HomeMy WebLinkAboutEnergy Projects 1984 Appendix D-1Bethel Area Power Plan
Feasibility Assessment
APPENDIX D-1
HYDROLOGY INVESTIGATIONS
DRAFT
Prepared for the
Alaska Power Authority
by
Harza Engineering Company
Draft
April 1984
TABLE OF CONTENTS
Chapter
I CHIKUMINUK LAKE HYDROLOGY
Introduction
Scope of the Study
Setting
Climate
Streamflow
Diversion Flood
Probable Maximum Flood (PMF)
Unit Hydrographs
Probable Maximum Precipitation (PMP)
Infiltration
Probable Maximum Flood
Reservoir Sedimentation
II HYDROLOGIC INVESTIGATIONS FOR ALTERNATIVE
HYDROELECTRIC SITES
Introduction
Streamflow
Design Floods
Probahle Maximum Flood
Diversion Flood
REFERENCES
FXHIBITS As)ioOnnnnnnnniry1|NNOBWWWNYHPRPRPe
LIST OF TABLES
Table No.Title -
I-1 Unit Hydrograph Parainmeters
I-2 Alaskan Unit Sediment Yield Values
II-1 Average Annual Streamflow Estimates
II-2 Alaskan PMF Studies
II-3 PMF and Diversion Flood Estimates
- ii-
II-3
II-4
Exhibit No.
1
2
LIST OF EXHIBITS
Title
Chikuminuk Lake Drainage Area Map
Monthly Sequences of Flow,Allen River
at Chikuminuk Lake Outlet
Unit Hydrographs,Chikuminuk Lake
Sub-basins
PMP Depth --Duration,Sub-basins 1,2
and Lake Area
PMP Depth Duration,Sub-basin 3
Critically Arranged Distribution of Probable
Maximum Precipitation,Sub-basins 1,2
and Lake
Critically
Probable
PMF Inflow
Area
Arranged Distribution of
Maximum Precipitation,Sub-basin 3
Hydrograph,Chikuminuk Lake Site
-iii-
Chapter I
CHIKUMINUK LAKE HYDROLOGY
Introduction
Scope of Study
The scope of the hydrologic investigations was to derive
the site-specific probable maximum flood (PMF)hydrograph and
estimate the diversion flood,mean annual and monthly stream-
flows,and reservoir sedimentation for the Chikuminuk Lake
site.
Setting
The Chikuminuk damsite is located at the outlet of
Chikuminuk Lake on the Allen River (Lat.60°09'Long.158°44"').
The 348 square mile drainage basin was divided into four areas
for the purpose of these investigations (See Exhibit 1):
Chikuminuk Lake,a surface area of about 40 square miles,
north of the lake,about 89 square miles (Sub-basin 1),
south of the lake,about 69 square miles (Sub-basin 2),and
west of the lake,about 150 square miles (Sub-basin 3).0000The sub-basins north and south of the lake consist of steep
mountains covered with moss,lichens and other typical tundra
vegetation.The soils are generally poorly drained and under-lain with discontinuous masses of permafrost (1,2)4/,A
thick layer of organic peat covers these soils.
The west sub-basin consists of steep mountains and wide
valleys with rolling hills.The soils and vegetative cover
conditions are similar to the sub-basins north and south of the
lake.The Chikuminuk glacier exists on the south-eastern por-
tion of the sub-basin;and Hart and Cascade Lakes lie within the
sub-basin.Neither the glacier nor the lakes was considered to
affect the hydrology of the sub-basin.
Climate
The climate of the Chikuminuk Lake area can be classified
as transitional,between maritime and continental,as described
in Appendix B.The prevailing storm winds are out of the
L/Numbers in parenthesis refer to references at end of text.
I-1l
southeast.The average annual precipitation over the basin is
39.7 inches (3).
Streamflow
The average annual and average monthly streamflows at the
Chikuminuk site were determined by transposing data from a
nearby gage using a regression analysis of concurrent monthly
flow records.The derived monthly sequences of flow at the
damsite are given on Fxhibit 2.The U.S.Geological Survey
(USGS)gaging station "Nuyakuk River near Dillingham"(No.
15302000,Drainage Area =1,490 square miles,period of record
1954 to present)exists at the outlet of Tikchik Lake (Exhibit
1).A discontinued USGS gage "Allen River near Dillingham"
(period of record June 1963 to September 1966)existed at the
outlet of Chikuminuk Lake,within a few thousand feet of the
Chikuminuk dam site.l/The average monthly flows for the
period of July 1963 to September 1964 (39 months)of both gages
were plotted on a rectilinear plot.The Nuyakuk River gage
flows were plotted on the x-axis,the Allen River gage flows on
the y-axis.The following relationship was determined by linear
regression:
Allen River Flow (cfs)=(9.2625)x Nuyakuk River Flow
(cfs)-168
The coefficient of correlation was 0.956.
This relationship was applied to the monthly flows at the
Nuyakuk River gage for the 28-year period of record.The
resulting estimated mean Allen River flow,or Chikuminuk site
streamflow,was 1,432 cfs.
Another method of transposition of streamflows from the
Nuyakuk gage to the Chikuminuk site was also considered.A
transposition ratio based on drainage area and average annual
precipitation ratios was determined.This method gave a mean
annual streamflow of 1550 cfs.However,some uncertainty exists
in average annual precipitation values in this area.There are
no precipitation stations in the mountainous area in which
Chikuminuk Lake is located.Therefore,the estimate based on
the regression analysis was adopted.
l/The USGS reported a drainage area of 278 mi2 at the gage;the value determined for these studies was 348 mi2,
Diversion Flood
A return period of 25 years was selected for the diversion
flood.A 25-year flood peak flow for the Chikuminuk site was
computed using two independent methods;the greater of the two
values was used.
The first method involved multiple-regression analyses
relating peak flood flows with a given frequency to physical and
climatic characteristics of the basin (4).The second method
involved applying a log-Pearson Type III flood frequency analy-
sis to the 28 years of data at the Nuyakuk River gage.The 25-
year flood from this analysis was transposed to the Chikuminuk
site using the drainage area ratio raised to the 0.5 power.The
multiple-regression analyses yield a peak discharge of 11,800
cfs.The transposed log-Pearson analysis resulted in a peak
flow of 14,100 cfs.Therefore,the selected diversion flood
peak is estimated to be 14,000 cfs.
Probable Maximum Flood
Unit Hydrographs
Unit hydrographs were computed for three sub-basins (Exhi-
bit 3).The Soil Conservation Service (SCS)dimensionless
curvilinear method,as outlined in Design of Small Dams (5),was
used.Table I-1 lists the unit hydrograph parameters for each
sub-basin.
The time of concentration,t.,for each sub-basin was
computed using Kirpich's formula:
t.=|zs?|0.385CcH
where tg =time of concentration in hours,
L =the stream course length in miles,and
H =the elevation difference from outlet to
divide in feet.
The unit hydrograph unit duration,D,normally should be less
than t,/4.For sub-basin 1,D =1.2/4 =0.3 hr.However,since the minimum practical D value recommended by the Sscs
(5)is 0.5 hrs,D =0.5 hrs was used.The time to peak,tp,and peak discharge,Qe were computed using:
1/2 D +(0.6)tetp(484)DA/tpQp
I-3
Table I-1l
UNIT HYDROGRAPH PARAMETERS
Sub-Sub-Sub-
Basin l Basin 2 Basin 3
Drainage area,DA (mi2)89.0 68.8 149.8
Streamcourse length,L (mi)7.3 7.0 32.5
Elevation difference from
outlet to divide,H (ft)2,900 2,050 4,200
Time of concentration,to (hr)1.2 1.3 5.8
Unit hydrograph
unit duration,D (hr)0.5 0.5 0.5
Time to peak,tp (hr)0.97 1.03 3.73
Peak discharge,Qp (cfs)44,400 32,300 19,400
tp and Q»are then multiplied by the SCS dimensionless unithydrographordinatest/tp and 0/Qp (5),respectively,toobtainthesub-basin unit hydrographs (Exhibit 3).
Probable Maximum Precipitation (PMP)
The U.S.Weather Bureau "Technical Paper No.47"(TP 47)
was used to determine the PMP (6).TP 47 gives values of PMP's
for durations of up to 24 hours.The 6-hour and 24-hour point
PMP values for the Chikuminuk basin are 9.0 and 14.0 inches,
respectively.A storm duration of 24 hours was used because of
the relatively large basin size.
The PMP was centered over sub-basins 1,2,and the lake
area to give a more critical runoff hydrograph.The areal
reduction factors for the point PMP were taken from TP 47.The
depth duration curves for sub-basins 1,2,and the lake area are
shown on Exhibit 4,and for sub-basin 3 on Exhibit 5.
The two depth-duration curves were tabulated in 0.5 hour
increments (unit duration of the unit hydrographs).These
increments were ranked and rearranged into a critical pattern
(7).The critically arranged PMP's for sub-basins 1,2 and the
lake area are shown on Exhibit 6,and for sub-basin 3 on Exhibit
7.
Infiltration
The rainfall excess was determined by subtracting the
infiltration rate from the critically arranged incremental PMP.Based upon soil descriptions by AEIDC (1)and a _phoneconversationwiththeSCSinAnchorage(8),Hydrologic Soil
Group C was assigned.Infiltration rates ranging from 0.08 to
0.15 inches/hour are recommended for Group C soils.The
discontinuous masses of permafrost underlying the basin tend to
limit infiltration.Therefore,a value of 0.08 inches/hour was
used.
Probable Maximum Flood
The PMF was determined by summing:
-the three hydrographs resulting from applying the PMP
excess to the unit hydrographs for each sub-basin,
-the lake runoff (precipitation on the lake),and,
-an assumed baseflow.
The unit hydrograph ordinates were multiplied by the
rainfall excess increments to obtain the direct runoff
hydrographs.The lake runoff was computed by the following:
Incremental _(Incremental PMP)x (Lake Area)
Lake Runoff (PMP Time Increment)
The baseflow was assumed to be the maximum historic flow
observed at the damsite.This flow was derived by transposing
the maximum daily flow at the Nuyakuk gage using the drainage
area ratio.This somewhat conservative approach was used
because antecedent and subsequent storms were not considered in
the analysis.
The maximum recorded daily flow at the Nuyakuk gage was
32,100 cfs on July 2-3,1977.Baseflow was computed as fol-
lows:
.2Baseflow=348 mi",32,100 cfs =9800 cfs1140mi2
Baseflow and lake runoff were added to the three sub-basin
hydrographs to obtain the PMF hydrograph (Exhibit 8).This
resulted in a peak flow of 284,000 cfs (Creager's "C"=119)and
a volume of 233,320 acre-feet.
Reservoir Sedimentation
An estimate of sediment deposition was not made because the
available data is extremely limited.Table I-2 shows the data
gathered to estimate the unit sediment yield.This data indi-
cates a maximum sediment yield of about 1.3 acre-feet per square
mile per year.However,this value is representative of a
drainage basin with glaciers which contribute large quantities
of sediment.The drainage basin upstream of the Chikuminuk site
has a glacier which is quite small compared to the total size of
the basin.Therefore,the sediment yield of the Chikuminuk
basin would be expected to be much lower.
In addition,Chikuminuk Lake is believed to have a rather
large natural lake volume,all of which would serve as sediment
storage.The surface area,as mentioned earlier,is about 40
square miles.The mean depth is not known,but AEIDC personnel
have determined that the maximum depth is greater than 180 feet.
Based on this,it was concluded that even the transposed sedi-
ment yield rates in Table I-2 would have minimal effect on the
project.
Table I-2
ALASKAN UNIT SEDIMENT YIELD VALUES
Unit Sediment Yield
Sediment Transposed to
Location and Source Yield Chikuminuk Sitel/
(ac ft /mi4/yr)(ac-ft/mi "/yr)
USGS "Nushagak River at -Nata too limited;no
Fkwok"suspended sediment estimate made -
data (DA=9,850 mi2)2/
Susitna River at Watana 0.93 1.32
Damsite (DA=5,180 mi2)
(Reference 9)
Terror Lake,Kodiak Island 0.068 0.049
(DA=23.7 mi?)(Reference 10)
Chow,summary of cont.US -1.01
observed sed.production
rates for DA's between 100
and 1,090 mi2 (Reference 11)
Using ratio of contributing drainage areas raised to the
-0.125 power.Contributing drainage area at Chikuminuksiteis308mi2(total area minus lake area).
14 sediment samples in three year period (1979-1981);
Chikuminuk site is within this basin.
I-7
Chapter II
HYDROLOGIC INVESTIGATIONS
FOR ALTERNATIVE HYDROELECTRIC SITES
Introduction
Screening studies were conducted to select the preferred
hydroelectric site,as described in Appendix D,Hydrologic
analyses were performed to develop streamflow estimates,proha-
ble maximum floods and diversion floods at each site.
Streamflow
Synthesized average annual and monthly streamflow were
estimated by transposing the average annual and monthly stream-
flows of the Nuyakuk River gage near Dillingham to each site
using a Transposition Ratio.The Transposition Ratio was deter-
mined using the following relationship:
Transposition Ratio =Drainage Area of the Basin
Drainage Area of Nuyakuk Gage Basin
Average Annual Precip.over the Basin
x Average Annual Precip.over Nuyakuk
Gage Basin
The average annual precipitation values for the basins were
derived from an isohyetal map published hy the Arctic Fnviron-
mental Information and Data Center (AEFIDC),University of
Alaska,December 1977.As a check,the precipitation values
were estimated over the drainage areas of three existing USGS
gages (years of record:2,7,and 27 years)in the Kilbuck
Mountains using the AEIDC map.When compared to the runoffs at
the gages,the ratios,or runoff coefficients,ranged from 154%
to 205%.The runoff records are generally rated good except for
the winter months.Contacts were made with personnel of the
USGS,National Weather Service (NWS),and other agencies respon-
sible for hydrologic data collection in this region to assess
the accuracy of this.data.The use of the map was judged
acceptable because the error in the ratio of two precipitation
values will be insignificant.
Table II-1 shows the drainage area,precipitation,transpo-
sition ratio and average annual streamflow at each site.
Average monthly streamflow was then estimated by applying the
transposition ratio for a site to the average monthly Nayakuk
gage data.
II-1
Table II-1l
AVERAGE ANNUAL STREAMFLOW ESTIMATES
Average Average
Annual Trans-Annual
Drainage Precipi-position Stream-
Site Area tation Ratio flow
(sq.mi)(inches)(cfs)
Nayakuk River Gage 1,490 36,5 -6,113
Chikuminuk Lake 348 39.7 0.254 1,550
Kisaralik River
(Lower Falls)500 34.6 0.318 1,945
Kisaralik River
(Upper Falls)271 36.8 0.183 1,120
Kipchuk River 224 37.1 0.153 935
Upnuk Lake 105 40.0 0.077 470
Milk Creek
(Upper Site)100 39.6 0.072 440
Milk Creek
(Lower Site)113 39.2 0.081 495
Salmon River 230 29.8 0.126 770
Tulksak River 144 27.0 0.072 440
Izavicknik River 231 39.8 0.169 1,035
Fog River 92 24.0 0.041 250
Design Floods
Initial screening of the twelve sites (Table II-1)reduced
the number of sites to six.Flood analyses were performed for
these six sites.The results are presented helow,
Prohable Maximum Flood
The preliminary estimate of the PMF for each site was based
on Creager's formila (12).Table II-2 shows the Creager's "C"
values computed for the Kisaralik,Tazimina,and Terror Lake
II-2
sites.The Kisaralik and Terror Lake studies were made by R.W.
Retherford Associates and the Tazimina studies were performed by
Dames and Moore.
Table II-2
ALASKAN PMF STUDIES
PMF Drainage PMF Creager's
Site Study By Area Peak "c"
(sq.mi)(cfs)
Kisaralik R.W.Retherford
Assoc.500 480,000 163
Tazimina Dames and
Moore 327 225,000 106
Terror Lake R.W.Retherford
Assoc.15.1 41,800 106
The Tazimina site is in the Bristol Bay region and the
Terror Lake site is on Kodiak Island,about 170 miles southwest
of Bristol Ray.The Kisaralik PMF study was a reconnaissance
level study and assumptions and procedures were judged to be
conservative.The Tazimina and Terror Lake studies each used a
conventional hydrometeorological approach generally followed in
feasibility studies.The Creager's "C"values for the latter
studies were considered to be representative of the Bristol Bay
area.However,after reviewing the physical characteristics of
these watersheds and comparing them to the Bethel region sites,
the value of Creager's "C"utilized for the study sites was
expected to be somewhat higher.Therefore,a Creager's "C"of
120 was judged to he reasonable to derive preliminary PMF's.
The preliminary PMF estimates are shown in Table II-3.
Diversion Flood
The preliminary diversion floods were based on a 25 year
return period and a multiple-regression analyses that relates
peak flood flow with a given frequency to physical and climatic
characteristics of each basin (4).The estimates of diversion
floods are presented in Table II-3.
II-3
Table II-3
PMF AND DIVERSION FLOOD ESTIMATES
Drainage Creager's PMF Peak Diversion
Site Area "cr"Inflow Flood Peak
(sq.mi.)(cfs)(cfs)
Chikuminuk Lake 348 120 287,000 11,800
Kisaralik River
(Lower Falls)500 120 341,000 19,500
Kisaralik River
(Golden Gate Falls)550 120 356,000 21,900
Kisaralik River
(Upper Falls)271 120 254,000 11,600
Kipchuk River 224 120 230,000 8,800
Upnuk Lake 105 120 154,000 3,500
II
10.
ll.
12.
REFERENCES
"Preliminary Summary of Environmental Knowledge of the
Bethel Area Power Plan Feasibility Assessment Project",
Arctic Environmental Information and Data Center (AEIDC)
for Harza Engineering Company,Anchorage,Alaska,
April 26,1982.
"Water Resources Data for Alaska",USGS,Anchorage,Alaska,
1977.
"Mean Annual Precipitation (inches)",AEIDC,University of
Alaska,December 1977.
"Flood Characteristics of Alaskan Streams",USGS in co-
operation with Alaska Department of Transportation and U.S.
Department of Transportation,Water Resources Investiga-
tions 78-129,Anchorage,Alaska,1979,
Design of Small Dams,Second Edition,U.S.Bureau of Recla-mation,Washington,D.C.,1977.
U.S.Weather Bureau,"Technical Paper No.47,ProbableMaximumPrecipitationandRainfall-Frequency Pata for
Alaska,for Areas to 400 Square Miles,Purations to 24
Hours and Return Periods from 1 to 100 Years,"prepared by
John F.Miller,Washington,D.C.,1963.
Frederick,Ralph H.,""Interstorm Relations in Pacific
Northwest,"ASCE Journal of the Hydraulics Division,vol.
104,No.HY12,December 1978.
Phone conversation with Mr.Louis Fletcher,Soil Conserva-
tion Service,Anchorage,Alaska,September 23,1982,
"Susitna Hydroelectric Project,Reservoir Sedimentation,"
prepared by R&M Consultants,Inc.Anchorage,Alaska for
Acres American Inc.,Buffalo,New York,January 1982,
"Terror Lake Hydroelectric Project,Kodiak Island,Alaska,
Definite Project Report,"Robert W.Retherford Associates,
Anchorage,Alaska,International Engineering Company,Inc.,
San Francisco,California,Pecember 1978,
Chow,V.T.(ed.),Handbook of Applied Hydrology,McGraw-
Hill,New York,1964,
"Hydroelectric Handbook,"hy W.P.Creager and J.D.Justin,
Second Edition,J.Wiley and Sons,Inc.,New York,1950,
EXHIBIT 1
Lionge a Fs.Six R Ney 4Ss
a
IN3SU
aged
aera(Ce 00)
9)SBT neg SS Res>ana 5 |ee.Chauekuktulji \-
SN SaASE:Ay dalekZRINEYoan3?ALASKA POWER AUTHORITY
BETHEL AREA POWER PLAN
FEASIBILITY ASSESSMENTSCALE6I2316MILES
CHIKUMINUK LAKE
DRAINAGE AREA MAP
HARZA ENGINEERING COMPANY
December 1982
MONTHLY SEQUENCES OF FLOW
ALLEN RIVER AT CHIKUMINUK LAKE OUTLET
(CFS)
Year Oct.Nov.Dec.Jan.Feb.Mar.Apr.May June July Aug.Sept.
1954 1383 816 541 383 278 200 173 861 2552 1615 1195 1299
55 1160 1436 890 620 462 383 357 713 2927 5192 3174 1797
56 1562 830 462 305 200 173 200 698 4211 2951 1671 2267
57 1248 707 549 305 200 200 278 1094 3266 1730 844 2861
58 1801 2165 1115 645 450 383 370 844 5056 5568 2840 1695
59 1474 738 515 436 278 173 200 728 3423 2696 1364 1216
60 2207 1169 669 500 319 212 42 1167 4203 3147 2346 1687
61 1546 1298 908 666 541 357 278 1378 4014 3103 2410 2145
62 1513 885 406 330 305 252 252 924 3938 3867 1593 1237
63 1204 751 593 646 672 630 331 764 3567 2846 1487 2659
64 1591 634 317 199 161 120 121 283 4129 3439 1859 1832
65 1765 941 541 357 252 278 436 684 4796 3754 2261 3604
66 2856 1223 646 357 278 252 226 360 2693 3210 2222 2112
67 2580 1381 860 603 454 358 313 442 2793 2359 1511 1557
68 1411 613 341 245 216 200 200 867 2727 1747 1958 1440
69 834 507 406 327 252 226 226 906 5946 3969 1555 1311
70 2843 2081 831 432 323 278 252 977 3804 3271 2318 2039
71 1049 634 382 285 234 216 206 755 3541 4649 3462 1720
72 1536 1032 626 433 308 225 208 487 3040 4050 1927 1719
73 1605 1424 810 522 370 282 234 768 3599 4066 1858 1985
74 1644 798 456 313 240 226 228 925 2943 2391 1334 1537
75 1634 1032 547 375 306 269 266 823 3470 3630 1678 1545
76 1862 1094 543 301 186 92 70 505 2717 2683 2001 2418
77 2825 1472 878 676 529 364 258 592 4720 6715 6182 2421
78 1387 816 494 444 377 331 333 2804 3741 3898 2150 2088
79 1417 1230 1005 707 445 326 438 1651 4709 3649 3617 2205
80 2993 2245 1064 572 535 487 487 1603 5124 4728 2580 1375
1981 1796 1452 631 555 580 518 487 1666 4654 3090 2090 1523
Mean 1740 1120 644 448 348 286 267 938 3746 3499 2196 1903
Mean annual streamflow -1,432 cfs
EXHIBIT 2
ALASKA POWER AUTHORITY
BETHEL AREA POWER PLAN
FEASIBILITY ASSESSMENT
MONTHLY SEQUENCES
OF FLOW ALLEN RIVER
AT CHIKUMINUK LAKE OUTLET
HARZA ENGINEERING COMPANY
December 1982
DISCHARGE,cfsx1000DISCHARGE,cfsx1000DISCHARGE,cfsx1000woWwOoaow&(=)NoN°-_oiokOoao= NNaoo=oato]'a Peak discharge =44,400 cfs
EXHIBIT 3
\SUB-BASIN 1
SN
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 #18 «19
J Peak Discharge =32,300 cfs TIME,HOURS
\
SUB-BASIN2
NN eeewbeem
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 #1 #17 «#«+'18 =«19
TIME,HOURS
Peak Discharge =19,400 cfs
LIN NXN SUB-BASIN 3
Ba _-
1 2 3 4 5 6 7 8 9 10 11 12 13 14 #=%116 16 #17 «18 «19
TIME,HOURS ALASKA POWER AUTHORITY
BETHEL AREA POWER PLAN
FEASIBILITY ASSESSMENT
UNIT HYDROGRAPHS
CHIKUMINUK LAKE
SUB-BASINS
HARZA ENGINEERING COMPANY
December 1982
DEPTH,INCHESEXHIBIT4
7 8 9 10 11 #12 #13 =«214
DURATION,HOURS
15 16 17 18 19 20 21 22 23 24
ALASKA POWER AUTHORITY
BETHEL AREA POWER PLAN
FEASIBILITY ASSESSMENT
PMP DEPTH-DURATION
SUB-BASINS 1,2 AND LAKE AREA.
HARZA ENGINEERING COMPANY
December 1982
DEPTH,INCHES12
10
EXHIBIT 5
DURATION,HOURS
20 21 22 23 24
ALASKA POWER AUTHORITY
BETHEL AREA POWER PLAN
FEASIBILITY ASSESSMENT
PMP DEPTH-DURATION
SUB-BASIN 3
HARZA ENGINEERING COMPANY
December 1982
INCREMENTALPRECIPITATION.INCHPERHALFHOUREXHIBIT 6
1.2
1.0
0.8
0.4
0.2
0 5 10 15 20 25
TIME,HOURS
ALASKA POWER AUTHORITY
BETHEL AREA POWER PLAN
FEASIBILITY ASSESSMENT
CRITICALLY ARRANGED
DISTRIBUTION OF PROBABLE
MAXIMUM PRECIPITATION
SUB-BASINS 1,2,AND LAKE AREA
HARZA ENGINEERING COMPANY
December 1982
INCREMENTALPRECIPITATION,INCHPERHALFHOUR1.2
1.0
0.8
0.6
0.4
0.2
EXHIBIT 7
10 15
TIME,HOURS
20 25
ALASKA POWER AUTHORITY
BETHEL AREA POWER PLAN
FEASIBILITY ASSESSMENT
CRITICALLY ARRANGED
DISTRIBUTION OF PROBABLE
MAXIMUM PRECIPITATION
SUB-BASIN 3
HARZA ENGINEERING COMPANY
December 1982
DISCHARGE,cfsx1000280
260
240
NOSo[=]=fo?)Oo=reeo80
40
|||||
PMP DURATION 24 HOURS
-284,000 cfs
\
L
VA IN
N
=,
EXHIBIT 8
ALASKA POWER AUTHORITY
6 8 10 12 14 16
TIME,HOURS
18 20 22 24 26 28 30 32 34 36 38 40 42 44 BETHEL AREA POWER PLAN
FEASIBILITY ASSESSMENT
PMF INFLOW HYDROGRAPH
CHIKUMINUK LAKE SITE
HARZA ENGINEERING COMPANY
December 1982