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Susitna-Watana Hydroelectric Project Document
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Title:
Fluvial geomorphology modeling approach : draft technical memorandum
SuWa 2
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Prepared by Tetra Tech
AEA-identified category, if specified:
Geomorphology
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Series (ARLIS-assigned report number):
Susitna-Watana Hydroelectric Project document number 2
Existing numbers on document:
Published by:
[Anchorage, Alaska : Alaska Energy Authority, 2013]
Date published:
May 3, 2013
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Prepared for Alaska Energy Authority
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Draft
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Draft technical memorandum
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79 p.
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Notes:
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/
Susitna-Watana Hydroelectric Project
(FERC No. 14241)
Fluvial Geomorphology Modeling Approach
Draft Technical Memorandum
Prepared for
Alaska Energy Authority
Prepared by
Tetra Tech
May 3, 2013
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page i May 2013
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DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page ii May 2013
TABLE OF CONTENTS
1. INTRODUCTION ............................................................................................................................... 1
1.1 Background.................................................................................................................. 2
1.1.1 Reach-Scale Issues .............................................................................................. 3
1.1.2 Local-Scale Issues ................................................................................................ 4
1.2 Objectives .................................................................................................................... 4
2. OVERALL MODELING APPROACH ............................................................................................ 7
3. SELECTION OF HYDRAULIC AND BED EVOLUTION MODELS ........................................... 9
3.1 1-D Models .................................................................................................................. 9
3.1.1 Overview of 1-D Model Development ...................................................................10
3.1.2 Selection Criteria for 1-D Models .........................................................................12
3.1.3 Potential 1-D Models ............................................................................................12
3.1.4 Selection of 1-D Model .........................................................................................14
3.2 2-D Models .................................................................................................................14
3.2.1 Overview of 2-D Model Development ...................................................................15
3.2.2 Selection Criteria of 2-D Models ..........................................................................17
3.2.3 Potential 2-D Models ............................................................................................18
3.2.4 Selection of 2-D Model .........................................................................................22
4. MODEL APPLICATION................................................................................................................. 25
4.1 Models and Survey Extent ..........................................................................................25
4.1.1 Reach-Scale Model..............................................................................................25
4.1.2 Local-Scale Focus Area Models ..........................................................................26
4.1.3 Other Tributary Models ........................................................................................27
4.2 Reach-Scale 1-D Modeling .........................................................................................27
4.2.1 Bed Evolution and Hydraulic Modeling .................................................................27
4.2.2 Large Woody Debris Effects ................................................................................29
4.2.3 Ice Effects ............................................................................................................31
4.2.4 Summary of Reach-Scale Model Results .............................................................31
4.3 Local-Scale 2-D Modeling ...........................................................................................31
4.3.1 Hydraulic and Bed Evolution Modeling .................................................................32
4.3.2 Large Woody Debris Effects ................................................................................34
4.3.3 Ice Effects ............................................................................................................35
4.3.4 Summary of Local-Scale Model Results ...............................................................37
4.4 Other Tributary Modeling ............................................................................................37
5. CONSULTATION DOCUMENTATION ........................................................................................ 38
6. REFERENCES .................................................................................................................................. 40
7. TABLES............................................................................................................................................. 44
8. FIGURES ........................................................................................................................................... 50
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page iii May 2013
LIST OF TABLES
Table 2-1. 1-D versus 2-D model capabilities .............................................................................. 44
Table 3-1. Evaluation of 1-D models ............................................................................................ 45
Table 3-2. Evaluation of 2-D models ............................................................................................ 46
Table 4-1 Tributary modeling ....................................................................................................... 47
Table 4-2. Large woody debris digitizing within geomorphic features ....................................... 48
LIST OF FIGURES
Figure 4-1. Survey and Model Cross Sections from PRM 30 to PRM 47 .................................... 52
Figure 4-2. Survey and Model Cross Sections from PRM 45 to PRM 66 .................................... 53
Figure 4-3. Survey and Model Cross Sections from PRM 63 to PRM 81 .................................... 54
Figure 4-4. Survey and Model Cross Sections from PRM 79 to PRM 99 .................................... 55
Figure 4-5. Survey and Model Cross Sections from PRM 98 to PRM 116 .................................. 56
Figure 4-6. Survey and Model Cross Sections from PRM 114 to PRM 131 ................................ 57
Figure 4-7. Survey and Model Cross Sections from PRM 131 to PRM 148 ................................ 58
Figure 4-8. Survey and Model Cross Sections from PRM 142 to PRM 154 ................................ 59
Figure 4-9. Survey and Model Cross Sections from PRM 166 to PRM 179 ................................ 60
Figure 4-10. Survey and Model Cross Sections from PRM 176 to PRM 188 .............................. 61
Figure 4-11. Proposed Cross Section Layout for Chulitna and Talkeetna Rivers, 1-D Model
Tributary Reaches ................................................................................................................. 62
Figure 4-12. FA104 (From R2 Resource Consultants Inc. 2013) ................................................. 63
Figure 4-13. FA113 (From R2 Resource Consultants Inc. 2013) ................................................. 63
Figure 4-14. FA115 (From R2 Resource Consultants Inc. 2013) ................................................. 64
Figure 4-15. FA128 (From R2 Resource Consultants Inc. 2013) ................................................. 64
Figure 4-16. FA138 (From R2 Resource Consultants Inc. 2013) ................................................. 65
Figure 4-17. FA141 (From R2 Resource Consultants Inc. 2013) ................................................. 65
Figure 4-18. FA144 (From R2 Resource Consultants Inc. 2013) ................................................. 66
Figure 4-19. FA151 (From R2 Resource Consultants Inc. 2013) ................................................. 66
Figure 4-20. FA128 (From R2 Resource Consultants Inc. 2013) ................................................. 67
Figure 4-21. FA184 (From R2 Resource Consultants Inc. 2013) ................................................. 67
Figure 4-22. Middle River Tributary Locations Relative to Geomorphic Reach and Focus Areas
............................................................................................................................................... 68
Figure 4-23. Lower River Tributary Locations Relative to Geomorphic Reach ......................... 69
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page iv May 2013
Figure 4-24. Example of Fine Mesh Applied in Whiskers Slough Focus Area ........................... 70
Figure 4-25. Example of Coarse Mesh Applied in Whiskers Slough Focus Area ....................... 71
Figure 4-26. Example of Ice Blockage Altering Flow Distribution in Multiple Channel Reaches
(Zabilansky et al. 2003) ........................................................................................................ 72
Figure 4-27. Ice Jam Locations at FA104 ..................................................................................... 73
Figure 4-28. Ice Jam Locations at FA113 ..................................................................................... 74
Figure 4-29. Ice Jam Locations at FA115 ..................................................................................... 75
Figure 4-30. Ice Jam Locations at FA128 ..................................................................................... 76
Figure 4-31. Ice Jam Locations at FA138 ..................................................................................... 77
Figure 4-32. Ice Jam Locations at FA141 ..................................................................................... 78
Figure 4-33. Ice Jam Locations at FA144 ..................................................................................... 79
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page v May 2013
ACRONYMS AND ABBREVIATIONS
1-D one dimensional
2-D two dimensional
AEA Alaska Energy Authority
AOW additional open water
BEI Bank Energy Index
BG background
cfs cubic feet per second
DHI Danish Hydraulic Institute
FAs Focus Areas
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
PRM Project River Mile [
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
USACE U.S. Army Corps of Engineers
USGS U.S. Geological Survey
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 1 May 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:
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);
Location and extent of one- and two-dimensional geomorphology and aquatic habitat
modeling in project reaches, focus areas, and other study sites;
Rationale and criteria for model selection including an overview of model
development;
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
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.
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 2 May 2013
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.
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. This draft technical memorandum is a planned topic for the May 21,
2013, TWG meeting and will be finalized for a June 30, 2013, submittal to FERC. 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
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 3 May 2013
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:
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. 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 will be performed using 1-D models, as they are well suited for long term
simulations over long river reaches. The 1-D models will be used to assess reach-scale sediment
transport conditions, potential changes in bed and water surface elevations, 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 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.
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.
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.
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 4 May 2013
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, side channels, 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
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
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 5 May 2013
approach, including which models will be used, for the Susitna River fluvial geomorphology
modeling and provide opportunity for other study team members and licensing participants to
provide feedback on modeling approaches to ensure that the needs of all parties are being met, to
the extent practical.
Specific objectives include:
Identify the 1-D and 2-D sediment transport models that will be used for reach- and
local-scale modeling.
Provide the rationale and criteria for model selection.
Specify the selected models.
Provide 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.
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 6 May 2013
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DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 7 May 2013
2. OVERALL MODELING APPROACH
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
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.
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.
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 8 May 2013
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 streamwise 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 side channel 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.
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 9 May 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 variet y 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
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 10 May 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:
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.
Locations of tributaries that will be modeled as reaches. Note: These will include
the Chulitna and Talkeetna rivers based on agency comments and SPD
recommendations.
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.
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.
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.
Develop bed-material gradation and layer information.
Surface sampling conducted throughout the channel network,
Subsurface sampling, and
Bank material samples.
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 11 May 2013
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.
Develop sediment inflow rating curves based on tributary models or gaging
station records that include sediment measurements.
Other considerations.
Bridge constrictions and geometries,
Ineffective flow areas around bridges and other rapid expansion and contraction
areas, and
Use of depth- or flow-variable roughness input.
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 significant change or instability in hydraulic results.
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 side
channels, and
o High water marks reported from extreme flood events.
Test the sediment transport model.
Conduct a sediment transport time-step sensitivity analysis to evaluate appropriate
computational time steps for different flow magnitudes.
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:
o Gage station measurements sediment loads, specific gage plots, flow area,
width, depth, and velocity measurements,
o Comparison cross sections, and
o Longitudinal profiles.
Run and evaluate the results of the sediment transport simulations.
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 12 May 2013
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:
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 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
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 13 May 2013
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
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, limited number of sediment
size classes (eight) to represent the range of sediments in the Susitna River and tributaries, 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
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 14 May 2013
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
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, its
broader range of project applications, and capability to simulate a larger number of sediment size
classes. 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 significant variability. A subset of
finite element models uses a curvilinear grid, which shares advantages and disadvantages of both
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 15 May 2013
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, side channels, 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,
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 significant 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:
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.
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FERC Project No. 14241 Page 16 May 2013
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.
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.
Refine the network in areas of significant change or areas of significant habitat
interest.
Determine the node elevations from the geometric data. Note: It is important to
refine the network prior to determining elevations in order to not simply refine the
geometry from the coarser network.
Review mesh quality to assure that element size transitions and other modeling
requirements are reasonably met.
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 are used to incorporate internal flow stresses and
reasonable values depend on each model’s numerical representation of these
stresses.
Develop bed material gradation and layer information.
Surface sampling conducted throughout the channel network,
Subsurface sampling, and
Bank material samples.
Develop inflow hydrographs and sediment inflows for existing and with-project
conditions.
For fully unsteady model, 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.
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 17 May 2013
Other considerations.
Ice jam breakup hydrographs,
Ice jam blockage of main channel or side channels causing redistribution of flow,
LWD as obstructions or changes in roughness, and
Erodibility of floodplain areas.
Test the hydraulic model over a range of flow conditions.
Evaluate mesh quality and the need for additional mesh refinement.
Calibrate and validate the hydraulic model.
Adjust flow resistance input values (within reasonable limits) to calibrate the
hydraulic results. 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. Calibration and
validation will be performed using available data including:
o Measured water-surface elevations throughout the focus areas during site
survey.
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 surface elevations collected at other flows.
o Water level loggers.
o Discharge distribution between main channel and side channels.
o High water mark information if available.
Test the sediment transport model.
Conduct a sediment transport time-step sensitivity analysis to evaluate appropriate
computational time steps for different flow magnitudes.
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:
o Main channel bed level changes observed in the 1-D modeling,
o Comparison cross sections, and
o Longitudinal profiles.
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 Susitna River. 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
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 18 May 2013
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.
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
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 19 May 2013
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
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. 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 significantly
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
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 20 May 2013
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
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.
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 21 May 2013
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-
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 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
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 22 May 2013
cannot be simulated. This is a significant shortcoming for evaluating potential Project effects.
This model is commercially available.
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 base 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 analyses 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. 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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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 23 May 2013
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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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 24 May 2013
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FERC Project No. 14241 Page 25 May 2013
4. MODEL APPLICATION
The selected models will be used to address hydraulic, 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, 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.
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 three 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, and (3) cross section locations on side 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 in-channel areas (similar to the 2012 cross sections) will be
surveyed directly. The remaining extent will be developed using detailed LiDAR data.
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 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
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 26 May 2013
modeled for purposes of this study with the 1-D sediment transport model (Figure 4-11). Figure
4-11 shows the cross sections that will be surveyed in the downstream approximately four miles
of these tributaries. 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 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.
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 below the
top-of-bank. Some floodplain survey will also be performed, but the primary topographic data
source for the floodplain areas will be the LiDAR data. 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.
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.
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 27 May 2013
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.
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
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 28 May 2013
bed lowering. Similarly, the Talkeetna River as a modeled reach will allow simulation of
changes in both water surface elevations and channel geometry.
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, side channels, 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. 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. The rate of width adjustment may be greatest in the initial years after closure,
so the time interval may be shorter during the initial periods of the simulation and, increase with
time during the simulation.
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. Inclusion of at least one large, relatively rare (i.e., 50- to 100-year
recurrence interval) event that does not occur in the selected model period will also be
considered. 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.
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 29 May 2013
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.)
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
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 30 May 2013
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.
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.
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 31 May 2013
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.
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.
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
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 32 May 2013
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 the 1-D modeling
results.
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 significant topographic
or flow resistance variability. 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 4-24 shows and example of a fine mesh. Based on input 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. For the 2-D sediment transport
models, the mesh sizes will be as large as possible, but with sufficient detail to represent
variability in bathymetry, topography, roughness, and bed composition. An example of a coarse
mesh is shown in Figure 4-25. These general guidelines represent starting values for the spatial
resolution that will be refined and adjusted as necessary throughout the model development
phase. These mesh examples do not fully meet the element size criteria outlined above because
greater detail will be incorporated in the side channels and habitat areas.
4.3.1 Hydraulic and Bed Evolution Modeling
4.3.1.1 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. Based on discussions with the Fish and Aquatics Instream Flow team, it is
anticipated that model output will be required for the range of flows from 10,000 cfs up to
approximately 60,000 cubic feet per second (cfs) in approximate 5,000 cfs increments for the
Focus Areas (W. Miller, pers. comm., 2013). This range can be adjusted, as necessary, during
the modeling phase. From discussions with Miller, it is anticipated that 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
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 33 May 2013
will be adjusted to represent the projected channel form 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 considers Project-related changes in the effective
discharge analysis, riparian vegetation, and sediment supply. Projected changes in side channels
and sloughs 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
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 up to 3 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 up to 3 operational scenarios to represent
conditions 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.
4.3.1.2 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. 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
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 34 May 2013
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
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 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 side
channels and sloughs 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.
4.3.2 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).
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 35 May 2013
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. 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 side 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. 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
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 36 May 2013
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
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 4-26, 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 4-27 through 4-33
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.
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 37 May 2013
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 maintenance of side
channels and sloughs.
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.
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, side channels, and floodplains.
Hydraulic parameters to estimate Bank Energy Index and bank erosion rates.
4.4 Other Tributary Modeling
In the Lower River, local-scale sediment transport models will be developed for the mouth 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.
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.
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 38 May 2013
5. CONSULTATION DOCUMENTATION
This section will be developed to document the review, consultation, and revision process.
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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,
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
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 41 May 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.
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.
Miller, William. 2013. Personal communication. Ecological Consultants, Inc., Fort Collins,
Colorado. April 23.
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.
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.
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.
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 42 May 2013
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.
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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.
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Journal of Hydraulic Engineering. 129(2), 120-128.
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
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 45 May 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 8 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.
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
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FERC Project No. 14241 Page 46 May 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 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
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.
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 47 May 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
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 48 May 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.
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Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 49 May 2013
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8. FIGURES
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Figure 4-1. Survey and Model Cross Sections from PRM 30 to PRM 47
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
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Figure 4-2. Survey and Model Cross Sections from PRM 45 to PRM 66
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 54 May 2013
Figure 4-3. Survey and Model Cross Sections from PRM 63 to PRM 81
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FERC Project No. 14241 Page 55 May 2013
Figure 4-4. Survey and Model Cross Sections from PRM 79 to PRM 99
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 56 May 2013
Figure 4-5. Survey and Model Cross Sections from PRM 98 to PRM 116
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FERC Project No. 14241 Page 57 May 2013
Figure 4-6. Survey and Model Cross Sections from PRM 114 to PRM 131
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 58 May 2013
Figure 4-7. Survey and Model Cross Sections from PRM 131 to PRM 148
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 59 May 2013
Figure 4-8. Survey and Model Cross Sections from PRM 142 to PRM 154
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 60 May 2013
Figure 4-9. Survey and Model Cross Sections from PRM 166 to PRM 179
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 61 May 2013
Figure 4-10. Survey and Model Cross Sections from PRM 176 to PRM 188
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 62 May 2013
Figure 4-11. Proposed Cross Section Layout for Chulitna and Talkeetna Rivers, 1-D Model Tributary Reaches
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
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Figure 4-12. FA104 (From R2 Resource Consultants Inc. 2013)
Figure 4-13. FA113 (From R2 Resource Consultants Inc. 2013)
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
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FERC Project No. 14241 Page 64 May 2013
Figure 4-14. FA115 (From R2 Resource Consultants Inc. 2013)
Figure 4-15. FA128 (From R2 Resource Consultants Inc. 2013)
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 65 May 2013
Figure 4-16. FA138 (From R2 Resource Consultants Inc. 2013)
Figure 4-17. FA141 (From R2 Resource Consultants Inc. 2013)
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
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FERC Project No. 14241 Page 66 May 2013
Figure 4-18. FA144 (From R2 Resource Consultants Inc. 2013)
Figure 4-19. FA151 (From R2 Resource Consultants Inc. 2013)
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 67 May 2013
Figure 4-20. FA128 (From R2 Resource Consultants Inc. 2013)
Figure 4-21. FA184 (From R2 Resource Consultants Inc. 2013)
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 68 May 2013
Figure 4-22. Middle River Tributary Locations Relative to Geomorphic Reach and Focus Areas
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
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FERC Project No. 14241 Page 69 May 2013
Figure 4-23. Lower River Tributary Locations Relative to Geomorphic Reach
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
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FERC Project No. 14241 Page 70 May 2013
Figure 4-24. Example of Fine Mesh Applied in Whiskers Slough Focus Area
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 71 May 2013
Figure 4-25. Example of Coarse Mesh Applied in Whiskers Slough Focus Area
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
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FERC Project No. 14241 Page 72 May 2013
Figure 4-26. Example of Ice Blockage Altering Flow Distribution in Multiple Channel Reaches (Zabilansky et al. 2003)
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
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FERC Project No. 14241 Page 73 May 2013
Figure 4-27. Ice Jam Locations at FA104
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
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FERC Project No. 14241 Page 74 May 2013
Figure 4-28. Ice Jam Locations at FA113
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
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FERC Project No. 14241 Page 75 May 2013
Figure 4-29. Ice Jam Locations at FA115
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
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FERC Project No. 14241 Page 76 May 2013
Figure 4-30. Ice Jam Locations at FA128
DRAFT TECHNICAL MEMORANDUM FLUVIAL GEOMORPHOLOGY MODELING APPROACH
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Figure 4-31. Ice Jam Locations at FA138
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Figure 4-32. Ice Jam Locations at FA141
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FERC Project No. 14241 Page 79 May 2013
Figure 4-33. Ice Jam Locations at FA144