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Susitna-Watana Hydroelectric Project Document
ARLIS Uniform Cover Page
Title:
SuWa 242
Fluvial geomorphology modeling below Watana Dam study (Study 6.6),
decision point on fluvial geomorphology modeling of the Susitna River
below PRM 29.9, technical memorandum
Author(s) – Personal:
Author(s) – Corporate:
Tetra Tech, Inc.
AEA-identified category, if specified:
September 26, 2014 technical memorandum filings
AEA-identified series, if specified:
Series (ARLIS-assigned report number): Existing numbers on document:
Susitna-Watana Hydroelectric Project document number 242
Published by: Date published:
[Anchorage, Alaska : Alaska Energy Authority, 2014] September 2014
Published for: Date or date range of report: Alaska Energy Authority
Volume and/or Part numbers:
Final or Draft status, as indicated:
Attachment C
Document type: Pagination:
Technical memorandum vi, 66 p.
Related work(s): Pages added/changed by ARLIS:
Cover letter to this report: Susitna-Watana Hydroelectric
Project, FERC Project no. 14241-000; Second set of 2014
technical memoranda for Initial Study Plan meetings. (SuWa
239)
Attachments A-B (SuWa 240-241) and D-G (SuWa 243-246)
Added cover letter
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/
September 26, 2014
Ms. Kimberly D. Bose
Secretary
Federal Energy Regulatory Commission
888 First Street, N.E.
Washington, D.C. 20426
Re: Susitna-Watana Hydroelectric Project, Project No. 14241-000
Second Set of 2014 Technical Memoranda for Initial Study Plan Meetings
Dear Secretary Bose:
As the Alaska Energy Authority (AEA) explained in its September 17, 2014 filing
with the Federal Energy Regulatory Commission (Commission or FERC) for the
proposed Susitna-Watana Hydroelectric Project, FERC Project No. 14241 (Project), the
June 3, 2014 Initial Study Report (ISR) provided for AEA to prepare certain technical
memoranda and other information based on 2014 work. In accordance with Commission
Staff direction, on September 17, 2014, AEA filed and distributed the first set of
technical memoranda and other information generated during the 2014 study season.
With this letter, AEA is filing and distributing the second set of technical
memoranda generated during the 2014 study season, as described below. As part of its
continued implementation of the study plan, AEA expects to file a third set of technical
memoranda prior to October 1, 2014.
This second set of technical memoranda includes:
• Attachment A: Geomorphology Study (Study 6.5) - Updated Mapping of
Aquatic Macrohabitat Types in the Middle Susitna River Segment from 1980s
and Current Aerials Technical Memorandum. This technical memorandum
updates the Middle Susitna River Segment portion of the aquatic macrohabitat
mapping results previously provided in the technical memorandum titled
Mapping of Aquatic Macrohabitat Types at Selected Sites in the Middle and
Lower Susitna River Segments from 1980s and 2012 Aerials (Tetra Tech
2013a).
• Attachment B: Geomorphology Study (Study 6.5) - Mapping of Geomorphic
Features and Turnover within the Middle and Lower Susitna River Segments
from 1950s, 1980s, and Current Aerials Technical Memorandum. This
technical memorandum updates the geomorphic mapping and assessment of
channel change that were initially provided in Mapping of Geomorphic
2
Features and Assessment of Channel Change in the Middle and Lower Susitna
River Segments from 1980s and 2012 Aerials (Tetra Tech 2013a). The initial
technical memorandum provided the results from tasks identified in Revised
Study Plan Study 6.5 Section 6.5.4.4. This update extends the previous 30
year analysis between the 1980s and 2012 by an additional 30 years with
aerial photography from the 1950s, and also provides a short term analysis of
geomorphic changes by comparing 2012 with 2013 aerial photography.
• Attachment C: Fluvial Geomorphology Modeling below Watana Dam Study
(Study 6.6) - Decision Point on Fluvial Geomorphology Modeling of the
Susitna River below PRM 29.9 Technical Memorandum. This technical
memorandum describes the decision of whether to extend the downstream
limit of the 1-D bed evolution model below Susitna Station at PRM 29.9.
• Attachment D: Fluvial Geomorphology Modeling Below Watana Dam (Study
6.6) - Winter Sampling of Main Channel Bed Material Technical
Memorandum. The overall purpose of this technical memorandum is to
quantify main channel bed material gradations at selected sites in the Upper,
Middle, and Lower Susitna River Segments. The data obtained from this
study serves as input for the 1-D and 2-D bed evolution modeling efforts
being conducted under the Fluvial Geomorphology Modeling Study (Study
6.6).
• Attachment E: Cook Inlet Beluga Whale Study (Study 9.17) - 2014 Cook Inlet
Beluga Whale Prey Study Implementation Technical Memorandum. This
technical memorandum summarizes activities implementing the Cook Inlet
Beluga Whale Study (Study 9.17) conducted in 2014 that tested methods to
document Cook Inlet Beluga Whale prey and prey habitat in the Susitna River
delta.
• Attachment F: River Productivity Study (Study 9.8) - 2013 Initial River
Productivity Results Technical Memorandum. This technical memorandum
provides a preliminary review and summary of 2013 river productivity sample
results based on laboratory data received after the ISR submittal in June 2014.
• Attachment G: River Productivity Study (Study 9.8) - 2014 Field Season River
Productivity Progress Report Technical Memorandum. This technical
memorandum presents an update on activities conducted during the Spring
field sampling event in June 2014, which was focused on data collection to
support the needs of the trophic modeling and stable isotope analysis
objectives of the River Productivity Study.
3
AEA appreciates the opportunity to provide this additional information to the
Commission and licensing participants, which it believes will be helpful in determining
the appropriate development of the 2015 study plan as set forth in the ISR. If you have
questions concerning this submission please contact me at wdyok@aidea.org or (907)
771-3955.
Sincerely,
Wayne Dyok
Project Manager
Alaska Energy Authority
Attachments
cc: Distribution List (w/o Attachments)
Attachment C
Fluvial Geomorphology Modeling below Watana Dam Study (Study 6.6) - Decision Point on
Fluvial Geomorphology Modeling of the Susitna River below PRM 29.9
Technical Memorandum
Susitna-Watana Hydroelectric Project
(FERC No. 14241)
Fluvial Geomorphology Modeling
below Watana Dam Study (Study 6.6)
Decision Point on Fluvial Geomorphology
Modeling of the Susitna River below PRM 29.9
Technical Memorandum
Prepared for
Alaska Energy Authority
Prepared by
Tetra Tech, Inc.
September 2014
TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
TABLE OF CONTENTS
Summary ....................................................................................................................................... vi
1. Introduction ........................................................................................................................1
2. Objective .............................................................................................................................2
3. Study Area ..........................................................................................................................2
4. Methods ...............................................................................................................................3
4.1. Overview ..................................................................................................................3
4.2. Existing Information ................................................................................................3
4.2.1. Study Area Extents ..................................................................................... 3
4.2.2. Sediment Transport ..................................................................................... 4
4.2.3. Pre-Project and Post-Project Hydrology ..................................................... 5
4.2.4. Changes in Channel Width ......................................................................... 6
4.2.5. 1-D Bed Evolution Model ........................................................................... 6
4.3. Decision Point Criteria .............................................................................................9
4.3.1. Approach for Changes in Flow at Susitna Station and Associated Potential
for Channel Width Adjustment ................................................................. 10
4.3.2. Approach for Change in Sediment Transport Mass Over the Open-Water
Flow Period ............................................................................................... 11
4.3.3. Approach for Difference in Modeled Bed Elevations Represented by
Channel Aggradation or Degradation ....................................................... 11
4.3.4. Approach for Estimated Change in Flow Depth and Velocity in the Lower
Portion of Geomorphic Reach LR-6 Inferred from Changes Modeled in
the Upper Portion of LR-5 ........................................................................ 12
4.4. Variances................................................................................................................12
5. Results ...............................................................................................................................12
5.1. Changes in Flow at Susitna Station and Associated Potential for Channel Width
Adjustment .............................................................................................................13
5.2. Change in Sediment Transport Mass Over the Open-Water Flow Period .............15
5.3. Difference in Modeled Bed Elevations Represented by Channel Aggradation or
Degradation ............................................................................................................15
5.4. Estimated Change in Flow Depth and Velocity in the Lower Portion of
Geomorphic Reach LR-6 Inferred from Changes Modeled in the Upper Portion of
LR-5 .......................................................................................................................17
6. Discussion..........................................................................................................................18
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page i September 2014
TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
6.1. Changes in Flow at Susitna Station and Associated Potential for Channel Width
Adjustment .............................................................................................................18
6.2. Changes in Sediment Transport Mass over the Open-Water Flow Period ............19
6.3. Difference in Modeled Bed Elevations Represented by Channel Aggradation or
Degradation ............................................................................................................20
6.5. Decision Point Recommendation ...........................................................................21
7. References .........................................................................................................................22
8. Tables ................................................................................................................................25
9. Figures ...............................................................................................................................29
LIST OF TABLES
Table 4.2-1. Average non-vegetated widths by Geomorphic Reach in the Lower River. ........... 26
Table 4.2-2. Calibration and validation data for the 1-D Bed Evolution Model. ........................ 26
Table 5.1-1. Flow frequency and width comparisons for the Lower Susitna River. ................... 26
Table 5.1-2. Variability of 2-Year discharge (Q2) and associated potential for width adjustment
at Susitna Station. .................................................................................................................. 27
Table 5.2-1. Comparison of Sediment masses (sand and larger materials) transported past
Susitna Station. ..................................................................................................................... 27
Table 5.3-1. Reach average bed elevation change for existing conditions. ................................. 27
Table 5.3-2. Reach average bed elevation change for Max LF OS-1b conditions. ..................... 28
Table 6.1-1. Summary comparison of flow duration curves ....................................................... 28
LIST OF FIGURES
Figure 3.0-1. Study area ............................................................................................................... 30
Figure 4.2-1. Comparison of observed and simulated water-surface elevations at Sunshine for
the calibration event .............................................................................................................. 31
Figure 4.2-2. Comparison of observed and simulated water-surface elevation at Susitna Station
for the calibration event ........................................................................................................ 31
Figure 4.2-3. Middle River comparison of observed point-in-time water-surface elevations to
simulated elevations for the calibration event ...................................................................... 32
Figure 4.2-4. Lower River comparison of observed point-in-time water-surface elevations to
simulated elevations for the calibration event. ..................................................................... 32
Figure 4.2-5. Comparison of observed and simulated flows at Gold Creek for the validation
event ...................................................................................................................................... 33
Figure 4.2-6. Susitna River near Talkeetna comparison of measured and modeled total bed
material loads ........................................................................................................................ 33
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page ii September 2014
TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 4.2-7. Susitna River near Talkeetna comparison of measured and modeled transported
bed material gradations ......................................................................................................... 34
Figure 4.2-8. Susitna River at Sunshine comparison of measured and model total bed material
loads ...................................................................................................................................... 34
Figure 4.2-9. Susitna River at Sunshine comparison of measured and model transported bed
material gradations ................................................................................................................ 35
Figure 4.2-10. Susitna River at Susitna Station comparison of measured and model total bed
material loads ........................................................................................................................ 35
Figure 4.2-11. Susitna River at Susitna Station comparison of measured and model transported
bed material gradations ......................................................................................................... 36
Figure 5.1-1. Susitna River active channel width measurements from PRM 29.5 to 13.0. ......... 37
Figure 5.1-2. Susitna River active channel width measurements from PRM 16.0 to 2.5. ........... 38
Figure 5.1-3. Susitna River non-vegetated channel width below PRM 29.5. .............................. 39
Figure 5.1-4 Comparison of annual flow duration curves for full open-water flow period. ........ 40
Figure 5.1-5. Range of flow duration curves for the full open water flow period. ...................... 40
Figure 5.1-6. Range of flow duration curves from start of open water flow period to end of May.
............................................................................................................................................... 41
Figure 5.1-7. Range of flow duration curves for June ................................................................. 41
Figure 5.1-8. Range of flow duration curves for July. ................................................................. 42
Figure 5.1-9. Range of flow duration curves for August ............................................................. 42
Figure 5.1-10. Range of flow duration curves for September ..................................................... 43
Figure 5.1-11. Range of flow duration curves for October to end of open water flow period .... 43
Figure 5.2-1. Comparison of sediment mass (sand and larger materials) transported past Susitna
Station by OWFP under existing conditions and Max LF OS-1b ........................................ 44
Figure 5.2-2. Time series of sediment mass (sand and larger materials) transported past Susitna
Station under existing conditions and Max LF OS-1b .......................................................... 45
Figure 5.3-1. Bed elevation and change in 50-years for existing conditions ............................... 46
Figure 5.3-2. Sediment stored in the bed and reach averaged bed change for existing conditions
............................................................................................................................................... 46
Figure 5.3-3. Bed material transport and bed storage for existing conditions ............................. 46
Figure 5.3-4. Bed elevation and change in 50-years for Maximum LF-OS1B conditions .......... 47
Figure 5.3-5. Sediment stored in the bed and reach averaged bed change for Maximum LF-
OS1B conditions ................................................................................................................... 47
Figure 5.3-6. Bed material transport and bed storage for Maximum LF-OS1B conditions ........ 47
Figure 5.3-7. Comparison between existing and Max LF OS-1b conditions incremental (at a
cross section) and cumulative sediment stored in the bed of the Lower River ..................... 48
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page iii September 2014
TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 5.4-1. Comparison of channel hydraulic depth at PRM 29.9 under existing conditions and
Max LF OS-1b ...................................................................................................................... 49
Figure 5.4-2. Comparison of channel hydraulic depth at PRM 30.8 under existing conditions and
Max LF OS-1b ...................................................................................................................... 50
Figure 5.4-3. Comparison of channel hydraulic depth at PRM 31.6 under existing conditions and
Max LF OS-1b ...................................................................................................................... 51
Figure 5.4-4. Comparison of channel hydraulic depth at PRM 32.4 under existing conditions and
Max LF OS-1b ...................................................................................................................... 52
Figure 5.4-5 Comparison of channel average velocity at PRM 29.9 under existing conditions and
Max LF OS-1b ...................................................................................................................... 53
Figure 5.4-6. Comparison of channel average velocity at PRM 30.8 under existing conditions
and Max LF OS-1b ............................................................................................................... 54
Figure 5.4-7. Comparison of channel average velocity at PRM 31.6 under existing conditions
and Max LF OS-1b ............................................................................................................... 55
Figure 5.4-8. Comparison of channel average velocity at PRM 32.4 under existing conditions
and Max LF OS-1b ............................................................................................................... 56
Figure 5.4-9. Time series of channel hydraulic depth at PRM 29.9 under existing conditions and
Max LF OS-1b ...................................................................................................................... 57
Figure 5.4-10. Time series of channel hydraulic depth at PRM 30.8 under existing conditions
and Max LF OS-1b ............................................................................................................... 58
Figure 5.4-11. Time series of channel hydraulic depth at PRM 31.6 under existing conditions
and Max LF OS-1b ............................................................................................................... 59
Figure 5.4-12. Time series of channel hydraulic depth at PRM 32.4 under existing conditions
and Max LF OS-1b ............................................................................................................... 60
Figure 5.4-13. Time series of channel average velocity at PRM 29.9 under existing conditions
and Max LF OS-1b ............................................................................................................... 61
Figure 5.4-14. Time series of channel average velocity at PRM 30.8 under existing conditions
and Max LF OS-1b ............................................................................................................... 62
Figure 5.4-15. Time series of channel average velocity at PRM 31.6 under existing conditions
and Max LF OS-1b ............................................................................................................... 63
Figure 5.4-16. Time series of channel average velocity at PRM 32.4 under existing conditions
and Max LF OS-1b ............................................................................................................... 64
Figure 6.5-1. Tide levels at Anchorage and stage data from PRMs 10.5 to 20.5 ........................ 65
Figure 6.5-2. Detailed plot of tide levels at Anchorage and stage data from PRMs 10.5 to 20.5 66
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page iv September 2014
TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
LIST OF ACRONYMS AND SCIENTIFIC LABELS
Abbreviation Definition
1-D One-dimensional
2-D Two-dimensional
AEA Alaska Energy Authority
cfs cubic feet per second
CIBW Cook Inlet Beluga Whale
FERC Federal Energy Regulatory Commission
ft foot
ft/s feet per second
HEC Hydrologic Engineering Center
HEC-RAS Hydrologic Engineering Center River Analysis System
HEC-ResSim Hydrologic Engineering Center Reservoir Simulation System
ISR Initial Study Report
LiDAR Light Detection and Ranging
LR Lower River
Max LF OS-1b Maximum Load Following Operation Scenario 1B
MVUE Minimum Variance Unbiased Estimator
OWFP Open-Water Flow Period
OS Operation Scenario
PRM Project River Mile
RSP Revised Study Plan
USGS U.S. Geological Survey
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page v September 2014
TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
SUMMARY
This technical memorandum describes the decision of whether to extend the downstream limit of
the 1-D bed evolution model below Susitna Station at PRM 29.9. An earlier decision to extend
the modeling from PRM 87.9 (Sunshine) to PRM 29.9 was made based on appreciable changes
occurring at Sunshine and the potential for appreciable change below that location.
As presented in Initial Study Report (ISR) 6.6 Section 7.1.1.1.2, the primary reason to consider
extending the fluvial geomorphology modeling below PRM 29.9 is to assist in describing the
relationship between river flows, water surface elevation and Cook Inlet Beluga Whale (CIBW)
foraging habitat in the Susitna River. Without actually modeling the area below Susitna Station,
the changes occurring upstream of that location are used to evaluate potential downstream
change.
The metrics that were evaluated included hydrology (peak flows and flow duration curves),
sediment transport (volumes of bed materials comprised of sands and larger sizes), channel
morphology (bed elevation changes and channel width adjustments), and hydraulic conditions
(changes in channel flow velocity and depths). The changes in these variables between existing
conditions (pre-Project) and the Maximum Load Following Operational Scenario 1B (Max LF
OS-1b) were characterized within the context of the natural variability under existing conditions.
If the expected changes due to Project operations are small relative to the range of natural
variability then potential impacts are considered as minor and extension of the 1-D fluvial
geomorphology modeling downstream is not warranted.
The Project-induced changes during the open water flow period are generally reduced flows,
sediment transport, water surface elevations, flow depth, and velocities. The Lower Susitna
River Segment was determined to be generally aggradational for existing conditions. The
reduced flows and sediment transport combined to maintain the aggradational trends of the
Lower River, but at a slightly reduced rate. The Susitna River channel is also expected to narrow
slightly due to changes in channel forming flows.
Although there are consistent reductions in the variables, they are predominantly within the
range of natural variability, both spatially and temporally. Values fall outside (below) the range
of natural variability infrequently and by small amounts. The ranges of variability for Max LF
OS-1b are also similar to the ranges of natural variability based on pre-Project conditions. Based
on the results regarding change and variability, combined with the finding that the Middle River
will continue the aggradational trends, AEA does not recommend extending the fluvial
geomorphic modeling or associated 1-D hydraulic modeling below PRM 29.9.
Because the tide range in Cook Inlet is extreme and highly variable, the expected relative
changes in hydrology, hydraulics, sediment transport, and channel morphology will be reduced
further when the tidal zone is reached. The large range in tides combined with continually
varying river flows means that the location of significant tidal influence varies greatly under
existing conditions. Because expected changes resulting from Project operations will be very
small compared to the large range of natural variability in the tidal zone, AEA recommends that
no tidal hydrodynamic modeling be conducted in the lowest portions of the Susitna River.
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page vi September 2014
TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
1. INTRODUCTION
The Alaska Energy Authority (AEA) is preparing a License Application that will be submitted to
the Federal Energy Regulatory Commission (FERC) for the Susitna-Watana Hydroelectric
Project using the Integrated Licensing Process. The Project is located on the Susitna River, an
approximately 320-mile-long river in the Southcentral region of Alaska. The Project’s dam site
will be located at Project River Mile (PRM) 187.1. The results of this study will provide
information needed to support the FERC’s National Environmental Policy Act analysis for the
Project license.
On December 14, 2012, AEA filed its Revised Study Plan (RSP) with the FERC for the Susitna-
Watana Hydroelectric Project (FERC Project No. 14241), which included 58 individual study
plans (AEA 2012). Included with the RSP is the Fluvial Geomorphology Modeling below
Watana Dam Study (RSP Study 6.6). RSP Study 6.6 focuses on the modeling planned for
assessing the effects of the proposed Project and its operations on the fluvial geomorphology of
the Susitna River. There are three study components to the Fluvial Geomorphology Modeling
Study below Watana Dam (Fluvial Geomorphology Modeling Study):
1. Bed Evolution Model Development, Coordination, and Calibration
2. Model Existing and with-Project Conditions
3. Coordination and Interpretation of Model Results
Several decision points have been reached in the Bed Evolution and Model Development,
Coordination, and Calibration study component, and one significant decision point remains. The
past decision points include:
Selection of Focus Areas for the 2-D Bed Evolution Model
Extension of the 1-D Bed Evolution Model downstream from PRM 79 to PRM 29.9
Selection of Middle River tributaries to complete the tributary delta modeling
Selection of Lower River tributaries to support investigation of potential changes in
geomorphology that might influence adult salmon habitat at their mouths, in particular,
holding and ability to access the tributaries (Study 9.12)
Selection of 1-D and 2-D Bed Evolution Modeling software
As presented in the Initial Study Report (ISR) Section 7.1.1.1.2, the last decision point remaining
in RSP Study 6.6 Section 6.6.3.2 for this study component is the potential need to extend the
downstream limit of the 1-D Bed Evolution Model below Susitna Station (PRM 29.9). This
decision will be based on characterizing geomorphic and hydraulic effects of the Project using
specific numerical criteria. The geomorphic criteria represent the potential for long-term channel
change related to alteration of flows and sediment supply due to Project operations that could
result in changes to habitat. The hydraulic criteria represent the potential for immediate changes
in habitat related to alteration in flows due to Project operations. The magnitudes of change in
the criteria, reflecting differences between pre-Project (existing) conditions and the maximum
load-following operation scenario (Max LF OS-1b) during the open-water period, were
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 1 September 2014
TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
compared to the range of natural variability, which is largely driven by hydrology. The range of
natural variability was characterized by considering spatial and temporal changes in the existing
conditions.
The four decision point criteria are:
1. Changes in flow at Susitna Station (PRM 29.9) and associated potential for channel width
adjustment
2. Change in sediment transport mass (sand and larger materials) over the open-water period
3. Difference in modeled bed elevations represented by channel aggradation or degradation
4. Estimated change in flow depth and velocity
Since the current downstream limit of the 1-D Bed Evolution Model is PRM 29.9, the model
could not be used directly to evaluate Project effects downstream of PRM 29.9. Instead, the
results of the model in Geomorphic Reach LR-4 (PRM 44.6 to PRM 32.3) and the portion of LR-
5 (PRM 32.3 to PRM 23.9) upstream of PRM 29.9 were compared to estimate the potential for
changes to the geomorphology and hydraulics downstream of PRM 29.9. The lower portion of
LR-6 is of particular interest in estimating the potential for change because this is the area of
most interest in terms of Cook Inlet Beluga Whale (CIBW) foraging habitat.
2. OBJECTIVE
The objective of the analyses documented in this technical memorandum is to provide a rational
basis for deciding whether to extend the 1-D Bed Evolution Model downstream of PRM 29.9 to
assist in describing the relationship between river flows, water-surface elevations, and CIBW
foraging habitat in the Susitna River.
3. STUDY AREA
As shown in Figure 3.0-1, the Susitna River, located in Southcentral Alaska, drains an area of
approximately 20,010 square miles and flows about 320 miles from its headwaters at the Susitna,
West Fork Susitna and East Fork Susitna glaciers to the Cook Inlet (Curran 2012). The Susitna
River basin is bounded on the west and north by the Alaska Range, on the east by the Talkeetna
Mountains and Copper River Lowlands and on the south by Cook Inlet. The highest elevations
in the basin are at Mt. McKinley at 20,320 feet while its lowest elevations are at sea level where
the river discharges into Cook Inlet. Major tributaries to the Susitna River between the
headwaters and Cook Inlet include the Chulitna, Talkeetna and Yentna Rivers that are also
glacially fed in their respective headwaters. The basin receives, on average, 35 inches of
precipitation annually with average annual air temperatures of approximately 29oF.
The overall study area extends from Cook Inlet to the Maclaren River confluence at PRM 261.3.
Within the geomorphology study area, the Susitna River has been subdivided into three segments
whose general characteristics are governed by the basin geology as described by Wilson et al.
(2009). The segments are referred to as the Upper, Middle, and Lower Susitna River Segments
and are identified in Figure 3.0-1 with the associated extents:
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 2 September 2014
TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
• Upper Susitna River Segment: Maclaren River confluence (PRM 261.3 / RM 260)
downstream to the proposed Watana Dam site (PRM 187.1 / RM 184).1
Middle Susitna River Segment: Proposed Watana Dam site (PRM 187.1 / RM 184)
downstream to the Three Rivers Confluence (PRM 102.4 / RM 98.5).
Lower Susitna River Segment: Three Rivers Confluence (PRM 102.4 / RM 98.5)
downstream to Cook Inlet (PRM 3.3 / RM 0).
The study area for this technical memorandum encompasses the Lower and Middle Susitna River
Segments.
4. METHODS
This section describes the methods used to analyze criteria to decide whether to extend the 1-D
Bed Evolution Model downstream of PRM 29.9.
4.1. Overview
The methods section begins with a summary of existing information, then describes how metrics
for each criterion were quantified, and how natural variability was characterized. The existing
information uses the most current information available including information in recently
submitted (2014) Technical Memorandums. The memorandum describing the development and
calibration of the 1-D bed evolution models of the Middle and Lower Susitna River Segments
(from Watana Dam site to PRM 29.9) and 2-D bed evolution model in the Middle River Focus
Areas will be submitted in Q4 2014 (Study 6.6 ISR Sections 7.2.1.2.1 and 7.2.1.2.2). Therefore,
a brief summary of the development, calibration and results of this model are included in this
section.
4.2. Existing Information
The analyses of the decision point criteria were related to multiple previous and ongoing efforts.
A summary of the existing information related to the analyses is presented below.
4.2.1. Study Area Extents
In March 2013, R2 Resource Consultants, Inc. (R2) prepared a Technical Memorandum
documenting Selection of Focus Areas and Study Sites in the Middle and Lower Susitna River for
Instream Flow and Joint Resource Studies- 2013 and 2014 (R2 2013). One objective of this
technical memorandum was to discuss the rationale and criteria considered for extending the IFS
related studies into the Lower River Segment. The Revised Study Plan described the
downstream boundary of the Study Area as RM 75 (PRM 79) because information indicated that
1 Note: Project River Miles (PRMs) are the river mile system used for the current Susitna-Watana Project. River Miles (RMs)
were the river mile system used in the 1980s project. The PRM delineation starts about 3 miles farther into Cook Inlet than the
RMs and has a slightly different thalweg than that of the 1980s. Thus, PRM values are generally 3 to 4 miles higher than the RM
values. Because this analysis is a temporal comparison, both systems are referenced.
Susitna-Watana Hydroelectric Project Alaska Energy Authority
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
the hydraulic effects of the Project below the Three Rivers Confluence are attenuated (See RSP
Study 8.5 Section 8.5.3). As noted in RSP Study 8.5 Section 8.5.3, the extent of the studies
conducted in the Lower River Segment was based upon consideration of the following six
criteria.
1. Magnitude of daily stage change due to load-following operations relative to the range of
variability for a given location and time under existing conditions (i.e., unregulated flows)
2. Magnitude of monthly and seasonal stage change under Project operations relative to the
range of variability under unregulated flow conditions
3. Changes in surface area (as estimated from relationships derived from LiDAR and
comparative evaluations of habitat unit area depicted in aerial digital imagery under different
flow conditions) due to Project operations
4. Anticipated changes in flow and stage to Lower River off-channel habitats
5. Anticipated Project effects resulting from changes in flow, stage and surface area on habitat
use and function, and fish distribution (based on historical and current information
concerning fish distribution and use) by geomorphic reaches in the Lower River Segment
6. Initial assessment of potential changes in channel morphology of the Lower River based on
Project-related changes to hydrology and sediment supply in the Lower River.
As a result of analyses of these six criteria, AEA confirmed that studies should be expanded in
the Lower River Segment. During the February 14, 2013 TWG meeting, this decision was noted
and an initial plan presented for commencing such studies in 2013 and completing the studies in
2014 (R2 2013).
The sixth criterion considered in defining the downstream extent of the study area in the Lower
River Segment was based on the Reconnaissance Level Assessment of Potential Channel Change
in the Lower Susitna River Sediment (Tetra Tech 2013a). This technical memorandum evaluated
potential Project-related changes in morphology of the Lower River to determine whether
portions of the Fluvial Geomorphology Modeling Study and other studies need to be extended
downstream in the Lower River. Results from the evaluation served as the basis for the
conclusion that the 1-D Bed Evolution Modeling should be extended approximately 50 miles
farther downstream to Susitna Station (PRM 29.9). This conclusion was based largely on initial
results suggesting the portion of the Lower River Segment below Sunshine could tend toward
degradation and channel narrowing, which warranted more detailed analyses to further
investigate potential Project effects below Sunshine (PRM 88). Subsequent to these analyses,
the sediment rating curves, post-Project hydrology, and bed material gradations used in the
analyses were updated, and results of more detailed analyses are presented and discussed in
Section 5 and Section 6.
4.2.2. Sediment Transport
Preliminary estimates of the overall sediment balance in the Middle and Lower River segments
under pre-Project conditions and the potential magnitude of the changes that could occur under
Maximum Load Following Operation Scenario (OS)-1 hydrologic conditions are presented in
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Development of Sediment-Transport Relationships and an Initial Sediment Balance for the
Middle and Lower Susitna River Segments (Tetra Tech 2013b). The sediment load rating curves
and preliminary estimates of the overall sediment balance under pre-Project conditions were
updated in 2014 Update of Sediment-Transport Relationship and a Revised Sediment Balance for
the Middle and Lower Susitna River Segments Technical Memorandum (Tetra Tech 2014a). The
updated sediment load rating curves reflect additional measurements collected by the U.S.
Geological Survey (USGS) in 2012 and 2013; while the USGS will collect additional
measurements in 2014 that will be reviewed, AEA does not anticipate further revision of the
sediment rating curves. Linear regressions of sediment transport and discharge were calculated
at five USGS gaging stations along the mainstem Susitna River: Denali, Cantwell (Vee Canyon),
Gold Creek, Sunshine, and Susitna Station, and on the three largest tributaries: the Chulitna,
Talkeetna, and Yentna Rivers. The minimum variance unbiased estimator (MVUE) technique
was used to convert the sediment load regression lines into unbiased sediment load rating curves.
The sediment rating curves were applied to develop the sediment balance along the Susitna River
mainstem, and to quantify sediment inflows to the 1-D Fluvial Geomorphology Modeling.
Unlike the 2013 analyses, operational conditions were not considered as part of the 2014
analyses because the effect of these operational conditions will be based on results of simulations
carried out using the 1-D Bed Evolution Model.
4.2.3. Pre-Project and Post-Project Hydrology
The pre-Project and Maximum Load Following OS-1b hydrology for the mainstem Susitna River
and tributaries was prepared within the Fish and Aquatics Instream Flow Study (Study 8.5) as
described in ISR Study 8.5 Part C Appendix K. Project operations will cause hourly, daily, and
seasonal changes in the Susitna River flows because releases from the Project are driven by
power generation requirements and constrained by flow requirements to protect non-power
resources such as fish and aquatic habitat. A HEC-ResSim model was developed to simulate the
releases from the Project and to simulate flow and stage hydrographs downstream from the
Project under pre-Project and post-Project conditions (MWH 2012). A 61-year period extending
from Water Years 1950 through 2010 was established as the period of record for the Project;
daily flow records were developed along the mainstem and major tributaries over this period
(Curran 2012). The extended flow series data were used to estimate pre-Project daily average
flows at the dam site by drainage-area-scaling the flows at Gold Creek and Cantwell. The
extended flow series were also input to the HEC-ResSim model, and the post-Project releases
were based on Max LF OS-1b. The pre-Project and post-Project daily flows were converted to
hourly flows to preserve the daily average flow and provide smooth and continuous flow
hydrographs. Lateral inflow hydrographs were calculated first on a daily basis, and then on an
hourly basis. Potential diurnal flow fluctuations were not synthesized in version 1 of the flow
records; version 2 accounts for potential diurnal flow fluctuations; version 3 will incorporate
measurements collected in 2013 and 2014.
Initially the HEC-ResSim model (MWH 2012) was used to simulate flow and stage hydrographs
downstream from the Project under pre-Project and post-Project conditions. These simulations
were subsequently replaced by the development and application of an unsteady-flow HEC-RAS
Open-water Flow Routing Model. The Open-water Flow Routing Model simulates the
translation and attenuation of changes in flow during the open-water period (defined as May 23
through October 27) associated with Project operations to locations downstream of the dam site.
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Version 1 of the Open-water Flow Routing Model extended from the dam site at PRM 187.1
downstream to PRM 80.0. Version 2 of the Open-water Flow Routing Model extends
downstream to Susitna Station (PRM 29.9). Results from Version 1 of the model showed that
Project effects were still apparent at the PRM 80.0; consequently, Version 2 of the model was
extended downstream. Version 3 of the Open-water Flow Routing Model will be completed in
2015; this will be the final version of the model and it will include (1) surveys and calibration
data collected in 2014, (2) revisions to tributary lateral inflows (i.e., Version 3), (3) LiDAR data
collected in 2014, and (4) incorporation of potential diurnal flow fluctuations.
While the Open-water Flow Routing Model assumed a constant duration open-water period each
year (May 23 through October 27), it is known that the spring break-up and fall freeze-up occur
at different times each year in response to climatic conditions. Study 7.6 Ice Processes in the
Susitna River provided specific dates of break-up and freeze-up for the selected 50 years of
open-water periods for use in the 1-D Bed Evolution Modeling.
4.2.4. Changes in Channel Width
In the Mapping of Geomorphic Features and Turnover within the Middle and Lower Susitna
River Segments from 1950s, 1980s and Current Aerials Technical Memorandum (Tetra Tech
2014b), measurements of non-vegetated channel width were calculated by geomorphic reach.
The total non-vegetated channel area per geomorphic reach was divided by the reach length to
calculate the average non-vegetated channel width. Changes in average width of Lower River
geomorphic reaches are apparent between the 1950s aerials and the 1980s aerials, as well as
between the 1980s aerials and the current aerials (Table 4.2-1). This analysis shows fluctuations
in channel width over the two successive three-decade periods between the 1950s and current.
4.2.5. 1-D Bed Evolution Model
As described in the Fluvial Geomorphology Modeling Approach Technical Memorandum (Tetra
Tech 2013c), a numerical 1-D Bed Evolution Model was developed to simulate reach-scale
sediment transport and morphologic change in the Susitna River and tributaries associated with
Project operations over the 50-year term of a FERC license. The development, calibration, and
application of this model will be documented in a technical memorandum submitted at the end of
2014; a brief summary is presented below since the model results are a key component of the
analyses carried out to support the determination of whether to extend the 1-D Bed Evolution
Model downstream of PRM 29.9.
4.2.5.1. Model Development
The 1-D Bed Evolution Model was developed using the U.S. Army Corps of Engineers
Hydrologic Engineering Center River Analysis System (HEC-RAS) version 5.0.0 Beta (August
2014 computational engines). This software was made available by U.S. Army Corps of
Engineers Hydraulic Engineering Center (HEC) because of the new unsteady-flow sediment
routing routines, which are ideally suited to the Susitna River and tributaries. The 1-D Bed
Evolution Model is comprised of four main reaches: (1) Middle Susitna River (PRM 187.1 to
PRM 107.1); (2) Lower Susitna River (PRM 107.1 to PRM 29.9); (3) the lower extent of the
Chulitna River (PRM 18.1 to PRM 0.0); and, (4) the lower extent of the Talkeetna River (PRM
4.7 to PRM 0.0). Each reach is defined by cross-sections perpendicular to the primary flow path
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and located to capture key hydraulic controls of each system. Three secondary flow paths (flow
splits) are included in key locations in the Middle River and two in the Lower River. The
Middle River is defined by 166 cross-sections with an average spacing of about 3,000 feet. The
Lower River is defined by 93 cross-sections with an average spacing of about 4,000 feet. The
Talkeetna River is defined by 14 cross-sections with an average spacing of about 1,800 feet. The
Chulitna River is defined by 34 cross-sections with an average spacing of about 2,800 feet.
Overbank topography for the Middle and Lower River was available from LiDAR mapping
collected between May 2011 and October 2011 (ISR Study 6.6, Part A, Section 5.1.9.5). LiDAR
for the Talkeetna and Chulitna overbank topography was collected in September 2013 (ISR
Study 6.6, Part A, Section 5.1.9.5). Bathymetric data for each reach were surveyed at each
cross-section by Geovera (ISR Study 8.5, Part A, Section 5.3.1). Survey efforts for the
Talkeetna and Chulitna reaches were conducted in August 2013. The Middle and Lower River
reaches were surveyed between June 2012 and August 2013. Between PRM 165.9 and PRM
154.6 (Devils Canyon), the Middle River is inaccessible to surveyors so a simplified trapezoidal
geometry developed as part of ISR Study 7.6 Ice Processes in the Susitna River was used for
bathymetry in this area. Throughout all of the model reaches, at each cross-section the
bathymetric surveys were merged with the LiDAR topography to create continuous geometry.
Following guidance provided by staff at HEC for using the mobile-bed capabilities within HEC-
RAS, the number of points that define each cross-section was filtered to reduce the potential for
bed adjustment issues while still preserving the essential geometry. In areas with dramatic
topographic changes between cross-sections, additional sections were included to keep
conveyance ratios within reasonable limits. Geometry for these new cross-sections was
estimated using a feature within HEC-RAS to linearly interpolate geometry between bounding
surveys. Ineffective flow areas were used in the Middle River along the tops of islands to
simulate the high roughness and low conveyance over these features. A simplified scheme of
overbank and channel roughness values was initially based on field observations and refined
during the calibration of each model.
The hydraulic model is the foundation of the sediment routing model; additional functionality
and associated inputs were specified to develop the sediment routing model. Sediment supplies
at model boundaries, bed material gradations and layering, the bed sorting method, the sediment
transport function, and the fall velocity method are the main components of the sediment routing
model that need to be specified. The bias-corrected sediment rating curves developed as part of
the 2014 Update of Sediment-Transport Relationship and a Revised Sediment Balance for the
Middle and Lower Susitna River Segments Technical Memorandum (Tetra Tech 2014a) were
specified as the bed material rating curves at major inflows (i.e., the dam site, the Chulitna River,
the Talkeetna River, and the Yentna River). Bed material gradations for surface and subsurface
materials were based on field measurements documented in both ISR Study 6.6 Part A and the
Winter Sampling of Main Channel Bed Material Technical Memorandum (Tetra Tech 2014c).
Due to the armored channel bed throughout the Middle River the Exner 5 bed sorting method
was selected and it was coupled with a user-defined sediment transport function based on the
Ackers-White function. Given the sand- and gravel-dominated bed material in the Lower River
Segment, the Exner 7 bed sorting method was selected and it was coupled with the Ackers-White
transport function (Ackers and White 1973; Ackers 1993).
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4.2.5.2. Hydraulic Calibration and Validation
After development and initial testing of the 1-D Bed Evolution Model, the simulated hydraulics
were calibrated and validated. Since flood flows have the greatest potential to mobilize bed
material and transport sediment, it was important to focus the calibration and validation efforts
on flood hydrographs. Calibration was carried out using measured flow and water-surface
elevations during periods of high flow conditions (Table 4.2-2). Calibration periods were
selected based on availability of measured data and the magnitude of the flood event. Since
calibration data in the Middle River and Lower River were available for different periods, these
two reaches of the model were calibrated independently. The hydraulic calibration was then
validated using flow data collected at the USGS stations between July 1981 and September 1981
(Table 4.2-2), which is the representative wet year. Calibration of the Talkeetna River reach and
Chulitna River reach was not conducted because data for the calibration and validation time
periods is only available for these rivers at the USGS gaging stations, which are used as
boundary conditions. The Chulitna and Talkeetna River reaches will be calibrated using
surveyed water surfaces when these tributaries are included in the final 1-D model.
The required boundary conditions for the calibration and validation events came from a variety
of sources. The mainstem Susitna River inflow at the dam site (PRM 187.1) came from the
USGS gage above Tsusena Creek (Gage No. 15291700) for the calibration event and from
results of Study 8.5 for the validation event. Ungaged tributary flows along the Middle and
Lower River were calculated for the calibration events following the method described
previously in ISR Study 8.5, Part C, Appendix K; tributary inflows for the validation event were
based on results from Study 8.5. USGS gages on the Talkeetna River (Gage No. 15292700) and
Chulitna River (Gage No. 15292410) provided additional input hydrographs for these two model
reaches, respectively, for the calibration event; for the validation event the inflows were provided
by Study 8.5. The downstream boundary condition was created by extending the model below
PRM 29.2 along a normal depth slope of 0.0003492 (ft/ft). Roughness for these extended cross-
sections was then varied with discharge so the modeled water-surface elevation at PRM 29.9
matched the current rating curve published by the USGS for Susitna Station (Gage No.
15294350).
Calibration of the simulated hydraulics to the observed data was achieved primarily through
adjustments to the base channel roughness coefficient. In the Middle River reach, where it was
supported by observed data, further refinement was achieved by allowing the channel roughness
to decrease with increasing discharge, reflecting the reduction in grain resistance due to the
greater submergence of the bed material. Overbank roughness values were not varied during
calibration and ranged between 0.13 and 0.15. Consistent with the Open-water Flow Routing
Model (ISR Study 8.5, Part C, Appendix K) the final calibrated main channel roughness values
varied between 0.032 and 0.035 in the Middle River, except through Devils Canyon where
higher roughness values (0.035 to 0.050) were necessary to maintain model stability. In the
Lower River, the calibrated main channel roughness values varied between 0.025 and 0.032.
Results indicate good correlation between observed and simulated flow and stage hydrographs
(Figure 4.2-1 and Figure 4.2-2). Additionally, point-in-time water-surface elevation
measurements from the ground surveys were also compared with model results. In the Middle
River simulated water-surface elevations were within one foot of the observed value at 33 of the
36 (92%) locations (Figure 4.2-3). The average difference for all 36 observed points was -0.13 ft
with an RMS of 0.65 ft. In the Lower River, 91 of the 119 (76%) modeled water-surface
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elevations were within one foot of the observed value (Figure 4.2-4), with a total average
difference of -0.08 ft and RMS of 0.95 ft. The calibrated model was validated, and the results
compare favorably with observations; an example of the validation at Gold Creek is provided in
Figure 4.2-5. Based on the agreement between the simulated and observed hydraulics, the
hydraulic calibration and validation were judged successful. The model provided a solid
foundation for incorporating the sediment routing capabilities.
4.2.5.3. Sediment Routing Calibration
The sediment routing was calibrated separately for the Middle River and Lower River reaches.
In the Middle River reach, the sediment rating curve simulated at the USGS gaging station at
Gold Creek was compared to measured sediment transport rates as described in the 2014 Update
of Sediment-Transport Relationship and a Revised Sediment Balance for the Middle and Lower
Susitna River Segments Technical Memorandum (Tetra Tech 2014a); the gradation of the
simulated bed material load was also compared to the measured gradations. Additionally,
simulated changes in bed profile were compared to observed changes in profile documented in
the Susitna River Historical Cross Section Comparison, 1980s to Current Technical
Memorandum (Tetra Tech 2014d). In the Lower River reach, the simulated sediment rating
curve and transported gradations at the USGS gaging stations at Sunshine and Susitna Station
were compared to measured sediment transport rates and gradations presented in Tetra Tech
(2014a).
The observed changes in bed profile along the Middle River between the early 1980s surveys
and the current surveys are described in Tetra Tech (2014d). Most of the profile changes are less
than about 2 feet with many portions of the profile showing only minor changes of less than one
foot. Spatially consistent patterns of degradation or aggradation are not apparent, indicating that
the thalweg profile has generally been dynamically stable over the past three decades.
For the simulation of the selected 50 open-water flow periods in the Middle River, the bed
material rating curve simulated at Gold Creek closely matches measured transport rates (Figure
4.2-6) and the average transported gradation over the entire simulation is similar to measured
gradations over a range of flows (Figure 4.2-7). The simulated changes in bed elevation ranged
from 1.8 to -2.0 feet, with 90 percent of the cross sections exhibiting changes between 0.9 and -
1.1 feet; these changes compare favorably with the trends presented in Tetra Tech (2014d)
comparing the 1980s surveys to current surveys. In the Lower River, the bed material rating
curve simulated at Sunshine also closely matches measured transport rates (Figure 4.2-8) and the
average transported gradation over the entire simulation is similar to the measured gradations
over a range of flows (Figure 4.2-9). The same patterns are evident at Susitna Station (Figure
4.2-10 and Figure 4.2-11). Based on the model similarities to the measured transport rates and
gradations, and the similarity in the simulated and observed changes in bed profile in the Middle
River, the sediment routing models were successfully calibrated and thus judged suitable for
simulating changes in the sediment balance and geomorphology of the Susitna River associated
with proposed Project operations.
4.3. Decision Point Criteria
As presented in the Initial Study Report (ISR) Section 7.1.1.1.2 the four decision point criteria
for evaluating whether to extend the 1-D Bed Evolution Model below PRM 29.9 are:
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1. Changes in flow at Susitna Station (PRM 29.9) and associated potential for channel width
adjustment
2. Change in sediment transport mass (sand and larger materials) over the open-water period
3. Difference in modeled bed elevations represented by channel aggradation or degradation
4. Estimated change in flow depth and velocity
The decision point criteria were selected to assist in describing the relationship between river
flows, water surface elevation and Cook Inlet Beluga Whale (CIBW) foraging habitat in the
Susitna River. The evaluation of these criteria includes determining the magnitude and natural
variability of metrics associated with each of the criteria. The magnitude of the change in each
of the metrics due to the Maximum Load Following OS-1b scenario, as well as variability, is
determined using consistent methods. Each of the metrics and the methods for their
determination are described in the following sections.
4.3.1. Approach for Changes in Flow at Susitna Station and Associated
Potential for Channel Width Adjustment
Hydraulic geometry relationships (Leopold and Wolman 1953; Langbein 1964; Emmett 1972;
Parker 1979; Andrews 1984; Hey and Thorne 1986; Julien and Wargadalam 1995) correlate
channel width to dominant discharge, which is often considered to be approximated by flows in
the range of 1.5- to 5-year average annual recurrence interval. Flow frequency analysis was used
with simulated annual peak flows at Susitna Station (PRM 29.9) under existing conditions and
Max LF OS-1b hydrology. The annual peak flows area based on the hourly maximum
discharges simulated over the 61-year period of record using the version of the 1-D Bed
Evolution Model with calibrated hydraulics but without sediment routing. Based on the changes
in 1.5-, 2- and 5-year recurrence interval flows, and the typical hydraulic geometry relationship
that channel width is proportional to approximately the square-root of the dominant discharge,
channel width changes were computed using Equation 4.1, where Qi denotes the i-year peak
flow:
𝐶ℎ𝑎𝑛𝑛𝑒𝑙 𝑊𝑖𝑑𝑡ℎ𝑤𝑖𝑡ℎ−𝑃𝑟𝑜𝑗𝑒𝑐𝑡𝐶ℎ𝑎𝑛𝑛𝑒𝑙 𝑊𝑖𝑑𝑡ℎ𝐸𝑥𝑖𝑠𝑡𝑖𝑛𝑔= �𝑄𝑖(𝑤𝑖𝑡ℎ−𝑃𝑟𝑜𝑗𝑒𝑐𝑡)𝑄𝑖(𝐸𝑥𝑖𝑠𝑡𝑖𝑛𝑔)
The natural variability of existing channel width was evaluated in several ways. Because the
decision is whether to extend below PRM 29.9, the non-vegetated channel width was measured
from PRM 29 down to PRM 2.5 at 0.5 mile intervals. This provides information on spatial
trends in channel width, but also the variability in the area of interest. Temporal trends in
channel width were based on measurements of active channel width throughout the Lower
Susitna River for the 1950s, 1980s, and 2012 (Tetra Tech, Inc. 2014b). Another method for
evaluating temporal variability in channel width is to review temporal variability in discharge.
For this method the Q2 was computed at 10-year intervals throughout the 61-year period of
record. Periods of higher discharge would tend to cause channel widening and lower flows
would produce channel narrowing.
Open-water flow period (OWFP) flow duration curves at Susitna Station were developed to
understand the temporal change in flow distribution. Monthly and OWFP curves were
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developed using simulated instantaneous hourly flows for Existing conditions and Max LF OS-
1b for each of the 50 years used in the 1-D Bed Evolution Model. These 50 curves were
summarized using the maximum, minimum and quartiles of the flows over the range of flow
exceedance values. The curves are used not only to compare with-Project conditions to Max LF
OS-1b, but also to evaluate the range of variability in both conditions.
The method for developing each flow duration curve followed the standard practice of
determined the amount of time any level of flow was exceeded. Although flow duration curves
are typically developed for long-term periods, annual curves were developed to evaluate the
range of flow conditions that would occur under operational versus pre-Project (existing)
conditions. Flow duration curves were developed for monthly as well as the full open-water
flow period. Each curve represents the amount of time flows are exceeded for each year in the
50 open-water flow periods over the period of interest (month or total). This process not only
allows for comparisons between existing conditions and Max LF OS-1b, but also provides
information on the range of temporal variability within each of the operational scenarios.
4.3.2. Approach for Change in Sediment Transport Mass Over the Open-Water
Flow Period
The sediment load transported past Susitna Station (PRM 29.9) under existing conditions and
Max LF OS-1b during the open-water flow period of the representative wet, average, and dry
years was simulated using the 1-D Bed Evolution Model. The model was run for the 50 selected
open-water flow periods, which allowed the results for the representative years to be extracted
along with the average over all 50 years.
To provide a means for interpreting the effect of Project operations under Max LF OS-1b on
transported sediment mass over the open-water flow periods, the expected range of natural
variability in this parameter was quantified. There is no spatial component to the variability; the
analysis is restricted to the location of Susitna Station because (1) this is the most downstream
location, (2) measured data are available at the USGS gaging station, and (3) this location
captures the hydrologic and sediment influences of the Yentna River. To capture temporal
variability, the masses transported over the 50 open-water flow periods were compared.
4.3.3. Approach for Difference in Modeled Bed Elevations Represented by
Channel Aggradation or Degradation
The 1-D Bed Evolution Model was run for 50 years of open-water flow periods to simulate
changes in bed elevations throughout the Lower River. Daily output was generated that includes
both instantaneous (final hour of each day) and cumulative results for each cross section in the
model. These results are used to evaluate the spatial and temporal trends in sediment loads and
channel aggradation and degradation. Aggradation and degradation are evaluated in terms of
mean bed elevation (i.e., mean elevation of all points within the bank stations) change at
individual cross sections, geomorphic reach-averaged bed elevation change, and the volume
(mass) storage along the river.
The 1-D Bed Evolution Model was used to simulate existing and Max LF OS-1b conditions over
the selected 50-year period of record. For the Max LF OS-1b conditions no sand or gravel load
was included with these flow releases at the Watana Dam site.
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4.3.4. Approach for Estimated Change in Flow Depth and Velocity in the Lower
Portion of Geomorphic Reach LR-6 Inferred from Changes Modeled in
the Upper Portion of LR-5
Flow depth was quantified using the simulated channel hydraulic depth; velocity was quantified
using the simulated channel average velocity. Only the upper portion of LR-5 between PRM
29.9 and the Yentna River confluence was considered. The Yentna River on an average annual
basis contributes approximately 40 percent of the flow and approximately 55 percent of the bed
material load passing Susitna Station (Tetra Tech 2014a). Estimates of the potential for changes
to depth and velocity in LR-6, which includes inflow and sediment loading from the Yentna
River, are most similar to the modeled conditions downstream of the Yentna River confluence.
Therefore, the changes in depth and velocity were characterized using all model results between
PRM 29.9 and the Yentna River confluence. Four surveyed cross sections within this reach were
included in the 1-D Bed Evolution Model: PRM 29.9, PRM 30.8, PRM 31.6, and PRM 32.4.
While it was assumed that hydrologic effects of Project operations would be the primary factor
that contributes to flow depth and velocity changes, the 1-D Bed Evolution Model was used to
simulate the effects of Project operations. Flow depth and velocity were compared under
existing conditions and Max LF OS-1b for the representative wet, average, and dry years. The
detailed hydraulic results, at 2-hour intervals, were extracted during the representative years from
the 50-year simulation of open-water flow periods. This allowed for geomorphic effects to be
combined with the hydrologic effects of Project operations when making comparisons.
To provide a means for interpreting the effect of Project operations under Max LF OS-1b on
flow depth and velocity, the expected range of natural variability in these parameters was
quantified. By considering four cross sections (i.e., PRM 29.9, 30.8, 31.6, and 32.4), which
reflect substantial variability in channel morphology, spatial variability in flow depth and
velocity was inherent in this analysis. To capture temporal variability, the flow depth and
velocity were compared over the open-water flow period during the representative wet, average,
and dry years. Because the duration of the open-water flow period varies, the temporal
variability was considered relative to days from the start of the open-water flow period.
4.4. Variances
There are no variances from the planned methods presented in ISR Study 6.6 Section 7.1.1.1.2
(AEA 2014).
5. RESULTS
This section presents the results of the analyses of the metrics associated with the four decision
point criteria.
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5.1. Changes in Flow at Susitna Station and Associated Potential
for Channel Width Adjustment
Table 5.1-1 includes 1.01- through 100-year recurrence interval flows for Sunshine and Susitna
Station for Existing and Max LF OS-1b scenarios. Over this range of recurrence intervals in
response to Project operations, flows decrease by 21 percent to 15 percent at Sunshine and by 13
percent to 5 percent at Susitna Station. The largest changes are for more frequent flows (1.01- to
5-year). The percent changes are smaller at Susitna Station due to the influence of tributary
flows between Sunshine and Susitna Station, with the Yentna River providing the largest
contributions. The expected reduction in width at Sunshine is approximately 10 percent and at
Susitna Station is less than 6 percent (Table 5.1-1, 1.5- to 5-year recurrence interval flows).
These width changes would likely require decades to occur, so these should be viewed as trends
that would be established by a new flow regime.
Temporal variability in width can result from changes in flow when higher flows would tend to
cause widening and lower flows would tend to cause channel narrowing. Table 5.1-2 shows
decadal variations in the 2-year flow (as a surrogate for the dominant discharge) at Susitna
Station. This table shows that channel would tend to widen and narrow over the 61-year period
as the 2-year discharge varies. Changes in the decadal 2-year discharges range from -8 to +11
percent of the long-term average resulting in width changes ranging from -3 to +5 percent. The
values are also shown for Max LF OS-1b conditions, which are also based on comparisons with
the long-term Existing conditions value. These show 2-year discharge ranging from -19 to +6
percent and corresponding width change ranging from -10 to +3 percent. Table 5.1-2 is intended
to show that channel widths are likely to adjust (widen or narrow) over time. Widening can
occur progressively or in single, extreme events. Narrowing may require longer time periods for
vegetation growth and sediment accumulation to occur.
Spatial variability of existing channel width below PRM 29.9 was evaluated by measuring the
non-vegetated width from 2012 aerial photographs at 0.5 mile intervals. The locations of the
measurements are shown on LiDAR base maps in Figures 5.1-1 and 5.1-2. Where there are
multiple channels the total width is based on the sum of the individual channel widths: vegetated
islands are excluded. These measurements and a power-fit trend line are shown in Figure 5.1-3.
Width generally increases with distance downstream of Susitna Station but varies considerably
between adjacent locations. Width increases rapidly below PRM 10, doubling between PRM 10
and PRM 5 and more than doubling again from PRM 5 to PRM 2.5.
Although channel width increases with increasing channel-forming discharge, other factors
influence channel width along a river. These include bed and bank materials, vegetation,
geologic controls, aggradation/degradation, and channel gradient. Below PRM 30, the width
increases are probably due to decreasing gradient, sediment deposition and tidal backwater. The
rapid increase below PRM 10 in channel width in the downstream direction is probably related to
these factors plus increasing channel forming discharge due to combined river flows and tidal
currents.
In Figure 5.1-3, a downward shift of 6 percent in the trend line is shown to represent the
tendency to narrow under Max LF OS-1b. The 6 percent value is a conservative, long-term
average and would only apply to the river-dominated areas. Where tidal flows (flood and ebb
currents) add to the river discharge the relative changes in flow and channel width would be
reduced.
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Flow duration curves were developed in order to understand the temporal change in flow
distribution, as well as the variability (pre- and post-Project) in flow conditions. Individual flow
duration curves for each of the 50 years were developed for monthly and open-water flow
periods. The model results used to develop the flow duration curves were hourly instantaneous
flows at Susitna Station (PRM 29.9). Figure 5.1-4 shows all 50 annual (full open-water flow
period) flow duration curves for pre- and Max LF OS-1b conditions. Below 50,000 cfs, the
curves for the two conditions are nearly coincident and for flows greater than 50,000 cfs Max LF
OS-1b results in generally lower flow for a specific exceedance percentage.
For better visualization, 50 individual flow duration curves were not plotted; rather, the
maximum, minimum and quartiles of the flows were computed from the 50 curves. The
quartiles for the full open-water flow period are shown in Figure 5.1-5. Solid lines are used for
existing conditions and dashed lines with the same color are shown for Max LF OS-1b. The
maximum and minimum curves are generated from maximum and minimum values at each
exceedance percentage from each of the 50 individual curves, so the enveloping curves are not
the result from a single year. Similarly, the inner quartiles are developed from the 50 flow
values for each exceedance percentage. Two other curves are included in Figure 5.1-5 as dotted
lines. These curves show the approximate percentage of time one conditions falls outside the
range of the other condition. For example, Max LF OS-1b flows are within the range of existing
conditions 96 percent of the time (i.e. lower flow approximately 2 out of every 50 years). The
dotted line at the high range shows that existing conditions flows are higher than would occur
approximately 4 percent of the time compared to Max LF OS-1b. The dotted lines were
determined by selecting the value from flow duration curves that best matched the minimum or
maximum lines for other condition. Figures (5.1-6 through 5.1-11) for individual months of the
OWFP were developed and discussed below. Each figure uses the same line types and colors
and dotted lines are when flows for one condition occur outside the range of the other condition.
Figure 5.1-6 shows the flow duration curves for May. The curves for May include earlier
portions of the open-water flow period when it starts in April. All of the individual flow duration
curves would plot between the computed minimum and maximum curves and 25 percent of the
values plot between successive quartiles. There is generally more than a 100,000 cfs range of
flows for the majority of exceedance values under Existing conditions. Flows between 25,000
and 50,000 cfs occur fairly often in May and the majority of flows are less than 150,000 cfs. The
flows are lower for Max LF OS-1b conditions and the range is slightly compressed.
The primary difference between May and both June and July (Figures 5.1-7 and 5.1-8) is that
flows are greater in the later months. June and July flows always exceed 50,000 cfs and often
exceed 150,000 cfs. Under Existing conditions June also has a slightly greater range. As with
May, Max LF OS-1b conditions are consistently lower and the range is smaller than Existing
conditions. For Existing conditions, August (Figure 5.1-9) shows greater extremes, but with
narrower interquartiles than the earlier months. Similar to June and July, flows greater than
150,000 cfs are relatively common, but unlike June and July flows less than 50,000 cfs also
occur. For Max LF OS-1b, the extremes are nearly the same as Existing conditions, but the
interquartiles are shifted lower.
August appears to be a transitional month between the characteristics of July and the latter part
of the OWFP. As shown in Figures 5.1-10 and 5.1-11, September and October show generally
reduced flows. Note that October includes flows through the end of the OWFP. In September
and October, flows greater than 150,000 cfs are uncommon and flows below 50,000 cfs occur
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frequently. October is unusual in that it has predominantly low flow (<40,000 cfs) but also
includes the highest flows in the record (approximately 280,000 cfs, which occurred in 1986).
5.2. Change in Sediment Transport Mass Over the Open-Water Flow
Period
The change in transported sediment mass (sand and larger materials) over the open-water flow
period due to Project operations under Max LF OS-1b is presented in Figure 5.2-1. This figure
plots the sediment mass transported past Susitna Station under Max LF-OS1B as a function of
the sediment mass transported past Susitna Station under existing conditions. There are 50
points on the figure to represent each of the 50 open-water flow periods. Symbols identify the
representative wet, average, and dry years, as well as the 50-year-average (i.e., the average over
the 50 open-water flow periods). A 1:1 reference line is shown along with reference lines
showing +/- 10 percent and -15 percent from the 1:1 reference.
In general, Figure 5.2-1 shows Project operations under Max LF OS-1b produce a 10 to 15
percent reduction in the sediment load (sand and larger materials) transported past Susitna
Station. Table 5.2-1 summarizes the effects of Max LF OS-1b on the representative wet,
average, and dry years, as well as on the 50-year-average. The average reduction in load is 12.6
percent, with a maximum reduction of 21.5 percent and a minimum reduction of 8.9 percent.
These reductions in transported mass are consistent with expectations because under existing
conditions on an average annual basis the Middle Susitna River delivers approximately 11
percent of the bed material load passing Susitna Station (Tetra Tech 2014a) and the proposed
Watana Dam is expected to trap 100 percent of incoming bed material load.
The expected natural variability in transported sediment mass (sand and larger materials) over
the open-water flow period during the representative wet (1981), average (1985), and dry (1976)
years is shown in Figure 5.2-2. This figure shows the time series of simulated sediment mass
(sand and larger materials) transported over the open-water flow period past Susitna Station.
The median values and the bounding quartile values are included. Figure 5.2-2 shows the
influence of Projection operations under Max LF OS-1b is to reduce the sediment mass
transported past Susitna Station. The reduction is consistently about 13 percent between the
interquartile thresholds. The range of sediment transport is from 0.61E7 to 1.62E7 tons/year for
existing conditions and from 0.56E7 to 1.39E7 tons/year for Max LF OS-1b. These extremes are
approximately +/- 40 to 50 percent of the long-term averages for the two conditions.
5.3. Difference in Modeled Bed Elevations Represented by Channel
Aggradation or Degradation
The differences in bed elevations represented by channel aggradation or degradation simulated
using the 1-D Bed Evolution Model for the 50 years of existing conditions are shown in Figures
5.3-1 through 5.3-3.
Figure 5.3-1 shows the channel bed profile at the beginning (dark blue) and end of the (red) of
the simulation. The initial bed profile is mostly obscured by final bed profile, except in areas of
greater change, which occur in LR-4 at and above the Yentna River confluence. The profiles
show gradually decreasing gradients in the downstream direction with a significant upward
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convexity between profile low-points at PRMs 30 and 45. This convexity indicates historical
aggradation between these two locations of lateral channel constriction.
The individual cross section changes are shown along the river in light blue. Where there are
two lines (around PRMs 50 and 70) the model includes split flow reaches, so one line represents
the main channel and the other the secondary channel. The model indicates that the bed is
generally aggradational with 50-year bed elevation changes between approximately -0.3 and +5
ft with the largest values occurring in LR-4 and LR-5. Channel aggradation is slightly higher
from approximately PRM 100 to PRM 85 (up to 2 ft in 50 years), reducing downstream between
PRMs 85 and 45 (generally less than 1 ft in 50 years), and dramatically increasing below PRM
45 (ranging from 1 to 5 ft in 50 years). These trends are expected based on the shape of the
profile and the form of the river. The frequent areas of braiding, multiple channels, islands and
bars are all indicative of an aggrading channel. The lower 2 miles of the Yentna River also
include multiple channels, islands, and active bars indicating sediment accumulation similar to
the Susitna River LR-4 and upper LR-5 geomorphic reaches. The rates of sediment
accumulation are very reasonable, though in the vicinity of the Yentna River confluence are
probably slightly high because aggradation in the lower portion of the Yentna River is not
accounted for in the loads input to the 1-D Bed Evolution Model.
Figure 5.3-2 shows reach-average bed change (all aggradation) and the amounts of sediment
stored in the bed on an average annual basis. The average bed elevation change in LR-5 of 0.071
ft/yr is probably slightly exaggerated because it includes some sediment that would likely have
deposited in the lower two miles of the Yentna River. The bed change in LR-4 (0.038 ft/yr) and
upstream reaches ranging from 0.013 to 0.023 ft/yr are not unreasonable considering the river
form.
Sediment transport along geomorphic reaches MR-8 and LR-1 through LR-5 (representing PRM
107.1 to PRM 29.9) are presented on Figure 5.3-3. This figure shows the sediment storage and
the total bed material (sand and gravel) transport. The bed profiles are also shown for reference.
The sediment transport line (green) shows split flow reaches where flow and sediment are
conveyed in the primary and secondary channels. The large vertical changes in sediment
transport reflect major tributary inputs from the Chulitna (PRM 102), Talkeetna (PRM 100) and
Yentna (PRM 32) Rivers. The gradual drop in the sediment transport along the channel is due to
sediment stored in the bed.
Figures 5.3-4 through 5.3-6 are equivalent figures to Figures 5.3-1 through 5.3-3, except they
represent the Max LF OS-1b simulation. The rates and ranges are all slightly less than for
Existing conditions. Individual cross section bed elevation changes range from -0.7 ft to +4.5 ft
in 50 years (Figure 5.3-4). The reach average rates of bed elevation change (Figure 5.3-5) are
0.064 ft/yr for LR-5, 0.028 ft/yr for LR-4, and upstream Lower River reaches range from 0.009
ft/yr to 0.022 ft/yr. The Lower Susitna River remains aggradational for Max LF OS-1b
conditions, but at a slightly slower rate. Figure 5.3-6 shows that sediment transport for Max LF
OS-1b is generally reduced over the Lower River, but the trends of sediment accumulation are
very similar to Existing Conditions. Although there is bed material being transported out of the
Middle River in this operational condition, the amount is insignificant.
Tables 5.3-1 and 5.3-2 show the reach-average bed elevation change for the 50-years and
individual decades. The decadal amounts are generally consistent between Max LF OS-1b and
Existing conditions with only LR-1 showing slightly more aggradation for the with-Project
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condition in two decades (second and fifth). These results indicate that the aggradational
character of the Lower Susitna River would be maintained throughout the extent of the Lower
River, through the entire 50-year licensing period, and through individual decades. This is
illustrated in Figure 5.3-7 where sediment storage is shown at each cross section and
cumulatively along the Lower River for Existing and Max LF OS-1b conditions. The primary
difference is that less sediment is stored (approximately 23 percent less) along the Lower River
for Max LF OS-1b conditions. Although the reduction in bed material sediment supply (zero at
Watana Dam site and minimal from the Middle River) is clearly evident, the reduction is not
enough to change the long-term aggradation trend of the Lower River. The change in supply at
Watana Dam site due to Max LF OS-1b appears to be sufficient to change MR-8 from slightly
aggradational (0.3 ft in 50 years) to very slightly degradational (-0.11 feet in 50 years), though
these amounts are likely undiscernible and within the limitations of the model.
5.4. Estimated Change in Flow Depth and Velocity in the Lower
Portion of Geomorphic Reach LR-6 Inferred from Changes
Modeled in the Upper Portion of LR-5
The changes in flow depth and velocity due to Project operations under Max LF OS-1b are
presented in a series of figures. The series of figures progresses through channel hydraulic depth
from PRM 29.9 to PRM 32.4 (Figure 5.4-1 through Figure 5.4-4), and then channel average
velocity following the same order (Figure 5.4-5 through Figure 5.4-8). Each figure plots the
parameter using instantaneous results at 2-hour intervals. The 2-hour interval was required due
to limitations in HEC-RAS file output size. The values under Max LF OS-1b conditions are
plotted as a function of the parameter under existing conditions. A 1:1 reference line is shown
along with reference lines showing +/- 10 percent from the 1:1 reference. The three
representative years are differentiated by symbol shape and color.
In general, the channel hydraulic depth and channel average velocity under Max LF OS-1b
during the representative wet, average, and dry years are within 10 percent of the values under
existing conditions. However, a bias is apparent corresponding to a reduction in both flow depth
and velocity under Project operations. Across all four locations over the open-water flow
periods, the median flow depth decreases approximately 4 percent and the median velocity
decreases nearly 2 percent. As was expected, the narrower sections (PRM 29.9 and PRM 30.8)
exhibit channel hydraulic depths approximately twice as great as the wider sections (PRM 31.6
and PRM 32.4). PRM 31.6 shows a narrow range of channel velocities, which is attributed to the
backwater caused by the downstream constriction to the narrow cross section at PRM 30.8.
The expected natural variability in flow depth and velocity over the open-water flow period
during the representative wet, average, and dry years is also illustrated in a series of figures.
Each figure shows the 2-hr instantaneous time series of simulated channel hydraulic depth or
channel average velocity with the solid lines reflecting existing conditions and the dashed lines
showing Project operations under Max LF OS-1b. The three representative years are
differentiated by color. Figure 5.4-9 through Figure 5.4-12 provide the channel hydraulic depth
for each of the four locations; Figure 5.4-13 through 5.4-16 show the channel average velocity.
These sets of figures show the changes resulting from Max LF OS-1b and the range of conditions
for both scenarios.
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6. DISCUSSION
This section presents the interpretation of the results of the analyses of the metrics associated
with the four decision point criteria.
6.1. Changes in Flow at Susitna Station and Associated Potential
for Channel Width Adjustment
The Susitna-Watana Hydroelectric Project would modify flows and sediment transport
downstream of the Watana Dam site. Changes relative to existing conditions would be greatest
in the Middle River and decrease as tributaries add flow and sediment. In the Lower River, three
major tributaries, the Chulitna, Talkeetna, and Yentna Rivers, are the major sources of flow and
sediment below Watana Dam. Therefore, flow frequency and flow duration relationships are
strongly influenced by these tributaries.
Below Susitna Station (PRM 29.9) channel forming discharges are reduced approximately 11
percent (Table 5.1-1). This results in an expected change in channel width of 5 to 6 percent. As
shown in Figure 5.1-3, a relative change in channel width of 6 percent is small in comparison
with the variability in channel width along the LR-5 and LR-6 geomorphic reaches.
Another way to consider the natural variability of channel width is to evaluate change over time
for historic conditions. Table 4.2-1 includes channels widths measured throughout the Lower
Susitna River from 1950s, 1980s, and current (2012) aerial photography. Also included in the
table are percent changes in width between the time periods. Along the Lower Susitna River,
width changes range from -15 to +12 percent for the intermediate time periods and between -20
and +12 percent for the total time period. The variability in LR-5 is between 0 and +12 percent
and for LR-6 is between -8 and +5 percent. The expected narrowing of up to 6 percent is within
the range of natural temporal variability in the Lower River in general and the geomorphic
reaches of interest.
As demonstrated in Table 4.2-1, channel width shows substantial variability over time. Periods
of high flow tend to increase hydraulic stress and increase channel widths; periods of lower
flows can result in channel narrowing. Short periods of high flows can cause significant change.
For both Existing and Max LF OS-1b conditions, 2-year recurrence flows computed on a decadal
interval vary appreciably (Table 5.1-2). Existing Q2 varies +11 to -8 percent from the long-term
value, and Q2 for Max LF OS-1b varies from +6 to -19 percent from the long-term pre-Project
value. Although the values for Max LF OS-1b indicate a long-term narrowing, the temporal
variability of flow and width would be maintained.
The results from the flow duration curves show similar trends. In general, flows during the open
water flow period (OWFP) are reduced, but the reductions are small compared to the wide range
of variability. Table 6.1-1 shows a comparative summary of the flow duration curves. The
values shown in the table are averages over the range of curves. Over the OWFP, the curves are
on average about 4 percent lower for Max LF OS-1b than for Existing conditions.
The OWFP range of variability for Max LF OS-1b is 95 percent of the range for Existing
conditions. The range of variability was calculated by determining the ratio of the range
(maximum minus minimum) for the two conditions at each exceedance percent and averaging
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the ratios over the exceedance percentages. May has the greatest change in range of variability,
with Max LF OS-1b maintaining 82 percent of the range for existing conditions.
Because there is a small downward shift in flows, Max LF OS-1b operations result in periods of
flow that are outside (less than) the existing range of variability. Other periods occur when
Existing conditions would have higher flows than with Max LF OS-1b operations. In each case
this occurs about 4 percent of the time (or 2 years out of 50). These percentages are shown in
Figure 5.1-5 as the “Max LF OS-1b 96 Percent” and “Existing 4 Percent” curves, which were
visually fit to approximate the extremes of the other condition. The monthly comparisons
(Figures 5.1-6 through 5.1-9) include similar comparisons when the range of one condition is
outside the range of the other condition. None of these results indicate a significant change over
the open-water flow period.
The flow duration curves were also compared on a monthly basis because CIBW and eulachon
activity vary by month. Primary use is in the May through July timeframe. As shown in Table
6.1-1, these three months have greater change than the total open-water flow period. June shows
the largest differences with a 12 percent downward shift in the flow duration curves, 82 percent
of the range of variability for Max LF OS-1b compared to pre-Project conditions, and 4 to 5
percent of the flows for one condition outside the range of flows of the other condition. In the
May through July period, flows for one condition fall outside the range of flows for the other
condition up to 6 percent of the time. When flows for Max LF OS-1b are less than the minimum
flow duration curves for Existing conditions, they average 3 percent lower during the full open
water flow period and 8 percent lower in June and July. When flows for Existing conditions are
higher than the maximum flow durations curves for Max LF OS-1b conditions, they average 5
percent higher for the entire open water flow season and 17 percent higher in June.
Given the infrequent occurrence of operational flows outside the natural range of natural
variability and the low magnitude of the differences when these excursions do occur, the flow
changes are not expected to noticeably affect the existing relationship between river flows,
water-surface elevations, and CIBW foraging habitat in the Lower Susitna River. Potential
reductions in channel width are also small and within the range of natural variability.
6.2. Changes in Sediment Transport Mass over the Open-Water
Flow Period
The simulated transported sediment mass (sand and larger materials) past Susitna Station during
the open-water flow period was considered to represent the potential for changes to transported
mass downstream of PRM 29.9. The simulated transported masses corresponding to wet,
average, and dry years (Table 5.2-1) illustrate the effects of Project operations under Max LF
OS-1b relative to existing conditions; however, of key importance is evaluating these effects in
the context of expected natural variability. Based on the consistently reduced sediment loads
transported past Susitna Station under Max LF OS-1b it is expected that the sediment masses
transported downstream of PRM 29.9 would similarly decrease. However, the range in sediment
masses (sand and larger materials) transported during the open-water flow periods over 50 years
under Max LF OS-1b is 83 percent of the range over the 50 years under existing conditions. In 2
years out of the 50, the sediment loads under Max LF-OS 1b are less than the minimum under
existing conditions, with differences of approximately 2 and 9 percent. In 5 years out of the 50,
the sediment loads under existing conditions are greater than the maximum load under Max LF
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OS-1b, with difference between approximately 1 and 17 percent. Since the effect of Project
operations under OS-1b is to decrease the sediment mass (sand and larger materials) transported
past Susitna Station, but (1) the range of annual variability relative to existing conditions is
largely preserved, (2) the occurrence of excursions outside the natural range of variability under
existing conditions is infrequent, and (3) the magnitude of the excursions is relatively small, it is
not expected that Project operations will appreciably affect the relationship between river flows,
water-surface elevations, and CIBW foraging habitat in the Lower Susitna River.
6.3. Difference in Modeled Bed Elevations Represented by Channel
Aggradation or Degradation
The results of the 50-year simulations using the 1-D Bed Evolution Model indicate that the
Lower River tends to be aggradational for both existing and Max LF OS-1b conditions, but less
so under the operational condition. In the Lower River, reach-average bed elevation change over
50-years ranges from 0.65 to 3.5 ft for existing conditions and from 0.43 to 3.2 ft for Max LF
OS-1b conditions. The reach-scale bed evolution model integrates the effects of hydrology,
sediment supply, hydraulics, and channel response throughout the Susitna River from the Watana
Dam site to Susitna Station. The aggradational character of the Lower River predicted by the
model is consistent with observations of the planform of the river as well as the trends in the
channel profile. These factors indicate that the Lower Susitna River is not strictly an equilibrium
channel, but the rates of change are so low that long-term change is not significant. The model
results differ from the initial assessment of potential channel change presented in Tetra Tech
(2013a), which indicates the potential for degradation and narrowing as a potential response in
the Lower River below Sunshine Station. The initial assessment was based on the assumption
that the channel is in equilibrium prior to the Project. Because the existing channel is
aggradational the combined influence of reduced flow and sediment under Max LF OS-1b does
not cause degradation, but instead continues aggradation at a slightly lower rate.
6.4. Estimated Change in Flow Depth and Velocity in the Lower
Portion of Geomorphic Reach LR-6 Inferred from Changes
Modeled in the Upper Portion of LR-5
The simulated channel hydraulic depth and channel average flow velocity in the upper portion of
LR-5 were considered to infer the potential for changes in flow depth and velocity in the lower
portion of LR-6. The simulated hydraulics for the representative wet, average, and dry years
(Section 5.4) illustrate the effects of Project operations under Max LF OS-1b relative to existing
conditions; however, of key importance is evaluating these effects in the context of expected
natural variability. What is apparent from Figures5.4-9 through 5.4-16 is that the influence of
Project operations under Max LF OS-1b is generally within and less than the natural variability
between wet, average, and dry hydrologic conditions. The results at PRM 32.4 may be the most
representative of conditions downstream of PRM 29.9 because the cross section morphology and
hydrology are most similar to the downstream conditions. At this cross section, the operation of
the Project tends to decrease channel hydraulic depth and decrease channel average velocity;
however, increases approaching 25 percent for channel hydraulic depth and 10 percent for
channel average velocity occur within the overall trend of decreases. When evaluated in context
of the temporal variability throughout the representative wet, average, and dry open-water flow
periods, and across the representative years, it is obvious that despite there being an effect of
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Project operations on both flow depth and velocity, the effect is less than the natural variability
within a single open-water flow period, and much less than the natural variability across
hydrologic conditions. While the geomorphic conditions at PRM 29.9 and PRM 30.8 are
substantially different than any locations downstream of PRM 29.9, comparing the simulated
hydraulics at these sections to PRM 32.6 and PRM 32.4 further expands the natural variability
through a spatial component.
Since the simulated effect of Project operations under Max LF OS-1b on flow depth and velocity
is approximately one order of magnitude less than the natural variability in these parameters, it is
not expected that Project operations will have a noticeable long-term effect on the relationship
between river flows, water-surface elevations, and CIBW foraging habitat in the Lower Susitna
River.
6.5. Decision Point Recommendation
Each of the decision point criteria were evaluated based on the amount of change relative to the
natural variability of the existing river. There are reductions in flows, sediment supply and
transport, channel width, aggradation, velocities and depths in the Lower River in general and in
the geomorphic reaches of primary concern related to this decision point (LR-4 and LR-5). The
changes predominantly occur within the range of natural variability and generally have a similar
range of variability as the existing river.
The changes in the Lower River provide the basis for making the decision whether to extend the
fluvial geomorphology modeling below PRM 29.9. The small amount of relative change
between Existing conditions and Max LF OS-1b indicates that the 1-D Bed Evolution Model
does not warrant extension. The changes are also small in comparison to the natural variability.
It is also important to note that the range of variability with operational conditions is very similar
to existing conditions.
Bed elevation change is the integration of many factors, including hydrology, sediment supply,
bed material composition, channel geometry, flow depths and velocities. Because the Lower
River is currently aggradational and remains aggradational under Max LF OS-1b conditions, the
character of the river will be unchanged. Minor channel narrowing is also expected below PRM
29.9, but the amounts are within natural spatial and temporal variability.
Depth and velocity were analyzed because they are indicative of both sediment transport
conditions and habitat availability. Although there are periods of reduced depths and velocities,
the representative years show far greater variability between years than occur within years. This
is also true over monthly and shorter periods within the representative years. Further, the effect
of the expected channel narrowing under the Max LF OS-1b conditions would somewhat offset
the reduced depths and velocities simulated without accounting for the narrowing; whether the
narrowing would fully offset the hydraulic effects is unknown, but will be addressed in future 1-
D Bed Evolution Modeling efforts.
Since the Max LF OS-1b represents the greatest potential hydrologic effects of Project
operations, it is likely that alternate operation scenarios would produce less pronounced effects
on the relationship between river flows, water-surface elevations, and CIBW foraging habitat in
the Lower Susitna River.
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 21 September 2014
TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Based on the results of these analyses, AEA recommends that bed morphology modeling not be
extended below PRM 29.9. AEA further recommends that associated 1-D hydraulic modeling
not be extended below PRM 29.9 as the range of hydraulic conditions is not appreciably
changed.
One area that is not addressed by the decision point criteria is tidally affected flow areas. Based
on the measured channel widths and observable channel form, it appears that tides begin to
dominate river flows downstream of about PRM 10. Figures 6.5-1 and 6.5-2 show tides levels
measured at Anchorage, and include stage records from pressure transducers located between
PRMs 10.5 and 20.5. These figures show that at PRM 10.5 tides or river flows can control the
water levels, but that only a small tidal influence reaches PRMs 15.6 and above. During spring
tides (new and full moon) the higher tides are more likely going to dominate water levels below
PRM 10.5. During neap tides (during the half moon periods) water levels are more likely to be
river-flow dominated.
The large range in tidal conditions combined with continually varying river flows means that the
location of significant tidal influence will vary greatly. The phases of the moon also mean that
periods of spring and neap tides do not occur during the same periods from year to year. The
large range in tide levels result in a highly variable zone of tidal influence over the range of river
flows. Because river flows are only slightly lower for Max LF OS-1b conditions (Section 5.1),
the changes in tidally influenced areas will be even less in comparison to the natural range of
variability. This conclusion is in agreement with the CIBW study (ISR Section 9.17.4.3) stating
“The degree of Project effects on instream flow and geomorphology in the Susitna River Delta is
likely insignificant or discountable compared to the high tidal flux in the delta.” Therefore, AEA
recommends that no tidal hydrodynamic modeling be conducted in the lowest portions of the
Susitna River.
7. REFERENCES
Ackers, P. 1993. Sediment transport in open channels: Ackers and White update. Proceedings
of the Institution of Civil Engineers, Water, Maritime, and Energy, Vol. 101 (4). Water
Board Technical Note 619. p. 247 – 249.
Ackers, P. and W.R. White. 1973. Sediment Transport: New Approach and Analysis.
American Society of Civil Engineers, Journal of the Hydraulics Division, HY11. p. 2041
– 2060.
Andrews, E.D. 1984. Bed-material entrainment and hydraulic geometry of gravel-bed rivers in
Colorado. Bull. Geol. Soc. Am. 95: 371–378.
Curran, J.H. 2012. Streamflow Record Extension for Selected Streams in the Susitna River
Basin, Alaska. U.S. Geological Survey Scientific Investigations Report 2012-5210. 36
p.
Emmett, W.W. 1972. The Hydraulic geometry of some Alaska streams south the Yukon River.
U.S. Geological Survey Open-file Report 72-0108: 110 p.
Hey, R D., and C.R. Thorne. 1986. Stable channels with mobile gravel beds. J. Hydraul. Eng.
112.8: 671–689.
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 22 September 2014
TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
IACWR. 1968. A Study of Methods Used in Measurement and Analysis of Sediment Loads in
Streams, Report No. 12 Some Fundamentals of Particle Size Analysis. Prepared by Inter-
Agency Committee on Water Resources (IACWR). U.S. Government Printing Office.
First printing 1958, second printing 1968.
Julien, P.Y., and J. Wargadalam. 1995. Alluvial channel geometry: Theory and applications. J.
Hydraul. Eng. 121.4: 312–325.
Langbein, W.B. 1964. Geometry of River Channels. J. Hydraulics Div. ASCE 90, HY2, 301-
312.
Leopold, L.B., and T. Maddock. 1953. The hydraulic geometry of stream channels and some
physiographic implications. U.S. Geological Survey Professional Paper 252; 57 p.
MWH Global (MWH). 2012. Preliminary Susitna River pre-Project and post-Project Flow
Stages. PowerPoint Presentation, Technical Workgroup Meeting on October 23, 2012.
Prepared for Alaska Energy Authority.
Parker, G., 1979. Hydraulic geometry of active gravel rivers. Journal of the Hydraulics
Division. v. 105. no. HY9: 1185-1201.
R2 Resource Consultants, Inc. (R2) 2013. Selection of Focus Areas and Study Sites in the
Middle and Lower Susitna River for Instream Flow and Joint Resource Studies- 2013 and
2014, Susitna-Watana Hydroelectric Project FERC Project No. 14241. Prepared for
Alaska Energy Authority. March 1, 2013.
Tetra Tech, Inc. 2013a. Reconnaissance Level Assessment of Potential Channel Change in the
Lower Susitna River Segment, Susitna-Watana Hydroelectric Project FERC No. 14241.
Prepared for Alaska Energy Authority. February 2013.
Tetra Tech, Inc. 2013b. Development of Sediment-Transport Relationships and an Initial
Sediment Balance for the Middle and Lower Susitna River Segments, Susitna-Watana
Hydroelectric Project FERC No. 14241. Prepared for Alaska Energy Authority.
February 2013.
Tetra Tech, Inc. 2013c. Fluvial Geomorphology Modeling Approach Technical Memorandum,
Susitna-Watana Hydroelectric Project FERC No. 14241. Prepared for Alaska Energy
Authority. June 2013.
Tetra Tech, Inc. 2014a. 2014 Update of Sediment-Transport Relationships and a Revised
Sediment Balance for the Middle and Lower Susitna River Segments. Technical
Memorandum. Susitna-Watana Hydroelectric Project FERC No. 14241. Prepared for
Alaska Energy Authority. September 2014.
Tetra Tech, Inc. 2014b. Mapping of Geomorphic Features and Turnover within the Middle and
Lower Susitna River Segments from 1950s, 1980s, and Current Aerials. Technical
Memorandum. Susitna-Watana Hydroelectric Project FERC No. 14241. Prepared for
Alaska Energy Authority. September 2014.
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 23 September 2014
TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Tetra Tech, Inc. 2014c. Winter Sampling of Main Channel Bed Material. Technical
Memorandum. Susitna-Watana Hydroelectric Project FERC No. 14241. Prepared for
Alaska Energy Authority. September 2014.
Tetra Tech, Inc. 2014d. Susitna River Historical Cross Section Comparison (1980s to Current).
Technical Memorandum, Susitna-Watana Hydroelectric Project FERC No. 14241.
Prepared for Alaska Energy Authority. September 2014.
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 24 September 2014
TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
8. TABLES
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FERC Project No. 14241 Page 25 September 2014
TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Table 4.2-1. Average non-vegetated widths by Geomorphic Reach in the Lower River.
Reach
Reach Bounds (PRM) Average non-Vegetated Width (ft) Width Change (%)
U/S D/S 1950s 1980s Current
1980s from
1950s
Current
from 1980s
Current
from 1950s
LR-1 102.4 87.9 3,495 3,488 3,340 -0.2 -4.2 -4.4
LR-2 87.9 65.6 3,890 3,671 3,120 -5.6 -15.0 -19.8
LR-3 65.6 44.6 4,290 4,395 4,040 2.4 -8.1 -5.8
LR-4 44.6 32.3 2,979 2,778 2,750 -6.7 -1.0 -7.7
LR-5 32.3 23.5 2,909 3,244 3,250 11.5 0.2 11.7
LR-6 23.5 3.3 5,443 5,722 5,280 5.1 -7.7 -3.0
Table 4.2-2. Calibration and validation data for the 1-D Bed Evolution Model.
Reach Period Available Data
Middle River Calibration: 9/12/2012 – 10/15/2012 1. Water-surface elevation hydrographs from 7 ESS
locations and 2 USGS gages
2. Flow hydrographs from 2 USGS gages
3. Point-in-time water-surface elevations
Middle River Validation: 7/5/1981 – 9/4/1981 Flow hydrographs from 2 USGS gages
Lower River Calibration: 5/24/2013 – 8/31/2013 1. Water-surface elevation and flow hydrographs
from 2 USGS gages
2. Point-in-time water-surface elevations
Lower River Validation: 7/5/1981 – 9/4/1981 Flow hydrographs from 2 USGS gages
Table 5.1-1. Flow frequency and width comparisons for the Lower Susitna River.
Recurrence
Interval
(yrs)
Sunshine Susitna Station
Pre-
Project
Q (cfs)
Max
LF-OS1B
Q (cfs)
Q
Diff.
(%)
Width
Diff.
(%)1
Pre-
Project
Q (cfs)
Max
LF-OS1B
Q (cfs)
Q
Diff.
(%)
Width
Diff.
(%)1
1.01 66,100 52,300 -21 n/a 128,400 112,200 -13 n/a
1.5 91,200 73,800 -19 -10.0 171,600 152,500 -11 -5.7
2 99,700 81,300 -18 -9.7 185,500 166,000 -11 -5.4
5 121,100 100,200 -17 -9.0 219,400 199,800 -9 -4.6
10 135,500 113,100 -17 n/a 241,400 222,300 -8 n/a
20 149,500 125,800 -16 n/a 262,300 244,000 -7 n/a
50 168,000 142,600 -15 n/a 289,400 272,600 -6 n/a
100 182,200 155,700 -15 n/a 309,800 294,600 -5 n/a
Notes:
1 Width difference is based on the square-root of change in discharge.
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FERC Project No. 14241 Page 26 September 2014
TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Table 5.1-2. Variability of 2-Year discharge (Q2) and associated potential for width adjustment at Susitna Station.
Period
2-Year Discharge, (cfs)
Q2 Difference from
Existing 61-yr Average (%)
Width Difference from
Existing 61-yr Average (%)1
Existing Max LF OS-1b Existing Max LF OS-1b Existing Max LF OS-1b
60-yr Average 185,500 166,000 0.0 -10.5 0.0 -5.4
1950-1959 186,100 165,100 0.3 -11.0 0.2 -5.7
1960-1969 200,800 178,100 8.2 -4.0 4.0 -2.0
1970-1979 174,000 150,200 -6.2 -19.0 -3.1 -10.0
1980-1989 205,700 197,400 10.9 6.4 5.3 3.2
1990-1999 170,700 151,500 -8.0 -18.3 -4.1 -9.6
2000-2009 175,900 157,600 -5.2 -15.0 -2.6 -7.8
Notes:
1 Width difference based on the square-root of difference in decadal discharge and long-term Existing conditions discharge.
Table 5.2-1. Comparison of sediment masses (sand and larger materials) transported past Susitna Station.
Year
Existing Conditions (tons) OS-1b (tons) Percent Change
Wet (1981) 1.444E+07 1.243E+07 -13.9
Average (1985) 1.130E+07 9.768E+06 -13.6
Dry (1976) 9.225E+06 8.104E+06 -12.1
50-year-average 1.100E+07 9.615E+06 -12.6
Table 5.3-1. Reach average bed elevation change for existing conditions.
Reach
Reach average aggradation(+) and degradation (-) (ft)
Years
0-10
Years
10-20
Years
20-30
Years
30-40
Years
40-50 Max. Avg. Min.
50-yr
total
MR-8 0.11 0.08 0.03 0.02 0.06 0.11 0.06 0.02 0.30
LR-1 0.31 0.27 0.24 0.23 0.13 0.31 0.23 0.13 1.17
LR-2 0.12 0.14 0.17 0.14 0.14 0.17 0.14 0.12 0.70
LR-3 0.07 0.11 0.14 0.16 0.17 0.17 0.13 0.07 0.65
LR-4 0.78 0.33 0.31 0.30 0.18 0.78 0.38 0.18 1.90
LR-5 1.60 0.72 0.49 0.30 0.43 1.60 0.71 0.30 3.53
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Table 5.3-2. Reach average bed elevation change for Max LF OS-1b conditions.
Reach
Reach average aggradation(+) and degradation (-) (ft)
Years 0-10 Years 10-20 Years 20-30 Years 30-40 Years 40-50 Max. Avg. Min. 50-yr total
MR-8 -0.04 -0.03 -0.02 -0.01 -0.01 -0.01 -0.02 -0.04 -0.11
LR-1 0.29 0.29 0.14 0.23 0.18 0.29 0.22 0.14 1.12
LR-2 0.08 0.10 0.12 0.10 0.13 0.13 0.11 0.08 0.53
LR-3 0.04 0.07 0.08 0.12 0.12 0.12 0.09 0.04 0.43
LR-4 0.59 0.28 0.22 0.21 0.11 0.59 0.28 0.11 1.41
LR-5 1.53 0.70 0.40 0.20 0.37 1.53 0.64 0.20 3.21
Table 6.1-1. Summary comparison of flow duration curves.
Open
Water Period
Start-
May June July Aug. Sept. Oct.-
End
Percent difference in Max LF OS-1b
compared to Existing Conditions -4% -7% -12% -9% -3% 0% 2%
Relative variability
(Range of Max LF OS-1b/
Range of Existing Conditions)
0.95 0.85 0.82 0.91 1.00 1.00 1.01
Approximate Percent time
Max LF OS-1b within
range of Existing Conditions
96% 96% 96% 94% 99% 100% 100%
Approximate Percent time Existing
Conditions would exceed maximum
flow duration curve of Max LF OS-1b
4% 6% 5% 2% 2% 0% 0%
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
9. FIGURES
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 3.0-1. Study area.
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 4.2-1. Comparison of observed and simulated water-surface elevations at Sunshine for the calibration event.
Figure 4.2-2. Comparison of observed and simulated water-surface elevation at Susitna Station for the calibration event.
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 4.2-3. Middle River comparison of observed point-in-time water-surface elevations to simulated elevations for
the calibration event.
Figure 4.2-4. Lower River comparison of observed point-in-time water-surface elevations to simulated elevations for
the calibration event.
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 4.2-5. Comparison of observed and simulated flows at Gold Creek for the validation event.
Figure 4.2-6. Susitna River near Talkeetna comparison of measured and modeled total bed material loads.
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 4.2-7. Susitna River near Talkeetna comparison of measured and modeled transported bed material gradations.
Figure 4.2-8. Susitna River at Sunshine comparison of measured and model total bed material loads.
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 4.2-9. Susitna River at Sunshine comparison of measured and model transported bed material gradations.
Figure 4.2-10. Susitna River at Susitna Station comparison of measured and model total bed material loads.
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 4.2-11. Susitna River at Susitna Station comparison of measured and model transported bed material
gradations.
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 5.1-1. Susitna River active channel width measurements from PRM 29.5 to 13.0.
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 5.1-2. Susitna River active channel width measurements from PRM 16.0 to 2.5.
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 5.1-3. Susitna River non-vegetated channel width below PRM 29.5.
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 5.1-4 Comparison of annual flow duration curves for full open-water flow period.
Figure 5.1-5. Range of flow duration curves for the full open water flow period.
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Figure 5.1-6. Range of flow duration curves from start of open water flow period to end of May.
Figure 5.1-7. Range of flow duration curves for June.
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 5.1-8. Range of flow duration curves for July.
Figure 5.1-9. Range of flow duration curves for August.
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 5.1-10. Range of flow duration curves for September.
Figure 5.1-11. Range of flow duration curves for October to end of open water flow period.
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 5.2-1. Comparison of sediment mass (sand and larger materials) transported past Susitna Station by OWFP under existing conditions and Max LF OS-1b.
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 5.2-2. Time series of sediment mass (sand and larger materials) transported past Susitna Station under existing conditions and Max LF OS-1b.
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 5.3-1. Bed elevation and change in 50-years for existing conditions.
Figure 5.3-2. Sediment stored in the bed and reach averaged bed change for existing conditions.
Figure 5.3-3. Bed material transport and bed storage for existing conditions.
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 5.3-4. Bed elevation and change in 50-years for Maximum LF-OS1B conditions.
Figure 5.3-5. Sediment stored in the bed and reach averaged bed change for Maximum LF-OS1B conditions.
Figure 5.3-6. Bed material transport and bed storage for Maximum LF-OS1B conditions.
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 5.3-7. Comparison between existing and Max LF OS-1b conditions incremental (at a cross section) and
cumulative sediment stored in the bed of the Lower River.
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 5.4-1. Comparison of channel hydraulic depth at PRM 29.9 under existing conditions and Max LF OS-1b.
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 5.4-2. Comparison of channel hydraulic depth at PRM 30.8 under existing conditions and Max LF OS-1b.
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 5.4-3. Comparison of channel hydraulic depth at PRM 31.6 under existing conditions and Max LF OS-1b.
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 5.4-4. Comparison of channel hydraulic depth at PRM 32.4 under existing conditions and Max LF OS-1b.
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 5.4-5. Comparison of channel average velocity at PRM 29.9 under existing conditions and Max LF OS-1b.
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 5.4-6. Comparison of channel average velocity at PRM 30.8 under existing conditions and Max LF OS-1b.
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 5.4-7. Comparison of channel average velocity at PRM 31.6 under existing conditions and Max LF OS-1b.
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 5.4-8. Comparison of channel average velocity at PRM 32.4 under existing conditions and Max LF OS-1b.
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 5.4-9. Time series of channel hydraulic depth at PRM 29.9 under existing conditions and Max LF OS-1b.
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Figure 5.4-10. Time series of channel hydraulic depth at PRM 30.8 under existing conditions and Max LF OS-1b.
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 5.4-11. Time series of channel hydraulic depth at PRM 31.6 under existing conditions and Max LF OS-1b.
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 5.4-12. Time series of channel hydraulic depth at PRM 32.4 under existing conditions and Max LF OS-1b.
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Figure 5.4-13. Time series of channel average velocity at PRM 29.9 under existing conditions and Max LF OS-1b.
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TECHNICAL MEMORANDUM DECISION POINT ON FLUVIAL GEOMORPHOLOGY MODELING BELOW PRM 29.9
Figure 5.4-14. Time series of channel average velocity at PRM 30.8 under existing conditions and Max LF OS-1b.
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Figure 5.4-15. Time series of channel average velocity at PRM 31.6 under existing conditions and Max LF OS-1b.
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Figure 5.4-16. Time series of channel average velocity at PRM 32.4 under existing conditions and Max LF OS-1b.
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Figure 6.5-1. Tide levels at Anchorage and stage data from PRMs 10.5 to 20.5.
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Figure 6.5-2. Detailed plot of tide levels at Anchorage and stage data from PRMs 10.5 to 20.5.
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