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Susitna-Watana Hydroelectric Project Document
ARLIS Uniform Cover Page
Title:
SuWa 237
Geomorphology study (Study 6.5), Susitna River historical cross section
comparison (1980s to current), technical memorandum
Author(s) – Personal:
Author(s) – Corporate:
Tetra Tech, Inc.
AEA-identified category, if specified:
September 17, 2014 technical memorandum filings
AEA-identified series, if specified:
Series (ARLIS-assigned report number): Existing numbers on document:
Susitna-Watana Hydroelectric Project document number 237
Published by: Date published:
[Anchorage, Alaska : Alaska Energy Authority, 2014] September 2014
Published for: Date or date range of report: Prepared for Alaska Energy Authority
Volume and/or Part numbers:
Final or Draft status, as indicated:
Attachment I
Document type: Pagination:
Technical memorandum iv, 27, 20 p.
Related work(s): Pages added/changed by ARLIS:
Cover letter to this report: Susitna-Watana Hydroelectric
Project, FERC Project no. 14241-000; Initial filing of September
2014 technical memoranda. (SuWa 228)
Attachments A-H and J (SuWa 229-236 and 238)
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 17, 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
Initial Filing of September 2014 Technical Memoranda
Dear Secretary Bose:
By letter dated January 28, 2014, the Federal Energy Regulatory Commission
(Commission or FERC) extended the procedural schedule for the preparation and review
of the Initial Study Report (ISR) for the proposed Susitna-Watana Hydroelectric Project,
FERC Project No. 14241 (Project).1 In particular, the Commission’s January 28 letter
established a deadline of June 3, 2014 for the Alaska Energy Authority (AEA) to file the
ISR, and provided a 120-day period for licensing participants to review the ISR prior to
the ISR meetings, which are scheduled to begin the week of October 13.2 The purpose of
this filing is to provide several technical memoranda to Commission Staff and licensing
participants prior to the ISR meetings.
As required by the Commission’s January 28 letter, AEA filed the ISR with the
Commission on June 3. Among other things, the ISR detailed AEA’s planned work
during the 2014 field season.3 As AEA was preparing this 2014 work plan, it recognized
that data gathered during the 2014 field season, together with other study work conducted
prior to the October 2014 ISR meetings,could assist Commission Staff, AEA, and other
licensing participants in developing the Project’s licensing study program for 2015. For
this reason, the ISR provided for AEA to prepare certain technical memoranda and other
information based on 2014 work.
AEA recognizes that Commission Staff and licensing participants need a
reasonable amount of time prior to the ISR meetings to review this additional
information. AEA and licensing participants consulted with Commission Staff on this
1 Letter from Jeff Wright, Federal Energy Regulatory Commission, to Wayne Dyok, Alaska Energy
Authority, Project No. 14241-000 (issued Jan. 28, 2014)[hereinafter, “January 28 letter”].
2 The full schedule for the ISR meetings appears in Section 1.5 of the ISR, as well as on AEA’s
licensing website, http://www.susitna-watanahydro.org/meetings/.
3 E.g., Initial Study Report §1.3 & Table 3, Project No. 14241-000 (filed June 3, 2014) [hereinafter,
“ISR”].
2
matter, and Staff directed that any additional information should be filed with the
Commission and made available to licensing participants no later than 15 days prior to
the ISR meetings, consistent with the typically applicable deadline under the
Commission’s Integrated Licensing Process regulations.4
With this letter, AEA is filing and distributing the first set of technical
memoranda and other information generated during the 2014 study season, as described
below. As part of its continued implementation of the study plan, AEA expects to file
certain additional technical memoranda prior to October 1, 2014, in accordance with
Commission Staff direction.
This first set of technical memoranda and other information consists of the
following:
Attachment A: Proposal to Eliminate the Chulitna Corridor from Further
Study. As explained in the ISR, throughout the licensing process AEA has
continually evaluated its proposal for Project development based on
environmental review, technical feasibility, practical considerations, and other
factors. As part of this iterative process, AEA notified the Commission and
licensing participants in the ISR that it was evaluating whether to continue
study of the Chulitna Corridor.5 Attachment A details AEA’s conclusion that
development of the Chulitna Corridor is not a reasonable alternative, and
therefore AEA proposes to eliminate the corridor from further study. AEA
seeks any comments or information on this proposal from federal and state
resource agencies and other participants in the licensing process.
Attachment B: Ice Processes in the Susitna River Study (Study 7.6),Detailed
Ice Observations October 2013 –May 2014 Technical Memorandum. The
ISR indicated that AEA would provide a summary of the 2014 break-up
observations.6 This technical memorandum describes all field activities and
observations between October 16, 2013 and May 15, 2014 for the Ice
Processes in the Susitna River Study (Study 7.6).
Attachment C: Study of Fish Distribution and Abundance in the Upper
Susitna River (Study 9.5), Proposed 2015 Modifications to Fish Distribution
and Abundance Study Plan Implementation Technical Memorandum. Based
on AEA’s experience in implementing the study plan for the Study of Fish
Distribution and Abundance in the Upper Susitna River (Study 9.5)during
2014, this technical memorandum proposes to continue certain modifications
to the implementation of this study during 2015.
4 See 18 C.F.R. §5.15(c)(2).
5 See ISR, ISR Overview §1.4.
6 See id., Ice Processes in the Susitna River Study,Study Plan 7.6, Part C §7.2.
3
Attachment D: Study of Fish Distribution and Abundance in the Middle and
Lower Susitna River Study (Study 9.6), 2013-2014 Winter Fish Study
Technical Memorandum. At the time the ISR was filed, AEA was still in the
process of conducting data entry, quality control, and analysis of winter
sampling for this study. AEA reported in the ISR that it would develop plans
for completing this study in a technical memorandum to be filed with the
Commission.7 This technical memorandum fulfills this commitment and sets
forth AEA’s proposal for winter efforts, including proposed methodologies
and modifications.
Attachment E: Characterization and Mapping of Aquatic Habitats (Study
9.9), 2013 and 2014 Aquatic Habitat Mapping Field Season Completion
Progress Technical Memorandum. In the ISR, AEA reported that its 2014
activities for the Characterization and Mapping of Aquatic Habitats Study
(Study 9.9) would consist of various ground-truthing surveys and collection of
habitat information for the 12 lakes within the potential reservoir inundation
zone.8 This technical memorandum reports on these activities.
Attachment F: Eulachon Run Timing, Distribution, and Spawning in the
Susitna River (Study 9.16), 2015 Proposed Eulachon Spawning Habitat Study
Modifications Technical Memorandum.After reviewing the 2013 and 2014
results from the Cook Inlet Beluga Whale Study (Study 9.17) and discussing
the results with the National Marine Fisheries Service, AEA has determined
that additional data are needed regarding eulachon spawning habitats. This
technical memorandum describes a proposed modification to the Study of
Eulachon Run Timing, Distribution and Spawning in the Susitna River (Study
9.16)to include an assessment of eulachon spawning habitats.
Attachment G: Fish and Aquatics Instream Flow Study (Study 8.5),
Evaluation of Relationships between Fish Abundance and Specific
Microhabitat Variables Technical Memorandum. Consistent with the
Commission’s study plan determination,9 this technical memorandum
provides a detailed evaluation of the comparison of fish abundance measures
with specific microhabitat variable measurements where sampling overlaps.
This memorandum is used to determine whether a relationship between a
specific microhabitat variable and fish abundance is evident.
Attachment H: Fish and Aquatics Instream Flow Study (Study 8.5), 2013-
2014 Instream Flow Winter Studies Technical Memorandum.In the ISR,
AEA reported that it would distribute its finding concerning the 2013-2014
7 See id., Study of Fish Distribution and Abundance in the Middle and Lower Susitna River Study,
Study Plan 9.6, Part C §7.1.2.5.
8 See id., Characterization and Mapping of Aquatic Habitats, Study Plan 9.9, Part C § 7.1.
9 See Study Plan Determination on 14 Remaining Studies for the Susitna-Watana Hydroelectric Project,
Appendix B at B-84 to B-86, Project No. 14241-000 (issued Apr. 1, 2013).
4
winter activities in 2014.10 This technical memorandum describes the
methods applied, and data and information collected, as part of the Instream
Flow Study 2013-2014 winter studies.
Attachment I: Geomorphology Study (Study 6.5), Susitna River Historical
Cross Section Comparison (1980s to Current) Technical Memorandum. As
specified in Revised Study Plan Section 6.5.4.1.2.3, this technical
memorandum describes changes within the main and side channels of the
Susitna River by comparing historical survey data from the 1980s with survey
data from the current Project.
Attachment J: Geomorphology Study (Study 6.5), 2014 Update of Sediment-
Transport Relationships and a Revised Sediment Balance for the Middle and
Lower Susitna River Segments Technical Memorandum. The purpose of this
technical memorandum is to update the sediment load rating curves and
preliminary estimates of the overall sediment balance in the Middle and
Lower River segments under pre-Project conditions that were initially
provided in “Development of Sediment-Transport Relationships and an Initial
Sediment Balance for the Middle and Lower Susitna River Segments,” (Tetra
Tech, Inc. 2013a). This update is based on additional data collected by the
U.S. Geological Survey in 2012 and 2013.
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)
10 See ISR, Fish and Aquatics Instream Flow Study, Study Plan 8.5, Part C §7.5.2.
Susitna-Watana Hydroelectric Project
(FERC No. 14241)
Geomorphology Study (Study 6.5)
Susitna River Historical Cross Section Comparison
(1980s to Current)
Technical Memorandum
Prepared for
Alaska Energy Authority
Prepared by
Tetra Tech, Inc.
September 2014
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page i September 2014
TABLE OF CONTENTS
Summary .......................................................................................................................................iv
1.Introduction .......................................................................................................................1
2.Study Objectives ................................................................................................................1
3.Study Area ..........................................................................................................................2
4.Methods ..............................................................................................................................3
4.1.Development of 1980s Cross Sections ....................................................................3
4.2.Development of Current cross sections ...................................................................3
4.3.Comparison of 1980s and Current Cross Sections ..................................................4
4.4.Comparison of 1980s and Current Thalweg/Channel Profile .................................4
5.Results .................................................................................................................................4
5.1.1980s Cross Sections ...............................................................................................5
5.2.Current Cross Sections ............................................................................................5
5.3.Comparison of 1980s and Current Cross Section....................................................5
5.3.1.Change in Average Bed Elevation...............................................................5
5.3.2.Change in Area below 2-Year Return Period WSE ....................................6
5.4.Comparison of 1980s and Current Channel Thalweg Profiles ................................7
6.Summary and Conclusions ...............................................................................................8
7.References.........................................................................................................................10
8.Tables ................................................................................................................................11
9.Figures ..............................................................................................................................14
LIST OF TABLES
Table 4.3-1. Summary of compared cross sections and distances between cross sections..........11
Table 5.3-1. Summary of cross sections comparison results. ......................................................11
LIST OF FIGURES
Table 4.3-1. Summary of compared cross sections and distances between cross sections..........11
Table 5.3-1. Summary of cross sections comparison results. ......................................................11
Figure 3-1 Extents of 1980s HEC-2 model and current HEC-RAS model.................................15
Figure 4.3-1 Cross section locations for 1980s HEC-2 cross sections and current HEC-RAS
cross sections from PRM 187 to PRM 183...........................................................................16
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page ii September 2014
Figure 4.3-2 Cross section locations for 1980s HEC-2 cross sections and current HEC-RAS
cross sections from PRM 183 to PRM 176...........................................................................17
Figure 4.3-3 Cross section locations for 1980s HEC-2 cross sections and current HEC-RAS
cross sections from PRM 174 to PRM 164...........................................................................18
Figure 4.3-4 Cross section locations for 1980s HEC-2 cross sections and current HEC-RAS
cross sections from PRM 156 to PRM 149...........................................................................19
Figure 4.3-5 Cross section locations for 1980s HEC-2 cross sections and current HEC-RAS
cross sections from PRM 150 to PRM 138...........................................................................20
Figure 4.3-6 Cross section locations for 1980s HEC-2 cross sections and current HEC-RAS
cross sections from PRM 139 to PRM 128...........................................................................21
Figure 4.3-7 Cross section locations for 1980s HEC-2 cross sections and current HEC-RAS
cross sections from PRM 129 to PRM 118...........................................................................22
Figure 4.3-8 Cross section locations for 1980s HEC-2 cross sections and current HEC-RAS
cross sections from PRM 118 to PRM 108...........................................................................23
Figure 4.3-9 Cross section locations for 1980s HEC-2 cross sections and current HEC-RAS
cross sections from PRM 108 to PRM 97.............................................................................24
Figure 5.2-1 Example cross-sectional geometry from the 1980s HEC-2 model and the current
HEC-RAS model...................................................................................................................25
Figure 5.4-1 Comparison of thalweg profiles for the 1980s HEC-2 URX model and the current
HEC-RAS model, PRM 187.2 to PRM 169.5.......................................................................26
Figure 5.4-2. Comparison of thalweg profiles for the 1980s HEC-2 LRX model and the current
HEC-RAS model, PRM 153.7 to PRM 98.4.........................................................................27
LIST OF APPENDICES
Appendix A.1: Historical Cross Section Comparison Plots
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page iii September 2014
LIST OF ACRONYMS AND SCIENTIFIC LABELS
Abbreviation Definition
1-D one-dimensional
AEA Alaska Energy Authority
cfs Cubic feet per second
FERC Federal Energy Regulatory Commission
FIPS Federal Information Processing Standard
HEC Hydraulic Engineering Center
HEC-RAS Hydraulic Engineering Center River Analysis System
GIS Geographic Information System
GPS Global Positioning System
LiDAR Light Detection and Ranging
LRX 1980s Lower reach one-dimensional hydraulic model
NAD North American Datum
NAVD North American Vertical Datum
NGVD National Geodetic Vertical Datum
NGS National Geodetic Survey
PDF Portable document file
PRM Project River Mile
RM River Mile(s)
URX 1980s Upper reach one-dimensional hydraulic model
USACE United States Army Corps of Engineers
USGS U.S. Geological Survey
WSE Water Surface Elevation
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page iv September 2014
SUMMARY
Data describing the cross-sectional geometry of the Middle Susitna River channel are available
from two sources spanning three decades. During the 1980s studies of the Susitna Hydroelectric
Project, various efforts were carried out to characterize sediment supply and sediment transport
capacity.Modeling of hydraulics of the Susitna River below the proposed Project was
performed in the 1980s using a one-dimensional HEC-2 hydraulic model to calculate water-
surface profiles and channel hydraulics (Acres 1983). The river cross sections used in developing
the model provide information on the Susitna River channel dimensions and elevations in the
1980s. For the current Project, a one-dimensional bed evolution model is being developed as part
of the Fluvial Geomorphology Modeling Study (Study 6.6) to simulate fluvial geomorphic
processes that influence the morphology of the Susitna River channel and floodplain downstream
of Watana Dam (Tetra Tech 2014) as well as supporting 1-D models being developed in the Fish
and Aquatics Instream Flow Study (Study 8.5), the Ice Processes the Susitna River Study (Study
7.6) and the Water Quality Modeling Study (Study 5.6).
The bathym etry and topography surveyed in both the 1980s and for the current project permits
the opportunity to compare the cross-sectional geometry from the two sets of surveys collected
approximately three decades a part.For instances where a cross section from the 1980s and one
from the current effort were surveyed near each other, a comparison of these data provides a
valuable resource to quantify channel change over the past three decades, as well as providing an
additional data source for validation of the current 1-D bed evolution model.Thirty-eight pairs
of cross sections were compared and changes in average bed elevation and cross sectional area
were determined.Channel profiles determined from thalweg elevations were also compared to
assess longitudinal bed elevation differences.
Overall, the results show that the Middle Susitna River was generally stable in terms of bed
elevation change over the three decade period between the 1980s studies and the current effort.
Changes were typically on the order of several feet or less and there was not a consistent trend
toward aggradation (bed elevation increase) or degradation (bed elevation decrease) throughout
the Middle River segment. The area with the most consistent trend were geomorphic reaches
MR-7 and MR-8 in which the majority of the length was aggradational; however, there were still
cross sections within these two reaches that showed degradation over the period of comparison.
MR-2 showed the least change, with all cross sections having minimal change in bed elevation
and area with the exception of the cross section at PRM 179.5, which aggraded by 2.3 ft and
increased in area by 490 sq. ft. The remaining reaches had more of a mix between aggradation
and degradation.Results of this work were used in the validation of the 1-D bed evolution
model currently under development as part of the Fluvial Geomorphology Modeling Study
(Study 6.6) (Tetra Tech 2014).
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page 1 September 2014
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 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 provide
information needed to support the FERC’s National Environmental Policy Act analysis for the
Project license.
During the 1980s studies of the Susitna Hydroelectric Project, various efforts were carried out to
characterize sediment supply and sediment transport capacity. Modeling of hydraulics of the
Susitna River below the proposed Project was performed in the 1980s using a one-dimensional
HEC-2 hydraulic model to calculate of water-surface profiles and channel hydraulics (Acres
1983). The river cross sections used in developing the model provide information on the Susitna
River channel dimensions and elevations in the 1980s.
For the current Project, a one-dimensional bed evolution model is being developed as part of the
Fluvial Geomorphology Modeling Study (Study 6.6) to simulate fluvial geomorphic processes
that influence the morphology of the Susitna River channel and floodplain downstream of
Watana Dam (Tetra Tech 2014)as well as supporting 1-D models being developed in the Fish
and Aquatics Instream Flow Study (Study 8.5), the Ice Processes the Susitna River Study (Study
7.6) and the Water Quality Modeling Study (Study 5.6).
The bathym etry and topography surveyed in both the 1980s and for the current project permits
the opportunity to compare the cross-sectional geometry from the two sets of surveys collected
approximately three decades a part. For instances where a cross section from the 1980s and one
from the current effort were surveyed near each other, a comparison of these data provides a
valuable resource to quantify channel change over the past three decades, as well as providing an
additional data source for validation of the current 1-D bed evolution model.
This effort was identified in RSP 6.5.4.1.2.3 as part of the Geomorphic Characterization of the
Susitna River task. Channel profiles from the two periods were developed using the surveyed
thalweg at each cross section and the profiles compared. As the layout of the surveyed cross
sections differs between the two periods, locations with cross sections in close proximity of each
other were identified and cross-sectional geometries at these locations were compared. The
magnitude and locations of change within the Susitna River can be used to validate the one-
dimensional sediment-transport model currently under development (Tetra Tech 2014).
2.STUDY OBJECTIVES
The overall goal of the Geomorphology Study (Study 6.5) is to characterize the geomorphology
of the Susitna River,and evaluate the effects of the Project on the geomorphology and dynamics
of the river by predicting the trend and magnitude of geomorphic response. The objective of the
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page 2 September 2014
work detailed in this memorandum is to identify changes within the main and side channels of
the Susitna River by comparing historical survey data from the 1980s with survey data from the
current project.
3.STUDY AREA
The Susitna River, located in South-central 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 the 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 air temperatures of approximately 29°F.
In the 1980s, a one-dimensional, steady-state hydraulic HEC-2 model (U.S. Army Corps of
Engineers 1976) was developed by R&M Consultants (1982). The HEC-2 model consisted of
two reaches, as shown in Figure 3-1. An upper reach (URX) consisting of 23 cross sections
extended from the confluence with Deadman Creek at PRM 189.3 to the confluence with Devil
Creek at PRM 165.5. The upper cross sections were surveyed in March 1981, by drilling
through ice. A lower reach (LRX) consisted of 66 cross sections, and extended from below
Devils Canyon at PRM 153.6 to just above the confluence with the Chulitna River at PRM
102.3. The cross sections for the lower reach of the 1980s HEC-2 model were surveyed in the
fall of 1980. The lower model was later extended by the Harza-EBASCO Susitna Joint Venture
(1984) downstream to the USGS gaging station at Sunshine. The additional cross sections were
surveyed in 1982;however, downstream of PRM 98.4, the additional cross sections were
interpolated from topographic maps and are not suitable for comparison with the current
surveyed cross sections. To avoid confusion, it needs to be noted that the 1980s LRX and URX
reaches do not correspond to the current Lower Susitna River segment (downstream of PRM
102.4) and Upper Susitna River segment (upstream of the proposed dam site at PRM 187.1).
A one-dimensional unsteady hydraulic model is being developed using HEC-RAS version 5 beta
for the current Project as part of the Fluvial Geomorphology Modeling Study (Study 6.5). Cross
sections used in the current HEC-RAS model were surveyed between 2012 and 2013. The
current HEC-RAS model extends from PRM 187.2 just above the Dam site to PRM 29.9, just
downstream of the confluence with the Yentna River.
There is overlap between the extents of the two models, the 1980s and current, from PRM 187.2
to PRM 165.5, and from PRM 153.6 to PRM 98.4. It is in these two reaches of the Susitna River
that the cross-sectional geometries from the two time periods were compared and evaluated for
changes.
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page 3 September 2014
4.METHODS
Surveyed cross-sectional geometries are available from the 1980s in the form of survey n otes, as
well as printouts of HEC-2 input files. Cross section locations for the HEC-2 URX and LRX
models are available as maps. The survey notes, HEC-2 input files, and maps were digitized for
the purpose of comparing geometries to the surveyed cross se ctions used in the current HEC-
RAS model. As the cross-section locations differ between the two time periods,not all cross
sections were comparable. The cross sections best suited for comparison were selected between
the two models. Average bed elevation and cross-sectional areas below the water surface
elevation (WSE) modeled using 2-year return period discharge at Gold Creek were compared.
Also, channel thalwegs from each time period were compared to assess any changes within the
channel profile.
Development of 1980s cross sections4.1.
As part of the 1980s studies, a one-dimensional HEC-2 model was developed from cross sections
which had been field surveyed in 1980 -1982. In order to compare with current cross sections, i t
was necessary to determine the location of the 1980s cross sections in relation to the current
cross sections. To facilitate the cross section comparison, the horizontal locations of 1980s cross
sections were digitized in GIS from maps (Harza-EBASCO 1984) displaying their locations. As
the 1980s cross section locations were digitized from maps as opposed to surveyed coordinates,
the 1980s cross section locations are approximate.
Cross-sectional geometries were digitized from survey notes where available, and from the HEC -
2 input files where survey notes were unavailable. The 1980s cross sections were referenced to
the National Geodetic Vertical Datum of 1929 (NGVD29), and needed adjustment to be
consistent with the current cross sections, which are referenced to the North Ame rican Vertical
Datum of 1988 (NAVD88). As part of the 1980s stud ies, survey control points were placed
along the Susitna River (R&M Consultants, 1981b). A GIS shapefile of National Geodetic
Survey (NGS) control points in Alaska was acquired from the NGS website (NGS 2014), which
included the control points placed in the 1980s study, as well as any subsequently placed control
points. All of the control points which had surveyed elevations referenced to both NGVD29 and
NAVD88 were used to develop a correction between the vertical datums. Elevations in the
1980s cross-sectional geometries were then corrected based upon the difference in NGVD29 and
NAVD88 elevations at the control point nearest the particular cross section.The corrections
ranged from an addition of 5.83 to 5.95 feet to the NGVD29 elevation.
Development of Current Cross Sections4.2.
A one-dimensional unsteady bed-evolution HEC-RAS model is currently being developed as
part of the current Project and will be used to simulate fluvial geomorph ic processes in the
Susitna River. The HEC-RAS model geometry was developed using bathymetry surveyed
between 2012 and 2014, as well as LiDAR acquired in 2011 (Matanuska-Susitna Borough,2011)
and 2013 (Tetra Tech 2014). The cross sections were filtered to reduce the amount of
computational nodes while still maintaining an accurate representation of the surface. Each
current cross section with a comparable 1980s cross section was plotted with the 1980s section
for a visual comparison.
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page 4 September 2014
Comparison of 1980s and Current Cross Sections 4.3.
While there is overlap in the domains of the HEC-2 and HEC-RAS models, the layout of cross
sections in the two models differs. As such, not all 1980s cross sections were suitable for
comparison to current cross sections as some were located too far from current cross sections .
The 1980s cross section locations in relation to the current cross section locations is shown in
Figures 4.3-1 through 4.3-9. Any 1980s cross sections where the nearest current cross section
was deemed too far to provide an adequate comparison between the two time periods was not
included in this study. Also, while some cross sections were near each other, the skew across
channels occasionally differed significantly or crossed the leading or trailing end of a mid-
channel bar, and were not included in this study. All of the cross sections that were included in
this study are identified in Table 4.3-1, with the distances between the cross sections being
compared. Distances between compared cross sections varied between 25 ft and 350 ft.
Changes in cross-sectional geometry were evaluated in part by calculating the change in average
bed elevation between the left and right toe of bank in each channel. In cross sections with
multiple channels, average bed elevations between the left and right toe of bank for each channel
were calculated separately. Cross-sectional area below a specific water surface elevation (WSE)
was also calculated. As with the average bed elevation, cross -sectional area was only calculated
between the left and right toe of bank in each channel. The specific WSE was determined for
each cross section from a steady simulation of the current calibrated HEC-RAS hydraulic model
using the 2-year return period discharge at Gold Creek of 43,500 cfs (Study 6.5 ISR Section
5.6.1.1).
Comparison of 1980s and Current Thalweg/Channel Profile4.4.
Channel bed profiles between the 1980s and current conditions were also compared by plotting
channel thalwegs at each cross section for both time periods. Thalweg elevations for the 1980s
were surveyed in NGVD29 (R&M Consultants 1982) and were adjusted to account for
differences between NGVD29 and NAVD88 datums as described in Section 4.1 of this
document. As surveys for the 1980s URX cross sections were performed by drilling through
ice, thalwegs may not have been captured in the 1980s cross sections upstream of Devils
Canyon. Thalweg elevations for the current cross sections represent the lowest elevation
surveyed in each cross section.
5.RESULTS
Selected cross sections surveyed for the 1980s HEC -2 model were digitized from survey notes
and HEC-2 input files, and were compared to cross sections surveyed for the current HEC-RAS
model. Comparable cross sections were plotted for a visual comparison, and average bed
elevation and area below the WSE resulting from the 2 -year return period discharge at Gold
Creek were calculated.A total of 38 cross sections were compared. Of these 38 cross sections,
eleven had multiple channels with 10 having 2 channels and 1 having three channels. The
thalweg profiles were also compared.
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page 5 September 2014
1980s Cross Sections5.1.
Each 1980s cross section with a corresponding current cross secti on was digitized from survey
notes or HEC-2 input files and plotted per the methods described in Section 4.1 . An example of
a resulting cross section is shown in Figure 5.2 -1, while all cross section plots are included in
Appendix A.
Current Cross Sections5.2.
Current cross sections were extracted from the HEC -RAS model geometry per the methods
described in Section 4.2 and plotted along with the corresponding 1980s cross section . The
horizontal stationing along the cross section used in the 1980s survey differed from the stationing
used for the current survey, so an offset was applied to the plots of the current geometry. An
example of a current cross section is shown in Figure 5.2-1 while all cross section plots are
included in Appendix A.The cross section plots show both the 1980s and current cross section
overlaid.
Comparison of 1980s and Current Cross Sections5.3.
Changes in average bed elevation and cross-sectional area within channels were calculated for
each 1980s cross section and comparable current cross section per the methods described in
Section 4.3. When an increase (bed aggradation) or decrease (bed degradation) is indicated it
refers to the direction of change from the 1980s to current condition. When the change in bed
elevation was within plus or minus a foot it was considered to be minimal or no change. Twenty-
five of the cross sections showed no change in average bed elevation,and of the remaining cross
sections, eight cross sections had an increase (aggradation) of greater than 1 foot and five had a
decrease in elevation (degradation)of greater than one foot.The most consistent and some of
the largest elevation changes were noted in reaches MR -8 and LR-1 and corresponded to bed
aggradation.
Assessing the difference of area allowed inc orporation of the changes in channel width in the
consideration of channel change. C hanges in area of less than 450 sq. ft. are considered to be
minimal. Twenty-one of the cross sections had minimal change in cross-sectional area. Of the
remaining cross sections, only five had an increase in area while twelve had a decrease in area.
In general, the changes in area mimicked the trends in changes in average bed elevation, except
where channel width also changed by greater than ten percent.
5.3.1.Change in Average Bed Elevation
Average bed elevations were calculated for each channel and are listed in Table 5.3-1. In cross
sections with multiple channels, an overall average bed elevation was determined by calculating
the average bed elevation for each channel and then averaging the elevations weighted by the
width of the respective channel. Twenty-five of the 38 cross sections had minimal change in
average bed elevation. Of the remaining cross sections, five degraded and eight aggraded. Three
cross sections had a decrease of one to two feet, one had a decrease of three to five feet, and one
had a decrease of greater than five feet in elevation. One cross section had an increase of one to
two feet,four had an increase of two to three feet, and three had an increase of three to five feet
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page 6 September 2014
in elevation. The largest increase in average bed elevation of 4.8 ft was observed in the cross
sections at PRM 153.7, at the downstream end of Devils Canyon at the upstream limit of MR-5.
The cross sections being compared at PRM 153.7 are approximately 300 ft apart and are on the
outside of a bend. A large pool present in the 1980s was filled in the current cross section,
accounting for the large increase in average bed elevation.
Some trends can be seen in the average bed elevation changes. The average bed elevation for all
cross sections in MR-2, from PRM 184.9 to PRM 170.1,had a minimal change in elevation with
the exception of the cross section at PRM 179.5 which had an increase of 2.3 ft of aggradation
The magnitude of elevation changes tend to increase approaching the Three Rivers confluence in
MR-8, with all channels downstream of PRM 115.4 having a change of greater than one ft, with
the exception of cross section at PRM 104.1, which had a decrease of less than one foot. The
downstream most cross section, at PRM 98.4 in LR-1, had degradation in the channel of 3.6 feet.
However, due to the proximity to the Three Rivers Confluence, the Susitna River channels
changed substantially over the three decade period. When expanding the average bed elevation
beyond the channels to the entire braid plain at PRM 98.4, the cross section aggraded, with an
increase in elevation of 2.9 feet. Most changes throughout all cross section were minimal, with
the average change in bed elevation for all cross sections being an increase of 0.3 ft.
When looking at individual channels with cross sections containing multiple channels, the
maximum change in elevation was an apparent decrease of 7.8 ft in elevation (degradation)
within the main channel in the cross sections at PRM 130.9. The side channel at PRM 130.9 on
river right also had a significant decrease in elevation of 4.5 ft. The main channel in the cross
sections at PRM 104.1 changed the least, showing almost no change. However, the two side
channels at PRM 104.1 showed more change, with the left most channel degrading with a 1.9 ft
decrease in elevation and the middle channel aggrading with a 1.7 ft increase in elevation.
5.3.2.Change in Area below 2-Year Return Period WSE
The cross-sectional areas below the 2-year WSE were determined per the methods described in
Section 4.3 and the results are presented in Table 5.3-1. Changes in area of less than 450 sq. ft.
(average current width for all cross sections multiplied by 1 foot depth) are considered to be
minimal. In general, changes in cross-sectional area should mimic those in average bed
elevation. The exceptions occur where the elevation change was also accompanied by a change
in width. Overall area for cross sections with multiple channels was determined by summing the
area of each individual channel in the respective cross section.
Similar to the average bed elevation, a majority of the cross sections, 21 of 38, showed minimal
change in area. Of the remaining cross sections, five had an increase in area (aggradation) while
twelve had a decrease in area (degradation). Upstream of Devils Canyon, all cross sections in
MR-1 through MR-3 had minimal change in area, with two exceptions. The cross section at
PRM 185.2 showed a slight decrease in area of 490 sq. ft., a 13 percent reduction. At PRM
179.5 the cross sections showed an increase in area of 470 sq. ft., a 24 percent increase.
Twenty-four cross sections shared trends between change in average elevation and change in
area, where minimal changes in average bed elevation corresponded with minimal changes in
area for 16 cross sections, and greater than one-foot changes in average bed elevation
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page 7 September 2014
corresponded to greater than 450 sq. ft. change in area in eight cross sections. Of the remaining
cross sections, all showed a greater than ten percent change in channel width. For example, the
cross section at PRM 132.1 showed a minimal decrease in average bed elevation of 0.5 ft, had
the width not changed, the area would have increased. However, the channel also narrowed by
twenty two percent, which led to the area decreasing by 1,200 sq. ft., a reduction of 25 percent .
Overall, the average change in cross-sectional area for all channels was an increase in area of
only three percent.
When looking at individual channels in cross sections with multiple channels, t he channels that
showed the largest changes in area (greater than 200 percent) also had large decreases in average
bed elevation of greater than 2 ft. Similar to the overall cross sections, the area of each
individual channel followed the trend of changes in average bed elevation, except where the
width changed by greater than ten percent. Also, the channels in MR-8 showed typically a larger
magnitude of changes in area;however, they also showed large changes in width, with the
exception of the main channel at PRM 104.1.
Comparison of 1980s and Current Channel Thalweg Profiles5.4.
Thalweg profiles for the 1980s and current data above Devils Canyon (PRM 169.5 to 187.2) are
shown in Figure 5.4-1 and for below Devils Canyon (PRM 98.4 to PRM 153.7) in Figure 5.4-2.
In general, the trends mimic those of bed elevation and cross-sectional area comparisons. It
might be expected that the thalweg profile would be more dynamic than the channel change
identified by average bed elevations or the channel area, but this is generally not the case for the
data compared. Most profile changes are on the order of several feet or less with many portions
of the profile showing only minor changes of less than 1 foot.
Looking closer at the portion of the thalweg profile upstream of Devils Canyon, Figure 5.4 -1, no
large changes are apparent between the 1980s profile and the current profile .The current thalweg
is consistently at a lower elevation than the 1980s thalweg between PRM 170 and PRM 174 by
one to two feet. Other areas of difference are less consistent and switch between aggradation and
degradation on the order of one foot or less.
Similar to above Devils Canyon the thalweg profile below Devils Canyon (PRM 98.4 to PRM
153.7) provided in Figure 5.4-2, large changes are generally not observed. The majority of the
profile depicted shows changes of less than 2 feet and few areas of consistent aggradation or
degradation. Some of the more discernable and consistent differences are in the area of PRM
102 to PRM 122 where the trend is for no change or slight aggradati on for vast majority of this
area. The aggradation is only interrupted between PRM 112.5 to PRM 115 where up to four feet
of degradation is indicated. This area consists of geomorphic reaches MR-7 and MR-8.
The two largest exceptions to lack of large change below Devils Canyon in the thalweg profile
occur at the upstream and downstream extents of the figure. At PRM 153.7,at the very upstream
point of the profile,a pool that was on the order of 12 feet deep in the 1980s was filled in the
current cross section (Note: the cross sections are located 300 feet apart so some of the apparent
change may be attributable to the difference in locations). At the downstream end of the profile,
PRM 98.4, the opposite response is noted with about seven feet of degradation. This was the
only pair of cross sections compared in the Lower River. As discussed earlier, this is a location
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page 8 September 2014
where a much larger portion of the flow was confined to the main channel as secondary channels
were filled including portions of the braid plain becoming vegetated.
6.SUMMARY AND CONCLUSIONS
Cross sections were surveyed as part of the 1980s studies for the use in developing a one-
dimensional HEC-2 hydraulic model. Cross sections were also surveyed for the current Project
for use in developing a one-dimensional bed evolution model. The portion of the Susitna River
with the two sets of cross sections covered the Middle River segment and the very upper portion
of geomorphic reach LR-1 in the Lower River segment.A comparison of the two sets of cross
sections was performed and provides an additional resource in assessing geomorphic changes in
the Susitna River as well as for verification of the 1-D bed evolution model being developed in
the Fluvial Geomorphology Modeling Study (Study 6.6).
To develop the comparison, 1980s cross sections were digitized from survey notes and HEC-2
input files from documentation of the 1980s study efforts. The 1980s elevations were adjusted
from NGVD29 datum to the NAVD88 datum, to be consistent with the datum for current cross
sections. The 1980s and current cross sections were plotted together to provide a visual
comparison. Average bed elevations between the toe of banks were calculated for each channel
and time period to assess whether average bed elevation had increased or decreased. In order to
compare cross-sectional areas, a reference water surface elevation was assigned to each cross
section and the area below the reference elevation determined. The reference water surface was
used to calculate changes in cross-sectional area between the toe of banks and below the
simulated WSE. Lastly, channel profiles from the 1980s and current conditions were plotted
using thalweg elevations from each cross section.
Overall, the results show that the Middle Susitna River was generally stable in terms of bed
elevation change over the 3 decade period between the 1980s studies and the current effort.
Changes were typically on the order of several feet or less and there was not a consistent trend
toward aggradation (bed elevation increase)or degradation (bed elevation decrease) throughout
the Middle River segment. The area with the most consistent trend were geomorphic reaches
MR-7 and MR-8 in which the majority of the length was aggradational; however, there were still
cross sections within these two reaches that showed degradation over the period of comparison.
MR-2 showed the least change, with all cross sections having minimal change in bed elevation
and area with the exception of the cross section at PRM 179.5, which aggraded by 2.3 ft and
increased in area by 490 sq. ft. The remaining reaches had more of a mix between aggradation
and degradation.
Only one cross section was compared in the Lower River PRM 98.4 so data in the Lower River
were insufficient to make a generalization about trends the bed elevation or area changes.
Results of this work were used in the validation of the 1-D bed evolution model currently under
development as part of the Fluvial Geomorphology Modeling Study (Study 6.6) (Tetra Tech
2014). An additional 48 cross sections were surveyed in 2014 and of these, 3 are located
sufficiently close to 1980s cross sections to perform comparisons to assess channel change.
These cross sections will be used to update this technical memorandum in 2015. The results will
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page 9 September 2014
also be incorporated into the Comprehensive Geomorphology Technical Memorandum to be
developed in 2015 (Study 6.5 ISR Section 7.2.2.1.3).
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page 10 September 2014
7.REFERENCES
Acres.1983. Before the Federal Energy Regulatory Commission Application for License for
Major Project Susitna Hydroelectric Project. Volume 5A, Exhibit E, Chapters 1 & 2. Prepared
for Alaska Power Authority.
Harza-EBASCO Susitna Joint Venture. 1984. Susitna Hydroelectric Project, Water Surface
Profiles and Discharge Rating Curves for Middle and Lower Susitna River. Prepared for Alaska
Power Authority, Draft Report.
Matanuska-Susitna Borough. 2011. Matanuska Susitna Borough LiDAR/Imagery Project.
http://matsu.gina.alaska.edu.
National Geodetic Survey.2014. NGS Shapefiles RETRIEVAL PAGE.
http://www.ngs.noaa.gov/cgi-bin/datasheet.prl
R&M Consultants, Inc. 1981a. Alaska Power Authority Susitna Hydroelectric Project, Task 2
Surveys & Site Facilities Subtask 2.16 –Closeout Report Hydrographic Surveys.Prepared for
Acres American Incorporated.
R&M Consultants, Inc. 1981b. Alaska Power Authority Susitna Hydroelectric Project, Task 2
Horizontal and Vertical Control Surveys. Prepared for Acres American Incorporated.
R&M Consultants, Inc. 1982. Alaska Power Authority Susitna Hydroelectric Project, Task 3
Hydrology, Hydraulic and Ice Studies. Prepared for Acres American Incorporated.
Tetra Tech.2014. One-Dimensional Bed Evolution Model Development and Decision Point.
Susitna-Watana Hydroelectric Project. Technical Memorandum. Prepared for the Alaska Energy
Authority. Anchorage, Alaska.
U.S. Army Corps of Engineers (USACE). 1976. HEC-2 Water Surface Profiles User’s Manual,
CPD-2A.
U. S. Geological Survey (USGS).2012. Streamflow Record Extension for Selected Streams in
the Susitna River Basin, Alaska,Scientific Investigations Report 2012-5210.
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page 11 September 2014
8.TABLES
Table 4.3-1. Summary of compared cross sections and distances between cross sections.
1980s HEC-2
XS
Current
HEC-RAS
XS
Approximate
Distance (ft)
1980s HEC-2
XS
Current
HEC-RAS
XS
Approximate
Distance (ft)
URX-208 186.2 250 LRX-43 139 150
URX-109 185.5 75 LRX-41 138.1 350
URX-110 185.2 25 LRX-40 137.6 50
URX-111 184.9 275 LRX-35 134.1 25
URX-112 184.4 25 LRX-34 133.8 100
URX-113 183.3 200 LRX-31 132.1 350
URX-114 182.9 50 LRX-30 130.9 175
URX-115 181.6 25 LRX-29 129.7 50
URX-116 179.5 75 LRX-27 126.8 175
URX-120 170.1 100 LRX-26 126.1 25
LRX-68 153.7 350 LRX-21 122.6 250
LRX-59 148.3 200 LRX-20 120.7 175
LRX-56 145.5 225 LRX-19 119.9 150
LRX-54 144.3 75 LRX-18 116.6 300
LRX-51 142.2 350 LRX-15 115.4 225
LRX-50 141.9 100 LRX-9 107.1 25
LRX-49 141.7 350 LRX-7 105.3 175
LRX-45 140 75 LRX-5 104.1 300
LRX-44 139.8 325 0.4 98.4 125
Table 5.3-1. Summary of cross sections comparison results.
1980s
HEC-2 XS
Current
HEC-RAS
XS¹
1980s
Avg. Elev.
(ft)
Current
Avg. Elev.
(ft)
Diff.
(ft)
1980s
width
(ft)
Current
width (ft)
1980s Area
below 2-yr
WSE (ft²)
Current Area
below 2-yr
WSE (ft²)
%
Change
URX-208 186.2 1449.8 1449.1 -0.76 330 330 3320 3490 5%
URX-109 185.5 1443.7 1444.2 0.59 260 240 2580 2240 -13%
URX-110 185.2 L 1442.5 1441.2 -1.33 230 240 2080 2180 5%
URX-110 185.2 R 1442.8 1441.6 -1.21 140 70 1200 720 -40%
URX-110 185.2²1442.6 1441.3 -1.34 370 310 3280 2900 -12%
URX-111 184.9 1437.0 1437.7 0.70 350 320 3670 3180 -13%
URX-112 184.4 1428.1 1428.6 0.50 140 170 1720 1660 -3%
URX-113 183.3 1415.6 1416.0 0.39 400 400 3810 3660 -4%
URX-114 182.9 1411.3 1411.3 0.07 260 330 1910 2240 17%
URX-115 181.6 1393.6 1393.9 0.29 400 440 4010 3860 -4%
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page 12 September 2014
Table 5.3-1. Summary of cross sections comparison results (cont.).
1980s
HEC-2 XS
Current
HEC-RAS
XS¹
1980s
Avg.
Elev. (ft)
Current
Avg. Elev.
(ft)
Diff.
(ft)
1980s
width
(ft)
Current
width (ft)
1980s
Area
below 2-yr
WSE (ft²)
Current
Area below
2-yr WSE
(ft²)
%
Change
URX-116 179.5 1368.2 1370.5 2.31 150 230 1950 2420 24%
URX-120 170.1 1276.6 1276.8 0.14 390 440 3060 3150 3%
LRX-68 153.7 846.6 851.4 4.80 280 260 5010 3310 -34%
LRX-59 148.3 785.4 785.7 0.26 280 250 3500 2880 -18%
LRX-56 145.5 752.4 751.7 -0.70 370 400 3290 3430 4%
LRX-54 144.3 L 739.2 740.1 0.92 340 340 1780 1400 -21%
LRX-54 144.3 R 734.5 734.5 -0.02 390 320 4160 3230 -22%
LRX-54 144.3²736.7 737.4 0.70 730 660 5940 4630 -22%
LRX-51 142.2 710.4 709.5 -0.85 610 590 4720 4720 0%
LRX-50 141.9 702.4 703.1 0.76 360 360 4430 4270 -4%
LRX-49 141.7 700.2 703.6 3.41 160 190 2060 1720 -17%
LRX-45 140 683.0 682.9 -0.04 290 270 3840 3100 -19%
LRX-44 139.8 682.5 683.0 0.55 300 230 2970 1370 -54%
LRX-43 139 L 681.6 678.9 -2.73 180 130 60 180 200%
LRX-43 139 R 664.0 665.0 0.99 90 80 1710 1600 -6%
LRX-43 139²675.7 673.6 -2.15 270 220 1770 1780 1%
LRX-41 138.1 L 659.4 663.9 4.50 110 190 1340 460 -66%
LRX-41 138.1 R 662.0 663.5 1.47 360 440 3360 3050 -9%
LRX-41 138.1²661.4 663.6 2.21 460 630 4700 3510 -25%
LRX-40 137.6 656.8 657.8 0.97 500 600 4330 4450 3%
LRX-35 134.1 616.3 616.1 -0.15 300 330 3950 4420 12%
LRX-34 133.8 617.9 617.4 -0.47 530 530 3970 4380 10%
LRX-31 132.1 594.6 594.1 -0.52 400 310 4600 3470 -25%
LRX-30 130.9 L 588.5 584.0 -4.52 150 130 850 170 -80%
LRX-30 130.9 R 592.0 584.2 -7.77 280 150 270 1490 452%
LRX-30 130.9²590.8 584.1 -6.66 420 280 1120 1660 48%
LRX-29 129.7 571.7 573.1 1.38 490 540 5070 5140 1%
LRX-27 126.8 544.4 544.5 0.12 450 450 5140 5590 9%
LRX-26 126.1 L 543.8 542.4 -1.40 60 60 350 220 -37%
LRX-26 126.1 R 539.0 540.1 1.06 450 520 4690 4220 -10%
LRX-26 126.1²539.6 540.3 0.74 510 590 5040 4440 -12%
LRX-21 122.6 L 514.7 514.5 -0.14 110 110 640 490 -23%
LRX-21 122.6 R 510.6 509.0 -1.61 350 300 3660 2910 -20%
LRX-21 122.6²511.5 510.5 -1.09 470 410 4300 3400 -21%
LRX-20 120.7 L 491.1 495.2 4.10 140 200 2020 1260 -38%
LRX-20 120.7 R 496.6 497.7 1.04 160 320 1430 2380 66%
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page 13 September 2014
Table 5.3-1. Summary of cross sections comparison results (cont.).
1980s
HEC-2 XS
Current
HEC-RAS
XS¹
1980s
Avg.
Elev. (ft)
Current
Avg. Elev.
(ft)
Diff.
(ft)
1980s
width
(ft)
Current
width (ft)
1980s Area
below 2-yr
WSE (ft²)
Current Area
below 2-yr
WSE (ft²)
%
Change
LRX-20 120.7²494.0 496.7 2.68 300 520 3450 3640 6%
LRX-19 119.9 488.2 488.3 0.13 420 440 4800 4650 -3%
LRX-18 116.6 462.2 462.3 0.06 550 470 6150 4810 -22%
LRX-15 115.4 L 456.2 457.5 1.28 170 230 850 1010 19%
LRX-15 115.4 R 455.1 453.5 -1.58 680 610 4690 4730 1%
LRX-15 115.4²455.3 454.6 -0.72 850 850 5540 5740 4%
LRX-9 107.1 381.7 385.5 3.82 640 650 6730 5840 -13%
LRX-7 105.3 L 367.2 369.7 2.44 270 620 2790 4840 73%
LRX-7 105.3 R 369.0 370.4 1.43 160 320 1310 2200 68%
LRX-7 105.3²367.9 369.9 2.04 430 940 4100 7040 72%
LRX-5 104.1 L 362.4 360.5 -1.86 500 310 2800 1990 -29%
LRX-5 104.1 R 361.3 363.1 1.77 190 300 1290 700 -46%
LRX-5 104.1 M 360.6 360.6 -0.01 480 460 3490 3720 7%
LRX-5 104.1²361.5 361.3 -0.21 1170 1070 4780 4420 -8%
0.4 98.4 319.3 315.7 -3.61 130 320 800 2710 239%
Notes:
1 Cross sections with multiple channels are desginated as "L" for Left, "R" for right and "M" for main channel.
2 Overall changes in average elevation (weighted by width) and total cross sectional area are calculated for cross
sections with multiple channels.
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page 14 September 2014
9.FIGURES
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page 15 September 2014
Figure 3-1 Extents of 1980s HEC-2 model and current HEC-RAS model.
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page 16 September 2014
Figure 4.3-1 Cross section locations for 1980s HEC-2 cross sections and current HEC-RAS cross sections from PRM 187 to PRM 183.
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page 17 September 2014
Figure 4.3-2 Cross section locations for 1980s HEC-2 cross sections and current HEC-RAS cross sections from PRM 183 to PRM 176.
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page 18 September 2014
Figure 4.3-3 Cross section locations for 1980s HEC-2 cross sections and current HEC-RAS cross sections from PRM 174 to PRM 164.
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page 19 September 2014
Figure 4.3-4 Cross section locations for 1980s HEC-2 cross sections and current HEC-RAS cross sections from PRM 156 to PRM 149.
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page 20 September 2014
Figure 4.3-5 Cross section locations for 1980s HEC-2 cross sections and current HEC-RAS cross sections from PRM 150 to PRM 138.
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page 21 September 2014
Figure 4.3-6 Cross section locations for 1980s HEC-2 cross sections and current HEC-RAS cross sections from PRM 139 to PRM 128.
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page 22 September 2014
Figure 4.3-7 Cross section locations for 1980s HEC-2 cross sections and current HEC-RAS cross sections from PRM 129 to PRM 118.
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page 23 September 2014
Figure 4.3-8 Cross section locations for 1980s HEC-2 cross sections and current HEC-RAS cross sections from PRM 118 to PRM 108.
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page 24 September 2014
Figure 4.3-9 Cross section locations for 1980s HEC-2 cross sections and current HEC-RAS cross sections from PRM 108 to PRM 97.
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page 25 September 2014
Figure 5.2-1 Example cross-sectional geometry from the 1980s HEC-2 model and the current HEC-RAS model.
370
375
380
385
390
395
400
405
410
415
420
0 1000 2000 3000 4000 5000 6000Elevation (ft., NAVD88)Station (ft.)
LRX-09 (1980s)XS 107.1 (current)
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page 26 September 2014
Figure 5.4-1 Comparison of thalweg profiles for the 1980s HEC-2 URX model and the current HEC-RAS model, PRM 187.2 to PRM 169.5.
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2012 Thalweg
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1982 Thalweg
Profile
TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No.14241 Page 27 September 2014
Figure 5.4-2. Comparison of thalweg profiles for the 1980s HEC-2 LRX model and the current HEC-RAS model, PRM 153.7 to PRM 98.4.
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2012 Thalweg
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1982 Thalweg
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APPENDIX A HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 September 2014
APPENDIX A. HISTORICAL CROSS SECTION COMPARISON PLOTS
Susitna-Watana Hydroelectric Project
(FERC No. 14241)
Geomorphology Study (Study 6.5)
Susitna River Historical Cross Section Comparison
(1980s to Current)
Technical Memorandum
Prepared for
Alaska Energy Authority
Prepared by
Tetra Tech, Inc.
September 2014
APPENDIX A HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page A.1 September 2014
Figure A.1. Cross section comparison of 1980s URX-208 and current XS 186.2
Figure A.2. Cross section comparison of 1980s URX-109 and current XS 185.5
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URX-208 (1980s)XS 186.2 (current)
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URX-109 (1980s)XS 185.5 (current)
APPENDIX A HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page A.2 September 2014
Figure A.3. Cross section comparison of 1980s URX-110 and current XS 185.2
Figure A.4. Cross section comparison of 1980s URX-111 and current XS 184.9
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URX-110 (1980s)XS 185.2 (current)
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URX-111 (1980s)XS 184.9 (current)
APPENDIX A HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page A.3 September 2014
Figure A.5. Cross section comparison of 1980s URX-112 and current XS 184.4
Figure A.6. Cross section comparison of 1980s URX-113 and current XS 183.3
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URX-112 (1980s)XS 184.4 (current)
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URX-113 (1980s)XS 183.3 (current)
APPENDIX A HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page A.4 September 2014
Figure A.7. Cross section comparison of 1980s URX-114 and current XS 182.9
Figure A.8. Cross section comparison of 1980s URX-115 and current XS 181.6
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URX-114 (1980s)XS 182.9 (current)
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URX-115 (1980s)XS 181.6 (current)
APPENDIX A HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page A.5 September 2014
Figure A.9. Cross section comparison of 1980s URX-116 and current XS 179.5
Figure A.10. Cross section comparison of 1980s URX-120 and current XS 170.1
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URX-116 (1980s)XS 179.5 (current)
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APPENDIX A HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page A.6 September 2014
Figure A.11. Cross section comparison of 1980s LRX-68 and current XS 153.7
Figure A.12. Cross section comparison of 1980s LRX-59 and current XS 148.3
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LRX-68 (1980s)XS 153.7 (current)
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LRX-59 (1980s)XS 148.3 (current)
APPENDIX A HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page A.7 September 2014
Figure A.13. Cross section comparison of 1980s LRX-56 and current XS 145.5
Figure A.14. Cross section comparison of 1980s LRX-54 and current XS 144.3
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LRX-56 (1980s)XS 145.5 (current)
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LRX-54 (1980s)XS 144.3 (current)
APPENDIX A HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page A.8 September 2014
Figure A.15. Cross section comparison of 1980s LRX-51 and current XS 142.2
Figure A.16. Cross section comparison of 1980s LRX-50 and current XS 141.9
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LRX-51 (1980s)XS 142.2 (current)
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LRX-50 (1980s)XS 141.9 (current)
APPENDIX A HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page A.9 September 2014
Figure A.17. Cross section comparison of 1980s LRX-49 and current XS 141.7
Figure A.18. Cross section comparison of 1980s LRX-45 and current XS 140.0
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LRX-49 (1980s)XS 141.7 (current)
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APPENDIX A HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page A.10 September 2014
Figure A.19. Cross section comparison of 1980s LRX-44 and current XS 139.8
Figure A.20. Cross section comparison of 1980s URX-43 and current XS 139.0
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LRX-44 (1980s)XS 139.8 (current)
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LRX-43 (1980s)XS 139.0 (current)
APPENDIX A HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page A.11 September 2014
Figure A.21. Cross section comparison of 1980s LRX-41 and current XS 138.1
Figure A.22. Cross section comparison of 1980s LRX-40 and current XS 137.6
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APPENDIX A HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page A.12 September 2014
Figure A.23. Cross section comparison of 1980s LRX-35 and current XS 134.1
Figure A.24. Cross section comparison of 1980s LRX-34 and current XS 133.8
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APPENDIX A HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page A.13 September 2014
Figure A.25. Cross section comparison of 1980s LRX-31 and current XS 132.1
Figure A.26. Cross section comparison of 1980s LRX-30 and current XS 130.9
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LRX-31 (1980s)XS 132.1 (current)
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LRX-30 (1980s)XS 130.9 (current)
APPENDIX A HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page A.14 September 2014
Figure A.27. Cross section comparison of 1980s LRX-29 and current XS 129.7
Figure A.28. Cross section comparison of 1980s LRX-27 and current XS 126.8
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APPENDIX A HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page A.15 September 2014
Figure A.29. Cross section comparison of 1980s LRX-26 and current XS 126.1
Figure A.30. Cross section comparison of 1980s LRX-21 and current XS 122.6
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APPENDIX A HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page A.16 September 2014
Figure A.31. Cross section comparison of 1980s LRX-20 and current XS 120.7
Figure A.32. Cross section comparison of 1980s LRX-19 and current XS 119.9
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LRX-20 (1980s)XS 120.7 (current)
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APPENDIX A HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page A.17 September 2014
Figure A.33. Cross section comparison of 1980s LRX-18 and current XS 116.6
Figure A.34. Cross section comparison of 1980s LRX-15 and current XS 115.4
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APPENDIX A HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page A.18 September 2014
Figure A.35. Cross section comparison of 1980s LRX-09 and current XS 107.1
Figure A.36. Cross section comparison of 1980s LRX-07 and current XS 105.3
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APPENDIX A HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page A.19 September 2014
Figure A.37. Cross section comparison of 1980s LRX-05 and current XS 104.1
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TECHNICAL MEMORANDUM HISTORICAL CROSS SECTION COMPARISON
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page A.20 September 2014
Figure A.38. Cross section comparison of 1980s 0.4 and current XS 98.4
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