HomeMy WebLinkAboutAPA246ALASKA POWER AUTHORITY
SUSITNA HYDROELECTRIC PROJECT
TASK 3 -HYDROLOGY
HYDRAULIC AND ICE STUDIES
MARCH 1982
Prepared with:
ACRES AMERICAN INCORPORATED
1000 Liberty bank Building
Main at Court
Buffalo, New York 14202
Telephone (71'6) 853-7525
Prepared by:
R&M CONSULT ANTS, INC.
5024 Cordova Street
Anchorage, Alaska 99502
Telephone: (907) 279-0483
HARZA-EBJ.\SCO
Susitna Joint Venture
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5 -ICE PROCESS ANALYSES
Geographically, the Susitna River upstream from Talkeetna can be
divided into three major segments. The upper segment above the
Oshetna confluence, the middle segment reaching to just below ·
Devil Canyon, and the lower segment from near Indian River to
Talkeetna.
The upper segment flows southward from its various glacial origins
near latitude 63°30'N for some 90 miles to latitude 62°40'N. The
elevation in this segment decreases from about 2,600 feet at the
toes of the glaciers (which rise to above 10,000 'feet) to
2,150 feet. This is a fall of sime 450 feet at an average slope of
about 5 ft/mile. No notable concentration of fall occurs in this
segment.
The middle segment flows westward with a slight northward trend
for about 90 miles from the Oshetna River confluence near
longitude 147°25 1, .. 1 to the downstream end of Devil Canyon (Indian
River confluence) at 140°25'W and 62°50'N. The fall in this
segment is about 1,350 feet to elevation 800 feet, providing an
average slope of 15 ft/mile. Concentrations of the fall in the
middle segment occur in four reaches: at the Oshetna River, at
Vee Canyon, just below Devil Creek, and in particular, at Devil
Canyon.
The lower segment flows in a south-southwesterly direction for
about 40 miles from Indian River to near the town of Talkeetna at
150°05'W and 62°20'N. The fall in this segment is about 450 feet
to elevation 350 feet at an average slope of about 11 ft/mile. No
notable concentration of fall occurs in this segment.
Downstream to Talkeetna where the Susitna flow conJoins with
those of the Chulitna and Talkeetna Rivers, the Susitna1s course is
southward for 75 miles to sea level at Cook inlet at 61 °20' N with an
average slope of 5 ft/mile. No concentration of fall occurs in this
southern segment of the r·iver.
5.1 -Field Observations
The geographic orientation of. the river on the south slope of the
Alaska Range results in air temperatures increasing along its
course from the headwaters to the lower reaches. Whereas this
temperature gradient may be due in part to the 2-degree
latitudinal span of the river, it is probably due primarily to the
3,300-foot altitudinal difference from the lower to the upper
reaches, as well as to the proximity of climate-moderating ocean
waters to the lower reaches. In any case, the gradient gives rise
to a period of time in the early stages of freezeup (late October-
early November) in which the lower basin temperatures are
s14/L 5 - 1
above thP freezing point while the upper basin is at subfreezing
temperaturas. This was the situation observed on October 17,
1980, with late afternoon temperatures (4:30 p.m. ADT) above
Watana being 30°F or lower while the temperatur·e at Talkeetna was
at about 39°F. Presumably 1 a similar springtime period occurs
with temperatures straddling the freezing point at some inter-
mediate point in the basin. This point would move in an upstream
direction with upward trending temperatures (i.e. springtime) and
downstram with downward trending temperatures (i.e. autumn).
In both cases, this pattern of temperature affects the sequence
and timing of ice cover events.
As noted 1 on October 16 and 17, 1980, the foregoing autumn
temperature situation was seen to prevail. Glacial melt fr'om the
headwaters cooled enough in its course through the relatively mild
slope of the upper segment of the river to produce notable
quantities of frazil flock and slush pans (15-25 percent surface
coverage) under post-dawn air temperatures of 18°F to 14°F. In
the fast-flowing rapids of the middle segment, relatively large
quantities of frazil were being produced, which augmented the
inflow of ice from upstr'eam loading 1 to surface coverage of the
river as high as 75-85 percent. As inflows from major tributaries
along the source of the upper and middle segments had little, if
any 1 ice content, the near-freezing temperature of the glacial melt
is judged to be a significant determinant of the origin of ice from
which a cover will deve::iop and, therefore, of the timing of that
development. Thus, the summer collection and storage of heat in
the proposed re5ervoirs in the upper reaches of the river will
produce significant changes in the autumn temperature regime in
the downstream reaches and, therefore, in the timing and rate of
cover development.
Details of the obs(=rvations made during the river freeze-up (1980)
and breakup (198'1) may be found in the Ice Observations report
( R&M I 1981 b).
5.2 -Modeling. of Ice Processes
(a) Description of the Computer Models
Acres• in-house computer models HEATSIM and ICESIM were
used to simulate the ice processes in the river reach above
Talkeetna. HEATSIM simulates a daily heat balance in the
river reach to determine water temperature progressively
downstream. Details of the model and its calibration to
simulate Susitna river reaches are presented in Appendix A4
to the main Feasibility Report. The model is used to predict
water temperature in the river and to determine the
approximate location and time when water temperature reaches
32°F. This location is used as input to the ICESIM model
s14/L 5 - 2
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which simulates the formation and progression of an ice cover
and the water levels associated with the processes. The
following paragraphs describe the ICESIM model in some
detail.
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JCESJM Model Input
Input data to the ICESIM model include streamflow, river
cross-sectional details, and an estimate of ice flow into
the study reach. Physical coefficients such as ice
density, cohesion, and ice cover friction (Manning's n)
are also input to the program. Standard values available
in the literature are used in the model. Aspects of flow
characteristics such as ice erosion velocity and critical
Fr·oude number for ice cover progression should also be
defined. Based on the literature and field observations
made in 1980-81, values were estimated for these
parameters and are Jisted in Table 5. 1.
MadeJ Backwater Calculations
The ICESIM model includes a subroutine which calculates
backwater profiles in the river reach to assess water
levels at different cross-sections. The routine is similar
to the H EC-2 model described in Section 4 but is less
sophisticated with respect to hydraulic computations in
order to accommodate the complexities of the ice process
simulation. Effectively, this simplicity translates to less
precise water· level calculations (±1 to 2 feet), as
compared to HEC-2 modeling accur·acy which is to better
than ±1 1 , but it is considered adequate to provide
representative results. This model was calibrated
against the HEC-2 model results for a single river
discharge as discussed below.
Historically, freezeup has started at a river discharge of
around 4,000 cfs at Gold Creek in the end of October
and progressed above Talkeetna until late November/
early December when the discharge dropped below
3 1 000 cfs. Calibrating the backwater routine with
observed water levels for a river discharge of around
3,000 cfs at Gold Creek proved exceedingly difficult due
to critical or near-critical flow conditions encountered In
the river reach analyzed. Post-project winter discharges
will be considerably higher (around 10,000 cf.s) as
discussed in Appendix A1 to the main Feasibility Report.
It was therefore decided that the backwater routine
should be calibrated against the H EC-2. model results for
a discharge cfose to the 10 1 000 cfs. Field measurements
5 - 3
of water levels in the river reach had been made for a
natur~l streamflow of 9, 700 cfs (at Gold Creek) and have
been used in the calibration of the H EC-2 model
(Section 4). It was considered appropriate to use this
discharge to calibrate the backwater routine of the
ICESIM model as well. A comparison of the HEC-2 and
ICESIM routine calculations is presented in Table 5.2
which indicates a reasonable agreement between the two
model resu Its.
Modeling of Ice Cover Formation and Progression
The model simulates the formation and progression
upstream of the ice cover given the location of the
leading edge of ice cover and the time of its occurrence.
The model checks the stability of ice cover and adjusts
its thickness consistent with ice supply, river goemetry 1
and hydraulics of the flow. The ice thickness is
adjusted either by telescoping of the cover or by
thickening, and the model proceeds to the next section
upstream. Except for occasional minor bridging, the
steep river slope in the reach does not permit ice
progression by bridging. This is also generally
confirmed by the river observations in 1980-81.
Model Calibration
An attempt was made to calibrate the ice process
simulation model with the field data collected during the
1980 river freezeup period. It became apparent that the
model could not simulate numerous critical or near-
critical flows that occur in the river 1 due to the
relatively large lengths of sub reaches modeled. Several
intermediate river cross-sections were synthesized
between surveyed sections to reduce these subreach
lengths. Nevertheless 1 the model has been used to
simulate ice formation and progression at average
post-project winter flows. Several qualitative checks
have b~en made to assess the accuracy of this
simulation. These include general heat balance of the
river waters, river hydr·aulic characteristics as observed
in the field, and comparisons with similar studies
elsewhere in the northern climate.
It must be emphasized that precision of predicted water
levels in the river under post"·project ice conditions is
rather limited (±1 to 2 feet). However 1 the width of the
uncertainty band in the modeling does not have a
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significant impact on the simulation of the ice regime of
the river above the Talkeetna confluence, due to limited
· progression (see Section 5.3).
5.3 -Results of Simulation Studies
Studies wer·e conducted for the following stages of project
developments:
(a)
0
0
0
During construction of the Watana dam.
Only Watana development operational.
Both Watana and Devi I Canyon developments operational~
Watana Construction Stage
During this stage, no significant change in the river regime
is expected since natural f~ows in the river below the damsite
will be maintained with the proposed diversion facilities. No
simulation of this condition was carried out.
(b) Operation with Watana Development Only
Heat balance analyses were made using the HEATS I M model in
the 35-mile river stretch between Watana and Devil Canyon
damsites. The analyses indicated that the temperature of the
power flow from Watana would reach close to 32°F below Devil
Canyon by about the third week of November under average
climatic conditions. This is about a month later than under
natural conditions and would delay the ice progression above
Talkeetna by a similar interval. It was determined that an
ice cover will be formed above the Chulitna confluence around
the end of November with ice generated from the reach below
Watana damsite. Ice simulation studies indicated that the ice
front progressed upstream at roughly 0.3 miles/day, a rate
less than one-eighth of that observed in 1980 (Table 5.3).
The front reached some 15 miles upstream by the end of
January, after which a thermal decay of the ice cover is
expected due to increased air temperature and reduced
cooling of the power flow from Watana. The ice cover formed
in the reach above Talkeetna is expected to melt in place by
the end of March, and the decay will proceed further down-
stream thereafter·. It is unlikely that any ice jam of
significance will occur above the Chulitna confluence.
Below the confluence, it is speculated that ice cover formation
will be delayed by one to three weeks due to tower· and
delayed supply of ice from the Susitna, but progresDion of
the ice front would not significantly differ from that under
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natural conditions. However 1 the decay of the ice cover is
expected to start earlier 1 by the end of March, due to
warmer waters from the power development. Significant
increase :n water temperature from that under natural
conditions is not expected near the river mouth.
(c) Operation with Watana and Devil Canyon
When both developments are operational, the temperature of
power outflows from Devil Canyon is expected to be close to
39°F during the winter months (se1: Appendix A4 to the main
Feasibility Report). As it progresses downstream, water
becomes cooler from heat exchange with the atmosphere. By
early January, it is expected that this water will coal to
about 32°F near Talkeenta (see Figure 5.1). It is expected
that very little ice cover will be formed in the river reach
above Talkeetna under average weather conditions.
Due to the warmer water temperatures above the confluence,
ice cover formation and progress in the lower river will also
be delayed. It is expected that ice contribution from the
Chulitna, Talkeetna, and Yentna Rivers will cause an ice
cover to be formed in the lower river, but this cannot be
quantified at this time without field data on such ice
contributions and further observations of river· freezeup
phenomena.
(d) River Water Levels Under Ice Cover Conditions
Under natural conditions, significant staging occurs at several
points in the river during ice cover formation. Table 5:4
presents staging observations made during the 1980 freezeup
period for selected locations in the river. With increased
flows in the winter under post-project conditions, a
significant rise in water level during ice cover formation can
be expected near Talkeetna. Table 5. 5 presents estimated
water leveis under pre~~ and post-project conditions. Below
the confluence, the rise in water levels during ice cover
formation under post-project conditions will progressively
decrease downstream. . More detailed river cross-section
surveys and river freezeup observations will be necessary to
confirm these estimates and speculations.
5.4-Reservoir Ice Cover
lee cover formation and growth in the Watana and Devil Canyon
reservoirs will be substantially different from that in the
corresponding river reach under natural conditions. An
s14/L 5 - 6
assessment of the formation, growth, bank-ice deposition, and
eventual decay of the reservoir ice is presented below.
The initial ice cover on the reservoirs is expected to be formed
with some 100 freezing degree-days (°F) after the surface water
reaches 32°F. Based on available climatic data and the reservoir
thermal modeling (see Appendix A4 to the main Feasibility Report),
the in itiaJ ice cover wi II be formed towar·d the end of October
under average weather conditions. Once a stable cover is formed,
its growth is accomplished chiefly by conductive heat loss to the
atmosphere. Figure 5.2 presents estimated ice cover growth in the
Watana reservoir over an average winter season. Devil Canyon
reservoir ice cover would progress similarly. The only difference
would be that several miles of this reservoir immediately below
Watana dam may have open water year-round due to outflow
temperatures from Watana of 39° F or higher. Near the power
intakes at each developn·.ent, open water stretches will be present
because of larger ve!oc·ties, as well as significant mixing with
warmer (39°F) waters in the lower Jayers.
Under normal operation, the Watana and Devil Canyon reservoirs
will be drawn down by about 90 and 50 feet, respectively, toward
the end of winter. Thus, the ice cover formed on the surface
would be deposited on the banks as blocks, with sizes varying
from a few inches to about three feet. The deposits will be
generally irregular and cracked due to irregular bank slopes and
drawdown rates. Most of this bank ice is unlikely to melt until
about the end of June, or earlier if the reservoir level is raised
with spring floods.
The ice cover in the reservoir itself will essentially melt in place.
By late February or early March, the ice cover will slowly start to
disintegrate with higher air temperatures and increased solar
radiation on the surface. Operation of the power intakes may
slightly alteJ~ the disintegration of ice cover close to the intake
with convection mixing underneath the cover. It is expected that
the ice cover in the reservoirs will completely dissipate by the end
of IVIay or early June, with war·mer inflow waters and the onset of
spring. During the period between March and May, the ice cover
may become structurally weak due to the disintegration process,
though its thickness may still be two to three feet.
s14/L 5 - 7
TABLE 5.1
CALIBRATION COEfriCIENTS USED IN ICESIM
Manning•s 'n 1 of Ice
Critical Froude No. at Ice Front
Erosion Velocity
Density of J ce
Cohesion of Ice
s14/L 5 "' 8
0.050
0.120
6. 5 ft/sec
47.0 lb/ft3
0.145 psi
TABLE 5.2
COMPARISON OF HEC-2 AND fCESIM
BACKWATER ROUTINE RESULTS
Computed Water Surface
Elevation (ft, msl )*
Cross-Section No. HEC-2 ICESIM
LRX - 3 344.0
LRX-9 378.1 379.0
LRX -15 452.5 453.5
LRX -21 510.0 511.7
LRX -27 542.9 544.4
LRX -34 616.0 615.8
LRX -41 659.9 659.8
LRX -47 690.6 690.3
LRX -54 733.2 733.8
LRX -61 832.9 834.2
LRX -68 850.6
* For Gold Creek discharge of 9700 cfs.
s14/L 5 - 9
TABLE 5.3
ESTIMATED ICE COVER PROGRESSION ABOVE TALKEETNA
Post-Project Conditions -Average Year
Date
December 1
December 15
December 25
January 10
January 20
Location of Leading Edge*
Rivermiles
No lee Cover
102
105
109
112
Section
LRX-7
LRX-10
LRX-12'
LRX-15
* With Watana only operational.
s14/L 5 -10
. l
TABLE 5.4
OBSERVED RIVER STAGING DURING
ICE COVER FORMATION -1980
Location
Talkeetna (LRX-3)
Gold Creek
Downstream end of Devil Canyon
Devil Canyon Dam Site (Devil's Elbow)
Immediately upstream of Devil's Elbow
s14/t 5 -11
Approximate Maximum
Observed Staging Above
10/17/80 Open Water Level
3 -4 ft.
5 - 6 ft.
12 -15 ft.
10 -12 ft.
3 -15 ft.
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TABLE 5.5
ESTIMATED WATER LEVELS AT SELECTED RIVER SECT!ONS
River Water Surface Elevations (feet)
Cross-Section Natural Ice-Post Project Conditions 2
Number Cover Condition 1 Open Water 3
LRX-3 349.0 345.0
LRX - 5 N/A 358.0
LRX - 6 N/A 362.9
LRX - 7 N/A 366.9
LRX - 8 N/A 373.8
LRX - 9
381.0 379.0
LRX -10 395.0 391.6
LRX -12 421.0 421.9
LRX -13 437.0 437.4
LRX ... 15 450.0 453.5
LRX -16 457.0 456.1
LRX -19 N/A 486.9
1 1980 Freeze-up data.
2
3
4
N/A
Average discharge 9,700 cfs at Gold Creek.
With Watana and Devil Canyon both operational.
With Watana only operational.
Not Available.
s14/L 5 -12
With Ice Cover
352.7
362.4
366.5
370.8
378.1 .
389.8
405.4
430.4
445.4
455.8
457.2
no ice
4
It·
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45.0
40.0
lt..
0
11.1
cr
:::1 ...
c(
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11.1 a.
~ w ...
a:
11.1 ... < ~
35.0
30.0
RIVER MILES
LRX SECTION
c( z
!:a: ..Jw
:::1> x-ua:
3
100
6 9 10
w
"' :! u
110
w
N z
w!l<: 11.1!1<: ~~ ~~ zl!l :0:11.1 uw <eta: c(O: :::10:: :.:a: ..Ju :eu uu cnu
~------6 6
0~~
e __.....x
~
120
14 15 18 20 27 29
LONGITUDINAL THERMAL PROFILES
POST PROJECT AtlD NATURAl CONDITIONS
z
c(
::=: 0:.: z!l<:
0: c(IIJ
IIJ ..Jl!l -11.1
;r; g5 Oa: en i!::u
130 1'10
33 35 38 Ill '17 51 53 5'1
LEGEND
Q NOVEMBER
0 DECEMBER
X JANUARY
0 FEBUARY
6 MARCH
w (!):.:
j'!w a:W oa: n.U
NATURAL WATER' "fEMPERATURE
IS A CONSTANT 32.°F THROUGH-
TillS PERIOD
WITH WllTANA AND DEVIL
CANYON DEVELOPMENTS
53
150
60 02 68
FIGURE 5.1
RIVER M1!.ES
LAX SECTION
35
30
25
-z -
U) en
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Y: u
::c
1-
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15
10
5
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OCT. I
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30 60 90 120 150
DAYS
ESTIMATED ICE COVER DEVELOPMENT IN
WATANA ·RESERVOIR
FIGURE 5.2
6 -REFERENCES
Barnes, Harry, H., Jr., 1967. Roughness characteristics of
natural channels. Water-Supply Paper 1849. United States
Department of the Interior Geological Survey, Washington,
D.C.
Bray, Dale J., 1973. Regime relations for Alberta gravel-bed
river·s. In: Fluvial Processes and Sedimentation. National
Resear·ch Counci I of Canada, Ottawa, pp. 440-452.
Chow, Ven Te 1 1959. Open-Channel Hydraulics. McGraw-Hill
Book Company, New York, New York.
Croley, Thomas
Computations
pp. 710-718.
University of
E., II., 1977. Hydrologic and Hydraulic
on Small Programmable Calculators. Section 34,
Iowa Institute of Hydraulic Research,
Iowa, Iowa City, Iowa.
Hyd~ologic Engineering Center (HEC), 1~73.
profiles programmers manual. U.S.
H EC-2 water surface
Army Corps of
Engineers, Davis, California.
----~
1 1976. HEC-2 water surface profiles users manual with
supplement. U.S. Army Corps of Engineers, Davis,
CaHfornia.
Kel!erhals, R. and Dale I. Bray, 1971. Sampling procedures for
co~rse fluvial sediments. In: Journal 2f Hydraulics Division,
American Soc:sty of Civil Engineers, New York, Vol. 97,
No. HY8 (Aug.), pp. 1165-1180.
Limerinos, J. T., 1970. Determination of the Manning coefficient
from measured bed roughns•ss in natural channels. Water
Supply Paper 1898-B. United States Department of the
Interior Geological Survey 1 Washington, CL.C.
R&M Consultants, Inc., 19f:s1a. Hydrogr·,-:phic surveys. Task 2 -
Surveys and Site Facilities, Subta5k 2.16 -Clos@oUt Report.
Anchorage, Alaska. December.
R&M Consultants, Inc., 198'1b. Ice observations.
Hydrology 1 Subtask 3. 03 -Field Data Collection.
Alaska. August.
TasJ~ 3
Anchorage,
R&M Consultants, Inc., 1981c. Preliminary tailwater rating
curves, Letter Report by Richard Giessel to Acres American,
Incorporated. Anchorage, Alaska.
R&M Consultants, Inc., 1981d. Regional flood
Susitna Hydroelectric Feasibility Report,
Anchorage, Alaska. December.
susi6/y 6 - 1
peak studies.
Appendix B .4.
R&M Consultants, Inc., 1982. River morphology. Susitna Hydro-
electric Feasibility Report, Appendix 8 .9. Anchorage,
Alaska. Januat,y.
Sargent, R .J., 1980. Variation of Manning's n roughness
coefficient with flow in open river channels. (Original source
un knol.!vn).
U.S. Army Corps of Engineers, 1975. Southcentral raHbelt area,
Alaska, Upper· Susitna River Basin, J nter·im feasibility report.
Appendix 1, Part 1. Alaska District, Anchorage, Alaska.
susi6/y 6 .. 2
s14/m1
ATTACHMENT A
STAGE-DISCHARGE RATING CURVES, STUDY
R!'"ACH OBSERVATION SITES
----~,......,......-
NOTES ON RATING CURVES
1. Rating curves are provided for the following water level
observation sites:
URX-101
URX-106.3
URX-111
URX-121
LRX-68
LRX-62
LRX-45
LRX-35
LRX-28
LRX-24
LRX-9
LRX-4
Susitna River near Deadman Creek CSR*
Susitna River at Watana Damsite CSR
Susitna River near Watana Damsite Streamgage
Susitna River near Devil Creek CSR
Susitna River at Devil Canyon Staff Gage
Susitna River at Portage Creek CSR
Susitna River at Gold Creek Streamgage
Susitna River at Sherman CSR
Susitna River at Section 25 CSR
Susitna River at Curry CSR
Susitna River at Chase CSR
Susitna River at Chulitna River Confluence CSR
2. The sites are in order by upstr~am-to-downstream sequence,
with a space separating the Upp~r Susitna Study Reach from
the Middle Susitna Study Reach.
3. Streamflows plotted on the curves were determined by
adj~sting the recorded flow at the nearest streamgage by a
factor based on drainage area. The two streamgages used
were the USGS gage at Gold Creek and the R&M gage near
the Watana Damsite. Adjustment factors are given on each
rating cu;ve.
* 11 CSR 11 =crest-stage recorder.
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