HomeMy WebLinkAboutHydro Eval for the Bristol Bay Regi Power Plan of the APA in the Tazimina River 1982BRI
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LIBRARY COPY
PROPERTY OF: · ··~{i/i;.·
1\laska Power Authorit'i, . -:.::·
334 W. 5th Ave.
Anchorage, Alaska 99501
REPORT
ON
HYDROLOGIC EVALUATIONS FOR THE BRISTOL BAY REGIONAL
POWER PLAN OF THE ALASKA POWER AUTHORITY
IN THE TAZIMINA RIVER BASIN
FOR
STONE & WEBSTER ENGINEERING CORPORATION
Dames&Moore
Job. No. 12023-006-20
February 12, 1982
Dames & Moore
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HE\,;t:l
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ALA~ p : R AUf "RITY
1626 Cole Boulevard
Golden, Colorado 80401
(303) 232-6262
TWX: 910-931-2600 Cable address: DAMEM ORE
February 12, 1982
Messrs. Alan Bjornsen and Ted Critikos
Stone & Webster Engineering Corporation
Post Office Box 5406
Denver, Colorado 80217
Subject: Hydrologic Evaluations for the Bristol Bay Regional
Power Plan in the Tazimina River Basin
Gentlemen:
We have pleasure in submitting fifteen (15) copies of this report
on the hydrologic evaluations pertaining to the Tazimina River Hydro-
electric Project.
We thank you for your confidence in Dames & Moore and appreciate
the cooperation provided by the personnel of Stone & Webster Engineering
Corporation during the course of the study reported herein. We look
forward to working with you on similar projects in the near future.
AP:JBC:smh
Enclosures
Yours very truly,
DAMES & MOORE
~~ t----··\#
Anand Prakash, Ph.D., P.E.
Chief Water Resources Engineer
Brian Cundelan
Staff Engineer
-i-
TABLE OF CCJ\iTENTS
1.0 SUMMARY
2.0 INTRODUCTION
2.1 AUTHORIZATION
2. 2 OVERVIEW Al'ID BACKGROUND
3.0 GENERATION OF HEAN ~ONTHLY STREAMFLOWS
3.1 REVIEW OF AVAILABLE DATA
Page
1
3
3
3
6
6
3.2 ALTERNATIVE APPROACHES TO GENERATE MEAN MONTHLY FLOWS 9
3.2.1 METHOD 1 9
3.2.2 METHOD 2 14
4.0 RESULTS OF STREAMFLOW ANALYSIS 20
4.1 MEA.~ MONTHLY STRW!FLOWS OF TAZIMINA RIVER 20
4. 2 DAILY STREA1'1FLOWS FOR LOW FLOW PERIOD 20
5.0 PROBABLE MAXIMUM FLOOD
5.1 BASIN CHARACTERISTICS
5.1.1 PHYSIOGRAPHY
5 .1. 2 SOILS
5.1.3 VEGETATION
5 .1. 4 CLIMATE
5.2 PROBABLE ~ffiXIMUM PRECIPITATION
5.3 UNIT HYDROGRAPH
5.3.1 TIME OF CONCENTRATION
5.3.2 OTHER PA~TERS
5.4 PROBABLE MAXIMUM FLOOD HYDROGRAPH
5.4.1 SEQUENCE OF INC~ffiNTAL PRECIPITATION
5.4.2 DIRECT RUNOFF
5. 4. 3 SNm.JMELT RUNOFF
6.0 REFERENCES
29
29
29
29
31
32
33
33
36
37
37
37
40
45
50
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LIST OF TABLES
Table ---
3-1 REGRESSION EQUATIONS BETI{EEN TOTAL MONTHLY FLOWS OF
NEW"HALEN AND TA.:.\iAL!.AJ.'l" RIVERS 10
3-2 RESULTS OF MULTIPLE LINEAR REGRESSION BETIVEEN PRE-
CIPITATION, TEMPERATURE, A.:.'l"D STREAMFLOWS OF NEW~LEN
RIVER AT ILIAMNA 11
3-3 ESTIMATED ME&~ MONTHLY FLOWS OF THE TAZIMINA RIVER
USING METHOD 1 13
3-4 REGRESSION EQUATIONS BETWEEN TOTAL MONTHLY PRECIPITATIONS
AT PORT ALSWORTH AND ILIAMNA 15
3-5 REGRESSION EQUATIONS BETWEEN HEAN MONTHLY TEMPERATURES
AT PORT ALSWORTH AND ILIAMNA 16
3-6 RESULTS OF MULTIPLE LINEAR REGRESSION BETwEEN TOTAL :HONTHLY
FLOWS OF THE TANALIA."ii RIVER, AND TOTAL MONTHLY PRECIPITA-
TION AND HEAN MONTHLY TEMPERATURE AT PORT ALSWORTH
3-7 ESTIMATED MEAN HONTHLY FLOWS OF THE TAZIMINA RIVER USING
18
METHOD 2 19
4-1 ESTIMATED MEAN MONTHLY FLOi.JS OF THE TAZIMINA RIVER -
AVERAGE OF METHOD 1 AND METHOD 2
4-2 COMPARISON OF ESTIMATED AVERAGE MONTHLY STREAMFLOWS FOR
THE TAZL~INA RIVER
4-3 ESTIMATED DAILY STREA.:.~OWS OF THE TAZIMINA RIVER FOR
THE 10-YEAR LOW FLOW PERIOD
5-l PROBABLE MAXIMUM PRECIPITATION
TAZIMINA RIVER BASIN, ALASKA
5-2 TIMES OF CONCENTRATION -TAZIMINA RIVER BASIN
5-3 SYNTHETIC UNIT lffDROGRAPH P.~~TERS
TAZIMINA RIVER BASIN
5-4 CRITICALLY SEQUENCED PRECIPITATION INCREMENTS
TAZIMINA RIVER BASIN
5-5 COMPARISON OF PMF HYDROGRAPHS USING THE GENERALIZED
AND OPTIMAL SEQUENCES OF INCR&~NTAL EXCESS RAINFALL
5-6 RUNOFF CURVE NUMBER ANALYSIS BASED ON SOILS AtiD VEGETA-
TION DATA -TAZIMINA RIVER BASIN
21
22
28
35
38
39
41
42
46
-iii-
LIST OF FIGURES
2-1 SITE VICINITY MAP
4-1 PLOT OF MEAN MONTHLY FLOWS FOR THE MONTH OF JA~1U.~Y
FOR THE TAZL~INA RIVER
4-2 PLOT OF MEAN MONTHLY FLOWS FOR THE HONTH OF FEBRUARY
FOR THE TAZIMINA RIVER
4-3 PLOT OF 't<f;EAN MONTHLY FLOWS FOR THE HONTH OF MARCH
FOR THE TAZIMINA RIVER
4-4 PLOT OF MEAN MONTHLY FLmiJS FOR THE MONTH OF APRIL
FOR THE TAZIMINA RIVER
5-1 TAZIMINA RIVER BASIN
5-2 TAZIMINA RIVER BASIN -DEPTH-DURATION CURVE
5-3 TAZIMINA RIVER BASIN -HYDROGRAPH AND RAINFALL
AUGUST 1-10, 1981
5-4 TAZL~INA RIVER BASIN -HYDROGRAPH AND RAINFALL
August 11-20, 1981
5-5 TAZIMINA RIVER BASIN -PROBABLE 1:-f..A.'UMUM FLOOD
HYDROGRAPH
5
24
25
26
27
30
34
43
44
47
1.0 s~~Y
This report documents the methods used to perform a preliminary
hydrologic evaluation of the streamflows of the Tazimina River at the
location of a proposed dam site for hydroelectric development in Alaska.
These investigations were performed under a contract with Stone & Webster
Engineering Corporation. The results of this study are to be used to in-
vestigate the technical and economic feasibility of the above-mentioned
hydroelectric project of Alaska Power Authority. The hydrologic informa-
tion generated during the course of this study consists of three sets:
o Mean monthly flows of the Tazimina River at the proposed dam site
for a drainage area of 327 square miles for the period 1941 to
1977;
o 10-year low daily flows of the Tazimina River at the proposed
dam site for the low flow months of January, February, March
and April;
o Probable maximum flood hydrographs for the proposed reservoir
on the Tazimina River with a drainage area of 273 square miles
for the PMP event alone and for the PMP event coincident with
a reasonably severe snowmelt runoff.
Computations for the mean monthly and 10-year low daily flows have
been made for a drainage of 327 square miles which represents the catch-
ment of the Tazimina River at the proposed dam site. The inflow hydro-
graph has been developed at the outlet of Lower Tazimina Lake where the
drainage area is 273 square miles.
The estimated mean monthly flows for the period 1941 to 1977 are
presented in Table 4-1. The daily flows for January, February, March
and April are given in Table 4-3 and the PMF hydrographs are shown in
Figure 5-5. The estimated peak flows for the PMP event alone and for
the PMP event coincident with snowmelt are 190,000 and 225,000 cfs, re-
spectively.
-2-
A comparison of the mean monthly flows estimated in this study with
those obtained by previous investigators (Ref. 1) is shown in Table 4-2.
It is noted that the mean monthly flows estimated in this study are about
20 percent lower for the months of January, February, March, April and
November but are significantly higher £or the months of May, June~ July,
August, September and October than those obtained in the previous study.
The flows for December are only 9 percent higher.
There is very little information on recorded streamflows and cli-
matolotical parameters, i.e., precipitation and temperature, for the
Tazimina basin. Therefore, approximate correlations were developed using
regression analyses between streamflows, precipitation, temperature and
drainage areas for nearby streams. Even in these cases, the data available
were not sufficient for a satisfactory statistical analysis. Therefore,
the results presented herein should be treated as qualitative and approxi-
mate and should be updated by refined analyses after more site-specific
hydrologic and climatologic data have been collected.
-3-
2.0 INTRODUCTION
2.1 AUTHORIZATION
The hydrologic analyses and results documented in this report were
authorized through PR 14007-W034Y dated November 2, 1981 issued by Stone &
Webster Engineering Corporation, Denver, Colorado to Dames & Moore. The
scope of services to be provided m1der this contract included collection
and review of hydrologic data, streamflow development, and determination
of the probable maximum precipitation (PMP) and probable maximum flood (PMF)
applicable to the Tazimina River Hydroelectric Project for the Bristol Bay
Regional Power Plan of the Alaska Power Authority.
2.2 OVERVIEW AND BACKGROL~ID
The Phase I report on Bristol Bay Energy and Electric Power Potential
(Ref. 2) identified Lake Tazimina as a potential site for t~lopment of
hydroelectric power with an available head of approximate!/ 300 ~et and an
average flow of 1,440 cfs. A conceptual report on the Taz~River Hydro-
electric Project was prepared in January, 1980. This included the construc-
tion of a storage reservoir with a 45-foot-high dam at the mouth of the Lower
Tazimina Lake. Stone & Webster Engineering Corporation, with Dames & Moore
as the Environmental Consultant, is currently evaluating the environmental
and technical feasibility of this project. This report provides information
on the probable maximum flood hydrograph to be used in sizing and designing
the spillway capacity for the proposed reservoir and simulated mean monthly
streamflows to perform reservoir operation studies to determine the power
generation potential of the project.
The Tazimina River has its headwaters on the western slopes of the
Alaska Range north of Lake Iliamna. It flows westerly through two large
lakes, the Upper Tazimina Lake with its mouth at river mile 32.2 and the
Lower Tazimina Lake with its mouth at river mile 18. From the Lower Tazimina
Lake, it flows through four small lakes up to river mile 9.5 and then joins
-4-
the Newhalen River near the mouth of Lake Clark. The proposed dam site
is located at river mile 10.44. The drainage area of the Tazimina River
at the USGS gaging staLion near the proposed dam site is 327 square miles.
The drainage area at the outlet of Lower Tazimina Lake is 273 square miles
(Fig. 2-1.).
Hydrologic characteristics of the drainage basin and development of
the PMF hydrograph are described in Section 5.0.
For a feasibility-level evaluation, simulation of the mean monthly
flows of the Tazimina River for a period of approximately 36 years is con-
sidered adequate. Simulation of daily streamflows for 50 years or more
would be desirable for a detailed reservoir operation study. ~1ethods used
to develop sequential mean monthly flows for the Tazimina River at the pro-
posed dam site are described in Section 3.0 and the corresponding results
are presented in Section 4.0.
Mean monthly flows of the Tazimina River have been independently
estimated by R. W. Rutherford & Associates and E. Woody Trihey of AEIDC
(Ref. 1) using different approaches. Both these estimates were based on
the ratio of the drainage areas of the Newhalen and Tazimina rivers coupled
with appropriate refinements by judgement. The drainage area of the Newhalen
River at the water-stage recording station of the U.S. Geological Survey,
approximately 8 miles north of Iliamna, is 3,300 square miles. Because of
the large difference in the drainage areas of the two rivers, this variable
is not considered sufficient to define the streamflows of the two rivers.
Therefore, two prominent climatic variables, i.e., temperature and precipi-
tation, were also used in the correlations developed in this study in ad-
dition to the size of the drainage area. A detailed description of these
correlations is presented in Section 3.0.
-5-
I
+
ft
0 T r iangle_l /. 7 ' <;]
\" • Seal Is ':J
I \ "'! .., .
FIGURE 2-1
TAZ IM I N A RIVER BAS IN
SITE VICINITY MAP
-6-
3. 0 GENERATION OF HEAN MONTHLY STRE..<\MFJ' ... OWS
3.1 REVIEW OF AVAILABLE DATA
Review of available hydrologic and climatic data indicated that suf-
ficient information is not available to develop and calibrate a deterministic
streamflow model or to develop a stochastic model. Also, such sophisticated
models are not considered necessary for a feasibility-level evaluation.
Therefore, available information was assembled to perform appropriate cor-
relation and regression analyses to generate a sequence of monthly stream-
flows.
Pertinent available climatological data include:
(i) Monthly average temperature at Iliamna for the period 1941-
1977 (Ref. 3);
(ii) Total monthly precipitation at Iliamna for the period 1941-
1977 (Ref. 3);
The location of Iliamna is shown on Figure 2-1. The climato·-
logical station at Ilia~•a is still operative. Because of its
location in the drainage basin cf the Newhalen River, the records
at this station are assumed to be representative of that basin.
However, the available record is not complete and has approxi-
mately 14 percent of the total number of months of temperature
and precipitation records missing. Also, the station is located
near the dcW11stream edge of the drainage area cf the Newhalen
River, and so may not accurately reflect the climatology of the
upper part of the basin.
(iii) Monthly average temperature at Fort Alsworth for the period
1960-1977 (Ref. 3);
(iv) Total monthly precipitation at Port Alsworth for the period
1960-1977 (Ref. 3);
The location of Port Alsworth is also shown on Figure. 2-1.
This climatologic st~tion is still operative. It is located
-7-
in the Tanalian River basin which is approximately 200 square
miles in areal extent and lies directly north of the Tazimina
basin. The clima·cological records at this station are fairly
complete with only 2 percent of the total number of months
with missing data.
Pertinent available hydrologic data include:
(i) Daily streamflows of the Newhalen River near Iliamna for the
period October, 1956 to September, 1967 (Ref. 4);
The drainage area of the newhalen River at this station is ap-
proximately 3,300 square miles. This station has a water-stage
recorder located 8 miles north of Iliamna. At this station,
gage heights cannot generally be recorded during the low stream-
flow months of January, February, March, April and the first
half of May. The streamflows for such periods are estimated and
reported by the USGS on the basis of a few actual discharge
measurements, weather records, records for a stream-gaging
station on the Tanalian River near Port Alsworth, and records
of streamflows for other nearby streams.
(ii) Daily streamflows of the Tanalian River near Port Alsworth for
the period October, 1951 to September, 1956 (Ref. 14);
The drainage area of the Tanalian River at this station is
approximately 200 square miles. This station has a water-stage
recorder located 2 1/2 miles southeast of Port Alsworth and 3
miles east of Tanalian Point. At this station also, gage-heights
cannot be recorded during the low streamflow periods generally
including the first half of December, January, February, March,
April, and the first half of May. The streamflows for such
periods are estimated and reported by the USGS on the basis of
a few actual dis~~arge measurements, recorded ranges in stages,
weather records, records for the Newhalen River near Iliamna,
and records for other stations on nearby streams.
-8-
There is a large glacier in the headwaters of the Tanalian River
which is believed to act as a reservoir and tends to moderate
its flows. Meltwater flow from the glacier increases as the
summer advances and declines gradually with the approach of
fall and winter. The Tanalian basin is reported to be heavily
forested with spruce, birch and cottonwood (Ref. 1).
(iii) Daily streamflows of the Tazimina River for the period June 19,
1981 to September 9, 1981 (Ref. 14);
These records were supplied by the USGS and are provisional
and subject to revision. This stream-gaging station is located
near Nondalton and is designated as USGS Station Number 15200099.
It is to be recognized that the period for which streamflow data for
the Tazimina River are available is too small to be used for a reservoir
operation study. Therefore, two alternative approximate methods were in-
vestigated to develop regression equations between temperatur~, precipitation
and monthly streamflows of the Newhalen and Tanalian rivers. TI1e conputed
monthly flows of the Tanalian River were adjusted in the ratio of the drain-
age areas to estimate the monthly flows of the Tazimina River. A description
of these two methods .is presented in Section 3.2.
Dames & Moore
-9-
3. 2 ALTERNATIVE APPROACHES TO GENERATE C.IEAN ~10NTHLY FLOWS
3.2.1 METHOD 1
This method included the following sequential steps of computation:
(i) Develop regression equations correlating the total monthly flows
of the Newhalen and Tanalian rivers at Iliamna and Port Alsworth,
respectively, using available data for both rivers for the pericd
October, 1951 to September, 1956. The results of this analysis
are summarized in Table 3-1. The two types of equations given
in Table 3-1 (y = AXB and y = A + BX) were selected after exam-
ining the physical possibility of different mathematical re-
lationships. The choice between these two equations was based
on a comparison of the coefficients of determination for each
case. In view of the fact that only five to six data points
were available for regression, the coefficients of determination
are considered reasonable except for December. For this case,
the inverse relationship between the total monthly streamflows
of the two rivers appeared to be spurious and was rejected. As
an alternative, it was assumed that the December flows of the
two rivers are proportional to their respective drainage areas.
(ii) Develop regression equations correlating the total monthly ?re-
cipitation and mean monthly temperature at Iliamna for the
period October, 1951 to September, 1967 with the total monthly
flows of the Newhalen River for the same period at the USGS
stream-gaging station near Iliamna.
The results of this analysis are summarized in Table 3-2. The
computer analysis used for this multiple linear regression re-
sulted in unrealistic correlations for January, February, April,
Nay and July and was unsuccessful for the months of October and
December. For these months, a graphical method for multiple
linear regression was used (Ref. 5). The values of the co-
efficients A, B and C for these months shown in Table 3-2 were
-10-
TABLE 3-1
REGRESSION EQUATIONS -BETI<l'EEN TOTAL l10NTHLY FLmlS
OF NEWHALEN A..li{D TANAL IAN RIVERS
Selected Coefficient of A
Month Eguation Determination Coefficient
January Y=AXB 0.55 0.13
February Y=A+BX 0.55 90.19
March Y=AXB 0.56 1.20
April Y=AXB 0.33 0.79
May Y=A+BX 0.48 -6677.66
June Y=A+BX 0.83 -43620.10
July Y=A+BX 0.79 -8795.19
August Y=A+BX 0.48 -68914.18
September Y=AXB 0. 77 0.16
October Y=A+BX 0.46 -23799.24
November Y=AXB 0. 71 0.03
December Y=AXB 0.14 39182.82
Y =Total monthly flows of the Tanalian River in cfs-days.
X =Total monthly flow of the Newhalen River in cfs-day.
B
Coefficient
0.91
0.04
0.68
0. 72
0.09
0.22
0.11
0.17
0.92
0.11
1.02
-0.18*
*Theoretical regression analysis resulted in a physically unrealistic
relationship. Therefore, this equation was rejected and the mean
monthly flows of the two rivers for December were assumed to be pro-
portional to the respective drainage areas.
Month A
January* 12,000
February* 28,000
March 39,167.78
April* -212,228
May* -2,021,500
June -717,896.1
July* -6,450,000
August 232,734.6
September -653,968.1
October* -1,446,875
November 93,179.58
December* -23,000
TABLE 3-2
RESULTS OF MULTIPLE LINEAR REGRESSION BETWEEN PRECIPITATION,
TEMPERATURE, AND STREAMFLOWS OF NEWHALEN RIVER AT ILIAMNA
Std. Error
B c
41,905 1,053
7,451 1,241
4. 701.537 713.8943 13,900
27,692 7. 778
12,433 51,500
54,488.77 21,009.52 66,900
83,750 125,000
25,721.53 5,749.535 94,800
14,831.60 24,460.49 95,400
127,451 43,750
33,372.02 1,895.635 55,400
65,652 3,767
Y A + BX
1
+ cx2 ; where, Y = Total Monthly Flow in cfs-days.
x1 ~ Total Monthly Precipitation in lnehes.
x2 ~ Uean Monthly Temperature in degrees Fahrenheit.
*Regression equatJ.on based on graphical method.
St1l. Error
B
3,457.809
16.942.91
12,366.83
9,800.163
17,178.78
Std. Error
586.5202
6,623.244
15,891.18
13,181.51 I
1-'
1-'
I
2,436.528
-12-
obtained from this graphical analysis. The standard errors
of the dependent variable and the regression coefficients
were not computed for these months.
(iii) Compute the total monthly flows of Newhalen River for the
period 1941 to 1977 using the regression equations of Table 3-2
and the monthly precipitation and temperature data for the same
period at Iliamna.
These computations resulted in negative values of streamflows
for some cases, which is unrealistic. Also, climatological
data were not available for some months and flows could not be
computed. For such cases, the values for the preceding and
following years were averaged to estimate the missing total
monthly flows.
(iv) Compute the total monthly flows of Tanalian River at Port
Alsworth for the period 1941 to 1977 from those of the Newhalen
River using the regression equations developed previously. For
cases where the total monthly flows of the Newhalen River were
negative or could not be computed due to non-availability of
climatologic data, the regression equations of Table 3-1 were
not used to compute the total monthly flows of the Tanalian
River. Instead, the computed total monthly flows of the Tanalian
River for the closest preceding and following years were aYeraged
to estimate such missing values.
(v) Compute the mean monthly flows of the Tazimina River at the pro-
posed dam site (drainage area = 327 square miles) from those of
the Tanalian River at Port Alsworth (drainage area = 200 square
miles) using the drainage area ratio.
The resulting values of the mean monthly flows of the Tazimina
River for all months for the period 1941 to 1977 are given in
Table 3-3. It may be noted that this method does not make use
of the climatological data at Port Alsworth.
TABLE 3-3
ESTIMATED MEAN MONTIII.Y FLOWS OF TilE TAZIMINA RIVER USING METIIOU 1
(cfs)
!ar January" FeJ>!.'.!!!!Y March April ~ June July August October November December
141 688 3,154 3,607 1,589 1,917 1,727 381 571
42 206 184 113 216 1,316 **3,,236 **3,193 2,450 1,908 483 353 * 933
43 60 146 109 122 625 3,321 2, 778 2,571 1,535 3,297 509 1,295
44 207 179 LOS 76 666 2,603 3,888 2,940 1,627 1,693 402 561
1,5 238 157 107 60 * 441 1,871 3,081 2,597 1,645 2,466 303 210
46 243 126 110 128 441 3,133 3,182 2,637 1,687 5,815 335 620
47 178 11,2 114 109 216 1,817 3,340 1,711 1,594 *3,151 1,12 669
48 169 118 107 94 479 2,362 2,567 2,156 1,430 486 301 638
1,9 337 111 115 12 * 286 1,51,9 1,896 2,584 1,713 1,635 443 193
JSO 126 73 110 140 93 3,643 2,494 2,788 1,687 1,027 262 183
51 118 144 98 111 388 2,769 3,186 2,151, 1 '722 755 320 189
52 107 79 77 92 143 1,668 3,279 2,891 1,212 1,638 610 441 I
I-'
53 280 11,7 168 232 765 3,873 3,561 3,205 1,879 755 320 224 w
54 141, 106 90 101 517 1, 749 2,100 2,508 1,880 863 561, 292 I
55 288 148 132 121 217 1,576 3,957 2,634 1,477 473 237 160
56 98 83 82 96 268 1,694 3,143 2,376 1,607 298 21,0 310
57 154 105 88 105 171 3,321 2,831, 1,133 1,900 1,006 675 625
58 328 201 127 126 516 1,,319 3,485 2,298 1,252 374 250 244
59 175 120 88 90 84 2,048 2,611 1,807 1,964 386 320 345
960 223 155 107 80 444 2,917 2,804 2,311 1,490 871 505 537
61 243 159 101 85 204 2,646 3,418 3,035 2,203 2,275 545 334
62 178 155 120 145 287 3,118 3,580 1,395 1,1,56 101 287 322
63 209 178 127 108 491 2,171 3,666 3,397 2,034 1,151 352 326
64 161 105 88 85 8 4,627 3,1,11 1,244 1,382 419 370 389
65 230 172 131 140 207 1,371 2,486 1,884 1,841 1,704 505 498
66 243 152 101 94 60 2,072 2,828 2,543 1,513 1,128 550 561
67 278 181 119 123 560 3,935 3,857 4,570 1,806 * 955 640 699
68 158 133 uo 103 1,059 2,603 3,568 1,825 1,496 782 328 189
69 85 131 115 126 485 3,291 4,511 2,159 1,685 4,473 399 516
970 68 178 126 120 ** 306 **2, 714 **3,750 **2,162 **1.832 **4,092 **367 ** 433
71 **113 **152 **120 **125 ** 306 **2, 714 **3,750 **2,162 **1,832 **4,092 **367 ** 433
72 **113 **152 **120 **125 ** 306 **2, 714 **3,750 **2,162 **1,832 **4,092 335 ** 433
73 158 126 113 **125 ** 306 **2, 714 **3,750 2,165 **1,832 3,712 '•03 ** 433
74 **157 **116 **110 **125 ** 306 **2,7It, **3,750 **1,953 **1,832 2,379 *332 ** 433
75 *"<157 **116 110 **12'} ** 306 **2, 711, 2,990 1,740 1,978 1,046 261 351
76 156 106 107 131 127 2,137 3,445 1,860 1,646 *1,317 526 ** 259
77 211 191 108 121 229 3,669 3,953 2,164 1,827 1,588 266 168
*The estimated value was negati~re. The value given is the average of the pr~ceding and following "non-averaged" years.
*The cllmatologlcal data were not sufficient to develop an estimated flow. TI1e value given is the average of the preceding and following
"non-averaged" years.
-14-
3.2.2 METHOD 2
This method included the following sequential steps of computation:
(i) Develop regression equations correlating the total monthly pre-
cipitations and mean monthly temperatures at Port Alsworth to
those at Iliamna for the period 1961 to 1977.
After examining the physical possibility of different mathemati-
cal relationships, tvo physically viable equations, i.e.,
y = ~ and y = A + BX, were selected for the aforementioned
regression analyses. Between these two equations, the one re-
sulting in a higher coefficient of determination was adopted.
For cases where the coefficients of determination were found
to be equal, the simpler relationship, y = A+ BX, was selected.
The results of these regression analyses for precipitation and
temperature are summarized in tables 3-4 and 3-5, respectively.
Except for the months of February, April, June and December,
the coefficients of determination in Table 3-4 are reasonably
high and indicate fair correlations. The coefficients of de·-
termination in Table 3-5 indicate even better correlations
except for the month of July.
(ii) Compute the total monthly precipitation and mean monthly
temperature at Port Alsworth for the period 1941 to 1977 from
those at Iliamna using the regression equations of tables 3-4
and 3-5, respectively.
(iii) Develop regression equations between the total monthly flows
of Tanalian River near Port Alsworth and the total monthly
precipitation and mean monthly temperature at Port Alsworth
for the period 1951 to 1956.
There being only five to six data points, the compu~er program
for multiple linear regression could not be used. The regression
was performed using a graphical method for multiple linear re-
gression (Ref. 5). The results of this regression analysis
-15-
TABLE 3-4
REGRESSION EQUATIONS BETWEEN TOTAL MONTHLY PRECIPITATIONS
AT PORT ALSWORTH AND ILIAMNA
Coefficient of A B
Month Equation Determination Coefficient Coefficient
January Y=A+BX 0.72 0.04
February Y=AXB 0.31 0.45
March Y=AXB 0.83 0.47
April Y=A+BX 0.39 0.01
May Y=A+BX 0.49 -0.12
June Y=A+BX 0.28 o. 71
July Y=A+BX 0.51 0.89
August Y=AXB 0.54 0.47
September Y=A+BX 0.80 -1.07
October Y=AXB 0.57 0.86
November Y=AXB 0.79 0.76
December Y=AXB 0.37 0.60
Y = Total Monthly Precipitation at Port Alsworth in inches.
X = Total Monthly Precipitation at Iliamna in inches.
0.78
0.93
1.07
0.58
0.66
0.50
0.41
1.15
0.84
0.66
0.85
0.64
-16-
TABLE 3-5
REGRESSION EQUATIONS BETWEEN MEAN MONTHLY TEMPERATURES
AT PORT ALSWORTH AND ILIAMNA
Selected Coefficient of A B
Month Equation Determination Coefficient Coefficient
January Y=A+BX 0.98 -7.26 1.24
February Y=AXB 0.95 0.43 1.27
March Y=A+BX 0.96 -3.24 1.13
April Y=AXB 0.74 0.93 1.04
May Y=A+BX 0.94 -0.32 1.03
June Y=AXB 0.50 3.69 0.67
July Y=AXB 0.21 4.84 0.61
August Y=A+BX 0.79 1. 75 0.96
September Y=A+BX 0.88 -4.73 1.08
October Y=AXB 0.73 1.26 0.93
November Y=AXB 0.94 0.57 1.16
December Y=A+BX 0.97 -5.33 1.21
Y = Mean Monthly Temperature at Port Alsworth in Degrees Fahrenheit.
X = Mean Monthly Temperature at Iliamna in Degrees Fahrenheit.
-17-
are summarized in Table 3-6.
(iv) Compute the total monthly flows of Tanalian River at Port
Alsworth using the previously estimated total monthly pre-
cipitation and mean monthly temperature data at Port Alsworth
for the period 1941 to 1977 and the regression equations of
Table 3-6.
These computations resulted in negative values of streamflows
for some cases, which is unrealistic. Also, climatological
data were not available for some months and flows could not be
computed. For such cases, the values for the closest preceding
or following year were averaged to estimate the missing values.
(v) Compute the mean monthly flows of the Tazimina River at the
proposed dam site (drainage area = 327 square miles) from those
of the Tanalian River at Port Alsworth (drainage area = 200
square miles) using the drainage area ratio.
The resulting values of the mean monthly flows of the Tazimina
River for all months for the period 1941 to 1977 are given in
Table 3-7. This method does not make use of the streamflow
data for the Newhalen River.
-18-
TABLE 3-6
RESULTS OF MULTIPLE LINEAR REGRESSION BETWEEN TOTAL HONTHLY FLOWS OF THE TANALIAN RIVER,
~~m TOTAL MONTHLY PRECIPITATION AND MEAN MONTHLY TEMPERATURE AT PORT ALSWORTH
Month A B c
January 1,860 1,237 84
February 950 989 9.6
March 1,125 1,171 13.3
April 1,475 327 6.7
May -150,800 27,000 3,600
June -132,000 55,556 2,000
July -682,000 10,851 13,000
August -27,404 5,441 1,194
September -163,400 7,571 3,900
October -75,800 14,500 2,000
November -6,600 4,133 259
December 1,650 3,088 41
y = A + BX 1 + cx 2
y = Total Monthly Flow in cfs-days.
x1 = Total Monthly Precipitation in inches.
x2 = Mean Monthly Temperature in degrees Fahrenheit.
Year January February March
1941
42 290 101 115
43 100 93 97
44 227 105 106
45 312 93 99
46 253 87 161
47 113 85 135
48 151 72 100
49 284 83 111
1950 174 55 80
51 135 101 96
52 107 79 77
53 280 14 7 168
54 144 106 90
55 288 148 132
56 98 83 82
57 315 79 95
58 260 83 114
59 148 105 62
1960 326 81 72
61 301 87 73
62 192 98 83
63 305 74 235
64 214 111 128
65 11,8 83 182
66 168 135 90
67 155 68 140
68 182 88 78
69 113 87 106
1970 104 87 128
71 69 135 119
72 189 85 100
73 136 79 170
74 11·8 66 156
75 199 70 97
76 191 76 124
77 336 88 124
TABLE 3-7
ESTIMATED MFAN MONTHLY FLOWS OF THE TAZIMINA RIVER USING METHOD 2
{ds)
~H ~ June July August Se[!tember
2,399 2,700 3,534 2,213 2,552
86 2,627 **2,917 **3,301 2,849 2,403
101 652 3,134 3,069 2,911 1,440
91 2,766 2,071 3,668 3,275 1,847
95 1,070 1,212 3,282 2,948 2,190
108 1 ,814 3,930 3,265 2,961 1,864
94 1,377 1,631 3,310 2,235 1,833
97 513 2,309 2,846 2,528 1,079
93 81 1,713 2,650 2,924 1, 734
107 141 6,073 2,935 3,164 1,912
96 244 3,804 3, 387 2,571 2,350
92 143 1,668 3,279 2,891 1,212
232 765 3,873 3,561 3,205 1,932
101 517 1,749 2,100 2,508 1,880
121 217 1,576 3,957 2,634 1,477
96 268 1,694 3,143 2,376 1,607
99 996 1,044 3,549 2,511 1,857
100 1,619 3,319 2,918 2,583 1,248
103 1,117 1,043 1,783 3,140 **1,505
100 2,334 3,033 4,536 2,706 1,763
105 1,066 5,785 2,964 2,523 5,055
96 2,069 5,100 5,003 2,789 962
111 894 7,205 6,296 3,624 2,474
99 1,916 6,401 2,218 2,811 2,058
111 *1,556 1,511 2,291 2,813 4,415
102 1,196 *4,580 2, 717 2,816 1,971
109 615 7,649 5,755 3,794 2,061
100 2,678 1,988 3,320 2,526 739
92 555 1,249 3, ll4 2,415 1,171
111 894 3,162 1,581 2,658 616
94 * 717 2,455 1,430 4,727 1,563
93 541 833 3,136 2,384 2,446
91 206 7,009 1,688 3,258 1,079
105 889 632 3,081 2,553 2,488
101 1,080 6,259 3,489 2,108 3,054
95 290 262 4,087 2,094 2,279
141 3,156 651 3,108 2,486 3,206
October
1,063
563
1,675
1,058
1,346
2,523
212
558
1,048
802
755
1,638
755
863
473
441
1,425
893
539
1,354
1,755
1,248
600
2,240
*1,635
1,031
* 584
138
2,554
473
2,417
1,428
211
507
757
344
236
*The estimated value was negative. The value given ls the average of the preceding and following "non-averaged" years.
**The climatological data were not sufficient to develop an estimated flow. The value given is the average of the prededing and
"non-averagedu years4
November December
159 272
95 107
43 408
242 268
* 156 171
69 282
312 295
* 3:11 288
350 174
* 335 168
320 189
610 441
320 224 I
564 292 1-'
IJ:l
237 160 I
1 135
547 221
132 231
251 230
205 245
241 371
196 210
189 453
182 222
164 203
545 154
1,01\9 44
98 165
82 197
506 445
207 760
197 181
25 325
348 221
* 457 196
565 285
* 457 196
following
II
II
II
-20-
4.0 RESULTS OF STREAMFLOW ANALYSIS
4.1 MEAN MONTHLY STREAMFLOWS OF TAZIMINA RIVER
Mean monthly flows of the Tazimina River for the period 1941 to 1977
at the proposed dam site computed by the two methods described previously
are given in tables 3-3 and 3-7, respectively. As stated previously, the
first method does not utilize the climatological data at Port Alsworth,
and the second method does not utilize the streamflow data f or the Newhalen
River at Iliamna. To reflect both these sets of information in the final
result, the averages of the mean monthly streamflows obtained from the two
methods were computed. These values of mean monthly streamflows are pre-
sented in Table 4-1.
In view of the discussions provided in the previous sections, the in-
formation given in Table 4-1 is considered to be a reasonable estimate of
the mean monthly sequential streamflows of the Tazimina River at the pro-
posed dam site. A comparison of the mean monthly flows estimated in this
study with those obtained in a previous study (Ref. 1) is shown in Table 4-2.
It is noted that the mean monthly flows estimated in this study are about
20 percent lower than those obtained in the previous study for the months
of January, February, March, April and November, but are significantly
higher for the months of May, June, July, August, September and October.
The flows for December are only 9 percent higher.
4.2 DAILY STREAMFLOWS FOR LOW FLOW PERIOD
The mean monthly flows of the Tazimina River presented in Table 4-1
indicate that January, February, March and April are the months of critical
low flows. To determine the storage capacity of the proposed reservoir and
the corresponding firm power, information on the daily flows of the stream
during these critical low flow months is required. To generate a sequence
of daily low flows for these months, the following computational steps
were used:
Dames & Moore
Year Januarz
1941
42 248 143
43 80 120
44 217 142
45 275 125
46 248 107
47 146 114
48 160 95
49 3.11 97
1950 150 64
51 127 123
52 107 79
53 280 147
54 144 106
55 288 148
56 98 83
57 265 92
58 294 142
59 162 113
1960 275 118
61 272 123
62 185 127
63 257 126
6l. 188 108
65 189 128
66 206 144
67 217 125
68 170 111
69 99 109
1970 86 133
71 91 1114
72 151 119
73 147 103
74 153 91
75 178 93
76 174 91
77 304 140
TABLE 4-1
ESTUIATED MEAN MONTHLY FLOWS OF THE TAZIMINA RIVER -AVJ<:RAGE OF METHOD I AND METHOD 2
(cfs)
Mat"ch April ~ June July ~ugust September October
-~-~-
1,544 2,927 3,571 1,901 2,335 1,395
114 151 1,972 3,077 3,247 2,650 2,156 523
103 112 639 3,228 2,923 2,741 1,488 2,486
107 84 1, 716 2,337 3, 778 3,108 1,737 1,376
103 78 756 1,542 3,182 2, 773 1,918 1,906
136 u8 1,128 3,532 3,224 2,799 1,786 4,169
125 102 797 . 1, 724 3,325 1,973 1, 714 1,682
104 96 496 2,336 2,707 2,342 1,255 522
113 53 184 1.631 2,273 2,754 1, 724 1,342
95 124 117 4,858 2, 715 2,976 1,800 915
97 104 316 3,287 3,287 2,363 2,036 755
77 92 143 1,668 3,279 2,891 1,212 1,638
168 232 765 3,873 3,561 3,205 1,906 755
90 101 517 1,749 2,100 2,508 1,880 863
132 121 217 1,576 3,957 2,634 1,477 473
82 96 268 1,694 3,143 2,376 1,607 370
92 102 584 2,163 3,21,2 1,822 1.879 1,216
121 113 1,068 3,819 3,202 2,441 1,250 634
75 97 601 1,546 2,197 2,474 1,735 463
90 90 1,389 2,975 3,670 2,509 1,627 1,113
87 95 635 4,216 3,191 2, 779 3,629 2,015
102 121 118 4,109 4,292 2,092 1,209 675
181 llO 693 4,688 4,981 3,511 2,254 876
108 92 962 5,514 2,815 2,028 1, 720 1,330
157 126 882 1,441 2,389 2,349 3,128 1,670
96 98 628 3,326 2, 773 2,680 1,742 1,080
130 116 588 5,792 4,806 4,182 1,934 770
94 102 1,869 2,296 3,444 2,176 1,118 460
111 109 520 2,270 3,813 2,287 1,428 3,514
127 116 600 2,938 2,666 2,410 1,224 2,283
120 110 512 2,585 2,590 3,445 1,698 3,255
110 109 424 1, 774 3,443 2,273 2,139 2,760
142 LOS 256 4,862 2,719 2,712 1,456 1,962
133 115 598 1,673 3,416 2,253 2,160 1,443
104 113 693 4,487 3,240 1,924 2,516 902
116 113 209 1,200 3,766 1,977 1,963 831
116 131 1,693 2,160 3,531 2,325 2,517 912
November
270 422
226 520
276 852
322 415
230 191
202 451
362 482
316 463
397 181!
299 176
320 189
610 441'
320 224 I
564 292 N ......
237 160 I
121 223
611 423
191 238
286 288
355 391
393 353
242 266
271 390
276 306
335 351
548 358
855 372
213 177
241 357
437 439
287 597
266 307
214 379
340 327
359 274
546 272
362 182
COM PAR l SON Of' ESTU1ATED
Janu'!!}'. February March Apdl May
AEIDC
Study 240 190 170 1.70 420
This
Study 197 115 113 110 76]
TABLE 4-2
AVERAGE HONTIILY STREAM!' LOWS
(cfs)
June July August
1,260 1,890 1,980
2,889 3,254 2,560
FOR THE 1'!\ZIMINA RIVER
September October
1,620 990
1,844 1,388
November
570
350
December
320
350
I
N
N
I
-23-
(i) Rank the mean monthly flows for the months of January,
February, March and April for the period 1941 to 1977 in an
ascending order of magnitude.
(ii) Plot the four sets of data on norF~l probability paper using
Weibull's plotting position (Ref. 6). These plots are shown
on figures 4-1, 4-2, 4-·3 and 4-4.
(iii) From the probability plots of figures 4-1, 4-2, 4-3 and 4-4,
obtain the 10-year low flow for each month. These 10-year low
flows for each month are indicated on figures 4-1, 4-2, 4-3
and 4-4.
(iv) From the recorded daily flows of the Tanalian River for the
months of January, February, March and April for 1952, 1953,
1954, 1955 and 1956, obtain the 5-year average daily flows
for each month. This gives an array of 5-year average daily
values for each month.
(v) For each of these months ot low streamflows, compute the frac-
tion of the total monthly flow attributed to each day in that
month.
(vi) Use the above fractions to compute the daily flows for each
month from the 10-year low total monthly flows for these four
months computed previously.
The resulting daily flows for the 10-year low flows for January,
February, March and April are shown in Table 4-3.
(__J
4
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-28-
TABLE 4-3
ESTIMATED DAILY STREAMFLOWS OF THE TAZIMINA RIVER FOR THE 10-YEAR LOW FLOW PERIOD
(for the critical months of January, February, March and April)
Streamflows (cfs)
Day January February March April
1 107 94 86 80
2 107 94 86 80
3 107 94 86 80
4 107 94 86 80
5 107 94 86 80
6 107 94 86 80
7 107 94 86 80
8 107 94 86 80
9 107 94 86 80
10 107 94 86 80
11 107 94 86 83
12 107 94 86 83
13 107 94 86 83
14 107 94 86 83
15 107 94 86 83
16 99 93 85 99
17 99 93 85 99
18 99 93 85 99
19 99 93 85 99
20 99 93 85 99
21 99 93 86 101
22 99 93 86 101
23 99 93 86 101
24 99 93 86 101
25 99 93 86 101
26 84 93 86 101
27 84 93 86 101
28 84 93 86 101
29 84 86 101
30 84 86 101
31 84 86
-29-
5.0 PROBABLE MAXIML~ FLOOD
5.1 BASIN CHARACTERISTICS
Basin characteristics for the Tazimina River watershed at the mouth
of Lower Tazimina Lake are presented and discussed in the following sec-
tions. These characteristics are general in nature and represent physi-
ography, soils, vegetation, and climate. Subsequent sections of this
report utilize these characteristics in estirnating the probable maximum
flood (PMF).
5.1.1 PHYSIOGP~HY
Located within the Bristol Bay region of southwestern Alaska, the
drainage basin of the Tazimina River at the outlet of Lower Tazimina Lake
encompasses an area of approximately 273 square miles (figures 2-1 and 5-l).
Bounded on the north, east and south by mountainous terrain, the basin has
a maximum elevation of approximately 6,000 feet (MSL) and a minimum eleva-
tion of approximately 660 feet (MSL). The mean elevation is approximately
2,500 feet (MSL).
The drainage basin (Figure 5-l) is elongated in shape with a length
of approximately 32 miles and an average width of approximately 8.5 miles.
Two large lakes, Upper Tazimina Lake and Lower Tazimina Lake, account for
approximately 4 percent of the basin area. Average gradients for the
Tazimina River range from approximately 171 feet per mile near the head-
waters to 9.5 feet per mile for the reach between Upper and Lower Tazimina
lakes. The total length of the river including both lakes is approximately
36 miles with an overall average channel slope of 65 feet per mile.
5.1.2 SOILS
Geologically, soils within the basin are classified as humic cryorthods
FIGURE 5-1
T AZIMINA RIVER BASIN
I w
0
I
-31-
and rough mountainous lands. The humic cryorthod association occupies
the foot slopes of the mountains and moraine hills, while the rough
mountainous lands with thin and stony soils over the bedrock make up
the steep, rocky slopes (Ref. 7).
Within the mountain foot slope areas, silty volcanic ash, 10 to 24
inches thick, is underlain by very gravelly glacial fill. Valleys and
depressions consist of very poorly drained fibrous peat with shallow perma-
frost. Ridgetop soils are well crained, shallow over bedrock and consist
of silty volcanic ash containing rock fragments.
Conversation with Mr. Louis Fletcher of the Soil Conservation Service
(SCS) in Anchorage indicated that the soils in this area could be placed in
the B or C hydrologic soil classification.
5.1.3 VEGETATION
Vegetation within the basin consists of both wetland and upland types.
In the nuskeg wetland a=eas, which commonly occur within depressions and
vall~y bottoms, the vegetation is predominately sedges and mosses. Upland
vegetation includes forest ranges, woodland and shrubs. Forests of white
spruce and paper birch are dominant on steeper slopes while black spruce
is dominant on more gentle slopes. On high ridgetops and slopes above
treeline, the vegetation is dominated by dwarf birch, low shrubs, willow,
alder, grasses and mosses (Ref. 7).
Ground cover within the forested areas consists chiefly of a thick
moss carpet or dense lichen. Estimates of the percentages of this ground
cover range from 5 percent to 80 percent depending upon the type of forest.
Humus depths vary from 2 to 6 inches.
'Based on USGS topographic maps, the forested area including muskegs
was determined to be approximately 129 square miles or 47 percent of the
entire basin.
-32-
5.1.4 CLIMATE
The climatic characteristics of this region are influenced by both
maritime and continental characteristics and, as such, the region is
generally placed within the transitional zone. Though open to the ocean,
the waters of the Bering Sea are cooler than those of the North Pacific.
This, combined with the lack of major orographic barriers,precludes any
sharp boundary between maritime influences along the coast and continental
characteristics of the interior. The region loses much of its maritime
influence during the winter due to ice cover over a large part of the
Bering Sea.
Low-pressure systems moving northeastward across the Bering Sea are
primarily associated with heavy daily precipitation in the region. The
monthly distribution of large daily precipitation amounts indicates the
heaviest precipitation occurs during the months of July through October
(Ref. 8).
Currently, no weather recording stations are located within the
Tazimina River basin. However, two weather stations, one at Iliamna and
one at Port Alsworth, are located within the region. Both these stations
are located adjacent to large lakes. Lake Iliamna and Lake Clark, re-
spectively. As such, the climatic records of these two stations may be
somewhat affected by the lakes. However, because of the reasons stated
below, the climatologic records at Port Alsworth are assumed to be more
representative of the Tazimina River basin than Iliamna:
o Closer proximity of the station to the Tazimina basin compared
to the Iliamna station.
o Lake Clark being much smaller than Iliamna Lake, its influence
on the climatologic records at Port Alsworth is expected to be
less dominant.
o Port Alsworth is located in a basin which exhibits physiographic
characteristics similar to the Tazimina River Basin.
-33-
5.2 PROBABLE MAXIMUM PRECIPITATION
As input to the development of the PMF hydrograph, estimates of the
probable maximum precipitation (PMP) were made for the basin. Isohyetal
maps and procedures presented in the United States Weather Bureau, Tech-
nical Paper Number 47 (T.P. 47), "Probable Maximum Precipitation and
Rainfall Frequency Data for Alaska" (Ref. 8), were utilized.
Precipitation depths (point values) for 6-hour and 24-hour durations
were estimated from the isohyetal maus and plotted on a depth-duration
diagram obtained from T.P. 47 (Ref. 8). Using this diagram, precipitation
depths for intermediate durations between 6 and 24 hours were determined.
An additional depth-duration diagram from T.P. 47 was further utilized to
determine depths for durations less than 6 hours.
All of these values were then adjusted for the drainage area and a
site-specific depth-duration curve constructed. This curve is shown on
Figure 5-2 and a tabulation of precipitation depths for various durations
is presented in Table 5-l.
5.3 UNIT HYDROGRAPH
Because of a lack of streamflow data for the Tazimina River, synthetic
unit hydrograph methods were utilized to develop the PMF hydrograph. A
synthetic unit hydrograph was developed to represent streamflow conditions
at the outlet of Lower Tazimina Lake. In developing the synthetic unit
hydrograph, it is assumed that the precipitation event occurs within a
specified unit of time and is 'Jniformly spread over the contributing
drainage basin.
Procedures developed by the SCS as described in the United States
Bureau of Reclamation (USBR) publication "Design of Small Dams" (Ref. 9),
and the SCS "National Engineering Handbook, Section 4, Hydrology" (Ref. 10)
were used to develop the unit hydrograph for the basin.
-35-
TABLE 5-1
PROBABLE MAXIMUM PRECIPITATION
TAZI!HNA RIVER BASIN, ALASKA
Duration Precipitation Duration Precipitation
(hours) (inches) (hours (inches)
1 1.9 13 10.9
2 3.7 14 11.3
3 4.8 15 11.6
4 5.9 16 12.1
5 6.8 17 12.3
6 7.5 18 12.5
7 8.0 19 12.7
8 8.5 20 13.0
9 9.2 21 13.2
10 9.8 22 13.4
ll 10.2 23 13.6
12 10.5 24 13.8
-36-
5.3.1 TIME OF CONC&~TfuiTION
Initial input to the hydrograph development consisted of determining
the time of concentration or travel time for the basin. Because of the
presence of the two large lakes, the basin was divided into four separate
reaches and individual times of concentration were calculated and summed
to obtain the total time of concentration. The first reach consisted of
the Tazimina River from the headwaters to Upper Tazimina Lake, the second
reach consisted of the length of Upper Tazimina Lake, the third reach con-
sisted of the Tazimina River between Upper and Lower Tazimina Lake, and
the fourth reach consisted of the length of Lower Tazimina Lake.
For the first and third reaches (river sections), the following equa-
tion (Ref. 9) was used to develop the time of concentration:
where
{_11.9 L3)0.385
Tc =\ H
Tc = time of concentration in hours,
L = length of watercourse in miles, and
H = elevation difference in feet.
For the second and fourth reaches (lake sections), the time of con-
centration was calculated by first determining the flood wave velocity
through the lake and then dividing by the length of the lake. Since the
depth to length ratio for both lakes is less than 0.1, they can be con-
sidered as shallow and the following equation used (Ref. 11):
Vw=lgDm
where
Vw flood wave velocity in feet per second,
')
g =gravitational accelera~ion, 32.2 feet per second~, and
Dm = mean water depth in feet.
-37-
Summation of the individual times of concentration resulted in a
total time of concentration from the headwaters of Tazimina River to the
outlet of Lower Tazimina Lake of approximately 7.5 hours. The individual
reach parameters and times of concentration are presented in Table 5-2.
5.3.2 OTHER PARAMETERS
After determining the times of concentration, the remaining unit: hy-
drograph parameters were calculated. A triangular-shaped unit hydrograph
was assumed and the corresponding equations as presented in Reference 9
were utilized. These parameters are presented in Table 5-3. The shape of
the unit hydrograph and corresponding ordinates are shown on Figure 5-5.
5.4 PROBABLE MAXIMUM FLOOD HYDROGRAPH
The incremental PMP and unit hydrograph ordinates were combined to
produce the probable maximum flood hydrograph. Two PMF hydrographs were
developed; the first using the PMP event alone, and the second using the
PMP event combined with snowmelt. Both hydrographs are shown on Figure 5-5.
Discussions regarding their construction are presented in the following
sections.
5.4.1 SEQUENCE OF INCREME1~AL PRECIPITATION
Since the hourly sequence of a rainfall event cannot be predicted
with certainty, the hourly increments of the PMP hydrograph were rearranged
to produce the optimal sequence of precipitation that would produce the
maximum flood peak. Two separate methods were used:
Method A: Generalized sequence as proposed by the United States Army
Corp of Engineers (USACE) (Ref. 12);
Method B: Optimal sequence as proposed in the Journal of The Hydraulics
Division, ASCE, December, 1978, Technical Note, "Optimal
Sequence of Incremental Precipitation" (Ref. 13).
Reach
1
2
3
4
-38-
TABLE 5-2
TIMES OF CONCENTRATION
TAZIMINA RIVER BASIN
Length
Description (miles)
Tazimina River headwaters to 13.4
Upper Tazimina Lake
Upper Tazimina Lake 8.3
Tazimina River Between Lakes 6.3
Lower Tazimina Lake 8.0
Total 36.0
Times of Concentration
(hours)
2.7
0.2
4.5
0.2
7.6
-39-
TABLE. 5-3
SYNTHETIC UNIT HYDROGRAPH PARAMETERS
TAZL~INA RIVER BASIN
Parameter
Duration
Time of Concentration
Time to Peak
Time of Base
Peak Discharge
1 hour
7.5 hours
5 hours
13 hours
26,500 cfs
-40-
The sequences of incremental precipitation resulting from both the
methods are presented in Table 5-4. 'The peak of the PMF hydro graph using
Method B was approximately 1 percent higher than the one produced by using
Method A. As a result, the rainfall sequence utilizing Method B was used.
A comparison of the flood hydrograph ordinates resulting from each method
is presented in Table 5-5.
5.4.2 DIRECT RUNOFF
Because of infiltration, evaporation and transpiration, some precipi-
tation falling within the basin does not contribute to storm runoff. To
predict the amount of direct runoff resulting from the probable maximum
precipitation, the SCS "Runoff Curve Number Method" (Ref. 10) was utilized.
This method is based on antecedent moisture conditions and soils, vegetation
and runoff characteristics of the basin.
To estimate the curve number applicable for the Tazimina River basin,
the runoff characteristics and the soils and vegetation characteristics
were analyzed independently.
Discharge records from a USGS gaging station located on the Tazimina
River were compared with rainfall records at Port Alsworth. Although con-
tinuous discharge records were available only for the months of July,
August and September, 1981, two prominent storm event hydrographs werz
evident, August 1 to 10 and August 11 to 20. The hydrographs for each
storm are shown on figures 5-3 anc. 5-4, respectively (Ref. 14).
Assuming a baseflcw discharge for ea~h hydrograph, the storm runoff
volume was calculated. Rainfall depths at Par~ Alsworth for the corres-
ponding storm periods (figures 5-3 and 5-4) were assumed to cover the
entire drainage basin upstream of the USGS gage and the volume of rainfall
calculated. Using this volume and the runoff volume, the runoff coefficient
and applicable curve number were calculated for each storm. The first storm
-41-
TlJ3LE 5-4
CRITICALLY SEQUENCED PRECIPITATION INCREMENTS
TAZIMINA RIVER BASIN
Generalized Optimal
Time Sequence Sequence
(hours) (inches) (inches)
1 0.2 0.2
2 0.2 0.2
3 0.2 0.2
4 0.2 0.3
5 0.3 0.3
6 0.4 0.4
7 0.4 0.4
8 0.5 0.5
9 0.6 0.5
10 0. 7 0.7
11 1.1 0.9
12 1.8 1.1
13 1.9 1.8
14 1.1 1 .9
15 0.9 1.1
16 0.7 0.7
17 0.5 0.6
18 0.5 0.5
19 0.4 0.4
20 0.3 0.3
21 0.3 0.2
22 0.2 0.2
23 0.2 0.2
24 0.2 0.2
-42-
TABLE 5-5
COMPARISON OF PMF HYDROGRAPHS USING THE GENERALIZED
AND OPTIMAL SEQUENCES OF INCREMENTAL EXCESS RAINFALL
PMF H::tdrograph
6. Runoff 6. Runoff
Generalized Optimal Generalized Optimal
Time Rainfall Sequence Rainfall Sequence Sequence Sequence
(hrs) (inches) (inches) (cfs) (cfs)
0 0 0 0 0
1 0.01 0.01 53 53
2 0.10 0.10 636 636
3 0.13 0.13 1,908 1,908
4 0.16 0.24 4,028 4,452
5 0.25 0.26 7,473 8,374
6 0.36 0.37 12,740 14,171
7 0.38 0.37 19,160 21,068
8 0.48 0.48 27,004 29,389
9 0.58 0.49 36,544 38,240
10 0.69 0.69 47,588 48,509
11 1.08 0.88 61,256 60,256
12 1. 79 1.09 81,138 74,592
13 1.89 1. 79 106,904 94,283
14 1.10 1.90 133,537 119,857
15 0.89 1.09 159,276 145,596
16 0.70 0.70 179,855 167,897
17 0.50 0.60 188,198 184,785
18 0.50 0.50 183,741 189,769
19 0.40 0.40 173,124 181,735
20 0.30 0.30 157,690 167,129
21 0.30 0.20 139,407 149,144
22 0.20 0.20 119,798 128,675
23 0.20 0.20 99,228 107,244
24 0.20 0.20 79,846 86,341
25 63,809 66,463
26 51,452 50,790
27 41,016 44,662
28 31,806 30,481
29 23,192 21,867
30 22,198 15,241
31 10,934 10,272
32 6,959 6,627
33 3,977 3,977
34 1,989 1,989
35 663 663
36 0 0
u z .....
z .....
....J
....J
c:::r::
1..1..
z -
(./)
1..1..
u
z:
I.J..J
<:..0
ex:::
c:::r:: = u
(./) -0
-43-
o.-~---~~~~~---~--~~H .;q~: r·•·•·•··::•j
0. 5 -+--+--1--,-~ i . i ~ +,----+---4
1. 0 -+--..______,_0 0 -~----+-----!
1.5~-+-~-~;+]+-~+:~~~-~--+-~
~ ~ ~ ~ }~ ~~~ ~~ ~~ j
(./) 2.5 I.J..J = u z: .......
z: 2.0 -
....J
....J
<( l.S 1..1.. z -c:::r:: e:::
1.0
0
I..L.I . 1-
c:::r::
....J 0.5 ::>
::E
=:;)" u u
c:::r:: 0
2. 0 -+----!---+-.~ _,__...,___----+-~
l
i
' . i ..... : l 2.5._------~------_.--~--~--J
29 30 31 1 2 3 4 31 1 2 3
DATE DATE
HYETOGRAPH MASS-RAINFALL
3000
2000
1000
I IV'l'.~~~-1
r-.....J
1
1 LJ ESTIMATED BASE FLOW Lli , .. ~. -
I ~ I I Lf-I ! i ·• ....... -+---+1-~-T~I-ll -~ -~ I I ~~""'-~
1 \ASSUMED BEGINNING .ESTIMATED _..-/
I
I
RAINFALL EXCESS RECESSION CURVE
l
I
'
I 0 ~2~9~~3~0~3~1~~1~~2_._3~~4_._5~~6~--7~-8_._9--~10~-1-1~
JULY 1981 AUGUST 1981
HYDROGRAPH
DRAINAGE AREA =327 SQUARE MILES
VOLUME RAINFALL =2.07 INCHES
VOLUME RUNOFF =0.72 INCHES
5-DAY ANTECEDENT RAINFALL:0.38 INCHES
ESTIMATED RUNOFF CURVE NUMBER : 83
SOURCE:
RAINFALL FROM PORT ALSWORTH STATION
HYDROGRAPH FOR USGS RECORDS TAZIMINA
RIVER, STATION AT RIVER MILE 11.6
FIGURE 5-3
T AZIMINA RIVER BASIN
HYDROGRAPH AND
RAINFALL AUGUST 1-10
u z
z -
-
Vl
w_
u
z ,_
I..U
<:.lJ
0:::
<
-'-u
Vl ,_
Q
-44-
0
. .
0.5 = c .,_ .,_
..... 0 ..... --N
a~4,····:,:r1 i r·····:··
.,_1.,_1.,_
~ ~ r::;i:: . . • l •
1.0 0 0 0 o o o I o
1.5
2. 0 -t--+-----.--+----+--+----1-~
3000
2000
1000
8 9 10 ll 12 13 14 15
DATE
HYETOGRAPH
I
ASSUMED BEGINNING
RAINFALL EXCESS
Vl 2 . 5 ..,--..,.--.,.--,.__,,___,
I..U
:I: u z
<
0::: 1. 0 +-+--r-+--~~---i.~
Q
I..U
1-< :5 0 . 5 +--t-...i-r-.i..-f----1--+---4
:::E:
:::::l u u < Q..o;;;........._..__...._.__L...-J.--l.--1
.g 10 1112 13 14 15 16
DATE
MASS-RAINFALL
HYDROGRAPH AUGUST 1981
DRAINAGE AREA =327 SQUARE MILES
VOLUME RAINFALL =1.36 INCHES
VOLUME RUNOFF =0.33 INCHES
5-DAY ANTECEDENT RAINFALL=0.04 INCHES
ESTIMATED RUNOFF CURVE NUMBER = 84
SOURCE:
RAINFALL FROM PORT ALSWORTH STATION
HYDROGRAPH FOR USGS RECORDS T.A.ZIMINA
RIVER, STATION AT RIVER MILE 11.6
FIGURE 5-4
T AZIMINA RIVER BASIN
HYDROGRAPH AND
RAINFALL AUGUST 11-20
-45-
resulted in a curve number of 83 and the second in a curve number of 84.
The antecedent precipitation amounts for both storms were such that
AMC II conditions could be assumed. Since the occurrence of a PMF event
assumes AMC III (saturated) conditions, the curve numbers obtained were
adjusted for this condition. This resulted in a curve number of 93 for
the basin.
As an independent check, a curve number was calculated using the
available soils and vegetation data. AssuMing that 47 percent of the
basin is forested, 4 percent water-covered and the remaining 49 percent
mountainous, a weighted curve number of 79 was determined for AMC II con-
ditions. This corresponds to a curve numbe= of 91 for AMC III conditions,
slightly less than the 93 developed from the rainfall-runoff analysis. The
data and assumptions used in this analysis are presented in Table S-6.
To develop a conservative estimate of the probable maximum flood, the
curve number was adjusted upward to 95 to reflect frozen ground and/or
snow-covered conditions. Also, because of the shallow permafrost depth,
it was assumed that any deep percolation would be negligible.
The PMF hydrograph resulting from the PMP alone is shown on Figure 5-5
along with the corresponding rainfall hyetograph. The peak discharge is
approximately 190,000 cfs and the runoff volume is approximately 12.9 inches,
cr 187,800 acre-feet. This corresponds to a 24-hour rainfall depth of 13.8
inches.
5.4.3 SNOWMELT RUNOFF
Since the possibility exists for a PMP event to occur at a time when
the basin is covered by snowpack, a PMF hydrograph was developed combining
both rainfall and sncwmelt runoff. Because snowpack and water content
data are not available for the basin, a generalized equation developed by
the USACE was used to estimate snowment resulting from rainfall on snow.
Land Type
Forest
Lakes
·Mountains
-46-
TABLE 5-6
RUNOFF CURVE NDMBER ANALYSIS
BASED ON SOILS .~ VEGETATION DATA
TAZIMINA RIVER BASIN
Percent of Hydrologic Curve
Basin Soil Group Number
47 B 70
4 100
49 c 85
Weighted
Curve Number
33
4
42
Actual Weighted Curve Number 79
V) 0
w :c u z::
1-t 1 0
z::
1-t-
z::
0 2 0 1-t
I-c::t:
I-
1-t
0... 3 0 1-t u w
0::
0...
240
I 220 I
L
200
180
V) 160 LL..
u
0
0 140 0
.-I
z::
1-t 120
w
I t!J I I 0::
c::t: 100 :c
u
I I V)
I 1-t
0 80
60
40
20
0
I
t .; ,;,,. ~ ~ J l J. ' ' ~ < r -~ ~ ' ~ ~ ~ ~ ~ ! L:.~~ •• «. ~
.. ... ,.,.."',..,.. ,.,.., .,., r '---7-+ ' ~------'.--:...........; .. =--1 ~ ~ ~ l:: ,......r,o) ;;----, ...........
I r r
I'
f ·:; NOTES
,DRAINAGE AREA =273 SQUARE MILES
VOLUME RAINFALL =13 8 INCHES ~-~~T~~ 1 l
.... U~-RAINFALL
VOLUME RAINFALL+ SNOWMELT =18 6 INCHES
VOLUME RUNOFF RAINFALL =
0 2 4 6 8
._._._,_~ _sNOWMELT
I I
I I
10 12 14 16 18
DURATION IN HOURS
I I
20 22 24 26
12 9 INCHES 187800 ACRE-FEET
VOLUME RUNOFF RAINFALL+ SNOWMELT=
17 7 INCHES 257,700 ACRE-FEET
RUNOFF CURVE NUMBER = 95
ANTECEDENT MOISTURE CONDITION = III
24-HOUR SYNTHETIC STORM HYETOGRAPH
2 5,oob cFJ ~I
/ "' / '\
If \
I ~ 190,000 CFS~
I , i\ \ ,c
I 1/ \ \
I . ~-' I \ 1\ ~PROBABLE MAXIMUM FLOOD
I I \ y FOR RA
1
INFALL EVENT (PMP)
1
I I \I \
I I ' I I I I
\ ~PROBABLE MAXIMUM FLOOD
I \ FOR RAINFALL (PMP) +
I I \ ~ SNOWMELT EVENT
I I ~ \ ..
ll.
1/ / r\ ' J 'I e 1\ ' \
/ l \ \
)f I ' "., \ ~
/ / ~ ~ v ./ '. \. 1 .... '\.
/ / ~ ~
~/ ./
,. ..... '" r-...
~ 7' -~ ~ .
0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40
DURATION IN HOURS
24-HOUR PROBABLE MAXIMUM FLOOD HYDROGRAPH
\
I
V)
LL..
u
z
1-t
w
t!J
0::
c::t:
:c
u
V)
1-t
0
16
14
V) 12 w :c u :=; 10
z::
1-t
:c 8
1-
0...
~ 6
z
1-t
~ 4 co
w
~ 2
0:: w
~>
c::t: 0
30000
25000
20000
15000
10000
5000
0
I
,___, ~
PROBABLE MAXIMUM l.,.....--'
,..,---
~ PRECIPITATION " v ~ FROM U S WEATHER ./ r--BUREAU T P 47 /
/
/
/
ESTIMATED SNOWMELT / I
I RUNOFF \ / ---__.....
I _J-~ ----~
I~ ~ ~
L---L---
0 2 4 6 8 10 12 14 16 18 20 22 24
DURATION IN HOURS
DEPTH-DURATION CURVE
26500 CFS ~
1\ I
I ' I
\ TIME OF
/; CONCENTRATION ___
I \ = 8 HOURS
~ v \
I 1\
I ' ~
0 2 4 6 8 10 12 14 16
DURATION IN HOURS
UN IT HYDROGRAPH
ORDINATES
TIME DISCHARGE
(HOURS) (CFS)
1 5300
2 10600
3 15900
4 21200
5 26500
6 23188
7 19876
8 16564
9 13252
10 9940
11 6628
12 3316
13 0
1-HOUR SYNTHETIC UNiT HYDROGRAPH
FIGURE 5-5
TAZIMINA RIVER BASIN
PROBABLE MAXIMUM
FLOOD HYDROGRAPH
I
~
-....J
I
-49-
The PMF hydrograph resulting from this condition is shown on Figure 5-5
along with the corresponding rainfall plus snowmelt hyetograph. The peak
discharge is approximately 225,000 cfs, and the runoff volume is approxi-
mately 17.7 inches, or 257,700 acre-feet. This corresponds to a 24-hour
rainfall plus snowmelt depth of 18.6 inches.
-50-
6.0 REFERENCES
1. Methodology for Estimating Pre-project Streamflows in the Tazimina
River, Alaska, Arctic Environmental Information and Data Center
(AEID), Anchorage, Alaska, January, 1982.
2. Bristol Bay Energy and Electric Power Potential, Phase I, U.S.
Department of Energy, Alaska Power Administration, December, 1979.
3. Climatological Data, Alaska, Volume 67, National Oceanic and
Atmospheric Administration, Asheville, North Carolina, 1981.
4. Water Resources Data for Alaska, U.S. Geological Survey Water-
Data Reports for Different Water Years.
5. Statistical Analysis for Business Decisions, W. A. Spurr and C. P.
Bonini, Richard D. Irwin, Inc., Homewood, Illinois, Revised Edition,
1973.
6. Handbook of Applied Hydrology, V. T. Chow, McGraw-Hill Book Company,
1964, Sections 8 and 10.
7. Exploratory Soil Survey of Alaska, U.S. Department of Agriculture,
Soil Conservation Service, February, 1979.
8. Probable Maximum Precipitation and Rainfall Frequency Data for
Alaska, Technical Paper (T.P.) 47, U.S. Weather Bureau, 1963.
9. Design of Small Dams, U.S. Department of the Interior, Bureau of
Reclamation, Revised Print, 1977.
10. National Engineering Handbook, Section 4, Hydrology, U.S. Department
of Agriculture, Soil Conservation Service, August, 1972.
11. Open Channel Hydraulics, V. T. Chow, McGraw-Hill Book Company, 1959.
12. Standard Project Flood Determinations, EM 1110-2-1411, Civil Engineer
Bulletin No. 52-8, Department of the Army, U.S. Corps of Engineers,
Washington, D.C., March, 1965.
13. Optimal Sequence of Incremental Precipitation, Anand Prakash, Journal
of The Hydraulics Division, ASCE, December, 1978.
14. Provisional Streamflow Records (Subject to Revision), Tazimina River
near Nondalton, Station 152999000, Water Year 1981 (Personal Communi-
cation).