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27 Letter 6/5/13 Whitney
Wolff
Talkeetna
Community
Council, Inc.
Susitna
Dam
Committee
1.) How will 1D sediment transport analysis be
incorporated into the Chulitna (10 mile reach from
mouth) and Talkeetna (5 mile reach from mouth)
without “upstream sediment supply” data to assess
sediment capacity?
The 1-D sediment transport analysis will include
sediment rating curves at the upstream model
limits of the Chulitna and Talkeetna River
tributary reaches (Section 4.2.1). These rating
curves have been developed using data
collected by the USGS at the gaging station
locations. The locations of the upstream reach
limits were selected so the gage sediment
transport rates would be applicable. See RSP
Section 6.5.4.3 and Section 4.2 (Sediment Load
Rating Curves) of AEA’s 2012 Study Report,
Development of Sediment-Transport
Relationships and an Initial Sediment Balance for
the Middle and Lower Susitna River Segments.
28 Letter 6/5/13 Whitney
Wolff
Talkeetna
Community
Council, Inc.
Susitna
Dam
Committee
2.) Bank erosion rates are calculated through BEI
which requires both 1D and 2D modeling. How will
AEA achieve a “defined approach” without 2D
modeling? The approach currently depends on LIDAR
for the upper 6 miles of the Chulitna; will there be
study site work on the cross sections during the 2013
season?
Bank Energy Index can be calculated with either
1- or 2-D results and is calibrated by comparing
historic flow energy with observed bank erosion.
LiDAR will be used to develop the channel
geometry data for the Chulitna River because it
is a braided reach that will be relatively shallow
during the low flow conditions when the LiDAR
data is collected. Field work on the Chulitna
River in 2013 includes survey of two cross
sections, bed material sampling, bank material
characterization, and LWD characterization. See
TM at Section 4.1.1 and 4.2.2.2.
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29 Letter 6/5/13 Whitney
Wolff
Talkeetna
Community
Council, Inc.
Susitna
Dam
Committee
3.) How will such a dynamic study year be
representative of base line conditions?
Base line conditions are flow and sediment
supplies unaffected by Watana Dam and
operational scenarios include dam operations.
Base line conditions will simulate a continuous
50 year hydrologic record to represent the
license period in the 1-D modeling and will
include a range of hydrologic conditions (dry,
average, and wet) for open-water periods at
years-0, -25, and -50. AEA will coordinate with
the other study team members and review
agencies to select suitable hydrologic conditions,
which need to include a wide range of flow
conditions to develop representative results, but
may not necessarily include 2012 or 2013. See
TM at Section 2.2.
30 Letter 6/5/13 Whitney
Wolff
Talkeetna
Community
Council, Inc.
Susitna
Dam
Committee
TCCI supports FERC’s recent inclusion of a ROR
operations model for examining base line flows. This
TM should be updated to include this necessary
addition.
AEA will run of river scenario in the Fluvial
Geomorphology Modeling effort See RSP
Section 6.6 and TM at Section 1.1.
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CWA
31 Letter 6/5/13 Harold
Shepherd
Center for
Water
Advocacy
The Draft Memo does not mention FERC’s directive to
AEA in the SPD that AEA include a run-of-river
alternative in the operating scenario evaluations for
Study 6.6 (geomorphology modeling) and Study 7.6
(ice processes). This is a significant oversight because
although AEA filed a request for reconsideration of the
run of river requirement in the SPD due to its
contention that such a scenario did not have a nexus
to the proposed Project alternative and would have
been too expensive, ultimately, FERC concluded “that
we have insufficient basis for dismissing consideration
of a run-of-river alternative at this early stage in the
prefiling licensing process. Accordingly, staff will not
alter the requirement that AEA evaluate a run-of-river
operating scenario.”2
AEA will include a run of river scenario in the
Fluvial Geomorphology Modeling effort. See
RSP Section 6.6 and TM at Section 1.1.
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32 Letter 6/5/13 Harold
Shepherd
Center for
Water
Advocacy
The Technical Memo, therefore, should include
floodplain modeling that incorporates the use of two-
dimensional hydraulic models, to compare the
unimpaired and current frequency, magnitude and
duration of floodplain inundation. AEA should use a
two-dimensional model of the middle Susitna River,
from RM 184 downstream to the confluence with the
Chulitna River at RM 98, to determine how much
floodplain area is currently accessible. It should then
use current and unimpaired hydrology to determine
the frequency, duration, and magnitude of floodplain
inundation under both scenarios as well as the total
area and depth of inundation during the ecologically
important spring snowmelt season. Finally, AEA
should work collaboratively with ILP participants to
define additional, specific ecologically important time
periods for floodplain inundation modeling.
The reach-scale 1-D modeling incorporates
channel and floodplain features from Watana
Dam site down to Susitna Station gage at PRM
30. See TM at Section 4.1.1. Therefore,
information on floodplain inundation frequency,
duration and magnitude will be available
throughout the potentially affected downstream
areas for existing and alternative operational
scenarios. In addition, each of the focus area
models will include main channel, side channels,
other lateral features, islands and floodplain
areas. The FAs will be simulated for Year-0, -25,
and -50 conditions for a range of hydrologic
conditions (dry, average, & wet). From the 1-D
and 2-D analyses we anticipate having sufficient
information in both the channel and floodplain
areas. See TM at Section 2.2.
24
Com
# Comment
Format
Comment
Date
Licensing
Participant
Name
Licensing
Participant
Affiliation Comment Response
33 Letter 6/5/13 Harold
Shepherd
Center for
Water
Advocacy
In the Susitna River, year-to-year differences in the
timing and quantity of flow result in substantial
variability around any average flow condition.
Accordingly, as in the case of the Draft Memo,
modeling for the “average” condition can be
misguided……..In addition, the Technical Memo
should include modeling that develops quantitative,
river-specific standards, based on the reconstruction
of the natural flow regime.5 Restoration actions based
on such guidelines should be viewed as experiments
to be monitored and evaluated—that is, adaptive
management—to provide critical new knowledge for
creative management of natural ecosystem variability.
The comment is beyond the scope of the issues
addressed in this technical memorandum. AEA
will include an alternative analysis in its License
Application, and FERC’s environmental analysis
will include analysis of Project alternatives. The
existing information and the studies approved by
FERC will provide sufficient information to
support alternative analyses.
34 Letter 6/5/13 Harold
Shepherd
Center for
Water
Advocacy
The Technical Memo, therefore, should incorporate
modeling efforts that describes how experimental high
and steady flows during a specific time period, could
help and assess the long-term benefits to aquatic
habitat salmon and fine sediment along the Susitna
River downstream of Project Dam site. This would
allow for an adaptive management approach, wherein
the relationship between dam operations and down
stream resources was recognized as uncertain and an
active experimental approach was adopted
The comment is beyond the scope of the issues
addressed in this technical memorandum. AEA
will include an alternative analysis in its License
Application, and FERC’s environmental analysis
will include analysis of Project alternatives. The
existing information and the studies approved by
FERC will provide sufficient information to
support alternative analyses.
25
Com
# Comment
Format
Comment
Date
Licensing
Participant
Name
Licensing
Participant
Affiliation Comment Response
35 Letter 6/5/13 Harold
Shepherd
Center for
Water
Advocacy
To this end AEA should continue to use Watana
reservoir inflows developed directly from USGS
records as the basic hydrologic input dataset for the
reservoir operation and power studies. However, it
should also consider alternative hydrologic input
datasets, which account for potential future hydrologic
change.
Using the historic flow record is considered as a
sound approach for making comparisons
between existing conditions and the operational
scenarios. Introducing other variables,
especially variables with high levels of
uncertainty, may make it very difficult to separate
potential project effects from other influences.