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Susitna-Watana Hydroelectric Project Document
ARLIS Uniform Cover Page
Title:
Probable maximum flood (PMF) study, Study plan Section 16.5 : Initial
study report
SuWa 207
Author(s) – Personal:
Author(s) – Corporate:
Prepared by MWH
AEA-identified category, if specified:
Draft initial study report
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Series (ARLIS-assigned report number):
Susitna-Watana Hydroelectric Project document number 207
Existing numbers on document:
Published by:
[Anchorage : Alaska Energy Authority, 2014]
Date published:
February 2014
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Alaska Energy Authority
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Study plan Section 16.5
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Draft
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Pagination:
v, 41 p.
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Notes:
All reports in the Susitna-Watana Hydroelectric Project Document series include an ARLIS-
produced cover page and an ARLIS-assigned number for uniformity and citability. All reports
are posted online at http://www.arlis.org/resources/susitna-watana/
Susitna-Watana Hydroelectric Project
(FERC No. 14241)
Probable Maximum Flood (PMF) Study
Study Plan Section 16.5
Initial Study Report
Prepared for
Alaska Energy Authority
Prepared by
MWH
February 2014 Draft
INITIAL STUDY REPORT PROBABLE MAXIMUM FLOOD (PMF) STUDY (16.5)
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page i February 2014 Draft
TABLE OF CONTENTS
Executive Summary ...................................................................................................................... v
1. Introduction ....................................................................................................................... 1
2. Study Objectives................................................................................................................ 1
3. Study Area ......................................................................................................................... 1
4. Methods and Variances .................................................................................................... 2
4.1. Board of Consultants Review ........................................................................... 2
4.1.1. Variances......................................................................................... 2
4.2. Data Acquisition ............................................................................................... 2
4.2.1. Variances......................................................................................... 3
4.3. Historical Data Analysis ................................................................................... 3
4.3.1. Variances......................................................................................... 4
4.4. Review of Previous PMF Study Report ............................................................ 4
4.4.1. Variances......................................................................................... 4
4.5. Field Visit.......................................................................................................... 4
4.5.1. Variances......................................................................................... 5
4.6. Flood Hydrology Model Selection.................................................................... 5
4.6.1. Variances......................................................................................... 6
4.7. Flood Hydrology Model Initial Setup ............................................................... 6
4.7.1. Variances......................................................................................... 6
4.8. Flood Hydrology Model Calibration and Verification ..................................... 6
4.8.1. Variances......................................................................................... 6
4.9. Development of the Site-Specific PMP ............................................................ 7
4.9.1. Variances......................................................................................... 8
4.10. Coincident Conditions for the PMF .................................................................. 8
4.10.1. Variances......................................................................................... 8
4.11. Development of the PMF Inflow Hydrograph .................................................. 8
4.11.1. Variances......................................................................................... 8
4.12. Reservoir Routing of the PMF .......................................................................... 9
4.12.1. Variances......................................................................................... 9
4.13. Freeboard Analysis ........................................................................................... 9
4.13.1. Variances....................................................................................... 10
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4.14. Reporting......................................................................................................... 10
5. Results .............................................................................................................................. 10
5.1. Board of Consultants....................................................................................... 10
5.2. Field Visit........................................................................................................ 11
5.3. Basin Hydrologic Data .................................................................................... 11
5.4. Sub-Basin Definition ...................................................................................... 12
5.5. Historic Flood Records ................................................................................... 12
5.6. Seasonal Flood Distribution ............................................................................ 12
5.7. Snowpack Determination ................................................................................ 13
5.7.1. Snowpack Distribution.................................................................. 13
5.7.2. 100-Year Snowpack ...................................................................... 14
5.7.3. Probable Maximum Snowpack ..................................................... 14
5.8. Runoff Model Calibration Floods ................................................................... 15
5.9. Probable Maximum Precipitation ................................................................... 16
5.10. Review of Previous PMF Studies ................................................................... 18
6. Discussion......................................................................................................................... 18
7. Plans for 2014 .................................................................................................................. 19
8. Literature Cited .............................................................................................................. 19
9. Tables ............................................................................................................................... 20
10. Figures .............................................................................................................................. 30
LIST OF TABLES
Table 5.3- 1. USGS Streamflow Gages in the Susitna Watershed ............................................... 20
Table 5.3- 2. Snow Course and SNOTEL Stations In or Near the Susitna Watershed ................ 21
Table 5.4- 1. Area in Elevation Bands to Watana Dam ................................................................ 22
Table 5.4- 2. Susitna Watershed Land Cover ............................................................................... 23
Table 5.5- 1. Recorded Peak Flows – Susitna River at Gold Creek – 59 Years of Record .......... 23
Table 5.5- 2. Recorded Peak Flows – Susitna River at Cantwell – 18 Years of Record .............. 24
Table 5.5- 3. Recorded Peak Flows – Susitna River near Denali – 28 Years of Record .............. 24
Table 5.5- 4. Recorded Peak Flows – Maclaren River near Paxson – 28 Years of Record .......... 24
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FERC Project No. 14241 Page iii February 2014 Draft
Table 5.6- 1. Maximum Daily Flows for Each Month for the USGS Gage at Gold Creek .......... 25
Table 5.6- 2. Monthly Distribution of Annual Peak Flows .......................................................... 25
Table 5.7.1- 1. Monthly Precipitation by Month and Sub-Basin .................................................. 26
Table 5.7.2- 1. 100-Year Snowpack at Snow Course Stations ..................................................... 26
Table 5.7.2- 2. 100-Year All-Season Snowpack SWE ................................................................. 27
Table 5.7.3- 1. Probable Maximum Snowpack SWE ................................................................... 28
Table 5.9- 1. Short List of Storms................................................................................................. 29
Table 5.10- 1. Harza-Ebasco PMF Snowpack Estimate ............................................................... 29
LIST OF FIGURES
Figure 5.2-1. Susitna River near Deadman Creek on May 29, 2013 ........................................... 30
Figure 5.2-2. Susitna River near the Denali Highway Crossing on May 29, 2013 ...................... 30
Figure 5.3- 1. Susitna Watershed Boundary and USGS Gage Locations ..................................... 31
Figure 5.3- 2. Location of Snow Course and SNOTEL Stations .................................................. 32
Figure 5.4- 1. Susitna Watershed Sub-Basins ............................................................................... 33
Figure 5.4- 2. Susitna Watershed Elevation Bands ....................................................................... 34
Figure 5.4- 3. Susitna Watershed Land Cover .............................................................................. 35
Figure 5.6- 1. Historic Flow Frequency at the USGS Gold Creek Gage ...................................... 36
Figure 5.6- 2. Watana Reservoir Simulated Elevations ................................................................ 36
Figure 5.7.1- 1. Average October through April Precipitation ..................................................... 37
Figure 5.8- 1. Recorded Flows at USGS Streamflow Gaging stations for June 1971 .................. 38
Figure 5.9- 1. SPAS Storm Analysis Results for the August 1967 Storm .................................... 39
Figure 5.9- 2. Rainfall Center Locations for the Short List Storms .............................................. 40
Figure 5.9- 3. Example of a Trajectory Model Analysis .............................................................. 41
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LIST OF ACRONYMS, ABBREVIATIONS, AND DEFINITIONS
Abbreviation Definition
AEA Alaska Energy Authority
BOC Board of Consultants
DAD Depth-Area-Duration
FERC Federal Energy Regulatory Commission
GLOF glacial lake outburst floods
HMR hydrometeorological reports
ILP Integrated Licensing Process
ISR Initial Study Report
PMF Probable Maximum Flood
PMP Probable Maximum Precipitation
Project Susitna-Watana Hydroelectric Project
RSP Revised Study Plan
SPAS Storm Precipitation Analysis System
SSARR Streamflow Synthesis and Reservoir Routing
SST Sea Surface Temperature
SPD study plan determination
WMO World Meteorological Organization
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FERC Project No. 14241 Page v February 2014 Draft
EXECUTIVE SUMMARY
Probable Maximum Flood (PMF) Study 16.5
Purpose The purpose of the study is to develop the inflow design flood (the PMF) for
Watana Dam. The PMF inflow hydrograph will be routed through the
reservoir and be used to size the spillway and determine the crest level of the
dam to ensure flood safety of the dam. The PMF results from the Probable
Maximum Precipitation (PMP), which is also developed as a part of this
study, and other coincident conditions including snowmelt.
Status Many aspects of the PMF study were completed during 2013 and the
remaining tasks are in-progress. Key tasks to be completed during the second
study season include unit hydrograph verification, seasonal PMP values as
developed from the all-season PMP, and flood routing of the PMF
hydrographs.
Study
Components
The major study components include the following: (1) data acquisition of
meteorological, hydrological, and snow data, (2) historical data analysis, (3)
review of previous Susitna PMF studies, (4) a field visit, (5) study reviews by
a Board of Consultants, (6) runoff model selection, (7) runoff model
calibration and verification, (8) development of a site specific PMP, (9)
development of snowpack and other conditions coincident with seasonal PMP
storms, (10) reservoir routing of the PMF, (11) spillway sizing, (12) freeboard
analysis, and (13) reporting.
2013 Variances There were no variances from the RSP that would limit the accuracy,
effectiveness or utility of the PMP and PMF results. The most significant
variance from the RSP was to increase the number of calibration and
verification floods from the standard three to six floods. As the PMF study
progressed, it became clear that floods resulting from two different dominant
sources (rainfall and snowmelt) must be considered. Choosing three floods of
each type doubled the need for historic meteorological data development and
flood calibration and verification, but ensured the accuracy of the ultimate
controlling PMF hydrograph.
Steps to
Complete the
Study
As explained in the cover letter to this draft ISR, AEA’s plan for completing
this study will be included in the final ISR filed with FERC on June 3, 2014.
Highlighted
Results and
Achievements
Extensive meteorological data acquisition and analysis has resulted in the
development of nine candidate storms from which the all-season PMP and
seasonal PMP values can be determined. Data acquisition and analysis has
been completed for development of the 100-year seasonal snowpack values
and probable maximum snowpack. A detailed flood runoff model was
developed with calibration and verification nearing completion.
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1. INTRODUCTION
On December 14, 2012, Alaska Energy Authority (AEA) filed with the Federal Energy
Regulatory Commission (FERC or Commission) its Revised Study Plan (RSP), which included
58 individual study plans (AEA 2012). Section 16.5 of the RSP described the Probable
Maximum Flood (PMF) Study. This study focuses on developing a site-specific Probable
Maximum Precipitation (PMP) and modeling the PMF. RSP 16.5 provided goals, objectives,
and proposed methods for data collection regarding PMF.
On February 1, 2013, FERC staff issued its study plan determination (February 1 SPD) for 44 of
the 58 studies, approving 31 studies as filed and 13 with modifications. RSP Section 16.5 was
one of the 31 studies approved with no modifications.
Following the first study season, FERC’s regulations for the Integrated Licensing Process (ILP)
require AEA to “prepare and file with the Commission an initial study report describing its
overall progress in implementing the study plan and schedule and the data collected, including an
explanation of any variance from the study plan and schedule.” (18 CFR 5.15(c)(1)) This Initial
Study Report (ISR) on the Probable Maximum Flood (PMF) Study has been prepared in
accordance with FERC’s ILP regulations and details AEA’s status in implementing the study, as
set forth in the FERC-approved RSP (referred to herein as the “Study Plan”).
2. STUDY OBJECTIVES
The general goals and objectives of the PMF study are as follows:
• Develop a site-specific PMP to be used for the derivation of the PMF including both a
temporal and spatial distribution of rainfall;
• Model the runoff through the project drainage basin to produce the PMF inflow,
including snowmelt considerations for the Project reservoir;
• Route the PMF inflow through the Project to obtain the PMF outflow and maximum
flood elevation at the dam;
• Determine the required outlet capacity to safely route the PMF through the reservoir;
• Determine the freeboard allowance; and
• Use the Board of Consultants (BOC) for technical review during development and
performance of the site-specific studies.
3. STUDY AREA
As established by RSP Section 16.5.3, the study area is the entire watershed tributary to the
Watana Dam site, plus the additional drainage area between Watana Dam and the USGS gaging
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station at Gold Creek. The watershed drainage area is 5,180 square miles at the Watana Dam
site and 6,160 square miles at the Gold Creek USGS gage.
4. METHODS AND VARIANCES
The following sections describe the study methods and major tasks necessary to develop the
PMP and PMF for Watana Dam, including variances from the original study plan.
4.1. Board of Consultants Review
During the 2013 study season, AEA implemented the methods for the Board of Consultants
(BOC) as set forth in RSP Section 16.5.4.1, with no variances.
A BOC has been established for technical review of many aspects of the dam design. The BOC
review of the studies described herein has been primarily focused on the development of the site-
specific PMP but has also included other aspects of the PMF study. The BOC has met and will
continue to meet and review design progress at appropriate intervals and, when appropriate, can
co-opt specialists for particular topic review. The PMP and PMF study methods and tasks
described herein have been the subject of review by the BOC.
4.1.1. Variances
There are no variances to the BOC section of this study.
4.2. Data Acquisition
During the 2013 study season, AEA implemented the methods for data acquisition as set forth in
RSP Section 16.5.4.2, with the exception of variances explained below in Section 4.2.1.
A variety of historical recorded meteorological and hydrologic data are necessary to develop the
PMP and PMF. Data acquisition began at the earliest possible time as it was anticipated that
some data (e.g., streamflow data on a time increment less than daily) could take months to
retrieve.
Previous PMP and storm analysis work in the region were reviewed to identify storm events,
available rainfall data and techniques applicable the basin. PMP-type storm events which have
occurred in a region that were considered transpositionable to the basin were identified in the
storm search. A comprehensive list of significant storm events and the characteristics of the
PMP type storm(s) relevant to the basin was constructed. Storms identified included storms used
in previous PMP and hydrologic studies in the region. Nine storms have been identified and are
the short list storms. The resulting list of storms was used to derive the PMP values for the basin
for the all season PMP and the seasonally adjusted PMP.
Data was acquired to develop meteorological time series for use in rain on snow PMF modeling.
Information from six floods and the associated meteorological data was used in the runoff model
calibration efforts. Daily and hourly time series were developed for meteorological parameters
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(i.e. temperature, dew point, wind) required for snow melt modeling using data from surrounding
weather stations (e.g. NWS COOP, RAWS, SNOTEL, and various other networks).
Relevant watershed data was collected including the streamflow data, drainage area of sub-
basins, the area within elevation bands for snowpack and snowmelt estimation, channel slopes,
vegetation cover, lake area, and soil types.
4.2.1. Variances
Archived USGS hourly streamflow records for selected floods were requested but not received.
USGS daily streamflow records are available at all gages in the watershed for the period of
record, and 15-minute flow data is available for the September 2012 flood. Due to the large area
of the watershed and the significant snowmelt component of most floods, flood hydrographs
occur over periods of 10 to 20 days, or even longer. Instantaneous annual maximum peak flows
are also available. This amount of streamflow data is considered adequate to fully meet the
hydrograph calibration and verification objectives of the PMF study.
4.3. Historical Data Analysis
During the 2013 study season, AEA implemented the methods for the analysis of historical data
as set forth in RSP Section 16.5.4.3, with no variances.
Historical data analysis forms the basis of the PMP and PMF analysis and consisted of the
following.
Previous PMP and storm analysis work were reviewed. Significant rainfall storm events that
were previously identified were noted along with supporting data and techniques used in
PMP determination. Additionally, procedures used in other site-specific studies were
identified that could be used for the Susitna-Watana Dam site-specific PMP study.
A search to identify the most significant rainfall storm events in and surrounding the basin
was completed.
Storm rainfall analyses using the Storm Precipitation Analysis System (SPAS) were
completed for the nine identified extreme rainfall storm events.
A maximum Sea Surface Temperature (SST) climatology was developed for storm
atmospheric moisture source regions over the Gulf of Alaska and northern Pacific Ocean.
This climatology is the mean SST plus 2-sigma (2 standard deviations) and provides
maximum SSTs for the storm maximization and transpositioning procedures.
Historic peak flows were summarized for selection of major flood events for model
calibration and verification.
Flood frequency analyses were performed for up to at least the 100-year flood from historical
peak flow data.
Antecedent watershed conditions prior to the PMP were developed.
The 100-year snowpack and snow water equivalent was determined for various elevation
bands.
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The 100-year and probable maximum snowpack was developed based on October through
April average precipitation.
4.3.1. Variances
There are no variances to the historical data analysis section of this study.
4.4. Review of Previous PMF Study Report
During the 2013 study season, AEA implemented the methods for review of the previous PMF
study report as set forth in RSP Section 16.5.4.4, with the exception of variances explained
below in Section 4.4.1.
In support of the previous design and licensing effort for the APA Susitna Hydroelectric Project,
two PMF studies were performed (Acres 1982, and Harza-Ebasco 1984). These PMF studies
included developing a site-specific PMP and used generally accepted methods at the time. It is
notable that although many new data have become available in the 30-year interim since the
previous PMF study, all of the five largest floods of record at the Gold Creek USGS gaging
station were available for calibration and verification studies in 1982 (subject to change by the
June 2013 flood). Although few calculations and model input data, and no output are available,
the two PMF studies do contain useful information regarding final results and conclusions of the
analysis, including numerous tables and figures. The two PMF study reports were thoroughly
reviewed to gain applicable insights to be used in the current PMF study.
4.4.1. Variances
Subsequent to the Commission’s February 1 SPD, the 1984 Susitna PMF study became
available. It was included in the review of previous PMF studies in the same manner as the 1982
Susitna PMF study so that a comprehensive background of PMF studies for the Susitna could be
used to inform and verify the current study approach As a historical note, two of the largest
seven floods of record at the USGS streamflow gaging station at Gold Creek have occurred
(September 2012 and June 2013) since filing of the PMF study plan, although peak flow rates are
preliminary and subject to change by the USGS. The June 2013 flood is not used in this study
because it occurred after selection and meteorological data analysis began of the storms
associated with the floods used for runoff model calibration and verification.
4.5. Field Visit
During the 2013 study season, AEA implemented the methods for the field season as set forth in
RSP Section 16.5.4.5, with no variances.
In conformance with FERC’s recommendations for PMF studies (FERC 2001), AEA conducted
two field visits during the 2012 and 2013 study seasons. These occurred on September 27, 2012
and May 29, 2013. The site flyover on May 29, 2013 with the BOC included representatives
from the study team. The site flyover of September 27, 2012 was specifically for the study team
participants. The site visits were undertaken to observe significant topographical variations
within and adjacent to the basin. Observations made during the field visits were oriented toward
the following aspects:
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Manning’s “n” and general hydrologic and hydraulic characteristics of river channels;
Special features within the drainage basin such as marshes, lakes, and closed basins that may
delay or reduce runoff;
Constrictions such as bridge abutments that may influence flood routing characteristics,
although none were observed;
Areal extent of snow cover;
Large natural constrictions that could act as hydraulic control structures, but none were
observed; and
Areas that could result in locally different infiltration rates, including rock exposures, dense
forest, or high altitude meadows.
4.5.1. Variances
There are no variances to the field visit section of this study.
4.6. Flood Hydrology Model Selection
During the 2013 study season, AEA implemented the methods for flood hydrology model
selection as set forth in RSP Section 16.5.4.6, with no variances.
At least three flood hydrology models are available, and a key task was to select which to use to
develop the PMF. These models include:
Streamflow Synthesis and Reservoir Routing (SSARR). This model was developed by the
U.S. Army Corps of Engineers (USACE), North Pacific Division. The SSARR model was
used for the 1982 Susitna PMF study. In addition to its use by the USACE, the SSARR
model was used occasionally by consultants for flood simulation on major watersheds,
particularly in the Pacific Northwest. The SSARR model is no longer in general use. The
latest version of SSARR was modified in 1991 to run on IBM-compatible personal
computers. The USACE has noted that there will be no further program updates or
modifications to the SSARR files by the USACE, and no user support is available.
Flood Hydrograph Package (HEC-1). This model was developed by the Hydrologic
Engineering Center (HEC) of the USACE and was (possibly still is) the most widely used
model in PMF studies. HEC-1 is one of the two rainfall-runoff models recommended for
PMF studies (FERC 2001). Compared to other models, HEC-1 has the advantage of
including the recommended energy budget snowmelt method as well as fully documented
equations for calculating snowmelt in the model.
Hydrologic Modeling System (HEC-HMS). This model was also developed by the HEC and
is the Windows-based successor to HEC-1. HEC-HMS contains many of the same methods
as HEC-1 and is the other model recommended for PMF studies (FERC 2001). Snowmelt in
the HEC-HMS model is based on a method that uses temperature data only.
Flood hydrology model selection was reviewed with the BOC during the initial BOC meeting on
November 2, 2012. With BOC input from that review, AEA has selected the HEC-1 Flood
Hydrograph Package as the rainfall-runoff model for developing the PMF.
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4.6.1. Variances
There are no variances to the flood hydrology model selection section of this study.
4.7. Flood Hydrology Model Initial Setup
During the 2013 study season, AEA implemented the methods for the initial setup of the flood
hydrology model as set forth in RSP Section 16.5.4.7, with no variances.
The flood hydrology computer model initial setup includes sub-basin delineation, areas in
elevation bands for use in snowmelt calculations, lake areas, areas in various soil groups,
coincident base flow, and initial estimates of infiltration rates. Sub-basin delineation was aligned
with USGS stream-gaging station locations whenever possible to facilitate model calibration and
verification. River channel geometry was checked for areas that may warrant special
consideration for storage-outflow routing. Topographic mapping was developed using ArcGIS
software.
4.7.1. Variances
There are no variances to the flood hydrology model initial setup section of this study.
4.8. Flood Hydrology Model Calibration and Verification
During the 2013 study season, AEA implemented the methods for flood hydrology model
calibration and verification as set forth in RSP Section 16.5.4.8, with the exception of variances
explained below in Section 4.8.1.
This task includes calibration and verification of the sub-basin unit hydrographs to the extent that
available recorded streamflow and meteorological data allow. Calibration provides the important
adjustments to hydrograph parameters that are initially estimated from standard equations or
based on experience in similar watersheds. Two of the largest floods on record were planned to
be selected for calibration, with a third large historical flood used for verification. However, as
work proceeded in reviewing floods, three floods were chosen each for rainfall and snowmelt
flood events because it was not clear which flood type would be the dominant and critical
condition for the PMF. More storms would also be available if further calibration/validation is
required. The calibration points at the outlets of the sub-basins coincide with USGS stream-
gaging stations to the extent possible. The selection of storm periods to use in model calibration
and verification included the availability of data at multiple stream-gaging stations. Activities
under this task would also include estimating ungaged local runoff as necessary, base flow
separation, and a final estimate of infiltration loss rates.
4.8.1. Variances
The calibration and verification of hydrograph parameters normally involves the collection of
meteorological and flow data for three flood periods. Analysis of flood records as the PMF
study proceeded revealed that there have been two fundamentally different and seasonally
separated flood generating scenarios, one resulting primarily from rainfall, the other primarily
from snowmelt. It could not be determined in advance which of these flood generating scenarios
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would ultimately be the critical condition for the PMF. Therefore, three floods of each type
(total of six) were selected for calibration and verification. This resulted in an initially
unanticipated increase in data acquisition and calibration effort, although it was considered to be
necessary for a reliable determination of the PMF.
4.9. Development of the Site-Specific PMP
During the 2013 study season, AEA implemented the methods for flood hydrology model
calibration and verification as set forth in RSP Section 16.5.4.8, with the exception of variances
explained below in Section 4.9.1.
The applicable available National Weather Service (formerly the U.S. Weather Bureau) PMP
guidance document is Probable Maximum Precipitation and Rainfall-Frequency Data for
Alaska, Technical Paper No. 47 (Miller 1963). Technical Paper No. 47 is applicable to areas up
to 400 square miles and durations up to 24 hours. Because the drainage area at the Watana Dam
site is 5,180 square miles and current standards call for the PMP to have a duration of at least
72 hours, development of a site-specific PMP is necessary. The existing PMP studies are being
used to make comparisons to the 1982 Susitna site-specific PMP and the Technical Paper No. 47
PMP at the highest-intensity central 400-square-mile area and 24-hour duration of the new site-
specific PMP. Development of the site-specific PMP for the watershed tributary to the proposed
Watana Dam site required a substantially greater effort than is necessary for most other dams in
the USA because of new storm analyses, sparse data availability and cool season considerations.
The site-specific PMP study follows many of the methods (e.g., a storm-based approach) used to
develop the current National Weather Service PMP hydrometeorological reports (HMR). The
basic techniques for storm maximization and transposition are well-established. An additional
30 years of data and more advanced models and recent adjustments to methods are now available
for development of site-specific PMP (e.g. radar aided storm analyses, quantification of
orographic affects). Results include both a temporal and spatial distribution of the PMP for
durations appropriate to most accurately model the PMF. No predetermined maximum storm
sequence length is set so that the critical PMP sequence could be 96 hours or more. Long
duration, high volume events are among the candidate PMF cases evaluated to determine if they
constitute the critical storm event for the determination of the PMF maximum reservoir
elevation. In addition, guidance for alternative centerings of the PMP design storm are
determined based on the patterns of the actual storm events used to derive the PMP values.
NEXRAD data are used when available (generally after 1995) in all storm analyses.
A consultant with extensive experience in developing site-specific PMP was retained to perform
this task. The initial storm search included all twelve months of the year, so the months that are
potentially PMP drivers will naturally result from this process. Based on an analysis of historic
flow frequency, peak annual flood data, and anticipated seasonal reservoir levels, the PMP
development is expected to be focused on the months of May through October. The site-specific
PMP task also includes development of the 100-year precipitation temporal and spatial
distribution during a season coincident with the probable maximum snowpack.
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4.9.1. Variances
While the development of the site-specific PMP is well underway, it was not completed in study
year 2013. At the time the RSP was prepared, it was contemplated the development of the PMP
could be fully accomplished in the first study year, however due to the complexity of the
meteorology to capture both snowmelt and rainfall, and the need to accommodate the BOC
schedules for meetings leading into 2014, the PMP work was not finalized, but will be finalized
in 2014.. This will allow the study to meet the study objectives by allowing the BOC a full
review of the PMP work.
4.10. Coincident Conditions for the PMF
Developing coincident conditions includes the 100-year snowpack, the probable maximum
snowpack, necessary temperature, dew point, and wind speed sequences, and other data for
energy budget method as necessary. The 100-year precipitation is also being developed, because
one of the potential combinations of coincident conditions that can result in the PMF is the
probable maximum snowpack combined with the seasonally appropriate 100-year precipitation.
A determination of the maximum reservoir level during the 50-year flood may also be required,
as this may become the starting reservoir elevation for spillway operation.
4.10.1. Variances
While the development of the coincident conditions for the PMF work efforts were mostly
accomplished in 2013, the full effort was not completed in study year 2013. At time the RSP
was prepared, it was contemplated the site-specific PMP and PMF study could be completed in
2013, however due to the complexity of the hydrology to capture both snowmelt and rainfall
events, and the need to accommodate the BOC schedules for meetings leading into 2014, the
PMF work was not finalized, but will be finalized in 2014. This will allow the study to be ensure
it meets the study objectives by allowing the BOC a full review of the PMP work.
4.11. Development of the PMF Inflow Hydrograph
The PMF is being developed at the proposed Watana Dam site by combining sub-area runoff and
performing channel and reservoir routings for various cases and months. The energy budget
snowmelt method is being used. Routing of the PMF through the reservoir may account for use
of the fixed-cone outlet valves for discharges up to the 50-year flood and use of the spillway only
after the expected maximum level of the 50-year flood has been exceeded, but final flood
operating procedures are not yet finalized. While the development of the PMF Inflow
Hydrograph was initiated in 2013, work efforts will continue into 2014. This task also includes a
sensitivity analysis to test the effects of variation in parameters with relatively high uncertainty
that could potentially have more significant effects on the results. The PMF channel routing will
use the selected flood hydrology model.
4.11.1. Variances
As noted above, the full effort was not completed in study year 2013. At the time the RSP was
prepared, it was contemplated the site-specific PMP and PMF study could be completed in 2013,
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however due to the complexity of the hydrology to capture both snowmelt and rainfall events,
and the need to accommodate the BOC schedules for meetings leading into 2014, the PMF work
was not finalized, but will be finalized in 2014. This will allow the study to ensure it meets the
study objectives by allowing the BOC a full review of the PMP work.
4.12. Reservoir Routing of the PMF
Spillway capacity should be determined as part of the economical combination of spillway
capacity and surcharge storage. Surcharge storage is defined as the storage between the normal
maximum pool level (still water) and the maximum design flood water storage level.
Determining the economical combination of surcharge storage/spillway capacity requires
evaluation of the cost of increasing spillway capacity versus the cost of raising the dam height to
provide the required freeboard (routed maximum flood level plus any required allowance for
wind setup and wave run-up). Reservoir flood routing is used to determine the temporal and
water level variation of the hydrograph as the flood passes through the reservoir. Increasing the
spillway capacity will reduce the necessary surcharge storage (determined by flood routing),
thereby lowering the required height of the dam. Alternatives analysis are being performed to
optimize spillway capacity and flood surcharge. The PMF reservoir routing will use the selected
flood hydrology model. As outlined in the RSP, this task was expected to be part of the PMF
study that would all be accomplished in 2013, however this specific study component is being
deferred to 2014.
It is expected that the volume and distribution of potential future sedimentation in the reservoir
will form a PMF routing sensitivity case. AEA is evaluating the potential for glacial lake
outburst floods (GLOF). AEA will compare the potential for GLOF to the critical PMF inflow
hydrograph and will route the GLOF to determine the peak reservoir level if the GLOF
potentially forms the critical condition for spillway design.
4.12.1. Variances
As noted above, the full effort was not completed in study year 2013. At time the RSP was
prepared, it was contemplated the site-specific PMP and PMF study could be completed in 2013,
however due to the complexity of the hydrology to capture both snowmelt and rainfall events,
and the need to accommodate the BOC schedules for meetings leading into 2014, the PMF work
was not finalized, but will be finalized in 2014. This will allow the study to meet the study
objectives by allowing the BOC a full review of the PMP work.
4.13. Freeboard Analysis
Freeboard provides a margin of safety against the potential for overtopping of dams. Freeboard
and flood control storage are required to provide the capacity to store and/or route the design
storm through the reservoir considering inflows, precipitation on the reservoir basin, and wind
generated waves without hazardous overtopping of the dam. Although freeboard selection
involves more than simply the PMF water level, the freeboard selection will be made as part of
the subject study, based on wind setup, wave action, uncertainties in analytical procedures, and
uncertainties in Project function in combination with the most critical pool elevation (USACE
1991). The freeboard determination will be based on site-specific conditions that can be
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reasonably expected to occur simultaneously. Design criteria will be developed for logical
combinations of reservoir levels/precipitation and wind conditions for freeboard determination.
Wind setup and wave run-up will be determined with standard methods (USACE 1984 and
USACE 2003). As outlined in the RSP, this task was expected to be part of the PMF study that
would all be accomplished in 2013, however this specific study component is being deferred to
2014.
Normal freeboard is defined as the difference in elevation between the top of the dam and the
normal maximum pool elevation. Minimum freeboard is defined as the difference in pool
elevation between the top of the dam and the maximum reservoir water surface that would result
from routing the PMF through the reservoir. It is generally not necessary to prevent splashing or
occasional overtopping of a dam by waves under extreme conditions particularly for a concrete
dam. If studies demonstrate that the RCC dam can withstand wave overtopping without erosion
of foundation or abutment material, then minimum (or no) freeboard will be selected for the
PMF condition. In that case, only normal freeboard would be required. The study of freeboard
will take into account unusual circumstances.
4.13.1. Variances
As noted above, the full effort was not completed in study year 2013. At time the RSP was
prepared, it was contemplated the site-specific PMP and PMF study could be completed in 2013,
however due to the complexity of the hydrology to capture both snowmelt and rainfall events,
and the need to accommodate the BOC schedules for meetings leading into 2014, the PMF work
was not finalized, but will be finalized in 2014. This will allow the study to meet the study
objectives by allowing the BOC a full review of the PMP work.
4.14. Reporting
Two reports will be prepared, one covering the development of the site-specific PMP, the other
an overall PMF report for all aspects of the PMF study, including a summary of the site-specific
PMP. The PMF report will generally follow the outline suggested by FERC for PMF studies
(FERC 2001).
5. RESULTS
This section summarizes results completed to date.
5.1. Board of Consultants
The BOC meetings to date with regards to the PMP and PMF are summarized as follows:
November 1-2, 2012, Bellevue, WA – This was the initial meeting of the BOC. The PMP
presentation included a comprehensive overview of the site-specific PMP study process, and
a preliminary graphic analysis of selected historic storms, sea surface temperatures,
meteorological data, and storm tracks. PMF discussion focused on the availability of USGS
streamflow gaging data, historic seasonal flows and annual peak flows, data contained in
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previous Susitna PMF studies from the 1980s, PMF rainfall-runoff model selection,
snowmelt method selection, seasonal limits of the PMF study, and a review of the PMF study
plan.
March 7-8, 2013, Bellevue, WA – This BOC meeting focused on Susitna-Watana aspects
other than the PMP and PMF. Only a very brief update on the PMP and PMF studies was
presented.
April 3-4, 2013, Denver, CO – This meeting was exclusively a PMP and PMF workshop with
only the PMP and PMF experts from the BOC attending. PMP topics covered included
selection of the initial storm short list, initial historic storm analysis, the storm maximization
process, the limits of storm transposition, the orographic transposition factor, and the
meteorological time-series development process. PMF aspects discussed included sub-basin
segmentation, data acquisition using GIS, the months of occurrence of annual peak flows,
selection of floods for hydrograph parameter calibration and verification, and results of a
reconstruction of the 1982 PMF.
May 29-30, 2013, Anchorage, AK – This meeting was primarily a site visit for the full BOC,
but PMP and PMF updates were also included on May 30. On May 29, the PMP and PMF
BOC experts and consultants conducted a watershed over-flight in a single-engine airplane.
The PMP status update included a summary of work completed on storm analysis,
meteorological data for the runoff model, storm maximization and development of the
proportionality constant. The PMF update included a discussion of alternative methods for
required snowpack development, seasonal watershed precipitation and mapping, snowpack
data availability, and a runoff volume frequency analysis.
5.2. Field Visit
A field visit is a recommended part of the PMF study (FERC 2001) and was performed on
May 29, 2013 with the BOC. The PMP and PMF BOC experts and consultants conducted a
watershed over-flight in a single-engine airplane, beginning and ending at Talkeetna airport.
Numerous geo-referenced photographs were taken. All watershed observations were made from
the air as no landings were made within the watershed area tributary to Watana Dam.
The field visit occurred at an opportune time because a flood flow that equaled the maximum
flow of record occurred at the Gold Creek USGS gaging station on June 2, 2013. On May 29,
the day of the site visit, the high temperature was 83 degrees at Talkeetna. A colder than average
spring was followed by a rapid warming that resulted in a snowmelt flood without significant
concurrent rainfall. Figure 5.2-1 shows remnants of a river ice cover following the breakup.
Figure 5.2-2 shows the Susitna River in the vicinity of the Denali Highway crossing with
remaining snow cover on May 29, 2013.
5.3. Basin Hydrologic Data
Table 5.3-1 lists the data availability at USGS streamflow gaging stations in or near the Susitna
watershed. Figure 5.3-1 is a location map that shows the location of USGS gages and the
Susitna River watershed boundaries to Watana Dam and to the most downstream USGS gaging
station.
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Determination of snowpack and the resulting snowmelt is a particularly important part of the
PMF study. Figure 5.3-2 shows the locations of the snow course and SNOTEL stations in and
near the Susitna River watershed. Table 5.3-2 provides the latitude-longitude location, elevation,
and period of record for the snow course and SNOTEL stations. Data for the full period of
record was gathered at all of the stations.
5.4. Sub-Basin Definition
Figure 5.4-1 outlines the 29 sub-basins tributary to Watana Dam and the 5 additional sub-basins
between Watana Dam and the USGS gaging station at Gold Creek, which is the downstream
limit of the PMF study. Table 5.4-1 provides a summary of elevation-band area data for the 29
sub-basins tributary to Watana Dam. The watershed area in elevation bands is depicted on
Figure 5.4-2.
Figure 5.4-3 shows the type and distribution of watershed cover and Table 5.4-2 provides a data
summary for the watershed cover types. Shrub and scrub is the dominant watershed cover type,
totaling about 56% of the entire watershed. Forest covers about 18% of the watershed to the
Gold Creek USGS gaging station. Barren land makes up about 15% of the watershed cover,
while wetlands cover 3.9%, perennial snow/ice is 3.8% and open water covers 2.9% of the
watershed.
5.5. Historic Flood Records
For the four USGS gages upstream or near the proposed Watana Dam site, the ranked highest ten
peak flows of record for the Susitna River at Gold Creek, Cantwell, near Denali, and for the
Maclaren River near Paxson have been summarized in Tables 5.5-1 through Table 5.5-4,
respectively. Floods for the same date at different stations have been highlighted in the same
color. Floods with the largest recorded peaks at the most gages are favored for selection as flood
hydrograph calibration and verification floods. As would be expected, there is some variation in
the flood rankings from gage to gage, in part due to the period of record available for each gage.
5.6. Seasonal Flood Distribution
The determination of a 100-year snowpack for every month of the year is unnecessary because of
the highly seasonal nature of Susitna River flow. With 59 years of daily flow data available, the
USGS streamflow gage at Gold Creek provides an excellent long-term record of the seasonality
of Susitna River flow. Table 5.6-1 provides the maximum daily flow of record at Gold Creek for
each month. During the coldest months of November through March, a daily flow of as much as
10,000 cfs has never been recorded, indicating that these five months can be eliminated as
potentially maximum flood producing months.
Table 5.6-2 presents a summary of the month of occurrence of the annual peak flow at each of
the four USGS gages in or near the watershed tributary to the Watana Dam site. For the gaging
stations nearest the Watana Dam site, Gold Creek and Cantwell, June is the month during which
the annual maximum flows most frequently occur and the same is true at the Maclaren gage.
The Denali gage is most heavily influenced by glacier melt and annual peak flows occur most
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frequently at Denali during July or August. In 134 gage-years of daily flow data, an annual peak
flow has never been recorded during the months of October through April.
Additional flow frequency data at Gold Creek is provided on Figure 5.6-1, and simulated
maximum and median monthly Watana reservoir elevations are shown on Figure 5.6-2. Because
April is the month with the lowest reservoir elevations, and April flows exceed 10,000 cfs less
than 1 percent of the time, April can be eliminated from further consideration as the critical PMF
month for Watana Dam. Although October has never had an annual maximum flow, the
reservoir levels would be higher and it was therefore retained for further consideration as a
potentially critical month for the PMF.
5.7. Snowpack Determination
5.7.1. Snowpack Distribution
Maximum snowpack distribution data was developed in proportion to the October through April
average precipitation as has been previously suggested for the Yukon River (Weather Bureau
1966). GIS-based monthly precipitation was prepared using PRISM (Parameter-elevation
Regressions on Independent Slopes Model) an analytical tool developed at Oregon State
University that uses point data, a digital elevation model, and other spatial data sets to generate
gridded estimates of monthly, yearly, and event-based climatic parameters, such as precipitation,
temperature, and dew point.
Figure 5.7.1-1 graphically depicts the October through April average precipitation for the
drainage area above the Gold Creek USGS gaging station. This figure clearly shows the wide
variation in precipitation with lower total precipitation in the southeast part of the watershed and
higher precipitation in the northern and western portions of the watershed.
Historic snowpack data at available SNOTEL and snow course stations can be used to develop
the 100-year snowpack by season. The same ratio of the 100-year snowpack at a given snow
course station (or stations) for a given month to the seasonal precipitation (Oct-April) is being
used to develop the 100-year snowpack at all locations. Different ratios are used for different
months. For example, if the 100-year SWE at a snow course station (or stations) for May was
equal to 120 percent of the October through April average precipitation at the snow course
station (or stations) as determined from GIS precipitation maps, then the 100-year SWE at all
locations in the watershed for May would be equal to 120 percent of the Oct-Apr precipitation.
Table 5.7.1-1 provides the monthly average precipitation for each sub-basin and for the annual
and October through April totals. Also shown is the area-weighted average precipitation to
Watana Dam and to each of the four USGS gaging stations. The months of maximum
precipitation are July through September with April being the month with the minimum
precipitation. The average October through April precipitation varies from a maximum of
almost 20 inches for the West Fork Susitna River (sub-basin 6) to a minimum of 4.32 inches in
the area tributary to Susitna Lake and Tyone Lake (sub-basin) 14.
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5.7.2. 100-Year Snowpack
PMF combined events criteria call for using a 100-year snowpack coincident with the PMP
appropriate for the same month. The 100-year snow water equivalent was developed at several
stations based on monthly snowpack statistics and the following equation:
SWE = M + KS
where: SWE is the 100-year snow water equivalent (inches)
M is the mean snow water equivalent for a month (inches)
S is the standard deviation of the monthly snow water equivalent (inches)
K is a factor corresponding to a 100-year return period and the calculated skew of
the monthly snow water equivalent
Table 5.7.2-1 presents the calculated 100-year snow water equivalent values on or about the first
of the month from February through May. Also shown is the October through April average
total precipitation at the snow course locations as obtained from PRISM data. The last column
of this Table shows the ratio of the calculated May 1, 100-year SWE values to the October
through April total average precipitation. These are the key values used to distribute the 100-
year snowpack over the watershed.
The last column ratios in Table 5.7.2-1 for snow courses in areas tributary to Watana Dam range
from 1.51 to 1.94 and average 1.68. The data for the snow courses highlighted in red, which are
all outside the area tributary to Watana Dam, are all outside the 1.51 to 1.94 range and have
therefore been eliminated from further consideration. Therefore, the tributary area average factor
of 1.68 times the average October through April total precipitation was selected and was used to
develop the 100-year May and June snowpacks. Due to the potential for cold weather to persist
from April up to the start of June, the May and June snowpacks were considered to be equal.
The precipitation that falls during May would essentially offset any snowmelt that occurs. Table
5.7.2-2 presents the 100-year all season snowpack SWE averaged by sub-basin. The runoff
model separates the 100-year SWE values within each sub-basin by 1000-foot elevation bands.
5.7.3. Probable Maximum Snowpack
The evaluation of a 100-year precipitation on a Probable Maximum Snowpack is required in
areas where snowpack may make a significant contribution to the PMF (FERC 2001). In many
cases, it can be enough to simply assume an unlimited snowpack and if the resulting PMF is less
than for the PMP on 100-year snowpack case, then the Probable Maximum Snowpack scenario
can be dismissed, which is the usual result. A more reasonable Probable Maximum Snowpack is
developed for Watana Dam in this section.
The Yukon River watershed lies to the north and east of the Susitna River watershed and is in
places adjacent to the Susitna River watershed. The Weather Bureau (1966) has prepared a
hydrometeorological report (HMR 42) for the Yukon River and preparation of a Probable
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Maximum Snowpack for the Yukon River was a major part of the report. Results of the HMR
42 are applicable to the Susitna River watershed.
The HMR 42 Yukon River final result was that the Probable Maximum Snowpack was equal to
3.0 times the October through April cumulative average precipitation, based on an enveloping
analysis of historic October through April precipitation data. The Susitna River watershed
tributary to Watana Dam lacks this type of long-term precipitation data. In terms of May 1
recorded snow course SWE as a ratio to October through April average precipitation, the
maximum recorded year values for the area in or near the area tributary to Watana Dam are
significantly less than 3.0. Although it is a very approximate comparison, a snowpack of 3.0
times the average snowpack on May 1 would be more rare than a calculated 10,000-year event at
many of the snow course stations, which would be appropriately rare for a probable maximum
event.
The adopted Probable Maximum Snowpack for the watershed tributary to Watana Dam will be
3.0 times the average October through April precipitation. The method of snowpack distribution
over the watershed will be the same as for the 100-year snowpack. The average Probable
Maximum Snowpack SWE for each sub-basin is presented on Table 5.7.3-1. The average
Probable Maximum Snowpack SWE in the area tributary to Watana Dam is 27.9 inches, which
compares to the Weather Bureau result of 15.7 inches Probable Maximum Snowpack for the
upper Yukon River.
5.8. Runoff Model Calibration Floods
Preference for selection of historic floods for calibration and verification was based on:
The largest floods of record;
The floods with data at the most USGS gages
The floods with the most complete flow data near the peak flow
Distribution of floods in the May through October potential flood season
Storms used for calibration in the 1980s PMF studies
Storms used for PMP development
The flood periods selected for calibration and verification of hydrograph parameters are:
1. June 1964
2. August 1967
3. June 1971
4. August 1971
5. June 1972
6. September 2012
Consideration was given to the June 1964 flood because it has the largest peak flow and the
largest daily average flow of record at Gold Creek and is the second largest flood of record at
Cantwell. It was also the 10th largest flood of record on the Maclaren River, and the largest flow
of the year at Denali. Because of the magnitude of the flood at Gold Creek and Cantwell and the
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availability of flow data at all four USGS gages, June 1964 was selected for use in the calibration
and verification of hydrograph parameters.
The August 1967 period was used as a calibration flood in the 1982 Susitna PMF study and was
also selected for PMP analysis. It had the 5th highest peak flows at Gold Creek and Cantwell,
the 2nd highest peak flow of record at Denali, and the 3rd highest peak flow on the Maclaren
River. Although a peak flow value is available at Denali, no daily flow data is available. Data
availability and the magnitude of the flood are sufficient for selection of the August 1967 period
as one of the floods for calibration and verification of hydrograph parameters.
June 1971 was used as a calibration flood in the 1982 Susitna PMF study. Data is available at all
four USGS gages. It is the 7th largest partial duration flood of record at Gold Creek and has the
3rd highest partial duration flow of record at Cantwell. The shape of the hydrograph appears to
be well-suited for calibration. Because of data availability and flood magnitude, June 1971 is
selected. As an example of recorded flood data, the available flow data for June 1971 are shown
on Figure 5.8-1.
August 1971 was used as a calibration flood in the 1982 Susitna PMF study. Data is available at
all four USGS gages and it was also selected as a PMP evaluation storm. It is the maximum
flood of record at Cantwell, Denali, and the Maclaren River, and the 2nd largest flood of record
at Gold Creek. Because of data availability at all four USGS gages and because of the flood
magnitude, August 1971 is selected for calibration and verification of flood hydrograph
parameters.
June 1972 was one of the calibration floods in the 1982 Susitna PMF study. Data is available at
all four USGS gages. It is the 3rd largest peak flow of record at Gold Creek, the 4th largest at
Cantwell, and the 6th largest on the Maclaren River. Because of data availability and the
magnitude of the flood, the June 1972 period is selected for calibration and verification of
hydrograph parameters.
The September 2012 flood was selected for further PMP analysis. Data is currently available at
the USGS gages at Gold Creek, Denali, and the new gage below Tsusena Creek. The September
2012 flood was the largest flood at Gold Creek in the past 40 years, the 6th largest on record, and
by far the largest flow ever recorded in September. Because of the exceptional nature of this
September flood, and because of the availability of more meteorological data than for other
floods, the September 2012 flood is selected for calibration and verification of unit hydrograph
parameters.
5.9. Probable Maximum Precipitation
Previous PMP and storm analysis work in the region were reviewed to identify storm events,
available rainfall data and techniques applicable to the basin. Previous site-specific PMP studies
were reviewed for procedures that are applicable. Discussions were drafted for inclusion in the
final report to summarize the applicability of the previously used procedures.
PMP-type storm events which have occurred in a region that were considered transpositionable
to the basin were identified in the storm search. Storms identified included storms used in
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previous PMP and hydrologic studies in the region. The resulting list of storms was used to
derive the PMP values for the basin for the all season PMP and the seasonally adjusted PMP.
Table 5.9-1 presents the short list of storms used to determine the PMP values in this analysis. A
comprehensive list of significant storm events and the characteristics of the PMP type storm(s)
relevant to the basin was constructed.
All storms on the short list were fully analyzed using the SPAS. The SPAS program allowed for
the development of rainfall grids on a 1/3rd of a square mile resolution at hourly (or 5-minute
with NEXRAD) temporal increments. Figure 5.9-1 shows an example of the total storm
isohyetal pattern for a SPAS storm analysis. The program follows the same basic procedures
used in the HMRs to develop Depth-Area-Duration (DAD) and mass curve information. These
analysis results were used to develop the PMP values and provided the required information to
calculate the orographic transposition factor. Nine new SPAS storm analyses were completed.
Figure 5.9-2 shows the locations of rainfall centers associated with the nine storms.
Total adjustment factors were calculated for each of the storms on the short storm list. This
included an update of the 2 sigma sea surface temperature climatology that was completed for
other PMP studies along the West Coast. This update extended that climatology to include all of
the Gulf of Alaska and northern Pacific Ocean to ensure all areas that could supply atmospheric
moisture for extreme rainfall events were included. The total adjustment factor is a combination
of the in-place maximization factor, the moisture transposition factor, and the orographic
transposition factor. The in-place maximization factors for all short list storms were calculated.
This procedure follows the standard procedures as outlined in the HMRs and the World
Meteorological Organization (WMO) PMP manual. This procedure has been used during PMP
studies completed over the previous 15 years. Trajectory model analyses were completed for
each of the short list storms to provide guidance in determining the storm moisture inflow vector.
Figure 5.9-3 shows an example of a trajectory model analysis. Each storm was transpositioned
to the basin using standard procedures outlined in the HMRs and WMO manuals and previous
PMP studies conducted by the consultant. Upwind and within basin mountainous regions are
being analyzed to determine the effect on moisture availability and rainfall production within the
basin. This process can either enhance storm dynamics or deplete available atmospheric
moisture, thereby affecting the resulting rainfall. The orographic transposition factor is being
calculated to quantify the difference in orographic effects from the in-place storm location and
the Susitna-Watana drainage basin.
A meteorological time series was developed for use in rain on snow PMF modeling. Information
from six storms was used in model calibration efforts. Daily and hourly time series were
developed for meteorological parameters (i.e. temperature, dew point, wind) required for snow
melt modeling using data from surrounding weather stations (e.g. NWS COOP, RAWS,
SNOTEL, and various other networks). A data set was provided that represented the
environment which occurred prior to, during, and immediately following the storm events used
to derive the PMP values when rain on snow is a consideration. Storm dates associated with
rain-on-snow rainfall events in the region were used to develop the meteorological input
parameters for each hour of a period required for hydrologic modeling. The average thermal
structure during extreme rainfall events was provided as input for PMF modeling to support
snowmelt calculations. The meteorological variables associated with the pre-storm and storm
environment starting prior to the beginning of rainfall and continuing through the rainfall period
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were analyzed for the storms on the short list. These parameters were supplied to the hydrologist
for inclusion in the hydrologic model analyses and specifically for the energy budget equation
used to calculate the rate and amount of snowmelt which would reasonably be expected to occur
prior to and coincident with the PMP event. This allowed for an accurate representation of the
conditions that could be expected during a rain-on-snow PMP scenario based on actual storm
data and physically possible meteorological parameters.
The site-specific PMP values is being derived for the basin required for PMF calculations. These
results are being provided on a gridded basis (.025DD x .025DD resolution) similar to the grid
spacing used in several other PMP studies completed or in progress. Also, DAD tables were
provided in the same format as given in the HMRs for comparison and sensitivity purposes.
Results were also provided on a sub-basin level.
5.10. Review of Previous PMF Studies
A comparison of the current study snowpack results to those obtained during the 1980s Susitna
PMF studies is instructive. The 1982 Acres June PMF had a 51 inch SWE in the area tributary to
Watana Dam site, and a 49 inch SWE even after eliminating the glacier areas that were assigned
an essentially unlimited 99 inch SWE. The Harza-Ebasco May (maximum) snowpack shown on
Table 5.10-1 has an average SWE of 16.8 inches, which is comparable to the 15.7 inch May-June
100-year snowpack developed for the current study. The 1982 Acres PMF snowpack SWE
appears to be the result of excessive conservatism as it is about 75 percent greater than the
Probable Maximum Snowpack as determined in the current study and 5.5 times the average
October through April precipitation.
A site-specific PMP is being prepared for the current PMF study, but a comparison of the PMPs
from the 1980s studies with the snowpack SWE provides useful information on PMF runoff
volume potential. The 1982 Acres June PMP was an average of 8.7 inches over the watershed
tributary to Watana Dam, compared to the snowpack average SWE of 49 inches. The 1984
Harza-Ebasco July-August PMP was an average of 6.85 inches over the basin, the June PMP was
6.37 inches and the May PMP was an average of 5.00 inches, which combined with the average
May SWE of 16.8 inches to form the critical PMF runoff in that study. These values indicate
that snowmelt is likely to be the dominant factor in PMF runoff volume at Watana Dam.
6. DISCUSSION
Data gathering to develop the PMP has been completed and the available data was adequate.
Development of the all-season and seasonal PMP values are ongoing. The seasonal PMP values
and coincident meteorological conditions are expected to be available for review by the BOC in
March 2014.
Data gathering to develop the rainfall-runoff model to develop and route the PMF has been
completed and the available data was adequate. Development and distribution of the antecedent
snowpack has been completed. Calibration and verification of hydrograph parameters is
ongoing. Development of the monthly PMF inflow hydrographs and determination of the critical
PMF inflow condition will logically follow development of the seasonal PMP values.
INITIAL STUDY REPORT PROBABLE MAXIMUM FLOOD (PMF) STUDY (16.5)
Susitna-Watana Hydroelectric Project Alaska Energy Authority
FERC Project No. 14241 Page 19 February 2014 Draft
7. PLANS FOR 2014
[As explained in the cover letter to this draft ISR, AEA’s plan for completing this study will be
included in the final ISR filed with FERC on June 3, 2014.]
8. LITERATURE CITED
Acres. 1982. Feasibility Report, Susitna Hydroelectric Project, Volume 4, Appendix A,
Hydrological Studies, Final Draft.
AEA (Alaska Energy Authority). 2012. 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.
FERC (Federal Energy Regulatory Commission). September 2001. Engineering Guidelines for
the Evaluation of Hydroelectric Projects, Chapter VIII, Determination of the Probable
Maximum Flood.
FERC (Federal Energy Regulatory Commission). May 31, 2012. “Request for Studies and
Comments on Preliminary Study Plan”, letter from Jennifer Hill, Chief, Northwest
Branch, Division of Hydropower Licensing to Wayne Dyok, Project Manager, Alaska
Energy Authority.
Harza-Ebasco Susitna Joint Venture. January 1984. Probable Maximum Flood for Watana and
Devil Canyon Sites, Susitna Hydroelectric Project, Draft Report, Document No. 457.
Miller, John F., 1963. Probable Maximum Precipitation and Rainfall-Frequency Data for
Alaska, Technical Paper No. 47, Weather Bureau, U.S. Department of Commerce.
U.S. Army Corps of Engineers (USACE), 1991, Inflow Design Floods for Dams and Reservoirs,
ER 1110-8-2(FR), March 1, 1991.
U.S. Army Corps of Engineers (USACE), 1984. Shore Protection Manual, Coastal Engineering
Research Center, Waterways Experiment Station, Second Printing.
U.S. Army Corps of Engineers (USACE), July 31, 2003. Coastal Engineering Manual, EM-
1110-2-1100, Part II Coastal Hydrodynamics.
Weather Bureau. May 1966. Hydrometeorological Report No. 42, Meteorological Conditions
for the Probable Maximum Flood on the Yukon River Above Rampart, Alaska, U.S.
Department of Commerce, Environmental Science Services Administration, Office of
Hydrology, Hydrometeorological Branch.
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FERC Project No. 14241 Page 20 February 2014 Draft
9. TABLES
Table 5.3- 1. USGS Streamflow Gages in the Susitna Watershed
USGS
Gage
Number
Gage Name
Drainage
Area
(sq.mi)
Latitude Longitude
Gage
Datum
(feet)
Available Period of Record
15290000 Little Susitna River near Palmer 62 61°42'37"149°13'47"917 1948 - 2013
15291000 Susitna River near Denali 950 63°06'14"147°30'57"2,440 1957 - 1976; 1978 - 1986; 2012
15291200 Maclaren River near Paxson 280 63°07'10"146°31'45"2,866 1958 - 1986
15291500 Susitna River near Cantwell 4,140 62°41'55"147o32'42"1,900 1961 - 1972; 1980 - 1986
15291700 Susitna River above Tsusena Creek 5,160 62°49'24"147o36'17"1,500 2013
15292000 Susitna River at Gold Creek 6,160 62o46'04"149o41'28"677 1949 - 1996; 2001 - 2013
15292400 Chulitna River near Talkeetna 2,570 62°33'31"150°14'02"520 1958 - 1972; 1980 - 1986
15292700 Talkeetna River near Talkeetna 1,996 62°20'49"150°01'01"400 1964 - 1972; 1980 - 2013
15292780 Susitna River at Sunshine 11,100 62o10'42"150o10'30"270 1981 - 1986; 2012 - 2013
15292800 Montana Creek near Montana 164 62°06'19"150°03'27"250 2005 - 2006; 2008 - 2012
15294005 Willow Creek Near Willow 166 61°46'51"149°53'04"350 1978 - 1993; 2001 - 2013
15294010 Deception Creek near Willow 48 61°44'52"149°56'14"250 1978 - 1985
15294100 Deshka River near Willow 591 61°46'05"150°20'13"80 1978 - 1986; 1988 - 2001
15294300 Skwentna River near Skwentna 2,250 61°52'23"151°22'01"200 1959 - 1982
15294345 Yentna River near Susitna Station 6,180 61°41'55"150°39'02 80 1980 - 1986
15294350 Susitna River at Susitna Station 19,400 61°32'41"150°30'45 40 1974 - 1993
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Table 5.3- 2. Snow Course and SNOTEL Stations In or Near the Susitna Watershed
Station In Susitna R.Latitude Longitude Elevation Years of Available Snowpack
Number Watershed (1)(deg:min)(deg:min)(feet)Data In the Period of Record
Anchorage Hillside 1070 SNOTEL No N 61:07 W 149:40 2,080 8 years: 2006 - 2013
Bentalit Lodge 1086 SNOTEL Yes N 61:56 W 150:59 150 8 years: 2006 - 2013
Fairbanks F.O.1174 SNOTEL No N 64:51 W 147:48 450 31 years: 1983 - 2013
Granite Creek 963 SNOTEL No N 63:57 W 145:24 1,240 26 years: 1988 - 2013
Independence Mine 1091 SNOTEL Border N 61:48 W 149:17 3,550 16 years: 1998 - 2013
Indian Pass 946 SNOTEL No N 61:04 W 149:29 2,350 34 years: 1980 - 2013
Monohan Flat (4)1094 SNOTEL Border N 63:18 W 147:39 2,710 6 years: 2008 - 2013
Mt. Alyeska 1103 SNOTEL No N 60:58 W 149:05 1,540 40 years: 1973 - 2013
Munson Ridge 950 SNOTEL No N 64:51 W 146:13 3,100 33 years: 1981 - 2013
Susitna Valley High 967 SNOTEL Yes N 62:08 W 150:02 375 27 years: 1988 - 2013
Tokositna Valley 1089 SNOTEL Yes N 62:38 W 150:47 850 8 years: 2006 - 2013
Blueberry Hill 49N07 Snow Course Yes N 62:48 W 149:59 1,200 26 years: 1988 - 2013
Clearwater Lake 46N01 Snow Course Yes N 62:56 W 146:57 2,650 47 years: 1964 - 2013
E. Fork Chulitna River 47N02 Snow Course Yes N 63:08 W 149:27 1,800 26 years: 1988 - 2013
Fog Lakes 48N02 Snow Course Yes N 62:47 W 148:28 2,120 50 years: 1964 - 2013
Horsepasture Pass 47N02 Snow Course Border N 62:08 W 147:38 4,300 46 years: 1968 - 2013
Independence Mine 49M26 Snow Course Border N 61:48 W 149:17 3,550 25 years: 1989 - 2013
Lake Louise 46N02 Snow Course Yes N 62:16 W 146:31 2,400 50 years: 1964 - 2013
Monohan Flat 47O01 Snow Course Border N 63:18 W 147:39 2,710 49 years: 1964 - 2013
Monsoon Lake 46N03 Snow Course Border N 62:50 W 146:37 3,100 29 years: 1985 - 2013
Square Lake 47N01 Snow Course Yes N 62:24 W 147:28 2,950 50 years: 1964 - 2013
Susitna Valley High 50N07 Snow Course Yes N 62:08 W 150:02 375 19 years: 1988 - 2012
Talkeetna 50N02 Snow Course Yes N 62:19 W 150:05 350 47 years: 1967 - 2013
Tyone River 47N03 Snow Course Yes N 62:40 W 147:08 2,500 21 years: 1981 - 2011
Notes:
(1) Items in bold indicate the location is tributary to Watana Dam. Border indicates the station is on or near the watershed border.
Station Name Station Type
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Table 5.4- 1. Area in Elevation Bands to Watana Dam
Basin Area in Elevation Bands (sq.mi.) for Model with Reservoir % of
No.1-2000 2-3000 3-4000 4-5000 5-6000 6-7000 7-8000 8-9000 9-10000 10-11000 11-14000 Total Total
1 0.0 0.0 8.7 19.7 8.9 11.3 3.9 0.2 0.0 0.0 0.0 52.7 1.02%
2 0.0 16.4 105.6 65.3 32.3 7.0 0.4 0.0 0.0 0.0 0.0 226.9 4.39%
3 0.0 145.7 139.5 9.8 0.2 0.0 0.0 0.0 0.0 0.0 0.0 295.2 5.71%
4 0.0 3.5 18.2 28.5 34.4 32.5 17.1 9.2 3.8 1.4 0.8 149.4 2.89%
5 0.0 90.7 93.0 99.8 48.5 18.5 3.6 0.0 0.0 0.0 0.0 354.2 6.85%
6 0.0 3.6 23.1 39.8 37.0 29.8 14.0 3.4 1.5 0.9 0.4 153.4 2.97%
7 0.0 55.2 9.4 2.1 0.8 0.0 0.0 0.0 0.0 0.0 0.0 67.5 1.31%
8 0.0 54.3 60.4 59.5 15.8 0.1 0.0 0.0 0.0 0.0 0.0 190.1 3.68%
9 0.0 38.5 91.3 52.5 5.3 0.0 0.0 0.0 0.0 0.0 0.0 187.6 3.63%
10 0.0 180.0 113.2 28.1 5.5 0.0 0.0 0.0 0.0 0.0 0.0 326.9 6.32%
11 0.0 72.4 130.2 57.0 13.7 0.4 0.0 0.0 0.0 0.0 0.0 273.6 5.29%
12 0.0 48.7 23.7 2.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 74.7 1.45%
13 0.0 202.6 20.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 222.6 4.30%
14 0.0 131.5 3.7 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 135.2 2.61%
15 0.0 68.0 87.9 29.3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 185.2 3.58%
16 0.0 41.6 100.5 22.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 164.4 3.18%
17 0.0 223.2 27.3 2.6 0.2 0.0 0.0 0.0 0.0 0.0 0.0 253.3 4.90%
18 0.0 0.1 28.7 48.2 21.2 1.8 0.0 0.0 0.0 0.0 0.0 100.0 1.93%
19 0.0 0.6 45.9 77.9 62.9 14.4 0.5 0.0 0.0 0.0 0.0 202.2 3.91%
20 0.0 16.5 19.8 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 36.3 0.70%
21 0.0 7.2 48.4 52.3 42.3 11.6 1.0 0.0 0.0 0.0 0.0 162.7 3.15%
22 0.0 76.3 14.0 1.7 0.0 0.0 0.0 0.0 0.0 0.0 0.0 92.0 1.78%
23 0.0 41.0 88.7 35.1 4.0 0.0 0.0 0.0 0.0 0.0 0.0 168.9 3.27%
24 0.0 51.6 89.5 20.2 1.5 0.0 0.0 0.0 0.0 0.0 0.0 162.8 3.15%
25 0.0 5.3 42.0 72.4 54.0 10.2 0.1 0.0 0.0 0.0 0.0 184.0 3.56%
26 0.0 37.1 115.5 51.0 17.2 2.1 0.0 0.0 0.0 0.0 0.0 222.9 4.31%
27 0.0 141.0 92.5 33.3 2.8 0.1 0.0 0.0 0.0 0.0 0.0 269.6 5.21%
28 0.0 62.2 88.5 61.7 8.8 0.0 0.0 0.0 0.0 0.0 0.0 221.1 4.28%
29 0.0 36.8 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 36.8 0.71%
Total 0.0 1851.4 1729.1 972.2 417.6 139.8 40.6 12.8 5.3 2.3 1.3 5172.3 100.00%
0.00%35.79%33.43%18.80%8.07%2.70%0.78%0.25%0.10%0.04%0.02%100.00%
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Table 5.4- 2. Susitna Watershed Land Cover
Table 5.5- 1. Recorded Peak Flows – Susitna River at Gold Creek – 59 Years of Record
To Gold Creek without Reservoir Area % of
Code Description (sq. mi.)Total
52 Shrub/Scrub 2784.0 45.3%
42 Evergreen Forest 996.4 16.2%
31 Barren Land (Rocks/Sand/Clay)925.9 15.1%
51 Dwarf Scrub 652.9 10.6%
90 Woody Wetlands 238.9 3.9%
12 Perennial Ice/Snow 234.3 3.8%
11 Open Water 180.3 2.9%
43 Mixed Forest 56.4 0.9%
41 Deciduous Forest 54.2 0.9%
72 Sedge/Herbaceous 14.6 0.2%
95 Emergent Herbaceous Wetlands 2.9 0.0%
22 Developed, Low Intensity 1.7 0.0%
71 Grassland/Herbaceous 1.6 0.0%
21 Developed, Open Space 0.1 0.0%
23 Developed, Medium Intensity 0.01 0.0%
Total 6144.1 100.0%
Rank Date Peak Flow
(cfs)cfs/sq.mi.
1 June 7, 1964 90,700 14.7
2 August 10, 1971 87,400 14.2
3 June 17, 1972 82,600 13.4
4 June 15, 1962 80,600 13.1
5 August 15, 1967 80,200 13.0
6 September 21, 2012 72,900 11.8
7 July 12, 1981 64,900 10.5
8 June 6, 1966 63,600 10.3
9 August 25, 1959 62,300 10.1
10 August 20, 2006 59,800 9.7
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Table 5.5- 2. Recorded Peak Flows – Susitna River at Cantwell – 18 Years of Record
Table 5.5- 3. Recorded Peak Flows – Susitna River near Denali – 28 Years of Record
Table 5.5- 4. Recorded Peak Flows – Maclaren River near Paxson – 28 Years of Record
Rank Date Peak Flow
(cfs)cfs/sq.mi.
1 August 10, 1971 55,000 13.3
2 June 8, 1964 51,200 12.4
3 June 15, 1962 46,800 11.3
4 June 17, 1972 44,700 10.8
5 August 14, 1967 38,800 9.4
6 June 16, 1984 33,400 8.1
7 July 18, 1963 32,000 7.7
8 August 14, 1981 30,900 7.5
9 June 23, 1961 30,400 7.3
10 July 29, 1980 28,500 6.9
Rank Date Peak Flow
(cfs)cfs/sq.mi.
1 August 10, 1971 38,200 40.2
2 August 14, 1967 28,200 29.7
3 July 28, 2003 27,800 29.3
4 September 21, 2012 25100 26.4
5 July 28, 1980 24,300 25.6
6 August 9, 1981 23,200 24.4
7 August 4, 1976 22,100 23.3
8 July 12, 1975 21,700 22.8
9 June 7, 1957 18,700 19.7
10 July 7, 1983 18,700 19.7
Rank Date Peak Flow
(cfs)cfs/sq.mi.
1 August 11, 1971 9,260 33.1
2 September 13, 1960 8,920 31.9
3 August 14, 1967 7,460 26.6
4 July 18, 1963 7,300 26.1
5 July 2, 1985 7,190 25.7
6 June 16, 1972 7,070 25.3
7 August 10, 1981 6,650 23.8
8 August 5, 1961 6,540 23.4
9 June 14, 1962 6,540 23.4
10 June 7, 1964 6,400 22.9
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Table 5.6- 1. Maximum Daily Flows for Each Month for the USGS Gage at Gold Creek
Table 5.6- 2. Monthly Distribution of Annual Peak Flows
Gold Creek USGS Gage
Maximum Daily Flow (cfs)
January 2,900
February 3,700
March 2,400
April 24,000
May 55,500
June 85,900
July 60,800
August 77,700
September 70,800
October 36,200
November 8,940
December 4,400
Gold Creek Gage Cantwell Gage Denali Gage Maclaren Gage Total of All Gages
Month Annual % of Annual % of Annual % of Annual % of Annual % of
Peaks Total Peaks Total Peaks Total Peaks Total Peaks Total
January 0 0%0 0%0 0%0 0%0 0%
February 0 0%0 0%0 0%0 0%0 0%
March 0 0%0 0%0 0%0 0%0 0%
April 0 0%0 0%0 0%0 0%0 0%
May 8 14%1 6%0 0%1 4%10 7%
June 28 47%8 44%3 10%12 43%51 38%
July 9 15%5 28%12 41%6 21%32 24%
August 10 17%4 22%12 41%7 25%33 25%
September 4 7%0 0%2 7%2 7%8 6%
October 0 0%0 0%0 0%0 0%0 0%
November 0 0%0 0%0 0%0 0%0 0%
December 0 0%0 0%0 0%0 0%0 0%
Total 59 100%18 100%29 100%28 100%134 100%
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Table 5.7.1- 1. Monthly Precipitation by Month and Sub-Basin
Table 5.7.2- 1. 100-Year Snowpack at Snow Course Stations
Sub-Basin Basin Area Average Precipitation (inches)Oct-Apr
Number (sq.mi.)Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual Oct-Apr % of Year
1 52.6 1.73 2.61 2.07 1.54 1.67 3.46 4.36 5.85 5.61 4.32 2.01 2.64 37.88 16.92 44.7%
2 226.4 1.26 1.79 1.40 1.11 1.34 2.86 3.75 4.60 4.15 3.30 1.44 1.95 28.94 12.24 42.3%
3 295.4 0.81 0.71 0.61 0.59 1.10 2.34 2.93 2.85 2.19 1.92 0.84 1.18 18.08 6.66 36.8%
4 149.3 2.38 2.73 2.49 1.60 1.76 3.72 4.84 6.29 5.83 4.44 2.43 3.14 41.66 19.22 46.1%
5 354.0 1.61 1.97 1.55 1.14 1.37 3.04 4.10 4.73 4.21 3.29 1.62 2.26 30.91 13.45 43.5%
6 153.4 2.67 2.60 2.21 1.65 1.62 3.83 5.39 6.31 5.79 4.68 2.33 3.74 42.84 19.90 46.4%
7 67.5 1.43 1.24 0.92 0.81 1.11 2.93 3.98 3.59 2.78 2.35 1.14 1.65 23.93 9.54 39.9%
8 189.9 1.35 1.67 1.29 1.01 1.28 2.87 3.85 4.35 3.85 2.96 1.41 1.88 27.76 11.57 41.7%
9 187.7 1.42 1.32 1.00 0.97 1.30 3.11 4.20 4.24 3.57 2.75 1.34 1.72 26.93 10.50 39.0%
10 326.8 0.94 0.97 0.72 0.76 1.13 2.35 3.24 3.70 2.94 2.36 0.90 1.31 21.31 7.96 37.3%
11 273.5 1.02 1.06 0.87 0.84 1.17 2.57 3.33 3.71 3.18 2.62 1.07 1.47 22.91 8.95 39.1%
12 74.7 0.69 0.57 0.54 0.51 1.08 2.28 2.86 2.69 2.01 1.61 0.79 1.12 16.76 5.84 34.9%
13 222.5 0.54 0.45 0.44 0.32 1.04 2.31 2.68 1.82 1.55 1.22 0.77 1.05 14.20 4.79 33.7%
14 135.1 0.47 0.41 0.38 0.26 1.06 2.34 2.70 1.75 1.64 1.25 0.66 0.90 13.81 4.32 31.3%
15 185.1 0.61 0.56 0.60 0.44 1.14 2.48 2.94 2.18 1.68 1.32 0.95 1.28 16.17 5.75 35.6%
16 164.3 0.60 0.50 0.58 0.51 1.18 2.53 3.02 2.36 1.85 1.44 0.95 1.30 16.83 5.88 34.9%
17 253.2 0.57 0.47 0.51 0.35 1.05 2.24 2.71 2.17 1.71 1.32 0.79 1.08 14.97 5.09 34.0%
18 100.0 0.69 1.00 0.89 0.75 1.45 3.01 3.57 2.92 2.35 1.75 1.03 1.40 20.81 7.52 36.1%
19 202.2 0.77 1.01 0.91 1.15 1.99 3.30 3.84 3.35 3.19 2.33 1.12 1.55 24.52 8.85 36.1%
20 36.3 0.52 0.46 0.47 0.63 1.26 2.49 3.03 2.72 2.21 1.58 0.76 1.04 17.15 5.45 31.8%
21 162.7 0.79 0.81 0.78 1.29 1.87 2.94 3.84 3.71 4.08 2.70 1.21 1.57 25.59 9.15 35.8%
22 92.0 0.56 0.46 0.49 0.54 1.05 2.24 2.83 2.73 2.05 1.59 0.77 1.08 16.40 5.50 33.6%
23 174.2 0.67 0.58 0.57 0.86 1.39 2.57 3.34 3.57 3.02 2.21 0.90 1.22 20.91 7.02 33.6%
24 157.4 0.86 0.75 0.63 0.85 1.23 2.48 3.45 3.86 3.04 2.46 0.99 1.28 21.89 7.84 35.8%
25 184.0 1.16 1.02 0.80 1.66 1.76 3.50 4.72 5.59 5.76 3.96 1.72 1.92 33.57 12.24 36.5%
26 222.9 1.02 0.92 0.75 1.32 1.40 2.99 4.35 4.72 4.06 3.07 1.46 1.60 27.67 10.14 36.6%
27 269.6 1.08 1.04 0.84 0.94 1.18 2.62 3.66 4.00 3.19 2.28 1.39 1.42 23.63 8.99 38.0%
28 218.5 1.20 1.23 1.03 0.99 1.22 2.89 4.05 4.44 3.71 2.15 1.78 1.66 26.35 10.04 38.1%
29 36.8 0.76 0.73 0.60 0.75 0.99 2.19 2.99 3.25 2.58 1.78 1.03 1.06 18.70 6.71 35.9%
30 146.4 1.32 1.42 1.23 1.20 1.36 2.91 4.22 4.79 4.12 2.19 2.16 1.88 28.78 11.40 39.6%
31 181.9 1.03 1.08 0.87 1.29 1.30 3.05 4.05 4.77 4.14 2.27 1.64 1.37 26.87 9.55 35.6%
32 208.1 1.02 1.48 1.39 1.53 1.52 2.86 3.85 4.69 4.10 1.75 2.59 1.72 28.49 11.47 40.3%
33 273.4 1.57 1.67 1.59 1.49 1.48 2.97 4.13 5.04 4.40 2.16 2.57 2.21 31.29 13.26 42.4%
34 164.8 2.07 1.98 1.87 1.48 1.21 3.04 4.57 6.27 5.45 3.69 2.28 2.69 36.60 16.06 43.9%
To Gold Creek Gage 6,143 1.11 1.17 1.01 0.99 1.32 2.80 3.70 3.97 3.45 2.46 1.40 1.67 25.04 9.80 39.1%
To Watana Dam 5,168 1.05 1.10 0.93 0.91 1.31 2.77 3.61 3.76 3.26 2.48 1.24 1.61 24.03 9.32 38.8%
To Denali Gage 914 1.85 2.08 1.71 1.25 1.44 3.24 4.37 5.09 4.56 3.57 1.79 2.53 33.50 14.79 44.2%
To Maclaren Gage 279 1.35 1.94 1.52 1.19 1.40 2.97 3.86 4.84 4.42 3.49 1.55 2.08 30.62 13.12 42.8%
To Cantwell Gage 4,079 1.05 1.13 0.96 0.85 1.30 2.74 3.51 3.58 3.10 2.42 1.17 1.62 23.44 9.20 39.3%
Is Station Area 100-Year Snow Water Equivalent Oct-Apr Avg.Ratio May 1
Tributary to Elevation Feb. 1 Mar. 1 Apr. 1 May 1 Total Precip.100-Year /
Watana Dam (1)(feet)(inches)(inches)(inches)(inches)(inches)Oct-Apr (2)
Blueberry Hill No 1,200 24.0 32.8 36.5 33.8 16.9 2.01
Clearwater Lake Yes 2,650 8.1 8.2 9.8 11.6 6.0 1.94
E. Fork Chulitna River No 1,800 23.6 28.8 31.5 34.3 11.8 2.90
Fog Lakes Yes 2,120 11.6 12.1 12.9 11.9 6.7 1.78
Horsepasture Pass Yes/Border 4,300 9.4 11.8 12.5 12.8 7.0 1.82
Independence Mine No 3,550 39.6 48.1 50.1 50.1 24.5 2.05
Lake Louise Yes 2,400 6.7 7.1 8.2 7.2 4.4 1.63
Monohan Flat Yes/Border 2,710 12.7 13.8 14.7 12.0 8.5 1.40
Monsoon Lake Yes/Border 3,100 8.3 9.6 10.8 -----6.0 1.79
Square Lake Yes 2,950 6.0 6.5 7.4 7.2 4.8 1.51
Susitna Valley High No 375 13.6 15.5 16.5 19.0 13.3 1.43
Talkeetna No 350 11.3 15.9 18.4 16.7 12.0 1.39
Tyone River Yes 2,500 5.7 6.2 7.3 -----4.8 1.53
Average of non-red values 1.68
Notes:
(1) Border indicates that the stations are on or near the watershed boundary.
(2) Where May 1 data is missing, April 1 data was used.
Values in the red boxes were not used to determine the 100-year snowpack.
Station Name
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Table 5.7.2- 2. 100-Year All-Season Snowpack SWE
Basin Annual Oct-Apr 100-Year
Sub-Basin Area Precip.Precip.SWE
Number (sq.mi.)(inches)(inches)(inches)
1 52.6 37.9 16.9 28.4
2 226.4 28.9 12.2 20.6
3 295.4 18.1 6.7 11.2
4 149.3 41.7 19.2 32.3
5 354.0 30.9 13.5 22.6
6 153.4 42.8 19.9 33.4
7 67.5 23.9 9.5 16.0
8 189.9 27.8 11.6 19.4
9 187.7 26.9 10.5 17.6
10 326.8 21.3 8.0 13.4
11 273.5 22.9 9.0 15.0
12 74.7 16.8 5.8 9.8
13 222.5 14.2 4.8 8.0
14 135.1 13.8 4.3 7.3
15 185.1 16.2 5.8 9.7
16 164.3 16.8 5.9 9.9
17 253.2 15.0 5.1 8.5
18 100.0 20.8 7.5 12.6
19 202.2 24.5 8.8 14.9
20 36.3 17.1 5.4 9.2
21 162.7 25.6 9.2 15.4
22 92.0 16.4 5.5 9.2
23 174.2 20.9 7.0 11.8
24 157.4 21.9 7.8 13.2
25 184.0 33.6 12.2 20.6
26 222.9 27.7 10.1 17.0
27 269.6 23.6 9.0 15.1
28 218.5 26.3 10.0 16.9
29 36.8 18.7 6.7 11.3
30 146.4 28.8 11.4 19.1
31 181.9 26.9 9.6 16.1
32 208.1 28.5 11.5 19.3
33 273.4 31.3 13.3 22.3
34 164.8 36.6 16.1 27.0
To Gold Creek Gage 6,143 25.0 9.8 16.5
To Watana Dam 5,168 24.0 9.3 15.7
To Denali Gage 914 33.5 14.8 24.9
To Maclaren Gage 279 30.6 13.1 22.0
To Cantwell Gage 4,079 23.4 9.2 15.5
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Table 5.7.3- 1. Probable Maximum Snowpack SWE
Basin Annual Oct-Apr PMS
Sub-Basin Area Precip.Precip.SWE
Number (sq.mi.)(inches)(inches)(inches)
1 52.6 37.9 16.9 50.8
2 226.4 28.9 12.2 36.7
3 295.4 18.1 6.7 20.0
4 149.3 41.7 19.2 57.7
5 354.0 30.9 13.5 40.4
6 153.4 42.8 19.9 59.7
7 67.5 23.9 9.5 28.6
8 189.9 27.8 11.6 34.7
9 187.7 26.9 10.5 31.5
10 326.8 21.3 8.0 23.9
11 273.5 22.9 9.0 26.9
12 74.7 16.8 5.8 17.5
13 222.5 14.2 4.8 14.4
14 135.1 13.8 4.3 13.0
15 185.1 16.2 5.8 17.3
16 164.3 16.8 5.9 17.6
17 253.2 15.0 5.1 15.3
18 100.0 20.8 7.5 22.6
19 202.2 24.5 8.8 26.5
20 36.3 17.1 5.4 16.3
21 162.7 25.6 9.2 27.5
22 92.0 16.4 5.5 16.5
23 174.2 20.9 7.0 21.1
24 157.4 21.9 7.8 23.5
25 184.0 33.6 12.2 36.7
26 222.9 27.7 10.1 30.4
27 269.6 23.6 9.0 27.0
28 218.5 26.3 10.0 30.1
29 36.8 18.7 6.7 20.1
30 146.4 28.8 11.4 34.2
31 181.9 26.9 9.6 28.7
32 208.1 28.5 11.5 34.4
33 273.4 31.3 13.3 39.8
34 164.8 36.6 16.1 48.2
To Gold Creek Gage 6,143 25.0 9.8 29.4
To Watana Dam 5,168 24.0 9.3 27.9
To Denali Gage 914 33.5 14.8 44.4
To Maclaren Gage 279 30.6 13.1 39.4
To Cantwell Gage 4,079 23.4 9.2 27.6
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Table 5.10- 1. Harza-Ebasco PMF Snowpack Estimate
Harza-Ebasco Drainage Wtd. Avg.
Sub-basin Area Sub-Basin Vicinity SWE
Number (sq.mi.)(inches)
2 460 Watnana Creek 15.8
3 580 Kosina Creek 17.1
4 725 Black River 18.1
5 1,060 Tyone River 14.6
6 790 Coal Creek 15.7
7 188 W. Fork Susitna to Denali 17.0
8 762 Susitna R. above Denali 19.7
9 335 Maclaren R. below USGS gage 14.9
10 280 Maclaren R. above USGS gage 19.6
Total 5,180 Weighted Average 16.8
Location Name State Lat Lon Year Mon Day
Total
Rainfall Precipitation Source
OLD TYONEK AK 61.260 -151.860 2012 9 15 15.91 SPAS 1256 Zone 1
DENALI NP AK 62.829 -151.138 1986 10 8 11.01 SPAS 1267 Zone 1
SEWARD AK 60.113 -149.513 1986 10 8 20.80 SPAS 1267 Zone 2
MT GEIST AK 63.638 -146.971 1980 7 24 5.26 SPAS 1268 Zone 2
DENALI NP AK 62.954 -150.079 1980 7 24 7.33 SPAS 1268 Zone 1
BLACK RAPIDS AK 63.471 -145.479 1971 8 5 12.17 SPAS 1269 Zone 2
SUTTON AK 61.904 -148.863 1971 8 5 11.39 SPAS 1269 Zone 1
DENALI NP AK 62.846 -150.513 1967 8 2 12.45 SPAS 1270 Zone 2
FAIRBANKS AK 65.521 -147.329 1967 8 2 12.45 SPAS 1270 Zone 1
LITTLE SUSITNA AK 61.854 -149.229 1959 8 18 13.05 SPAS 1271 Zone 1
DONNELLY AK 63.496 -145.629 1958 7 25 7.06 SPAS 1273 Zone 1
DENALI NP AK 63.038 -150.471 1955 8 22 13.75 SPAS 1272 Zone 1
DENALI NP AK 63.029 -150.371 2006 8 17 SPAS 1303 Zone 1
Table 5.9- 1. Short List of Storms
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10. FIGURES
Figure 5.2-1. Susitna River near Deadman Creek on May 29, 2013
Figure 5.2-2. Susitna River near the Denali Highway Crossing on May 29, 2013
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Figure 5.3- 1. Susitna Watershed Boundary and USGS Gage Locations
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Figure 5.3- 2. Location of Snow Course and SNOTEL Stations
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Figure 5.4- 1. Susitna Watershed Sub-Basins
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Figure 5.4- 2. Susitna Watershed Elevation Bands
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Figure 5.4- 3. Susitna Watershed Land Cover
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Figure 5.6- 1. Historic Flow Frequency at the USGS Gold Creek Gage
Figure 5.6- 2. Watana Reservoir Simulated Elevations
0
10,000
20,000
30,000
40,000
50,000
60,000
70,000
80,000
90,000
100,000
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov DecDaily Flow (cfs)Maximum
1% Exceedance
5% Exceedance
50% Exceedance
90% Exeedance
Based on Historic
Recorded Daily Flows
in the Susitna River at at
Gold Creek -USGS
Gage 15292000
1800
1850
1900
1950
2000
2050
2100
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov DecReservoir Elevation (feet)Maximum
Median
Minimum Pool
Based on 56 years of simulated
daily reservoir elevations.
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Figure 5.7.1- 1. Average October through April Precipitation
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Figure 5.8- 1. Recorded Flows at USGS Streamflow Gaging stations for June 1971
0
10,000
20,000
30,000
40,000
50,000
60,000
70,000
6/3/19716/4/19716/5/19716/6/19716/7/19716/8/19716/9/19716/10/19716/11/19716/12/19716/13/19716/14/19716/15/19716/16/19716/17/19716/18/19716/19/1971Flow (cfs)Susitna River at Gold Creek
Susitna River at Cantwell
Susitna River at Denali
Maclaren River at Paxson
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Figure 5.9- 1. SPAS Storm Analysis Results for the August 1967 Storm
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Figure 5.9- 2. Rainfall Center Locations for the Short List Storms
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Figure 5.9- 3. Example of a Trajectory Model Analysis