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Susitna-Watana Hydroelectric Project Document
ARLIS Uniform Cover Page
Title:
Reconnaissance level assessment of potential channel change in the lower
Susitna River segment : 2012 study technical memorandum
SuWa 12
Author(s) – Personal:
Author(s) – Corporate:
Prepared by Tetra Tech, Inc.
AEA-identified category, if specified:
AEA-identified series, if specified:
Series (ARLIS-assigned report number):
Susitna-Watana Hydroelectric Project document number 12
Existing numbers on document:
Published by:
[Anchorage, Alaska : Alaska Energy Authority, 2013]
Date published:
February 2013
Published for:
Prepared for Alaska Energy Authority
Date or date range of report:
Volume and/or Part numbers:
Attachment F
Final or Draft status, as indicated:
Document type:
Technical memorandum
Pagination:
iv, 26 p.
Related work(s):
Cover letter (SuWa 6), Attachments A-E, G-I (SuWa 7-11, 13-
15)
Pages added/changed by ARLIS:
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/
March 1, 2013
Ms. Kimberly D. Bose
Secretary
Federal Energy Regulatory Commission
888 First Street, NE
Washington, DC 20426
Re: Susitna-Watana Hydroelectric Project, FERC Project No. 14241-000;
Filing of 2012 Baseline Environmental and Resources Study Reports
Dear Secretary Bose:
As explained in its Pre-Application Document and Revised Study Plan (RSP) for
the proposed Susitna-Watana Hydroelectric Project, FERC Project No. 14241 (Project),
the Alaska Energy Authority (AEA) carried out numerous baseline environmental and
resources studies related to the proposed Project during the 2012 field season. Because
the 2012 studies occurred prior to the commencement of the study phase of the licensing
effort under the Federal Energy Regulatory Commission’s (Commission) Integrated
Licensing Process, AEA was not required to complete these baseline studies. However,
AEA voluntarily undertook these studies for purposes of taking advantage of the 2012
field season to gather environmental data related to the proposed Project, and to help
inform the scope and methods of the licensing studies during 2013-14, as set forth in
AEA’s RSP.
As AEA has completed the study reports associated with these 2012 baseline
environmental and resources studies, it has made the study reports publicly available by
uploading them to the “Documents” page of its licensing website, http://www.susitna-
watanahydro.org/type/documents/. The purpose of this filing is to submit these study
reports to the Commission’s record for the above-referenced Project.
In particular, the following study reports are attached, all of which are relevant to
the Commission’s study plan determination scheduled for April 1, 2013:
• Attachment A: Adult Salmon Distribution and Habitat Utilization Study
(January 2013)
• Attachment B: Synthesis of Existing Fish Population Data (February 2013)
• Attachment C: Mercury Assessment and Potential for Bioaccumulation
(February 2013)
2
• Attachment D: Technical Memorandum, Susitna River Large Woody Debris
Reconnaissance (March 2013)
• Attachment E: Riparian Vegetation Study Downstream of the Proposed
Susitna-Watana Dam (February 2013)
• Attachment F: Technical Memorandum, Reconnaissance Level Assessment of
Potential Channel Change in the Lower Susitna River Segment (February
2013)
• Attachment G: Stream Flow Assessment (February 2013)
• Attachment H: Development of Sediment-Transport Relationships and an
Initial Sediment Balance for the Middle and Lower Susitna River Segments
(February 2013)
• Attachment I: Technical Memorandum, Initial Geomorphic Reach
Delineation and Characterization, Middle and Lower Susitna River Segments
(February 2013)
As the remaining 2012 study reports are finalized, AEA will continue to update its
website and submit them to the record.
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 F
Technical Memorandum, Reconnaissance Level Assessment of Potential Channel Change
in the Lower Susitna River Segment (February 2013)
Susitna-Watana Hydroelectric Project
(FERC No. 14241)
Reconnaissance Level Assessment of Potential
Channel Change in the Lower Susitna River Segment
2012 Study Technical Memorandum
Prepared for
Alaska Energy Authority
Prepared by
Tetra Tech
February 2013
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page i February 2013
TABLE OF CONTENTS
Summary ....................................................................................................................................... iv
1. Introduction ........................................................................................................................1
2. Study Objectives.................................................................................................................1
3. Study Area ..........................................................................................................................3
3.1. General .....................................................................................................................3
3.2. Specific Study Area .................................................................................................3
4. Methods ...............................................................................................................................4
4.1. Deviations from Study Plan .....................................................................................4
4.2. Analysis....................................................................................................................4
4.2.1. Stream Flow Assessment ............................................................................ 4
4.2.2. Sediment-transport Supply and Balance ..................................................... 5
4.2.3. Initial Assessment of Downstream Effects ................................................. 5
5. Results .................................................................................................................................9
5.1. Evaluation of the Sediment Supply Ratio, S* ..........................................................9
5.2. Evaluation of the Transport Duration Ratio, T* .....................................................10
5.3. Integration of S* and T*..........................................................................................10
6. Summary and Conclusions .............................................................................................11
6.1. Stream Flow Assessment .......................................................................................11
6.2. Development of Sediment-Transport Relationships and an Initial Sediment
Balance for the Middle and Lower Susitna River Segments .................................12
6.3. Reconnaissance Level Assessment of Potential Channel Change in the Lower
Susitna River Segment ...........................................................................................12
7. References .........................................................................................................................13
8. Tables ................................................................................................................................15
9. Figures ...............................................................................................................................19
LIST OF TABLES
Table 4.2-1. Average annual sediment loads and S* under pre-Project conditions. ..................16
Table 4.2-2. Average annual sediment loads and S* under Maximum Load Following
OS-1 conditions. ....................................................................................................17
Table 5-2-1. T* results for critical discharge estimates. .............................................................18
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page ii February 2013
LIST OF FIGURES
Figure 3.2-1. Susitna River Geomorphology Study Area and Large-scale River Segments .......20
Figure 4.2-1. Response domain for predicted channel adjustments. (Grant et al. 2003) .............21
Figure 4.2-2 Gravel Bed-Load Transport at Gold Creek Gage. .................................................22
Figure 4.2-3 Gravel Bed-Load Transport at Sunshine Gage. .....................................................22
Figure 5.1-1. S* for gravel material on the Middle and Lower Susitna River Reaches. .............23
Figure 5.1-2 S* for sand material on the Middle and Lower Susitna River Reaches. ................24
Figure 5.2-1 T* for the Middle and Lower Susitna River Reaches. ...........................................25
Figure 5.3-1 S* and T* on the Middle and Lower Susitna River Reaches. ...............................26
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page iii February 2013
LIST OF ACRONYMS AND SCIENTIFIC LABELS
Abbreviation Definition
AEA Alaska Energy Authority
APA Project Alaska Power Authority Susitna Hydroelectric Project
cfs cubic feet per second
ER Entrenchment Ratio
FERC Federal Energy Regulatory Commission
ft/mile feet per mile
ILP Integrated Licensing Process
LiDAR Light Detection and Ranging-based Topography
LR Lower Susitna River Segment
MC Multiple Channel Reach Classification
mi mile(s)
mm millimeter(s)
MR Middle Susitna River Segment
NEPA National Environmental Policy Act
OS operational scenario
Project Susitna-Watana Hydroelectric Project
PRM Project River Mile (the current, Susitna-Watana Project river-mile system)
RM River Mile (the 1980s APA Project river-mile system)
RSP Revised Study Plan
SC Single Channel Reach Classification
UR Upper Susitna River Segment
USGS United States Geological Survey
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page iv February 2013
SUMMARY
This effort synthesized results from the Development of Sediment-Transport Relationships and
an Initial Sediment Balance for the Middle and Lower Susitna River Segments technical
memorandum and the Stream Flow Assessment technical memoranda within an analytical
framework to perform an initial assessment of potential Project-related changes in channel
morphology of the Lower River. The overall goal was to determine whether portions of the
Fluvial Geomorphology Modeling Study and other studies need to be extended downstream in
the Lower River based on the potential for the Project to affect channel morphology. The 2012
reconnaissance level assessment involved analysis of pre-Project and post-Project conditions in
the Susitna River below Watana Dam. The pre-Project condition was based on the extended
flow record developed by the USGS. The post-Project condition was based on a hypothetical
operational scenario (OS) referred to as Maximum Load Following OS-1.
The assessment of potential channel change in the Lower River was successfully completed. The
limits of the one-dimensional sediment transport modeling effort were extended downstream
based on the results of this study. The assessment procedure will be used in 2013 and 2014 to
serve as check on the one-dimensional sediment transport modeling efforts conducted in the
Middle and Lower Rivers.
Results from three aspects of the study were combined to reach the conclusion that the one-
dimensional sediment transport modeling component of the Fluvial Geomorphology Modeling
Study should be extended approximately 50 miles farther downstream to Susitna Station. The
combination of the results from the stream flow assessment, the sediment balance, and the
analytical assessment framework suggested sufficient possibility of Project-related channel
change that more detailed analyses are warranted to further investigate potential Project effects
below Sunshine.
The stream flow assessment estimated reduction in peak flows at Sunshine and Susitna Station of
24 percent and 17 percent, respectively, under the Maximum Load Following OS-1 compared
with the pre-Project condition. The reductions were for flows in the channel forming range of the
1.5- to 5-year return interval. Discharges in this range are considered representative of the
channel forming or effective discharge to which the bankfull channel capacity adjusts.
Relationships between channel size and discharge suggest this level of peak flow reduction could
result in narrowing of the channel width by slightly greater than 10 percent below Sunshine, and
less than 10 percent below the Yentna River confluence.
The results of both the sediment balance and analytical framework were inconclusive as to
whether significant channel change would occur as a result of the Project. The results indicate
the portion of the Lower River above Sunshine will continue to be aggradational. The results of
the analytical framework in Lower River segment below Sunshine fell within the “Effects
Subtle” range suggesting the need for further investigation of the potential for channel change in
this portion of the Lower Susitna River.
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 1 February 2013
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 (Project) using the Integrated Licensing Process (ILP). The Project is located on the
Susitna River, an approximately 300-mile-long river in the South-Central Region of Alaska. The
Project’s dam site will be located at Project River Mile (PRM) 187.1. The results of this study
will inform the 2013-2014 formal study program, and Exhibit E of a license application, and
FERC’s National Environmental Policy Act (NEPA) analysis for the Project license.
This report provides the results of the Integrated Sediment Transport and Flow Results into
Analytical Framework subtask of the 2012 Reconnaissance-Level Geomorphic and Aquatic
Habitat Assessment of Project Effects on Lower River Channel (AEA 2012a). This effort
synthesizes results from the Sediment Load Comparison task and Stream Flow Assessment task
within an analytical framework (Grant et al. 2003) to develop an initial assessment of potential
Project-related changes in channel morphology of the Lower River. The downstream limit of the
1-D bed evolution model in the Revised Study Plan (RSP; AEA 2012b) is at PRM 79. This
effort was conducted to help inform the need to continue studies further downstream in the
Lower Susitna River Segment.
This effort compares pre-Project and post-Project conditions to identify potential Project effects,
based on information developed in the 2012 Geomorphology Study and presented in the
technical memoranda: Development of Sediment Transport Relationships and an Initial Sediment
Balance for the Middle and Lower Susitna River Segments (Tetra Tech 2013a) and Stream Flow
Assessment (Tetra Tech 2013b).
The pre-Project flows used for the analysis are based on the 61-year extended record developed
by the USGS (2012). The post-Project condition is a hypothetical operations scenario referred to
as Maximum Load Following Operation Scenario 1 (Maximum Load Following OS-1), a
simulated flow record developed with the Project-conditions flow-routing model (MWH 2012)
for the same 61-year period as the pre-Project records. Maximum Load Following OS-1 is based
on the assumption that the load fluctuation of the entire Railbelt would be provided by the
Susitna-Watana Project, and all other sources of electrical power in the Railbelt would be
running at base load. This assumed condition is not realistic for an entire year, and the results of
this condition should be conservative with respect to assessing downstream impacts of load
following.
2. STUDY OBJECTIVES
The specific objective of the effort presented in this technical memorandum is to perform an
initial assessment of anticipated Project effects on the Lower Susitna River Segment channel
type and morphology. The assessment was performed using the data developed for the pre- and
post-Project flood frequency, flow duration, and sediment load analyses to predict the
geomorphic response of the Susitna River in an analytic framework proposed by Grant et al.
(2003) along the longitudinal profile of the river system from the Middle Susitna River Segment
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 2 February 2013
downstream through the Lower Susitna River Segment, with particular emphasis on the effects
of the Chulitna and Talkeetna rivers at and downstream from the Three Rivers Confluence.
This work performed for this Technical Memorandum integrates the results of several earlier
tasks in the 2012 Reconnaissance-Level Geomorphic and Aquatic Habitat Assessment of Project
Effects on Lower River Channel Study (AEA 2012a) with several broader study objectives
including:
Assess potential changes to channel morphology and aquatic habitat pre- and post-Project.
Evaluate the relative magnitude of changes to the sediment regime pre- and post-Project, the
potential impacts on sediment/substrate gradations, and the vertical and lateral stability of the
channel.
This effort is one of several conducted to help evaluate the need to extend studies into the Lower
River Segment by addressing the sixth item in the criteria outlined in the Fish and Aquatics
Instream Flow Study (AEA 2012b, RSP Section 8.5.3). The criteria from the RSP are provided
below:
This assessment will include a review of information developed during the 1980s
studies and study efforts initiated in 2012, such as sediment transport (see Section
6.5), habitat mapping (see Sections 6.5 and 9.9), operations modeling (see Section
8.5.4.2.2), and the Mainstem Open-water Flow Routing Model (see Section
8.5.4.3). The assessment and the following criteria will be used to evaluate the
need to extend studies into the Lower River Segment and if studies are needed,
will identify which geomorphic reaches require instream flow analysis in 2013.
The criteria include: 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; and 6)
Initial assessment of potential changes in channel morphology of the Lower River
(see Section 6.5.4.6) based on Project-related changes to hydrology and sediment
supply in the Lower River. Results of the 2013 studies will then be used to
determine the extent to which Lower River Segment studies should be adjusted in
2014.
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 3 February 2013
3. STUDY AREA
3.1. General
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 (USGS 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.
3.2. Specific Study Area
For the Susitna-Watana Hydroelectric Project licensing effort the Susitna River from Cook Inlet
to the Maclaren River confluence at Project River Mile (PRM) 261.3, the river has been
subdivided into three segments (Tetra Tech 2103c) 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 (Figure 3.2-1):
Upper Susitna River Segment: Maclaren River confluence (PRM 261.3) downstream to the
proposed Watana Dam site (PRM 187.1)
Middle Susitna River Segment: Proposed Watana Dam site (PRM 187.1) downstream to the
Three Rivers Confluence (PRM 102.4)
Lower Susitna River Segment: Three Rivers Confluence (PRM 102.4) downstream to Cook
Inlet (PRM 3.3)
In addition to the segment boundaries, Figure 3.2-1 also shows the locations of gaging stations
where flow, and in some cases, sediment measurements are available. The upstream-most
segment, referred to as the Upper River (UR), extends from PRM 261.3 to PRM 187.1 at the
Watana Dam site. The morphologic characteristics of this segment of the river are dominated by
the products of Quaternary-age glaciation. The Middle River (MR) segment extends from the
Watana Dam site to the Three Rivers Confluence at about PRM 102.4. The general
characteristics of the river in this segment are heavily influenced by bedrock outcrop as well as
Quaternary-age glaciations. The Lower River (LR) segment extends from the Three Rivers
Confluence (PRM 102.4) to the tidal flats at Cook Inlet (PRM 3.3). The morphologic
characteristics of the river in this segment are dominated by the sediment loading from the major
tributaries and variable resistance to erosion of the Pleistocene-age, glacially-derived materials
including tills (moraines), glacio-fluvial sediments in various elevation outwash-surfaces and
glacio-lacustrine sediments that control the width of the valley.
The study effort presented in this Technical Memorandum is concentrated on the Lower Susitna
River Segment. However, the study area also includes the Middle Susitna River Segment since
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 4 February 2013
it is the altered hydrologic and sediment supply conditions in the Middle River that are the
drivers of potential channel change in the Lower River.
4. METHODS
4.1. Deviations from Study Plan
There were no deviations from the tasks in the 2012 study plan involving the Reconnaissance
Level Assessment of Potential Channel Change in the Lower Susitna River Segment.
4.2. Analysis
This section describes the methods used to evaluate potential channel change in the Lower
Susitna River based on the frequency and magnitude of flows that mobilize and transport bed
material sediment. The procedure is based on a relatively simple conceptual model relating
downstream channel changes due to dams to hydrogeomorphic change and geologic control
(Grant et al. 2003). The procedure is based on estimates of two ratios:
1. The ratio of sediment supply downstream of the dam to the above-dam sediment supply (S*).
2. The ratio of time when flow exceeds the threshold for bed-load transport under pre-Project
conditions to post-Project conditions (In this case, Maximum Load Following OS-1) (T*).
The sediment supply ratio (S*) was computed from the results of the sediment balance
investigation (Tetra Tech 2013a). S* was computed for both pre- and post-dam conditions. The
time ratio (T*) is computed using the flow-duration curves developed as part of the stream flow
assessment (Tetra Tech 2013b). The additional information required for the time ratio is an
estimate of the critical flow (Qcr) for initiation of sediment transport. For this assessment, the
critical flow was estimated by identifying the discharge that corresponds to a low, threshold bed
load transport rate, as defined by Parker et al. (1982) and Wilcock and Crowe (2003) using the
bed-load sediment discharge measurements from Knott et al. (1987) and more recent USGS
sediment discharge, along with USGS flow measurements at the gages.
4.2.1. Stream Flow Assessment
In the stream flow assessment (Tetra Tech 2013b) flow-duration and flood frequency analyses
were performed for Maximum Load Following OS-1 for the three mainstem Susitna River gages
(Gold Creek PRM 140, Sunshine PRM 88 and Susitna Station PRM 30). Data used in the
analysis were developed by MWH (2012) using the HEC-ResSim operations model of the
Project that uses the USGS 61-year extended record of mean daily flows as a long-term reservoir
inflow time series. The model run on which these data are based represents a preliminary
operation scenario that was developed by placing the entire variability of the Railbelt electricity
load on Susitna-Watana; thus, it represents a maximum (or worst-case) load-following scenario
(John Haapala, personal communication, January 24, 2013). The model was used to route the
reservoir outflows downstream through the Susitna River to the Sunshine Gage at PRM 88,
providing a 61-year period of simulated flows for Maximum Load Following OS-1 at Gold
Creek and Sunshine. A 61-year flow record for the Susitna Station gage was estimated by
adding the difference between the flows at the Sunshine and Susitna Station gages from the
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 5 February 2013
USGS (2012) extended record to the routed flows at Sunshine. Annual maximum hourly flows
from the HEC-ResSim routings were used for the peak flood frequency analysis at Gold Creek
and Sunshine as a surrogate for the instantaneous (15-minute) gage data that are typically used
for this type of analysis, since the maximum temporal resolution of the model output is 1 hour.
This approach is not considered to be a significant limitation in the analysis, since Susitna River
is relatively large, and the difference between the peak 15-minute and maximum hourly flows is
typically quite small. The frequency analysis for Susitna Station was performed based on the
annual maximum mean daily flows because sufficient information is not available at this time to
reliably estimate maximum flows at a higher temporal resolution. As will be shown in the
analysis, this is also not a significant limitation at this location on the river.
The data used from the Stream Flow Assessment report (Tetra Tech 2013b) are flow-duration
curves for the three mainstem gages (Gold Creek, Sunshine, and Susitna Station). The flow-
duration curves for all three stations are compared for pre-Project and Maximum Load Following
OS-1 in Figure 4.5-2 of the Stream Flow Assessment report (Tetra Tech 2013b). For each
mainstem gage, the critical discharge is estimated for initiation of motion of the bed material.
Once this discharge is estimated, the flow-duration curves are used to determine the amount of
time this flow is exceeded.
4.2.2. Sediment-transport Supply and Balance
The sediment-transport supply and balance assessment (Tetra Tech 2013a) provides an initial
sediment balance for the Susitna River system. The annual sediment loads at the three mainstem
gages were developed by combining the flow duration results with rating curves of each of the
major components of the sediment load (silt/clay, sand, and gravel). Sediment loads supplied
from gaged tributaries were determined using the same procedure. The sediment loads at the
Watana Dam site and the sediment supplied from ungaged tributaries was also estimated based
on the unit sediment yields from the available data. These results provide the basis for
computing the sediment supply ratio (S*) for pre-Project and Maximum Load Following OS-1
conditions.
4.2.3. Initial Assessment of Downstream Effects
Grant et al. (2003) describes a simple conceptual model of downstream channel changes due to
changes in sediment supply and duration of sediment-mobilizing flows caused by dams. The
model uses two dimensionless ratios (Figure 4.2-1). S* is the ratio of sediment supply below the
dam (SB) to the sediment supply upstream of the dam (SA):
(4.2-1)
For post-dam conditions, most of this sediment is trapped in the dam. For pre-dam conditions,
SA and SB are equal immediately below the dam site, and both SB and S* increase as other
sediment sources contribute downstream. For this analysis, it is assumed that 100 percent of the
sand and gravel supply is trapped in the dam; thus, SB=0 immediately below the dam under post-
dam conditions. Similar to pre-dam conditions, the values of SB and S* increase in the
downstream direction as other sources of sediment are introduced. Separate values of SB and S*
were calculated for the sand and gravel loads, as each is the dominant factor in determining
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 6 February 2013
sediment-transport conditions and the morphology of the Susitna River in specific portions of the
study area.
T* is the ratio of fractional time that bed sediments are mobilized under pre- and post-dam
hydrologic conditions. To compute T*, the fraction of time, T, that flows mobilize bed sediments
is computed separately for pre- and post-dam conditions:
∑
∑
(4.2-2)
For perennial channels the denominator of Equation 4.2-2 is the complete flow record, or 100
percent of the time in a flow-duration analysis. For ephemeral channel the denominator can
change from pre- to post-dam conditions. T* is then computed from the time fractions:
(4.2-3)
The channel adjustment model (Figure 4.2-1) shows three regions. Reaches of river with low
values of S* and high values of T* indicate that the upstream supply is dominant and mobilizing
flows occur frequently. These reaches will respond primarily by degrading (i.e., downcutting)
and/or armoring. Conversely, in reaches of river with high values of S* and low values of T*, the
downstream sediment supply is dominant but mobilizing flows occur infrequently under post-
dam conditions. These reaches will respond through aggradation, especially at the tributary
confluences, and, potentially, fining of the bed material. Along the central diagonal the effects
of the altered sediment and flow characteristics are much less extreme. At the origin, although
sediment supply is truncated, the frequency of sediment mobilizing flow is also truncated so
there is neither the supply of sediment nor the energy to alter the channel form. Reaches that fall
along the shaded diagonal may still adjust, but sediment supply and transport capacity are likely
to remain more closely in balance. The effects of changing hydrology on T* are obvious;
changing flow durations will likely have an impact on the amount of time that flows mobilize
bed material. Trapping sediment in a reservoir impacts the sediment supply at the dam, reducing
S*. Downstream tributary sediment supply is unaffected by the dam. However, changing
hydrology also impacts the mainstem transport capacity. In the case of the Susitna River,
Maximum Load Following OS-1 decreases sediment loads because of the reduced magnitude of
high flows, the corresponding increase in magnitude and duration of low flows, and the non-
linear form of the sediment transport versus flow relationship. These reduced loads become the
supply to reaches farther downstream.
4.2.3.1. Evaluation of the Sediment Supply Ratio, S*
Tables 4.2-1 and 4.2-2 show the annual sediment loads for pre-Project and Maximum Load
Following OS-1 conditions. These tables are the same as those presented in Tetra Tech (2013a),
but also include an estimate of the sediment supply for ungaged tributaries between Sunshine and
the Yentna River. This location was included because the supply of gravel above the Yentna
River confluence exceeds the capacity of gravel transport at Susitna Station, which is just
downstream of the Yentna River confluence. As a result, deposition of gravel is expected
between Sunshine and the Yentna confluence, even under pre-Project conditions. The results
also indicate deposition of sand between Sunshine and the Yentna River confluence. For both
pre-Project and Maximum Load Following OS-1 conditions, S* was computed using the pre-
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 7 February 2013
Project sediment supply at Watana Dam (1,197,000 tons/year for sand and 56,000 tons/year for
gravel) as the denominator of Equation 4.2-1. S* for both sand and gravel through the Middle
Susitna River is close to zero for the Maximum Load Following OS-1 condition and is between
1.0 and 1.2 for pre-Project conditions. This indicates that relatively little sediment supply comes
from tributary sources in the Middle River segment. At the Three Rivers Confluence, S*
increases dramatically for both sand and gravel sizes. S* for sand at this location is 5.2 for pre-
Project conditions and 4.2 for Maximum Load Following OS-1 conditions. S* for gravel at the
Three Rivers Confluence is 15.6 under pre-Project conditions and 14.5 for Maximum Load
Following OS-1 conditions. These large increases for both conditions result from sediment
supplied primarily by the Chulitna River, although the Talkeetna River is also a significant
source.
The area from the Three Rivers Confluence downstream nearly to Sunshine is braided and stores
considerable amounts of gravel (Tetra Tech 2013a, Tetra Tech 2013c). This is evident in the
significant drop in values of S* for gravel at Sunshine to 5.0 for pre-Project conditions.
Similarly, S* decreases to 2.5 for Maximum Load Following OS-1 conditions at Sunshine gage.
Under pre-Project conditions , S* for the sand load ranges between 5.1 and 5.5 between Sunshine
and the Yentna River, then increases significantly to 11.9 downstream of the Yentna River.
Under Maximum Load Following OS-1 conditions, S* for the sand load ranges from 4.2 to 4.6
between Sunshine and the Yentna River, and increases to 10.9 downstream of the Yentna River.
For pre-Project conditions, S* for the gravel load ranges from 5.0 to 5.9 between Sunshine and
the Yentna River, and decreases to 4.6 downstream of the Yentna River. For Maximum Load
Following OS-1 conditions, S* for the gravel load increases from 2.5 to 3.5 between Sunshine
and the Yentna River, then increases slightly to 3.7 downstream of the Yentna River. This
indicates that there may be a small amount of gravel accumulation upstream of the Yenta River
under pre-Project conditions, but the results for Maximum Load Following OS-1 conditions are
less certain.
4.2.3.2. Evaluation of the Transport Duration Ratio, T*
The parameter T, from Equation 4.2-2, is the fraction of time that flows are transporting
sediment related to the specific resource issue under consideration (Grant et al. 2003). For this
Project, the primary concerns are Project effects on the channel form that could alter the
important lateral habitats (side channels, side sloughs and upland sloughs), as well as on the
margins of the main channel that are used for adult salmonid spawning and juvenile rearing.
Since the study area is primarily gravel-bedded, effects on the gravel component of sediment
transport are the most important in addressing the resource issues of concern. It is also important
to note that gravel is not mobilized for all flows, perhaps only a small percentage of the time,
while sand sizes are mobile at essentially all flows. Therefore, T* can change considerably for
the gravels and remain unchanged for sand.
As indicated by Grant et al. (2003), the actual calculation of T* can be difficult and is generally
based on empirical or theoretical sediment-transport equations that consider only the median
(D50) sediment size. Although determining a critical discharge for incipient motion of a specific
D50 is useful, in practice, small amounts of sediment are typically in transport below this
“threshold” value. As a result, it is customary to define the threshold for gravel mobilization
based on a reference condition of very low transport rates (Parker et al. 1982, Wilcock 1988)
based on the dimensionless transport rate given by:
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 8 February 2013
(
) (4.2-4)
where:
Wi* = dimensionless transport rate,
s = specific gravity of the sediment,
g = gravitational acceleration,
qbi = volumetric transport rate of bedload per unit width for size fraction i,
Fi = proportion of size fraction i in the size distribution,
u* = shear velocity,
= boundary shear stress,
ri = reference shear stress for size class i, and
phi () = /ri.
Parker et al. (1982) and Wilcock (1988) defined incipient motion as a Wi* value of 0.002, for =
1.0.
Equation 4.2-4 was used to determine the critical discharge for bed mobilization (Qcr) in
Equation 4.2-2 using the available bed-load transport measurements at the Gold Creek/near
Talkeetna and Sunshine gages. The computational procedure involved estimating the value of
W* and the corresponding value of for each gravel-bed load transport measurement, adjusting
the assumed D50 of the bed material so that the expected relationship between W* and
computed using the Parker et al. (1982) bed-load transport equation, fits through the data. The
available measurements included the total bed-load transport rates and corresponding water
discharge, but did not include the hydraulic parameters (flow width, depth, slope, and velocity)
necessary to calculate the unit transport rates, shear stresses and corresponding values of . The
values of these parameters for each measurement were estimated from hydraulic geometry
relationships for velocity, depth and width as a function of discharge that were developed using
data from the USGS measurements that have been taken to calibrate the stage-discharge rating
curves at the gages. The gradient (S) at each of the gages that is required to estimate the shear
velocity (u*) was estimated based on the local slope from longitudinal profiles developed from
the 2012-surveyed cross sections at Gold Creek and the LiDAR mapping at Sunshine.
At Gold Creek, the channel slope is approximately 10.5 ft/mile and a D50 of 67 mm positions the
transport relationship through the measured data (Figure 4.2-2). The discharge, Qcr, associated
with =1 and W* =0.002 is approximately 25,000 cubic feet per second (cfs). The channel slope
at Sunshine is about 6 ft/mile, and a D50 of 40 mm positions the transport relationship through
the data, resulting in Qcr equal to about 16,000 cfs (Figure 4.2-3). While sufficient data are not
available to verify the estimated D50 values, they appear to be reasonable for purposes of this
preliminary analysis, based on field observations and the limited data that are available. Typical
of bed-load transport data, the plots show a plus/minus one log cycle of scatter in the data. The
plot for Gold Creek appears to be a better fit, in part because the range of values corresponding
to the measured data is greater. The Sunshine site shows more scatter because the sediment
supply is much more variable with sediment contributions from the three upstream rivers, and
the data were collected over a smaller range of values.
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 9 February 2013
A similar analysis could not be performed at the Susitna Station. For the sand-bed channel at
this location probably all flows mobilized the bed. For comparison, a low discharge of 4,000 cfs
was used as Qcr.
The values of Qcr for each location were used in combination with the flow duration curves to
determine the amount of time sediment is mobilized for pre-Project and Maximum Load
Following OS-1 conditions. The ratio of these durations may be more or less than 1.0 depending
on the flow value. If Qcr is high, then T* is likely to be less than 1.0 because the duration of bed-
mobilizing flows will be reduced. If Qcr is low, then T* will likely be greater than 1.0 because
the duration of bed-mobilizing flows will increase. If Qcr is very low, such as the conditions that
are believed to occur at Susitna Station, T* will be equal to 1.0 because the bed is mobile at
essentially all flows under both pre-Project and Maximum Load Following OS-1 conditions.
5. RESULTS
5.1. Evaluation of the Sediment Supply Ratio, S*
S* was computed for both the gravel and sand loads for pre-Project and Maximum Load
Following OS-1 conditions (Figures 5.1-1 and 5.1-2). S* for both the sand and gravel loads is
about 1 between the dam site and the Three Rivers Confluence under pre-Project conditions, but
is essentially zero in this reach under Maximum Load Following OS-1 conditions. Downstream
of Three Rivers, S* for gravel increases dramatically to approximately 15, with a slightly higher
value under pre-Project conditions. The ratio of S* between the two conditions (right axis on
Figure 5.1-1) exceeds 0.9 at the Three Rivers Confluence. The similarity in the without and
with-dam values could lead to the assumption that sediment impacts may not occur downstream
of this location. Moving downstream to Sunshine gage, however, S* for gravel decreases to a
value of 5 for pre-Project conditions, and even more dramatically to slightly less than 3 for
Maximum Load Following OS-1 conditions, resulting in a ratio between the two conditions of
about 0.5. The large spike in the S* values corresponds with the braided, aggradational area at
and below the Three Rivers Confluence resulting primarily from the inflows from the Chulitna
River. The more dramatic decrease in S* under Maximum Load Following OS-1 conditions
results from a combination of the changes in hydrology that decrease the duration of bed-
mobilizing flows (i.e., T*) and the significant increase in sediment supply. The altered hydrology
under Maximum Load Following OS-1 conditions reduces the transport capacity of the Lower
Susitna River Segment that becomes the supply for downstream reaches.
Moving downstream to the reach between Sunshine gage and the Yentna River confluence, S*
increases to about 6 for pre-Project conditions and 3.5 for Maximum Load Following OS-1
conditions, increasing the ratio to about 0.6. Values of S* approach 4 for both conditions
downstream of the Yentna River (Susitna Station gage) and the ratio of the two approaches 0.8.
The mild crest in the pre-Project curve upstream of the Yentna River indicates a tendency toward
accumulation of gravel in this reach.
For the sand load, an abrupt, but lower magnitude, increase in S* occurs for both pre-Project and
Maximum Load Following OS-1 conditions at the Three Rivers Confluence (Figure 5.1-2).
Unlike the gravel plot, values of S* always remain constant or increase in the downstream
direction and the ratio of sediment transport (Maximum Load Following OS-1 conditions divided
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 10 February 2013
by pre-Project conditions) is greater than 0.8 for the Susitna River below the Three Rivers
Confluence. This indicates that potential impacts related to sand are less significant than for
gravel. Sand is almost certainly supply limited in the Middle River Segment, and likely
transitions to capacity limited in the reach upstream of the Yentna River.
5.2. Evaluation of the Transport Duration Ratio, T*
Based on the flow-duration curves presented in Tetra Tech (2013b), the critical discharge (Qcr) of
25,000 cfs in the Middle River Segment occurs about 11 percent of the time under pre-Project
conditions, and this would decrease to about 1.8 percent of the time under Maximum Load
Following OS-1 conditions, resulting in a best-estimate value of T* of 0.16 (Table 5.2-1, Figure
5.2-1). Similarly, the estimated Qcr of 16,000 cfs at Sunshine occurs about 38 percent of the time
under pre-Project conditions, but would increase to about 44 percent of the time under Maximum
Load Following OS-1 conditions due to the increased duration of flows at or above this relatively
low level. Because of the uncertainty in determining incipient motion discharge (Qcr), a lower
and higher value of this discharge was also estimated. For purposes of this preliminary analysis,
the estimated value of Qcr at the Gold Creek and Sunshine gages was increased and decreased by
5,000 cfs to reflect this uncertainty. At the Susitna Station gage, a value of plus and minus 2,000
cfs was used. The resulting values of T* are also summarized in Table 5.2-1. In the Middle
River, T* is approximately 0.2 for the best-estimate and high and low values of Qcr , indicating
that bed mobilizing flows under Maximum Load Following OS-1 conditions would only occur
for about 20 percent of the duration at which they occur under pre-Project conditions.
Downstream of the Three Rivers Confluence to Susitna Station, T* is at or slightly greater than
1.0 for the best- and high estimate of Qcr, but could be as much as 1.5 at the Sunshine gage using
the lower estimate of Qcr.
5.3. Integration of S* and T*
The values of S* for gravel and T* values were plotted in the same conceptual format proposed
by Grant et al. (2003) (Figure 5.3-1). Although the ranges of S* and T* axes are not meant to be
absolute, the shaded area of “Effects Subtle” are from an example application by Grant et al.
(2003) for three rivers (Deschutes River, Oregon; Green River, Utah; and Colorado River,
Arizona). Although the term “Subtle” was used in the paper, it is probably better to consider this
area as being “not extreme” or “indeterminate”, at least applying this model to the Susitna River.
The Middle River Segment plots near the ordinate with sediment supply and time of bed
mobilization are each small compared to pre-Project conditions. In the area between the Three
Rivers Confluence and Sunshine, the results plot in an area of more extreme potential change,
where aggradation and textural shifts at confluences is indicated. This is already an area of
significant sediment accumulation, so the result does not actually represent a significant change
from pre-Project conditions. The best- and high estimate values for the Sunshine gage plot at the
lower range of the “Effects Subtle” area, as defined by Grant et al. (2003) , but the low estimate
value plot somewhat below this area. At Susitna Station, the values plot in a cluster in the
“Effects Subtle” area for all three values of Qcr, largely due to the sand bed character of this
location.
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 11 February 2013
6. DISCUSSION
The results of applying the Grant et al. (2003) conceptual model suggest that the impacts to the
channel form in the Middle River Segment would not be extreme, as both the sediment input and
the frequency of mobilizing flows will be significantly reduced. The impact of the significant
reduction in the frequency and duration of gravel mobilization on side channel and instream
habitat could, however, be significant. In this segment the planned sediment-transport modeling
will provide more complete analysis of potential effects (AEA 2012b, Section 6.6).
The S* and T* values at Sunshine gage plot at the lower limit of “Effects Subtle” range of Grant
et al. (2003), indicating that the portion of the Lower River Segment above Sunshine will
continue to be aggradational with respect to the gravel load, but is likely to see little impact
related to sand transport. Although these results are not extreme, at least in comparison to
upstream, the S*-T* values indicate that the portion of the Lower River Segment below Sunshine
could tend toward degradation and channel narrowing.
Because the bed material is presumed to be sand at Susitna Station (the single sample available
was dominated by sand), the results would indicate minor impact at this location because T* is
1.0 at Susitna Station and S* is nearly unchanged between pre-Project and Maximum Load
Following OS-1 conditions. It should be noted, however, that this conclusion is based on only
one bed material sample, so there is considerable uncertainty in the results for this location.
The conceptual model of downstream impacts proposed by Grant et al. (2003) is a relatively
simple way to assess the potential channel change impacts downstream of a dam. This model
incorporates sediment-transport magnitude and duration to identify areas of large potential
impact. It is not, however, a complete analysis of the potential impacts of channel change.
Considering the borderline results of the Grant et al. (2003) model for the Lower River between
Sunshine and the Yentna River confluence and the results from the stream flow assessment
(Tetra Tech 2013b) and initial sediment transport assessment (Tetra Tech 2013a), AEA will
investigate the potential Project-related effects downstream to the Susitna Station gage. This
investigation will include bed material and bed-load sampling, as well as 1-D sediment-transport
modeling to quantify and clarify the potential magnitude of the Project-related impacts.
6.1. Stream Flow Assessment
The primary basis for identifying the need to continue the 1-D bed evolution modeling effort
below the initially proposed downstream extent is based on interpretation of the results of the
potential changes in hydrology identified in the stream flow assessment technical memorandum
(Tetra Tech 2013b). A comparison of the annual peak flow frequency results between the
existing conditions and the Maximum Load Following Operations Scenario 1 indicates an
appreciable reduction in flows in the 1.5- to 5-year range of recurrence intervals in the Lower
River. Discharges in the range of the 1.5- to 5-year peaks are often representative of the channel
forming or effective discharge to which the bankfull channel capacity adjusts in streams such as
the Lower Susitna River Segment that have mobile bed material and a substantial sediment
supply (Wolman and Miller 1960, Wolman and Gerson 1978, Williams 1978, Andrews 1980).
For the 2-year event, the reduction at Sunshine and Susitna Station were estimated at 24 and 17
percent, respectively.
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 12 February 2013
Numerous researchers have identified hydraulic geometry relationships (i.e., relationships
between channel dimensions and discharge) that clearly demonstrate this linkage (Leopold and
Maddock 1953; Langbein 1964; Emmett 1972; Parker 1979; Andrews 1984; Hey and Thorne
1986; Julien and Wargadalam 1995). The channel width is typically proportional to about the
square-root of the discharge; thus, the indicated reductions in 2-year discharge suggest that the
channel could narrow by slightly more than 10 percent in the portion of the Lower River segment
below Sunshine, and less than 10 percent downstream from the Yentna River confluence. The
narrowing could occur through a combination of vegetation encroachment and sediment
deposition along the margins of the channel and by expansion of the mid-channel islands. Since
the channel margins, including the side sloughs are key habitat units, changes in these areas
could have implications to habitat.
6.2. Development of Sediment-Transport Relationships and an
Initial Sediment Balance for the Middle and Lower Susitna River
Segments
Results from this technical memorandum (Tetra Tech 2013a) indicate that the portion of the
Lower Susitna River Segment below Sunshine is aggradational under pre-Project conditions, and
it would likely remain aggradational under Maximum Load Following OS-1 conditions, although
the magnitude of the aggradational tendency would be somewhat reduced. The sediment balance
results are inconclusive as to whether significant channel change would occur as a result of the
Project. More accurate quantification of this change under Project conditions is necessary to
provide a basis for understanding the potential implications of the change in sediment balance to
both channel form and instream and channel-margin habitat. Extension of the 1-D bed evolution
model downstream to Susitna Station will help provide this understanding.
6.3. Reconnaissance Level Assessment of Potential Channel
Change in the Lower Susitna River Segment
The application of the Grant et al. (2003) conceptual model of channel change suggests that the
potential for significant change in the Lower River downstream from Sunshine is indeterminate;
thus, it cannot be concluded with certainty that the impacts of the Project would be acceptably
small. The S* and T* values at Sunshine gage plot at the lower limit of “Effects Subtle” range of
Grant et al. (2003), indicating that the portion of the Lower River segment above Sunshine will
continue to be aggradational with respect to the gravel load, but is likely to see little impact
related to sand transport. Although these results are not extreme, the S*-T* values indicate that
the portion of the Lower River Segment below Sunshine could tend toward degradation and
channel narrowing.
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 13 February 2013
7. REFERENCES
AEA (Alaska Energy Authority). 2012a. 2012 Reconnaissance-level Geomorphic and Aquatic
Assessment of Project Effects on Lower River Channel: Susitna-Watana Hydroelectric
Project FERC Project No. 14241. May 2012. Anchorage, Alaska. http://www.susitna-
watanahydro.org/type/documents/.
AEA. 2012b. Revised Study Plan: Susitna-Watana Hydroelectric Project FERC Project No.
14241. December 2012. Prepared for the Federal Energy Regulatory Commission by the
Alaska Energy Authority, Anchorage, Alaska. http://www.susitna-
watanahydro.org/study-plan.
Andrews E.D. 1980. Effective and bankfull discharges of streams in the Yampa River basin,
Colorado and Wyoming. Journal of Hydrology 46: 311–330.
Andrews, E.D. 1984. Bed-material entrainment and hydraulic geometry of gravel-bed rivers in
Colorado. Bull. Geol. Soc. Am. 95: 371–378.
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.
Julien, P.Y., and J. Wargadalam. 1995. Alluvial channel geometry: Theory and applications. J.
Hydraul. Eng. 121.4: 312–325.
Grant, G.E., J.C. Schmidt, and S.L. Lewis. 2003. A geological framework for interpreting
downstream effects of dams on rivers. AGU, Geology and Geomorphology of the
Deschutes River, Oregon, Water Science and Application 7.
Knott, J.M., S.W. Lipscomb, and T.W. Lewis. 1987. Sediment Transport Characteristics of
Selected Streams in the Susitna River Basin, Alaska: Data for Water Year 1985 and
Trends in Bed-load Discharge, 1981-95. U.S. Geological Survey Open-File Report 87-
229. Prepared in cooperation with the Alaska Power Authority. Anchorage, Alaska. 45 p.
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. 2012. Susitna-Watana Hydroelectric Project, Preliminary Susitna River Pre-Project and
Post-Project Flow Stages, presented at Technical Work Group Meetings, October 23-25.
Parker, G., 1979. Hydraulic geometry of active gravel rivers. Journal of the Hydraulics
Division. v. 105. no. HY9: 1185-1201.
Parker, G. P.C. Klingeman, and D.L. McLean. 1982. Bedload and size distribution in paved
gravel-bed streams. Journal of Hydraulic Engineering, ASCE, 108(4), 544-571.
Tetra Tech, Inc. 2013a. Development of Sediment Transport Relationships and an Initial
Sediment Balance for the Middle and Lower Susitna River Segments. Susitna-Watana
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 14 February 2013
Hydroelectric Project. 2012 Study Technical Memorandum. Prepared for the Alaska
Energy Authority. Anchorage, Alaska.
Tetra Tech, Inc., 2013b.Stream Flow Assessment. Susitna-Watana Hydroelectric Project. 2012
Study Technical Memorandum. Prepared for the Alaska Energy Authority. Anchorage,
Alaska.
Tetra Tech, Inc. 2013c. Initial Geomorphic Reach Delineation and Characterization, Middle and
Lower Susitna River Segments. Susitna-Watana Hydroelectric Project. 2012 Study
Technical Memorandum. Prepared for the Alaska Energy Authority. Anchorage, Alaska.
U.S. Geological Survey (USGS). 2012. Streamflow Record Extension for Selected Streams in
the Susitna River Basin, Alaska, Scientific Investigations Report 2012–5210. 46 p.
Wilcock, P.R. 1988. Methods for estimating the critical shear stress of individual fractions in
mixed-size sediment. Water Resources Research. 24(7) 1127-1135.
Wilcock, P.R. and J.C. Crowe. 2003. Surface-Based Transport Model for Mixed-Size Sediment.
Journal of Hydraulic Engineering, ASCE, 129(2), 120-128.
Williams GP. 1978. Bankfull discharge of rivers. Water Resources Research 14: 1141–1154.
Wilson, F.H., C.P. Hults, H.R. Schmoll, P.J. Haeussler, J M. Schmidt, L.A. Yehle, and K.A.
Labay. 2009. Preliminary Mapping of the Cook Inlet Region Alaska Including Parts of
the Talkeetna, Talkeetna Mountains, Tyonek, Anchorage, Lake Clark, Seward, Iliamna,
Seldovia, Mount Katmai, and Afognak 1:250,000 Scale Quadrangles. USGS Open-File
Report 2009-1108. 54p plus maps.
Wolman, M.G., and J.P. Miller. 1960. Magnitude and frequency of forces in geomorphic
processes. Journal of Geology 68: 54–74.
Wolman, M.G., and R. Gerson. 1978. Relative scales of time and effectiveness of climate in
watershed geomorphology. Earth Surface Processes 3: 189-208.
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 15 February 2013
8. TABLES
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 16 February 2013
Table 4.2-1. Average annual sediment loads and S* under pre-Project conditions.
Note: S* is computed by dividing annual loads by the supply at Watana (1,197,000 tons/year for sand and 56,000 tons/year for gravel).
Gage Drainage Area
(mi2)
Water Discharge
(acre-feet)
Average Annual Load, tons (and S*)
Wash Load Bed Material Total Load Silt/Clay Sand Gravel Total
Watana 5,180 5,803,000 1,684,000 1,197,000
(1.0)
56,000
(1.0) 1,252,000 2,936,000
Ungaged Tributaries 980 1,242,000 117,000 213,000 11,000 223,000 340,000
Supply above Gold Creek 6,160 7,045,000 1,800,000 1,409,000 66,000 1,475,000 3,276,000
Gold Creek/Susitna nr Talkeetna 6,160 7,045,000 1,800,000 1,409,000
(1.18)
66,000
(1.18) 1,475,000 3,276,000
Talkeetna 1,996 2,938,000 940,000 866,000 57,000 923,000 1,863,000
Chulitna 2,570 6,231,000 5,264,000 3,917,000 748,000 4,665,000 9,929,000
Supply above Sunshine 10,726 16,213,000 8,005,000 6,192,000
(5.17)
871,000
(15.55) 7,063,000 15,067,000
Sunshine 11,100 17,426,000 10,012,000 6,101,000
(5.10)
279,000
(4.98) 6,380,000 16,392,000
Ungaged Tributaries 2,120 3,654,000 2,366,000 534,000 53,000 587,000 2,953,000
Supply above Yentna 13,220 21,080,000 12,378,000 6,635,000
(5.54)
332,000
(5.93) 6,967,000 19,345,000
Yentna 6,180 14,102,000 7,162,000 8,205,000 180,000 8,385,000 15,547,000
Supply above Susitna Station 19,400 35,182,000 19,540,000 14,840,000 512,000 15,352,000 34,892,000
Susitna Station 19,400 35,182,000 19,534,000 14,278,000
(11.93)
260,000
(4.64) 14,538,000 34,072,000
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 17 February 2013
Table 4.2-2. Average annual sediment loads and S* under Maximum Load Following OS-1 conditions.
Note: S* is computed by dividing annual loads by the pre-Project supply at Watana (1,197,000 tons/year for sand and 56,000 tons/year for gravel).
Gage Water Discharge
(acre-ft)
Average Annual Load, tons (and S*)
Wash Load Bed Material Total Load Silt/Clay Sand Gravel Total
Watana Dam 5,785,000 168,000 0 (0.0) 0 (0.0) 0 168,000
Ungaged Tribs 1,209,000 117,000 213,000 11,000 223,000 340,000
Supply above Gold Creek 6,995,000 285,000 213,000 11,000 223,000 508,000
Gold Creek 6,995,000 285,000 213,000
(0.18)
4,000
(0.07) 217,000 502,000
Talkeetna 2,938,000 940,000 866,000 57,000 923,000 1,863,000
Chulitna 6,231,000 5,264,000 3,917,000 748,000 4,665,000 9,929,000
Supply above Sunshine 16,164,000 6,490,000 4,995,000
(4.17)
809,000
(14.45) 5,804,000 12,294,000
Sunshine 17,375,000 8,497,000 4,995,000
(4.17)
142,000
(2.54) 5,137,000 13,634,000
Ungaged Tributaries 3,654,000 2,366,000 534,000 53,000 587,000 2,953,000
Supply above Yentna 21,029,000 10,863,000 5,529,000
(4.62)
195,000
(3.48) 5,724,000 16,587,000
Yentna 14,102,000 7,162,000 8,205,000 180,000 8,385,000 15,547,000
Supply above Susitna Station 35,131,000 18,025,000 13,734,000 375,000 14,109,000 32,134,000
Susitna Station 35,131,000 18,019,000 13,040,000
(10.89)
207,000
(3.70) 13,247,000 31,266,000
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 18 February 2013
Table 5.2-1. Results for critical discharge estimates.
Location Estimated Qcr (cfs) Tpre TMax LF OS-1 T*
Best Estimate
Watana 25,000 11.0% 1.8% 0.16
Gold Creek 25,000 11.0% 1.8% 0.16
Sunshine 16,000 44.0% 50.0% 1.14
Susitna Station 4,000 100.0% 100.0% 1.00
Low Estimate
Watana 20,000 20.0% 4.5% 0.23
Gold Creek 20,000 20.0% 4.5% 0.23
Sunshine 11,000 46.0% 70.0% 1.52
Susitna Station 2,000 100.0% 100.0% 1.00
High Estimate
Watana 30,000 5.0% 0.7% 0.14
Gold Creek 30,000 5.0% 0.7% 0.14
Sunshine 21,000 38.0% 38.0% 1.00
Susitna Station 6,000 92.5% 99.2% 1.07
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 19 February 2013
9. FIGURES
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 20 February 2013
Figure 3.2-1. Susitna River Geomorphology Study Area and Large-scale River Segments.
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 21 February 2013
Figure 4.2-1. Response domain for predicted channel adjustments. (Grant et al. 2003).
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 22 February 2013
Figure 4.2-2. Gravel Bed-load Transport at Gold Creek Gage.
Figure 4.2-3. Gravel Bed-load Transport at Sunshine Gage.
0.00001
0.0001
0.001
0.01
0.1
1
10
0.5 1 2 4W*phi
0.0001
0.001
0.01
0.1
1
10
0.5 1 2 4W*phi
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 23 February 2013
Figure 5.1-1. S* for gravel material on the Middle and Lower Susitna River Reaches.
20406080100120140160180
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0
2
4
6
8
10
12
14
16
18
20
0 20 40 60 80 100 120 140 160
Project River Mile
S*-Max LF OS-1/S*preS*Miles Below Dam
Sand
Pre-Project
Max LF OS-1
S*-Max LF OS-1/S*pre
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 24 February 2013
Figure 5.1-2. S* for sand material on the Middle and Lower Susitna River Reaches.
20406080100120140160180
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0
2
4
6
8
10
12
14
16
18
20
0 20 40 60 80 100 120 140 160
Project River Mile
S*-Max LF OS-1/S*preS*Miles Below Dam
Gravel
Pre-Project
Max LF OS-1
S*-Max LF OS-1/S*pre
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 25 February 2013
Figure 5.2-1. T* for the Middle and Lower Susitna River Reaches.
20406080100120140160180
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
0 20 40 60 80 100 120 140 160
Project River Mile
T*Miles Below Dam
Estimated Qcr
Qcr-low
Qcr-high
2012 TECHNICAL MEMORANDUM POTENTIAL CHANNEL CHANGE/LOWER SUSITNA
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 26 February 2013
Figure 5.3-1. S* and T* on the Middle and Lower Susitna River Reaches.
0
2
4
6
8
10
12
14
16
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6S*T*
Estimated Qcr
Qcr-low
Qcr-high
Middle
Reach
Between Three Rivers Conflence
and Sunshine Gage
Sunshine Gage
Susiitna
Station