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Home My WebLink AboutAPA457I < ~~-·----------------------~--------------------~ SUSITNA HYDROELECTRIC PROJECT FED.:RAL ENERGY REG~;'lATORY COMMISSION PROJECT No. 7114 WATANA AND DEVIL CAN'tON SITES PROBABLE MAXIMUM FLOOD ~ l [M]~ifJZ£~=~~~~~© I I susJTNA JOINT ve,: ;uRe DRAFT REPORT JANUARY 1984 DOCUMENT No. 457 <-----~ALASKA POWER AUTHORITY _ ___. >< -:J z u l. l.. ~ SUSITNA HYDROELECTRIC PROJECT PROBABLE MAXIMUM FLOOD FOR WATANA AND DEVIL CANYON SITES Report by Harza-Ebasco Susitna Joint Venture Prepared for Alaska Power Authority Draft Report. January 1984 Document No. 457 TABLE OF CONTENTS (/) W -Ico«..... (f) ~-rtJ-J:><W 6-1 6...1 6-1 5-5 6-1 5-5 5-5 5-5 5-3 5-4 5-4 5-5 5-1 5--1 5-1 5-1 4-6 4-7 4-7 4-7 4-4 4-5 4-6 4-5 PAGE 4-5 4-5 4-6 Watershed Parameters Unit Duration Unit Hydrographs Watershed Parameters of the Basins Lag Tim-es Lag Cux-v,;:. Historic Flood Events Derivation of Dimensionless Graphs Serial Numbers Shortwave Radiation Long-wave Radiation Prior to the PMP During the PMP After the PMP --ii- 5.3.1 5.3.2 5.3.3 5.2.1 5.2.2 5.2.3 5.1.1 5.1.2 4.5.1 4.5.2 4.5.3 4.4.1 4.4.2 4.4,,3 4.3~1 Prior to and During the PMP 4.3.2 After the PMP TABLE OF CONTENTS (Cont'd) 5.3 Sub-basin Unit Hydrograph b"l Hydrologic Soil Groups 6.2 Permafrost 6 ..3 Infiltration Rates INITIAL ESTIMATE OF INFILTRATION RATES 5.2 Lag Curve 5.1 Dimensionless Graphs 4,,6 REC-l Snowmelt Sub-routine 4.5 Radiation 4.4 Dewpoints 6.0 5.0 UNIT HYDROGRAPHS SECTION/TITLE •• 7-5 7-5 7-6 7....6 7-5 7-1 7-3 7-3 7-4 7-4 7-4 8-1 7-1 7-2 7-2 7-2 9-1 PAGE 11-1 11 ....2 11-2 11-3 11-3 11-1 10-1 Observed Rainfall and Its Time Distribution Observed Hydrographs Unit Hydrographs Routing Coefficients Precipitation Infiltration Losses Unit Hydrographs Basef1 ".).f Channel ROtlting ....iii.... PMP Snowpack Water Equivalent Temper'4 t!re Sequences Rainf~Ll -Runoff Rel~tionship PMF 7.5.1 7~5.2 7.5.3 7.5.4 7.4.1 7.4.2 7.4.3 7.4.4 7.4.5 TABLE .OF CONTENTS (Cant'd) 11.1 11.2 11.3 11.4 11.5 7.5 Analysis of the Reconstitution 7.1 HEC-l Computer Program 7.2 Selection of Flood Events 7.3 Storm Precipitation 7~4 Reconstitution toO PROBABLE MAXIMUM FLOODS 9.0 COMPARISON WITH FLOODS OF RECORD APPENDIX A TA.BLES REFERENCES EXHIBITS 10.0 COMPARISON WITH 100-YEAR FLOOD 11.0 COMP~A.RISONWITH PREVIOUS PMF STUDIES 7.0 RECONSTITUTION OF HISTORIC FLOODS SECTION/TITLE -iv- • • ".'1#.. Variation in Greatest Observed One--Day Precipitation of Record for Stations near the Susitna Basin with 10 or More Years of Record Adjustment of Base NOl~-Orographic PMP for Orography in Watana Basin Variation of Greatest Monthly Precipitation Variation in Precipitable Water Associated with Maximum 12-Hour Persisting Dewpoints in the Susitna Basin Sub-basin Rainfall Variation of Snowpack with Elevation Maximum Daily Snowmelt Winds (mph)at Anemomete:c Level 3 Selected Stream Gaging Stations 1 Average Summer and Winter Flows at Selected Stream Gaging Stations 4 Snow Survey Stations 2 Average Precipitation and Temperatures at Selected Climatological Stations 6 6-Hour Accumulated and Incremental PMP LIST OF TABLES 5 Computation of Barrier Adjustment for Tra~sposing the August 12-14, 1967 Storm to the Susitna Basin NO.TITLE 9 7 8 10 11 12 13 14 Arranged Daily Snowmelt Air Tempera.tures,Dewpoints and Wind forMayPMP 15 Arranged Daily Snowmelt Air Temperatures,Dewpoints and Wind forJunePMP 21 Selected Flood Events NO.TITLE •• • -v- Comparison of PMF Studies for Watana Dam Site Maximum Instantaneous Historic Flood Peaks -South Central Alaska Summary of Probable Maximum Flood Initial Loss and Infiltration Rates Based on Reconstitution of His- toricFloods 31 32 33 29 Reconstitution of Sept.ember 11-22,1982 Flood LIST OF TABLES (Cont'd) 28 Reconstitutiol'of July 4-15,1981 Flood 30 27 Reconstitution of July 25-31,1980 Flood 25 Adopted Retention Rates for Sub-basins 26 Sub-basins Precipitation for Reconstitution of Historic Floods 24 Soils Description 23 Watershed Parameters of Sub-basins Upstream from Gold Creek 22 l-latershed Parameters of Basins above Selected Stream Gaging Sta- tions 20 Variation of Snow-pack and Mean Annual Precipitation with Elevation Zones in the Sub-basins. 18 Daily Net Radiation (Langleys)for May PMP 19 Daily Net Radiation (Langleys)for June PMP 16 Maximum Daily Temperatures Prior to Spring PMP 17 Dewpoints (OF)for 3-day Mid-MonthPMP Storm 14 Seasonal Variation of PMP • ."....--I.,,,.>2:.:~:~..,--..~~~-_.:-~t ! I,•.~I~IiiI:.:., 1 m I I I 1"fi h LIST OF EXHIBITS Dimensionless Graphs,Willow Creek ur.Willow "'vi- Ma~imum 24-Hour Winds for Snowmelt Season Dimensionless Graphs,Talkeetna River nr.Talkeetna Analysis of August 11-14,1967 Storm by Isopercental Method Analysis of August 23-25,1955 Storm by Isopercental Method Dimensionless Graphs,Eagle River at Eagle River Dimensionless Graphs,Maclaren River near Paxson Dimensionless Graphs,Caribou Creek near Sutton Depth--Duration Curve Mean Annual Precipitation Versus 6-Hour,10 square mile Probable Maximum Precipitation Elevations of Moisture Inflow Barriers Mean Annual Precipitation Annual Flood Peak Discharges,Susitna.River at Gold Creek Isohyeta.l Pattern of August 12-14,1967 Storm Climatic Stations and Snow Courses Location and Orientation of August 12-14,1967 Chena Storm over Susitna Basin Stream Gag::'ng Stations Devil Can7Ton and Watana Basins Susitna River Basin Streamflow Characteristics of Susitna River Above Gold Creek Sta- tion TITLE 6 9 8 7 4 5 2 3 1 21 20 19 17 16 18 13 15 12 10 11 • 'L.....:~'~.Q;.f.h .ul,a IE • Reconstitution of July 1980 Flood Reconstitution of July 1981 Flood Isopercent.al Map of September 11-22,1982 Storm Isopercental Map of July 4-15,.1981 Storm Isopercental Map of Ju.ly 25-31,1980 Storm Isohyetal Map of September 11-22,1982 Storm Isohyetal Map of July 4-15,1981 Storm Isohyetal Map of July 25-31,1980 Storm Soil Classification Sub-basin Unit Hydrogra.phs Lag Curve 36 35 34 33 38 Channel Routing Scheme with Muskingum Routing Coefficients -vii- 39 PMF Inflow Hydrographs at Watana 37 Reconstiution of September 1982 Flood 40 PMF InfloW Hydtographs at Devil Ca.nyon 41 Draina.ge Are,s,Versus Maximum Unit Discharge LIST OF EXHIBITS (Conttd) 42 Flood Frequency Curve,Su.sitna River at Gold Creek 44 Temperature Sequences for Snowmelt Computations 43 PMP for Watana Ba.sin 32 31 30 29 28 Sub-basin Permafrost Map 27 Hydrologic Soil GroupS 26 25 24 23 Average Dimensionless Graphs 22 Dimensionless Graphs,Deshka River nr.Willow NO.TITLE .. ....I-0:oa..wa: enw ..Jen<: I- SECTION 1 INTRODUCTION •• enw --Im«1- A.On PMP l~O INTRODUCTION • 1-1 3.It Was assumed that the August 8-17,1967 storm could OCCur in mid-June and was ma'Ximized to obtain the PMP.It was used to combine with art unlimited snow and ice water equivalent for glaCial sub-basins a~'1d a very conservative.snoWpack in Indicates reference at the end of te'Xt. 20 An isohyetal map of a single storm (July 1980)was used as the basis for preparing isohyetal maps of the storms used in theanalyr ~•The general practice is to use a mean seasonal or meart annual isohyetal pattern fnr this purpose. This report presents the results of a study made to estimate probable m.axi- m.Uin floods (PMFs)at two potential damsites,Watana and Devil Canyon,for the Susitna Hydroelectric Project.The derived PMFs will be used to select Q'design floods for spillways and other related facilities at the two sites. 1.Six storms which occurred wi.thin the Susi '~na River basin 01' its vicinity were used in the analysis.Transposition of storms from outside of the basin but withi.n the region of simi.lar hydrometeorological characteristics was not consi- dered.This limits the size of samples used In the analysis. A PHF study for the two sites was conducted by the U.S.Army Corps of Engi- neers (CaE)in 1975 (1)11 and 1979 (2).The CaE study was reviewed by Acres American Incorporated (ACRES)to determine its adequacy and it was concluded that a further anlaysis of PMF was required in view o.f its sensi- tivi ty to snowpack and probable m.aximum precipi tatlon (PMP).The revised PMF a.nalysis by ACRES,is presented in their feasibili ty report (3).This analysis,however.,has the following weaknesses: 11 1-2 1.1 SCOPE OF THE STUDY • ~ I I fI I•I .... other sub-basins.nlis tends to yield an overly conservative estimate of PMF. Probably,realizing the inaccuracy in the isohyetal pa ttern, the sub-basin average rainfalls were determined by Thiessen's l.1ethod. 4. L.The "Streamflow Synthesis and Reservoir Regulation (SSARR)" computer model (4)was used by ACRES to develop the PMF hy- drographs at Watana and Devil Canyon sites..The model was calibrated for the Susi tna basin (1,2)using two flood events of August 1967 and June 1972 and verified using the floc£i of 1971.The model utilizes seven empirical relation- ships involving 12 parameters to define the rainfall-runoff relationships I"A l:'eliable estimation of the parameter values requires the calibration of the model for more flood events. 2.The overland and channel routing scheme (4)used in the model to define flood hydrographs at desired locations also needs to be ascertained from a number of flood events .. B.On Rainfall-Runoff Relation Because of the above weaknesses,the present study was underta.ken to upgrade the ACRES 'Study for a better ~stimation of PMF which ca.n be confidently used for design purposes. The study ptovides major refinements ill the estimation of PMP and its areal and time distri.bution.Seasonal variabili ty of PMP also is considLl.-ed. SnoWll\elt criteria are prOvided to c01ll.b:tne snolilnelt With J:>MP during the snow- melt periods.Unit hydrographs are developed and refined through a calibra- Three major glaciers -West Fork Susitna,East Fork Susitna and Maclaren, exist in the basin.The landscape consists of barren bedrock mountains, glacial till-covered plains and exposed bedrock cliffs in canyons and along streams.Soils are typical of those formed in cold,wet climates and have developed from glacial till and out-wash.They include the acidic,sa tu- rated,peaty soils of poorly drained areas,the acidic relatively infertile soils of the forest and gravels and sands along the river(6).The baSin is generally underlain by discontinuous permafrost. 1.2 ..2 The River The Susitna River originates in the East Fork and West Fork Susitna Glaciers at an al titude of ab out 7,800 f t ,msl and travel s a di stance of ab out 318 miles before discharging into Cook Inlet.The head waters of the Susi tna River and the major upper basin tributaries are charac terized by broad" braided,gravel flood plains below the glaciers.Several glaci.erized streams e.."ti t from beneath the glaciers before they combine further down- stream.The l'1est Fork Susitna River joins the main river about 18 miles downs tream from the Susi tna Glaciers.Below this confluence,the river develops a split-channel configuration with numerous islands and is general- ly constrained by low bluffs for about 55 miles (6). The Maclaren River draining the Maclaren Glacier and a few small lakes,and the non-glacial Tyone River draining Lake Louise and swampy lowlands of the South Eastern part of the basin,join the main river downstream of Denali (Exhibit 2).Below this confluence,the river .flows "West for about 96 m.iles th~ough steep-walled canyons before reaching the mouth of DeVil Canyon.The major tributaries entering this reach are:Black Rivet',Goose Creek,Kosina Creek,Watana Creek and Tsusena Creek. River gradients average about 14 feet per mile (ft/mi)in a 54-m.ile reach upstream of Watana,about 10.4 ft/tni frOm Watana to the entrance of Devil Canyon and about 31 ft/llli in a 12-mile reach bel:l'leen Devil Creek and the outlet of DeVil CanYOn (6). 1-4 •II The Susitna River is a typical natural glaci.al river with high turbid summer flow and low,clear winter flow.The river generally sta.rts rising in early May.The high flows during July throaghS~ptember are associated with gen- eral frontal type or thunderstorm c:lctivities..The May through June flows are caused by snowrr..el t combined wi th rainfall.The average summer and win- ter flows at a few selected stream gaging stations are given in Table 1. Exhibit 2 shows the locations of the stream gaging stations.Exhibit 3 shows the general streamflow characteristics of the river (7).Recorded annual peak flows of the Susitna River at Gold Creek are shown on Exhibit 4. The river flbW rapidly decreases in November or December as the river fl"eezes..The break-up generally occurs in early May. The t'iver cat't'ies a significant amount of suspended sediment dUt'ing flood season.Glacial silt mostly contributes to higher concentration in summer. 1.2.3 Sub-basins The dt'ainage basin upstreanl of the Devil Canyon damsite was divided into ten sU;'~basins as shown on Exhibit 2.The boundades of the sub~basi.ns were selected at Watana damsite,at stt'eam glE\gi.ng stations upstrealll.ft'om Watana and downstream of major tt'ibut;lt'ies entet'ing thE!nla:l.n dYet'o The PUt'pose of the sub-division was: 1.To pt'oper1y acCOunt for thE!areal variatiou of the PMP as i.ndicated by the isohyetal pattern; 2.To diffet'entiate betwMn ptedominantly glacial area (S\lb~basins 8 and 10),SWampy atea with lakes (sub-basin 5)and 0 thet ateas; 3.to have sUb~basins of reasonablE!si!:E!for developing unit hydro- graphs;and 4.To acCOunt fot'storage effect of the main dYer channel. 1-5 ..II [ 'If Q ) I 1 f { I....'. tlCf.:..I~F'" I I", II, r t i~ 1'··1;. '~L~'.': li~ I 1 I L I I 1 i~o E 1..L. :( 1.3 CLI~1ATE The climate of the Susitna River basin is typical of interior Alaska.The winters are long,summers are short and there is considerable variation in daylight betwe.en these seasons. Mean annual precipitation is about 20 inches in most of the southeastern part,which increases to about 40 inches near Devil Canyon and to about 70 inches over the northeastern watershed divide near Susitna Glacier (Exhibit 5)•The mean temperature ranges between about -5 OF in Winter and 55 OF in summer.Table 2 gives mean monthly temperatures and precipitation at se- lected stations in the Vicinity of the basin.Exhibit 6 shows the locations of the climatologicel stations. 1.4 HYDROMETEOROLOGICAL NETWORKS 1.4.1 Stream Gaging Stations u.s.Geological Survey (USGS),Water Resources DiVision,Alaska is maintain- ing a number of stream gaging stations on the Susitna River and its major tributaries..The gaging stations selected to study '(;iinfall-runoff charac- teristics of the baSin and to develop unit hydrographs representative for the basin above Devil Canyon,are listed in Table 3 and shown on Exhibit 2. R&M Consultant Incorporated (R&M),Alaska established a gaging station on the SUs:ltna River at Watana as part of the ill1restigation program for Susitna Hydroelectric Project.'!'he data at this station are of short period and hence not used in this study..a&M also established a number of staff gages upstream frOm the Devil Canyon and Watana damsites for a short period to study hydraulic properties of the river. 1-6 1.4.2 Climatological Stations u.S.Na tiona!Weather Service (NWS)is maintaining a number of precipi tation and temperature stations in the.Susitna basin (Cook Inlet Climatic Sector) and its vicinity (Central and South Central Sectors).However,there is no station located in the Susitna basin upstream from the Devil Canyon damsite. The stations are located either along railroad and highway routes or places easily accessible by ground transportation.Some of these stations perti- nent to the current study are shown on Exhibit 6. R&M,as part of the investigation program for Susitna Hydroelectric Project, established seven climatological stations:Sherman,Devil Can.yon,Watana, Kosina Creek,Tyone,Denali and Susitna Glacier during 1980--1981.These stations also are shown on Exhibit 6.The climatic elements observed at each station include:precipi tatiOD (daily and hourly),solar radiation, temperatures,dewpoints,wind velocity and direction and sunshine hours. 1.4.3 Snow Stations and Snow Courses u.s~S011 Conservation Service (SeS)is maintaining a number of snow courses (see Table 4)wi{;hin or in the vicinity of the Susitna basin.The stations are shown on Exhibit 6.The data for the first six stations are available for about 14 to 19 years (up to 1981)while the other stations have only two to six years 0.£data. The NWSAlaska Region also has sc,me measurements of snow water equivalents at a number of stations 1vithin or in the vicinity of the Susitna basin.but the data at the stations of interest have frequent discontinuities. 1-7 •.. .. ....'.. .• II...,... SECTION 2 GENERAL APPROACH ..,......' I I . .. .. I '•.. .. •• C/) W ...Jco« I- (J).....-OJ-:t:><W ><-:l 2: U l. l. ( 2 •.0 GENERAL APPROACH The PMFs for the two damsites are.derived by estimating the PMP through a hydrometeorological analysis and applying the PMP to the unit hydrographs 0 This involved primarily estimation of PMP by transposition and moisture maximization of maj or historic storms)estimation of snowmelt based on snow course and meteorological data,estimation of infiltration rates based on soil type,vegetative cover and permafrost,derivation of unit hydrographs based on recorded rainfall and flood data,convolution of rainfall excess increments and unit hydrographs)routing of sub-basin flood hydrographs through the channel system and estimation of baseflow. For the purpose of this study,the Susitna River basin above Devil Canyon was divided into ten sub-basins as disc\1ssed under "Sub-basins".In the following discussions)the basins upstream of the Watana and Devil Canyon damsites are referred to as "Watana Basin"and "Devil Canyon Basin",re~ spectively.When both basins are involved)they are referred to as"the Basins".Exhibit 7 shows the general topography of the Basins. The P:iP estimates were derived based on transposition and moisture maximiza- tion of the Chena storm of August 12-14,1967 for a duration of 72 hours. Two storms could occur in sequence within a much longer period to result.in a larger total rainfall.While such a sequence may be critical for a larger basin,the duration of 72 hours was considered to be sufficiently long for the Basins under study. A depth-duratJJn relationship Was developed for the 72-'hour PMP to provide maximum depths for shorter durations.A "bell-shaped"distribution of PMP increments was adopted for sequential maximization..Seasonal variation of PMP also was estimated. Estimation of snowmelt was also necessary to determine the most critical combination of PlviP during the IT'.lelting period and the snowmelt.In the re- gions of high latitudes,such a combination in later spring Or early Summer 2-1 • ,I d.. • ., .It.. ..IJ 2-2 -may cause a.more severe flood than the all--seasonPHP (greatest rain of any sea.son,likely to occur in July-August)alone.Therefore,criteria were de-- veloped for snowmelt computations..These criteria included:sequences of critical air temperatures,dewpoint temperatures (indices to the air moist- ure content),winds and solar radiation.Values of those parameters were computed for the duration of the PMP storm as well as for critical days prior to and after the Pr-1P..In addition,the most severe snowpack that c.an be possible before the flIP storm was estimated. A unit hydrograph was derived for each sub-basin using the dimensionless graph-lag curve techniques.Infiltration rates were initially estimated ror each 3ub-basin using i.nformatton :on soil types,vegetative cover and perma- frost.These rates were refined by simulating historic flood events re- corded at the stream gaging stations on the Susitna River upstream from and at Gold Creek.The simulation was performed by using the HEC-l computer program (5). The HEC-lcomputer program (5 )was also used to derive the PMF at Watana and Devil Canyon sites resulting from the PMP. For comparison with the flood resulting from July-August PHP,floods also were derived using mid-May and mid-June PMP in combination wi th a critical snowmel t prior to,during and after the PMP.The most severe floods were adopted as thePMFs for the Basins. may cause a more severe flood than the all-season PHP (greatest rain of any season,likely to occur in July-August)alone.Therefore,criteria were de- veloped for snowmel t computations.These criteria included:sequences of critical air temperatures J dewpoint temperatures (indices to the air moist- ure content),winds and solarra.diation.Values of those parameters were computed for the duration of the PMP storm as well as for critical days prior to a.nd after the P~!P..In addition,the most severe snowpack that can be possible before the flIP storm was estimated .. A unit hydrograph was derived for each sub-basin using the dimensionless graph-lag curve techniques.Infiltration rates were initially estimated for each sub-basin using information 011 soil types,vegetative cover and perma- frost.These rates were refined by simulating historic flood events re- corded at the stream gaging stations on the Susitna River upstream from and at Gold Creek..The simulation was performed by using the HEC-!computer program (5). The IiEC-l computer program (5)was also used to derive the lnviF at Watana and Devil Canyon sites resulting from the PMP. For comparison with the flood resulting from JUly-August PHP,floods also were derived using mid-May and mid-June PMP in combination wi th a critical snowmelt prior to,during and after the PMP.The most severe floods were adopted as the PMFs for the Basins. 2-2 .. .. .. SECTION 3 PROBABLE MAXIMUM PRECIPITATION ow •to C/) W ...Jm<.... .- ! .. 3.0 PROBABLE MAXIMUM PRECIPITATION PMP is defined as "the theoretically greatest depth of precipitation for a given duration tha.t is physically possi.ble over a particular drainage area at a certain time of year"(8).A recent NW~publica tlon (9)gives a slightly modified version of this definition by substituting the words "over a given size storm area at a particular geographic location"for the words "over a particular drainage area."This modification,which has been agreed to by the COE and the u.s."Bureau of Reclamation (BDREC),is based on the consideration that a storm isohyetal pattern cannot coincide exac tly wi th the shape of a basin.There are as ye.t some unknowns wi th regard to meteo- rological parameters that are important to rainfalls.It is also uncertain how these paramete~s interact to cause extreme situations.Because of these short comings it is customary to refer to PMP values as estimates. 3.1 ESTI~~TION OF PMP 3.1.1 Methods of Estimation For the relatively flat regions typified by much of the U.S.east of the 105thmeridian,estimation of PMP has become quite standardized.First, major rains of record are thoroughly analyzed.Then the pertjnent ones for a particular region of concern are transposed to the region and adj11sted for possibly greater moisture than was available in the original storms.Smooth curves enveloping these storm rainfall depths for various durations,area. sizes and seasons are then constructed to give generalized estimates of PMP which have quite well stood the test of time.c;For the western states,the effects of mountains on rainfall magnitude ane distribution gt:eatly reduces the confidence in transposing storms.Therefore,an orographic.precipi ta- tion complttation model or adjustment of hon-orographic PMP by various tech- niques is most commonly used.Details of such procedures may be found in the series of reports published by the NWS (10)..They also can be found in a report published by World Meteorological Organization (11). 3-1 • ..lJ * r:<(to- j II.e'fJ'\ I Ii I I t IIv r~~. i~, fr • For the Basins,the topographic features (Exhibit 7)are important in set- ting the level of PMP.This makes it difficult to transpose storms from other regions.In spite of the difficulties,storms were transposed to the Basins to estimate the PMP because experience has shown that unless this is done the fliP estimate 'Will not stand the test of time.In other words,the storm.sample over any particular loca.tion is generally insufficient for a reliable PMP analysis.An additional problem for Alaska is that relatively few rainfall observations are available for high elevations.The problem of estimating how much rain fell in the mountains during severe storms at low elevations adds additional subjectivity to the PMP estimation. 3.1.2 Extreme Rainstorms The greatest areal rainfall of record in the general region of the Basins occurred on August 12-14,1967.It centered (as far as is known)on the Chena River,a tributary of the Tanana River,extending east of Fairbanks. An isohyetal map analyzed by the NWS (12)for the 3-day period is shown on Exhibit 8.All available rainfall catches,including several unofficial values obtained from a survey of the region soon after the storm,are also shown.The two greates t depths are an official 8.1 inches at Clear Airport and an unOfficial e.s inches to the north of the Chena River. Considerable analysis of the data,includ;ng streamflow runoff particular- ly for the tributaries of t.he Tanana,by the NWS resulted in the estimated r·ainfa11 depths in the mountains of up to 11 inches as shown on Exhibit 8. The streamflow measurements pt'ovided rainfall estimates for some tributaries where no rainie11 measurements were available. lhe hec:lviest rainfall (;enters were located on southwest facing slopes of the mountain to the north of the Tanana River where inflowing moist ai.r is lifted8nd rainfall intensified.Rainf;::a.il depths observed during the 3-day p:eriod,within orin the vicinitycf the Susitna basin reached 4.5 inches at McKinlcay Park.a~d 4.1 inches at Trims Camp.There is Ii ttle d,oubt;,tha"t somewhat greater depths Occurt'ed on the nearby ridges <;IS indicated on the 3 --2 • t:... .. .. • I -•I•I •• .. An arctic weather front in a pressure trough at approximately la- titude 65°N and dipping down to the Bering Sea;and 2. 3.A seri.es of low pre$sure centers,including that associated wi th typhoon "Hope",moV'ing along the Arctic front;Typhoon Rope was responsible for bringing much moisture into the system and itself may have been responsi.ble for the most ex:treme rain on August 12. 1.Abnormally high pressure in the Gulf of Alaska,wttich was helpful in s.etting up a strong pressure gradient that induced large quan- tities of mo-r.sture from the Pacific Southwest of Alaska into the low elevation "corridor"extending from about the mouth of the Yukon River to the extreme rainfall location; In a.PMP study for the Yukon River Basin (14),it vtdS found that an 8jttreme rainfall with a measured rainfall of over 5 inches occttrred near Roly Cross on September 10-12,1915.Thisstonn had many features si.;nilar to those of the Chena Storm"A high pressure prevailed in the Gulf of Ali:\ska and a low pressure center in the Bering Sea,~th an associated cold front extending to tbe south,quickly moved into the central portion of Alaska.Holy Cross 3-3 map (Exhibit 8).However,they most likely are less than the depths to the north of the Alaskan Range ~i~h bounds the Susitna drainage.This range rises up to above 6000 feet to the south of the "Chena"rainfall centers. The meteorological features associated with this extreme storm rainfall have beert studied in some detail by E.D.Diemer of the U.S.Weather Bureau.His work was incorporated in a U.s.Geological SurV'ey Water Supply paper (13), and is attached to this study as Appendix A.Some of the outstanding broad scale weather features of the storm were: The series of low pressures and associated rainfall persis'tedfrorn about August 8 to August 15,but exceptionally heavy precipi tation occurred pri- marily on August 12. is located near the center of the low elevation corridor along which moist- ure through greater depth can extend into Central Alaska.When this is ac- companied by other necessary atmospheric features,extreme rainfalls can occur.The 5 inches at Holy Cross included 3 inches in 3 hours on September 12.An isohyetal map of the storm is given in Figure 8-5 of the Yukon River Basin study (14). Efforts to find g;eater storms on the Susitna basin itself,led to the Augu'.i t 23-25,1955 s tom centered at Talkeetna where 4"66 inches of rain was measured"No measurements are available wi thin the Basins"However, with 2.27 inches at Summit near the northern divide,there could have been considerable rain in the Basins. 3.1 ..3PMP EstImation The foregoing discussion clearly points to the Chena.storm of Augus t 12-14, 1967 as being the most extreme known in the general vicinity of the Basins. Through transposition and adjustment,it constituted the basis for the cur- rent PMP estimates. In numerous PMP studies a base rainfall index is used to transpose a storm from one mountainc~s location to another.A mean seasonal or mean annual precipitation (MAP)map is often selected as the index.Some studies,how- ever,have used a rainfall frequency map such as that for lOO-year,24-hour -rainfall.The principle behind the use of such an index is that it more nearly reflects the orographic effects on ex:tt'eme rainfalls.A mean preci- pitation Il1apcan be the result of many "run of the mill"rains and,there- fore,is not necessarily a good measure of extreme rainfall. The basic.pt'ocedure in tidjusting and transposing storms using an index,is to multiply the observed rainfall by the ratio of the index in the trans- posed location to that whet'e the rainfall occurred. 3-4 .. .~ liN!\I~.·~.·..h~i~r I t' I"\r Ir~ 1 I . I>;·' 1 rim..·f··\·~.···J..<•••••..••..'.e"'.01}-L'm'.t'~IIII'~' •_,..if,l,'040 _..:.,,~4".~,l.'".'':•'..,_•"J • ••:' 1"t L_ r 1 • 1. .~, '\ "''Ii.,~.i. j I .. I, j I I 1$ ~ ~ • .. Because a reliable MAP map is not available for the Chena basin and reliable rainfall-frequency maps are not available for both the Chena and Susitna ba- sins,the Basins'terra.in was analyzed instead to transpose and adjust the storms..111is essentially involved the estimation of rainfall that could have occurred in the Basins rather tl'tan in the Chena basin,if the storm tracks and other meteorological features were optimtnn to the Susitna basin. A study (15 )by the Soil Conservation Services (SCS)gives a detailed l-fAP map (Exhibit 5)for the Susitna River basin.The map was based on stream- flow and snow course data as well as all recorded rainfall data.Unfortu- nately,such an analysis is not available for the Chena River basin and its vicinity where the Chena storm of 1967 occurred 0 3-5 A study by the W~S (16)gives rainfall frequency maps covering Alaska.Ho~ e";er,since they are base.d on.only a few and low-elevation rainfalls,they cannot be used with confidence in a PMI'estimate. The Basins are surrounded by higher mountains,the Alaska range to the north,the Talkeetna mountains to the west,the Chugach monnt.ains to the south and Wrangell mountains to the east.The lowest passes (down to about 2,000 ft ,mal)for atmospheric moisture inflow are in the Talkeetna moun- tains •The NWS study (16)includes a map of Alaska,showing generalized elevations (see Exhibit 9)that are barriers to moisture inflow. The barrj.er to moisture inflow for the entire Susitna basin averages 3,000 feet..'Th.e barrier to moisture inflow for the Chena basin where the 1967 storm centered isestirnated to be about 1000 ft,msl. The adjustment for a 3000 feet barrier was based on the atmospheric mOisture derived from tha maximum 1 OOO-mb,12--hour persisting dewpoint of 61°F ,as- suming a saturated atmosphere wi th a pseudo-adiabatic lapse rate..The temperature of 61°F was taken from the regionally and seasonally smoothed 12-hour persistingdewpoint maps covering Alaska (17)for the month of July, ~l I l I "r~liind •• for a location some distance towards the optimum moisture source for the Basins.The resulting adjustment is 0.79 as computed in Table 5. The time distribution of the 72-hour PMP giving maximum depths for shorter durations wasbase.d partially on experience wlth extreme rains in other re.... glons"'Ibis was necessary because hourly rainfa.ll for the Chena storm were not avallable~ 3-6 The average rainfall depths over Watana and Devil Canyon Basins,based on Exhibit 10,are 6.4 and 6.7 inches respectively.Multiplying these by the combined moisture maximization and barrier reduction factor of 1.07,the estimated 72-hour PMP of the two Basins are 6.8 and 7.2 inches respectively. 3.2 TIME DIStRIBUTION OF PMP The Chena storm isohyetal pattern was located and oriented in the Basins as shown on Exhibit 10.Both the 10-and II-inch centers are set in the re- gimes of highest MAP in the Susitna basin (see Exhibit 5). Additional adjustment was made for maximum atmospheric moisture (moisture maximization)•The de-wpoint representative of the stotnl moisture and the maximum dewpoint (both measured near McGrath,E.."iChibit 1)are 55°F and 610F, respectively.This gives moisture maximization adjustments of L.36 which agrees with the one used,by the NWS (14).The combined adjustment to the Chena stOr1ll is,therefore,1.07 (0.79 x 1.36). In relatively flat regions ,the magnitudes of 6/24....hour and 24/72-hour ra- tios of extreme rainfalls are related to rain area size;the smaller the area,the larger are the ratios.This relationship plus rainfall experience in the mountainous regions of the lower 49 states led to the adoption of rain ratios of 0.46 to 0.52 for 6/24-hour and 0 ..54 to 0.60 for 24/72-hour PMP for several basin sizes in the Tanana.River Study (12).The ratios for the sizes of the Basins are 0.50 for 6/24-hour and 0.60 for 24/72-hour pre- <--• ••------------•,0 •l'\~'.''\. •• .. 1.Adjus ted it fo r a 3000 fe et barrier.'I'hi s .esul ted in 4.5 inche s • (6.4 :x 0.7);and Some checks on the genera.l level of PMP for the Basins are in order. 3-7 A "bell shaped"distribution of the PMP increments (greatest value in the center of the storm period with next smaller valles al ternately on either side)was adopted.This,in general,agrees wi th the time distribution of rain in the Chena.storm. ApplYing the above selected ratios and fitting a smooth curve to the result- ing three PMP values,the 6-hour incremental PMP values were obtained as listed in Taple 6.The values are expressed in percent of 72-hour PMP as well as in inches. cipitation.During the August 12-14 storm,12 rainfall stations in the Chena area recorded oVer 3 inches of rainfall.The recorded data indicated 24/72-hour rainfall ratios of 0.41 to 0.75.The average ratio is 0.57 com- pared to the adopted ratio of 0.60. 3.3 CHECKS ON THE PMP ESTIMATES In the Yukon Report (14),the greates t depths of non-orographic rainfa~l found fo.the non-coastal regions of Alaska came from the September IO~12, 1915.3~day !!to:tln centered at Holy CrOs s •All 'a.infall depths repo.ted we re at low-level valley stations.For the sizes of the drainage areas above the Watanaand Devil Canyon,the tDoisturemrocimized rainfall for this storm is 6.4 inChes.This value is Somewhat gteate.than the estimated non~o.o graphic component of 5.9 inches (also moisture ma){imized)for the August 12- 14,1967 stonn (12). The following adjustments were made to the 6.4 inches rainfall: f f f f 2.Augmented it for orographic effects In the Basins.This was based on a relationship between 3-day PHP and elevation that was devel- oped for the Yukon Report (14,Fig ..2-9).Based on th:ts relation- ship and the area versus elevation curve for Devil Canyon Basin, the non-orographIc rainfall was augmented to have an ar~\\a1 average depth of 5.0 inches for a 3-day period..This is considerably smaller than the recommended PMPvalue of 7.2 inches for the basin. AnOther comoarison was made by adjusting the low elevation non-orographic.. rainfall by uSing a relationship of PMI'to MAP for the west facing slopes of the Sierra'Nevada in California.A reasonably high correlation ex:ists be- tween these ~~o parameters (see Exhibit 11).Table 7 shoWS the cOmputations to obtain orographic rainfall for the basin above Watana.Column (2)is taken ftom Exhibit 11 for the MAP bands in C01UD1n (1),Column (3)shows the PMP fot each MAP band ex:pressed in percent of PMI'for the MAp band of SDlall- er than 20 inches.The non~orographic rainfall of 4.5 inches based on the September 1915 storm ad.justed for barrier effects corresponds to an MAP of 20 inches in the .atana Basin.Using this base value and the percentages in Colllmn (3),the corresponding value for each MAP band is calculated in Co- lUDIO.(4).Colllmn (5)shows the percent of area in each band and Column (6) shows the weighted depths.The reSUlting rainfall depth for Watana Basin is 5.4 inches.Similar computations give 5.3 inches for the basin above DeVil Ca nyon Which is qUi te simi!ar to the 5.0 inches 0 btained •using the 3 -day PMP vs elevation relationship. A check on Whether the PMI'vallles are too high is based on desirable consis~ tency frOm location to lOcation.Consistency in itself does not lead neces- sarily to the best estimate of PMP.However,When OVer ten times the amount of effort is put into ustimates for nearby areas,there is a good reason to desire conSistency with such eStimates.Fur thetnlore,SUch consistency h a way of taking into account the thoughts of other experts having SOmewhat different philosophieS On acceptahle levels of PMI'.With this in mind,the eStimated PMI'valUes were compared with thOSe derived fOr the Tanana River basin by the l>lWs (12).~<u stUdy was made for three drainage areas of 317, 3-8 • I."" I !.~ilJiii..rt:l'It~'19, :!.'.:....t'" I !r i I.~~'\ I I I k"~. ~.:...~I:~E•.-.:•.'...-..•-••....••"'.•... J ,J ".~~.'. 1380,and 19,000 mi2 ,a.ll well exposed to the southwest corridor of moisture inflow with relatively low barriers.Part of these drainages was in the area of extreme.rainfall during the Chena storm of August 12-14,1967.The 6-,24~and 72-hour PMP depths for these drainages,TNhen plotted against area on semi-log paper,could be connected by a reasonably smooth depth-area curve.This indicates a consistency among the PI1Pestimates.The PMP values from the current study (6 ....,24-and 72-hours)are well enveloped by the smooth.curves of the Tanana basin.This is as it should be because the upwind barriers 'which practically surround the Basins,reduce moisture in- flow to the Basins.At the least,from this comparison,we can say that the estimates are not unreasonably high. Other checks,such as comparisons of ratios of PMP to lOa-year maximum rain- falls would be desirable.Unfortunately,as mentioned previously,reliable estimates of lOa-year rainfall covering all the region of concern are not available. 3.4 ANALYSIS OF OBSERVED STORM DEPTHS IN SUSITNA BASIN As discussed earl".er it is apparent that the rainfall of the Chena storm (A1Igust 12~14.1967)in the Susitna basin was less than that on the slopes north of thE'Alaska range..However,it may be of interes t to make an es- timate of the rainfall on the SUsitna basin making use of the.detailed available MAP for the basin (15).The steps taken to make this estimate by using isopercentaJ.maps,are described below: 1.Plot the storm rainfalls for all stations in and surrounding the Basins; 2.Express 'these rainfall amounts as a percent of the station MAP; 3.Prepare isopercental map based on the ~,cents obtained in Step 2 above;and 3-9 .. •• J 4..Multiply percents obtained from the isopercetltal map with MAP at the corresponding points to prepare the stonn isohyetal map or calculate the average stonn rainfall depth. The reliability of the estimated areal pattern and magnitude of storm rain- fall is dependent on the degree to which the patterns of major stonn rain- falls are similar to the pattern of MAP.It has been shown (18)that the patte'rn are qui te similar in strong orographic regions like the Sierra Neva- das of California where a relativley few winter storms contri~ute largely to the .HAP.In other oIographic r~gions,such similari ty mayor may not exis t depending on dominant rainfall patterns. The analysis using the isopercelltal techniques gives areal rainfall of 2 to 3 inches for the Basins during the August 1967 storm (Exhibit 12).When this is multiplied by the moisture maximization factor of 1.36,the result- i.ng rainfall is consider ably smaller than that obtained by transposing and adjusting the portion of the 1967 storm near Chena to the Basins. The August 23-25,1955 storm gave 4.66 inches of rainfall at Talkeetna. This is the second greatest 3-day rainfall found close to the Basins.In the same manner as jus t desc ribed for the Augus t •1967 storm,the average rainfall over the Susi tna basin was determir~ed for this storm.The esti- mated average depth is alsobetw'een2 and 3 inches.Exhibit 13 shows the analysis followed to reach this estimate. SEASONAL VARIATION OF PMP .;; One index to the seasonal variation of PMP is the variation from month to month of gteatest ptecipitation observed at stations located Wi thin or in the Vicinity of the Basins.The longer the period of record,the more stable such a tecotd and!the better the index.Table 8 shows the stations used and the reSUlting average seasonal variation of mlQl;imum One-day preci- pitation.All stations had their mlQl;imum values :I'.n June or July Wi th One exception;.Alpine where the maximum oCcurred in March. 3-10 .. • . .. ..v. ......... j ~'"' ;r I I), I 1:.eJ.lWl>rl)~12~,~ It- .I j, I I •.·.~.:c,t" f~•• r hr:':lr~ m1,_ 1.1:.l.'~'.""".."..I ....' I,.,' Ie' I I Ii.. 3-11 3.6 SUB-BASINRAINFALL Another index to seasonal variation is the maximum ava.ilable moisture in the Basins based on the maximum 12-hour persisting dewpoints (17).These values are shown in Table 9 for mid--month from March through August. Exhibit 14 shows the above described data along wi th the adopted seasonal variation.The adopted mid-April,mid-May and mid-June values are 53,73 and 93 percent respectively,of the all-season PMP. ... Yet another index is maximum monthly observed precipi tation at .stations of long record.The stations used in this comparison were the same as those used for maxi.mum one-day precipitation (see Table 10). Average rainfall Over each sub-basln resulting from the 72-hour PMP was determined by planimetering the areas between isohyets shown on Exhibit 10, averaging over the area of the sub-basin and mul tip1yi ng by the combined moisture maximization and barrier reduction factor of 1.07.The smoothed depth-duration curve (values expressed as percent of 72-hour PMP)~discussed under "Time Distribution of PMP"(Exhibit 15)was used to derive 3-hour in- cremental perc.::entages..These percentages were arranged sequentially as dis- cussed under "Time Distribution of PMP".The 3-hour duration was selected based on the lag times for the sub-basins (see section entitled "Unit Hydro- graphs'").Tablle 11 shoW's the resulting 3-hour rainfall incremC{lts for each sub-basin. J :1 J J "I ,) J J j ,I .1 ,I I SECTION 4 SNOWMELT .. • (fJ fo--m .5:><w ><-:l 2:u, t :( ! I . •...........'.r " •• ,,-, The snow Survey stations Within or in thE!vicinity of the Sl1sitna River basin are listed in Table 4.The data for the stations vary from 2 to 19years. ThE!100-YMr =imom snow wa tel'E!quivalents werE!considered to bE!a conser- vative estimate of available snow fol'mel t in cOmbination Wi th thE!PMPs in mid-May and mid-June.These were estimated based on available snow course data and adjusted for higher E!1evations using the factorsdevE!lopE!d by NWS in HMR 42 (14). 4-1 Basad on the sizes of T"atana an<t Devil Canyon Basins and other hydrologic chal'acteds tic s ,the SnoWlllE!l t computations we l'e considerE!d neCes Slll'y for the months of May and June.!he combined snowrne1 t and rainfall was computed for a 3-day period aSSUnliog the PM!'to begin on May 16.and on JunE!16.SnowmE!1 t Was also computed fol'the periods pr:f."r to and after the 3-day period. 4.1 MAXIMUM SNOW WATER EQUIVALENTS--------------,---.::------- 4 .0 SNOWMELT ThE!ma:dmom snow water equivalents on all courses WerE!:measured either on April 1 or May 1.HOWE!ver,the valUE!s on the two dates were not s ignifi- cant!y diffe rent •Th ereio re,the =i:mum wa ter E!qni val ents w-ere assUl)lE!d tooccuronMay1. Frequency analYSE!s werE!made to derive 100-year maximum Water equivalE!uts on snow courses With 14 to 19 years of data (fi1;"st six stations in Table 4). These value s ranged be twe en 6.0 to 12 •9 inches.A 95 pe r cent one-sid ed upper confidence lim:l.t was COIllPUted beCause 14 to 19 years of data are not Sufficien t!y long to pr oVide a reHab le es t ima te of 100-YE!ar wa te t'~q ni Va- 1ents.The stat.fons froIll which data Wel'e available are also too few for thE! rela tive1 Y large llas ins •The value s Wi th the confidence limi t ranged be- tWeen 7. 4 to 1 8. 1 inchE!s.lhe al titu<tE!of the five s ta tions varies betwe E!n 2,270 and 3,100 feet With the sixth station at 4,160 feet.The average of J . .~..,~'f/"•".• o the.six stations,about 14.0 inches,was considered representative for ele- vation between 2,000 and 3,000 feet. 4.1.1 Variation of Snowpack With Elevation In the absence of high-elevation precipitation records in the Susitna River. basin,the variation of snowpack with elevation.developed by NWS for the Yukon River Basin (HMR 42,Table 24)was adopted..The average value of 14.0 inches,representative for 2,000 to 3,000 ft elevation ZOne,was adjusted by the fac.tors given in HMR 42 (14).The estimated water equivalents thus derived are given in Table 12 for elevations between 1,000 to 9,000 ft. 4.1.2 Beginning Date of Melt May 12 was,used as the date of beginning of snowmel t for the 3-day PMP starting on May 16.This date was selected based on trials to have a com- bination of snowmelt and rainfall that would produce maximum flood with the adopted temperature sequences (see the sub-section entitled "Temperatures ")• For the PMP starting on June 16,the date of beginning of snowmelt was taken as May 15.However,the temperature sequence was selected such that minimum snolY'mel t occurred between May 15 and June II (see the sub-section entitled I'Temperatures·').This yAII allow maximum remaining snow for melt imm.ediate- ly prior to and during the PMP. 4.2 WINDS Wi.nd data are needed for various elevations in the snowmelt computation. Because upper air Wind observations are not available in.the Susitna basin, the 100-year maximum ~1inds for Fairbanks w'ere adopted for this study.The laO-year Fairbanks Winds were determined by the NWS (14)by Computing free ~:t:r:100-yea:r:>dIld Speed adjusteq to 24-hou:r du:ration for the 950-,850-, 800-and 700-mb levels (correspondj.ng approximately to 2000,5000,7000 and 10,000 ft,msl)~Fifteen years of data w-as used in the computation~Winds 4-2 .. • j c I I .. . •.....>.,.....H.J...<.\-!o.>-l.~~~s..,~~...... accompanied by sub-freezing temperatures (at the le\Tel being considered) were ex:cluded from the analysis.A comparison of the resulting 1 DO-year values with maximum winds during the Chena Storm of August 12--14,1967 at Fairbanks,indicated the fOlmer values exceeded the latter by 7 to 19 per-- cent depending on the level. The free air 24-hour wind speeds were then adjusted to anemometer level (usually about 40 feet above the ground surface)by comparisons of anemo- meter level 'Wind speeds at Gulkana Glacier,elevation 4,800 ft,msl,with concurrent free air wind speeds at F2i rbanks •Gulkana Glacier is about 135 miles to the.sgutheast of Fairbanks..The average ratio of the Gulkana wind speeds to the Fairbanks free air wind speeds at 4,800 ft was estimated to be 0.6.This ratio was applied to the Fairbanks free air winds at all levels to estimate surface wind speeds.Exhibit 16 shoYlS the 100-)'ear free air wind speeds at Fairbanks and the estimated surface wind speeds (anemo- meter level)over unforested terrain,both for a 24-hour duration. Variation.of wind speed with respect to duration up to 30 days were esti- mated in the referenced NWS study (14)..This was based on winds of 12-hour to 30-day durations during May and June When temperatures were about goc and above at 900 mb..Table 13 shows the wind speeds by 1000 feet f.Uevation bands from 2000 to 10,000 feet.The NW'S recommends the use of these data for the basins with lateral ~tents up to 60 nautical miles.The lateral extent of the Basins is about 55 nautical miles .. The three days of greatest wind speeds were assumed to coincide wi.th the 3- day PMP storm.For two days prior to the storm,the lowest speeds shown on Table 13 were used.This is the time ",~en "",ather condi tions are changing prior to the advent of the storm.Tables 14 and 15 show the arrangement of daily wind Speeds used in the Snowntelt computation for May and June :PMI', respectively. • I 1I~ ) r I ~ ___.....--.,..-------.--..--;;r-- 4.3 AIR TEMPERATUP~ 4.3,,1 Prior to and Du!!:2-.g the PMP Storm Daily temperature data at Anchorage were obtained from the National Climate Center,Asheville,North Carolina for the period from 1917 to 1981.These data were used to determine annual maximum for 1-,2-,3-,10-and 30-day temperatures for the months of April,May,and June respectively.Frequency analyses were then made for ea.ch of the five durations and for each of the three months to determine the laO-year maximum tempE~ratures.Smooth curves were drawn through the computed temperatures for I~ach month in order to estimate the 100-year values for each duration from 1 to 20 days.From these values,the daily temperatures ~t]ere computed for a.20-day period,as shown in.Table 16. These temperatures are applicable for elevations up to 2 OOOft ,IDsl.Above that,a decrease of 3°F for each 1000fee.t increase :tn elevation was as- sumed.The highest daily temperatures were placed two days prior to the beginning of the PMP as suggested by observed temperatnres prior to major storms.Tables 14 and 15 sho~the sequences of daily \:emperature used in the snowmel t com.putation for Hay and June PMP,r.espectively.In these se- quences ,daily temperatures 2 OF higher than the daily dew points are used far the period of the PMP. The beginning date of melt for June PMP was assumed to be l-fay 15 as dis- cussed under "Maximum Snow Water Equivalents".A sequence of temperatures was derived for May 15 through June 11 (prior to highest temperature a.ssumed to occur before the.PMP starting on June 16)based on minimum daily tempera- tures at Anchorage for the period 1917 through 1981 (Table 15).This pro- vided.m.inimum snowmelt during Hay 15 through June 11 so that maximum amount of snowpack would beavaila)le at the onset of PMP. 4-4 .. •Ii '.,I)"•'0,.'•.",".'-.''.,~,... •,••I t •'".• 4-5 4.3.2 After the PMP The tem.peratures after the third day of the PMP ~re arranged according to the procedures given in ~fR 42 (14).Starting with the highest temperature from Table 16,the temperature is decreased by 0.5°F for the next day and an additional O.SoF for each succeeding day (see.Tables 14 and 15). • 4 .4 .2 During the rMP 4.4 DEWPOINTS 4.4.1 Prier to the PMP Daily dewpoints were assumed to be 14 of lower than the daily air tempe ra- tures prior to the storm.This is based on conditions during major stor:ms in Alaska.However,during the several days immediately prior to the PMP, when the dewpoint temperature estimated based on this criterion exceeded those during the PMP,the dewpoints were reduced as shown in Tables 14 and 15.This is because the dew"P0ints during the PMP are based on the envelop- ing of observed values and should not be exceeded. These are based on maximum pers~.sting dewpoint maps analyzed by the NWS (17).All available statiop.8 in Alaska plus those .tn Canada between Alaska and Washington State inland t<)about 170°west longitude,"W'ere used in the analysis.Table 17 shows the 12-hour persisting values and the da.ily values for the three-day period of P}fP.They are ba.sed on the variation of ob- served maximum dev7points during major storms in Alaska and elsewhere.As wi th the temperatures,the dewpoirtts given in Table 17 atYe to be reduced by 3 '>F for ea.ch 1000 feet increase in elevation above 2000 feet.Tables 14 and 15 show the dewpoint:sequences adapted for vat·....ous elevation bands. • • • Ik ';), " . - -£;'A.J".'"......'~•."'6 oJ d.•O~Q .'•'.....• .:-"~.".#'.'..~-~~.~--_...__.._....'-.'.---"-'."'>.--...' Radiation reaching the snow surface as solar radiation (short-wave radia- t.ion)and infrared radiation (long-wave radiation)during the snowmel t season was com.puted using the data and procedures given in HMR.42 (14)..The net daily radiation (sum of short-wave and long-wave radiation)for mid-l-fay and mid-June EMP are given in Tables 18 and 19. Solar radiati.on data are available at eight climatological stations (Susitna Glacier,Denali,!yone River,Kosina Creek~Eklutrla,Watana,Devil Canyon and Sherman)wi thin the Susi tna basin (Exhibi t 6)for the period from July 1980 to September 1982.T'nese data could not be used effectively because of the short period of records. After the third day of the PMP,the dew points were assumed to be slightly lower than those during the PMPas shown in Tables 14 and 15. 4.4.3 After the PMP .. 4.5 RADIATION HMR 42 pro,rirfes detailed analyses of short-w-ave ""'ud long-wave radiation for the Yukon River Basin located between latitudes 59°and 69°.Since the Susitna basin is located betweell latitudes 62°and 64°,the mIlt 42 da.ta were cQnsidered applicable for the Basins. 4.5.1 Seri,al Numbers'-__-r .._~ Computations of short-or long-wave-radiation require serial numbers as- signed to all da.ys for which computations are made..The~e numbers are used to derive ratios c.f maximum observed ra.diation to clear-weather radiation (HMR 42)pages 53-.54)..Serial numbers prior to,d\lring and after the PHP "(ITere a.ssigned following tbe procedures given in lIMR 42. ! 1 1 ILr,;; f,"I~.•·.k.·./".l•.•.•~,.:w"~..Ii"_,~'': (N!!t...'.'.f.~J ..!C( ;J-r, • .' Melt during rainy period: SNWMT =COEF (..09 f-(.029 i-.00504 WIND i-.007 RAIN) (TMPR "'FRZTP» The long-wa.ve radiation was computed as typical downward radiation from the air,including water vapor,plus the radiation from partial cloud-cover minus upward radiatJ.ou.Elevation adjus tmsnts were not made because these were considered t"J be :f.nsignificant. Mel t dur.ing rainfree period: SrWMT :::COEF (.002 SOL (l-ALBDO)f-(.0011 WIND f-.0145) (TMPR-FRZTP)f-..0039 WI.ND (DEWPT-FRZTP) 4-7 The short-wave radi...tion was comp~ted 1.:,;,ding maximum daily clear-weather short-wave radiat.ion and the ratios of maximum observed radiation to clear- weather radiation.Since short-wave radiation intensity increases with elE- vation because of lessening optical air mass and lessening interception.by water vapor,variation of radiation with elevation also was considered using the data from HMR 42. 4.5.2 Short-wave Radiation 4.5.3 Long-wave Radiation 4.6 REC-I,SNOWMELT SUB-ROUTINE The snowmelt sub-\'outine of HEC-I ctllJ1puter program developed by COE (5)was used to c.ompute daily sDlowmsl t rates for rainy and rainfree periods.The program can compute daily snoWlIlel t rates either using degree-day or energy- bUdge.t method.The basic equations £()I'snowmelt are.taken from COE's publi- cation EMI110-1-I406(19).The simpLified energy-budget equations used by the program are given below: "'1t,, in which SNWMT =snolVDlel t in inches per day in an elevation zone of a basin; COEP =a dimensionless term,usually assumed 1.0; WIND =wind speed in mil~s per hour (mi/hr)at 50 feet above snow surface,average for the basin; RAIN =precipitation in inches per day,assumed to fall as snow if zone temperature is less than FRZTP plus 2°p;snow- mel t Is subtracted and snowfall is added to the snowpack in each zone; TMPR =air temperature at bottom of lowest elevation zone are adjusted to higher zones by lapse rate (TLAPS);a rate of 3°F per 1000 feet elevation difference was adopted for this study. FRZTP :::index temperature of at which snow will melt,assumed 32°F in this study; ALBDO =program computes values as (0.75/n·2)with maXimum value of 0.4,D is number of days since last snowfall; DEWPT=dewpoint OF at bottom of lowest ele7atiot;l zone,are ad- justed to higher ZOnes by 0.2 TLAPS;and SOL -.net radiation in langleys per day,average for the basin. The enetgy~budget method Wi<S use,d in this study.The TMl'lt,nswPT ~nd WINO dati<are g;l.ven in Tables 14 and 15 for May and June,respectively.The SOL 4-8 .. • (i.,'r'; 1° L 4-9 • .. M- data are given in Tables 18 and 19.Initial water equivalents and drainage areas in each 1000 feet elevation zone are given in Table 20."RAIN"is the PMP starting on May 16 or June 16.Based on seasonal variation of PMP (Ex- hibit 14)the PMP occurring on May 16-18 and on June 16-18,is a.bout 73 and 93 percent of July--August PMPrespectively.Table 11 shows sub-basin ra.in- falls based on July-August PMP.These values were adjusted by a.factor of 0.73 and 0.93 respectively to derive mid-May and mid-June PMP for combina- tion with the corresponding snowmelt. i" I 'I :1 ~J 'I I 'I: '-- II,I 'j • • • .. SECTION 5 ~JNIT HYDROGRAPHS I .. I I k"5 ="[j L L 1::i •• .. 5-1 5.1.1 Historic Flood Events Unit hydrographs were derived for 10 sub-basins upstream of the Devil Canyon site and the sub-basin between Devil Canyon and the stream gaging station on the Susitna River at Gold Creek (sub-basin 11).The dimensionless graph _ lag curve technique described :tn "Design of Small Dams"(2 0,pages 64-67) was used.Four average dimensionl es s graphs we re derived based on abse rved flood hyoTographs at selected stream gaging stations and used for sub-basins 1 and 4;2,3 and 7;5,6 and 9;and 8 and 10 respectively.One lag curve was used for all sub-basins~ 500 UNIT HYDROGRAPHS 5 .1 DlMENSIO~"'LESS GRAPHS Daily'streamflow records for the pe.l,·iod 1961 to 1981 at 19 stream gaging stations (Exhibit 2,Table 21)were examined to select high flood events for deriving dimensionless graphs.Prior to this period,the years of maxi- mUtll floods of record as reported in t:h\~USGS Water Resources Data,were examined.Da.ily rainfall data at key ste\tions also was reviewed to select flood events. The daily flows generally show risin.g trend in May J.High flows continue during the months of June through 8epteml.,e'r and occasionally high flows could occur in November.Eighty-four flood events were selected at 19 sta- tions (Table 21). 5.1.2 Derivation of Dimensionless Graphs- Hourly streamfloW'data for the selected flood evertts wet ~obtained from the USGS sub-district officeirt Anchorage.These data were examined and flood hydrographs With reasonably t¥ell defined peaks and recessions were selected. Gener\9.11y,floods from larger draina.ge baSins have mUlti""peaks whet"eas those ~I .~.' r • 2 3 2 6 3 4 2 2 No.0 f Dimen- sionless Graphs___S_t_r_~_C!m Gaging Station Eagle River at .Eagle RiveT.' Caribou Creek nr.Sutton Susitna Riv~r nr.Denali Little Susitna River nr.Palmer Maclaren River nr.Paxson Talkeetna River nr.Talkeetna Willow Creek nr.Willow Deshka.River nr.1'--Jillow from smaller basins show fluctua.tions on rising or falling limbs of hydro- graphs .. 5-2 A total of 24 dimensionless graphs were derived using procedures outlined in "Design of Small Dams"(20,pages 64-69).The selected flood events are marked wi th an asterisk in Table 21.The following table summarizes the number of dimensionless graphs derived for each gaging station. The dimensionless graphs at each of the above stations were carefully exam- ined.The graphs of the Little Susitna River nr.Palmer and the Susitna River nr.Denali were judged to be inconsiste1.1t in shapes and times to peaks,and hence Were not used in further analysis. The watershed characteristics (overland and channel slopes,vegetat:f.ve cover)soils,presence of glaciers and lakes,Ptc;)of the remai1'ting six basins an(:the sub-basins above the Susi tna River at Gold Creek were com- pared and the following dim.ensionless graphs were judged to be representa- tive for the sub-basins. I:·,..·.:.-.-----.-.ll...._.._.':_._.__.~_ ··1'..·.·· \'... '.,.'{ I :1 'I • ... •9 1m, •'<>•','~"~.,'.''.,".'..'..'. \"'. Dimensionless Gr~~p~h~.s .___ Average of 4 graphs derived for Talkeetna River Average of graphs derived for Eagle River,Caribou Creek and Willow Creek. {; _.,':.,_.*,:i>-"''''''=:'m''C T :=Lag time,defined as the time from the center of rainfall excess to half the volume of direct runoff,hours; L ::.:Length of longest stream from outlet to watershed diVide,miles; L c =Length of main stream from outlet to intersection of perpendicu- lar from centroid of the basin to stream alignment,miles; T ~A [:;:r 5-3 S ..Overall $lClpe Clf lClngeat $tream frem Clutlet to watershed diVide, ft/roi; Sub-basins 1,4 and 11 2,3 and 7 5,6 and 9 Average of graphs derived for Deshka River. 8 and 10 Average of graphs derived for Maclaren River. 5.2 LAG CURVE Exhibits 17 to 22 show the dimensionless graphs for various basins.Exhibit 23 shows the average dimensionless graphs. The hydrographs of the 24 flood events analyzed to derive the dimensionless graphs and the hourly rainfall da ta e,vailable in the vicinity of the basins above the stream gaging stations were used to develop a lag curve defined by the following equation: in which J.",. i ] J A =Coefficient,intercept on logarithmic scale;and B =Exponent 5.3.1 Watershed Paramete.rs of the Basins "" The w">!tet"!Ihed parameters,1.,Lc and S,of the basins above the gaging stations for which the dimensionless graphs were derived,were determined rt"Oill US GS topog raphic map 13 of 1:63 ,360 seal e •These parame ters are 1:1.13 t ed in Table 22. 5.2.2 Lag Times Hourly ra;l.J:lfall data for the period correspOnding to the flood events anal- y;e;ed for the diIllensionless graphs were obtained from the Na tional Technical Inr01:lllation Service,Springfield,Va.(NTIS)fot four stations,Anchorage, Big Delta,Gulkana and Talkeetna,in the 'Vicinity of the BasfilS.These data indicated that houdy tecords were Illissing for most of the flood events because of non-functioning of the the recording gages. Hourly rainfall data collected by :s&M during 1980-81 at stations Wi thh1 the Susitna basin also were re'~iewed to obtain data corresponding to flood events on the Maclaren River near Paxson or Susitna River near Dena.li. The avaUable hourly data could not he used to determine periods Or rainrall excess for the floOdS analy~ed ancl hence,the lag tiIlles for the basins could! no t be cle rived •Based on Ha r;e;a expe dence On sim:l.1at studies,the lag t:l.r4e was assomed to be equal to the tiIlle from rise of hydrograph to its peak. Depending upon the rainfall distribution pattern,this aSSumption COuld either underestimate or OVel:estimate the lag time wldch,for the PUJ;'pose of this study,is defined as the time ftOIll the center of "ainiall excess to half the volome of direct :runoff hydrograph.The estimatecl lag times for the basins are given in Table 22. 5-4 •• I I ! ·.. 5"'5 5.3.1 Watershed Parameters 0.32 The basin parameters (LL c ~IS)and estimated lag times are plot ted on Exhibit 24.A curve fitting most of the plotted points was assumed to be representative for the Susitna River basin.The resulting lag curlTe is: T =8.2 (LL -:-1/8)c 5.3 SUB--BASIN UNIT HYDROGRAPHS 5.2.3 Lag Curve The watershed parameters,L,Lc and S J of each sub-basin were detet1ni.l1ed from USGS topographic maps of 1:63,360 scale..These parameters are listed in Tabll:!23.This table also shows the lag times for the sub-basins based on the c.urve shown on Exhibit 24 .. 5.3.2 Unit Duration The unit duration i~generally recommended as less than one fourth of the lag time.The lag times for the sub-basins vary from 18 to 79 hours.A unit duration of 3 hours was adopted for all sub-basins for uniformity II This duration also was used to compute sub-basin incremental rainfalls. 5.3.3 Unit Hydrographs 3-nour unit hydrograpQs were derived for eacn sub-basin using lag time given in Table 23 and respective dimensionLess graph shown on Exnibits 23.Tne resulting unit hYdrographs are shown on Exhibit 25. I I! I: 11 11 [1 J~,~ ft:.• L L SECTION 6 I'NITIAL ESTIMATES OF INFILTRATION RATES •• (J) UJ ..J CO <C.... (f) I--CO-::c Xw .. tr I I " I fe, IIf' I, I • .. 6-1 6.1 HYDROLOGIC SOIL GROUPS The infiltration rates for the sub-basins were first estimated based on hydrologic soil groups in the sub-basins and the recommended infiltration rate associated wi th each hydrologj.c group as given in "Design of Small Dams··(20,page 64).The esti1l1ated rates were then refined as discussed under "Reconstitution of Historic Floods." "Exploratory Soil Survey of Alaska"(21)includes the Susitna River basin. Data included in this publication were used to identify the types of soils in each sub-basin.Table 24 give.s the description of soils and relevant soil numbers are shown on Exhibit 26. 6.0 INITIAL ESTIMATE OF INFILTRATION RATES Based on the soil descriptions,a hydrologic group was assigned to each soil type (Table 24).Exhibit 27 shows the hydrologic soil groups. 6·.2 ilERMAFROST The infiltration rates are affected by the presence of permafrost in the sub....basins 0 Exhibit 28 shows the permafrost map of the Susitna River basin. The 1l1ap is derived using a permafrost map prepared by USGS (22). The estimated percentages of hydrologic soil groups in each sub-basin are given in Table 25 to The recommended infiltration rates (20,page 64)for each group are: 6.3 !NFILTRATION RATES 'J....,.".;.'l~;-;e.;),mil-QW&l"rM-'""·K1#n'.,....'ttiie'-~'It":"**ran;~F,'1'Mtj,__.ifi9{'~.~..:i Oi.',""~'if'-.'<~,,,-.'0 -<,'i,'.k"M •--,;.~-",-~~,~,",,-'-''.--''';.:-•.;:::',;._.:,.~,,,"'~""-,,,.••.,."'-""'~'"'-".,.''--'.",'--'-..;,:~.'....•.•""~_.~-'-",,...•_.,.;;:,;.;;"..:"--~-,~,~,~,~".,,""'-"'"-;''"''-''~'.'_"~"""-'~,,_.-''-'<~, I, I I Because of the presence of permafrost,the lower limit of reco1llmended rates was used to derive weighted infiltration ra.te for each sub-basin.Table25 shows the sstimated rates. •• .. P~nge of Minimum Rates inch/hour 0.30-0.45 0.15-0.30 0.08-0.15 0.02-0.08 A B C D Hydrologic 80i1 Group I, -..'......~'.'.'..' --...----•~-~------~...-;:•rtI I·~•... 1<'.~_'.••._._.\.'.~..~.'.'."g'.~'....• •..• • SECTION 7 RECONSTITUTION OF HISTORIC FLOODS (J) W ...Jco' <C i- -----~.----_......._--------.. 7.Sv3 Unit Hydrographs Sensitivity analysis indicated that changing the shape of unit hydrographs will not improve the results of reconstitution.Hence the unit hydrographs shown on Exhibit 25 were adopted without further adjustments. 7.5.4 Routing Coefficients Changes in channel routing coefficients also did not improve the results of reconstitution.The adopted channel routing coefficients with routing scheme are shown on Exhibit 38. ~'"~ ,.1 1 ,~----i SECTION 8 PROBABLEMAX.IMUMFLOODS .. •II roo iW -I1m..« if- f I f)-Ii E<!J ~-.. I i ~• .. • • .. These ".~J ".•••~~_~•~__--:---_~._ ~~-..~~-~---...~.~..,~.~•-I ••.• 8-2 antecedent storm,glacier melt and base flow for July-August PMF.The transposed 1971 flood was combined with the flood due to the PMP to yield the PMF.The combination was made such tha.t the peak of the 1971 flood precedes the peak of the flood due to the PMP by 3 days.The resulting floods for Watana and Devil Canyon are summarized in Table 31 and shown on Exhibits 39 and 40. The May and June floods already include a substantial amount of runoff due to the critical sno'WlIlelt discussed under the section entitled "Snowmel t.II Therefore,nominal values of base flow were added to these floods. values W"e.e aSsumed to be the mean tnQnthly flows fo.May and June 'espec- tively.The resulting floods also are summarized in Table 31 a.nd shown on Exhibits 39 and 40. 11 U .151~j 'iIiLi SECTION 10 COMPARISON WITH 100-YEAR FLOOD Of •.. enw -Jm<: I- ~ -•I•I• .. , 1 , ! I .. 'r-. 10.0 COMPARISON laTH lOO-YEAR FLOOD 10 ....1 The lOO-year flood peak at Gold Creek (drainage area =6,160 mi 2 )is about 108,000 cfs.'This peak was transposed to the two dam sites on the basis of 0.5 power of the drainage area ratios.The resulting flood peaks are about 99,000 ¢f sand 105,000 cf s at Viatan;!and Devil Can1o n ,res pectively.The respecU'!e M;;ly PMl's ar",about 3.1 Umes the 100-year flood at Watana and about:304 times the lOa-year flood at Devil Canyon .. A flood frequency curve was.developed for the susitna River at Gold Creele based on al1nual ma:ltilll\$flood peaks for the period fr01ll 1949 through 1981 al1d using procedures given by United Stat~s Water Resources Coun¢i1 (24). The station skewnesS was used in the computations.Weibu11's formula.[M of (N tl)J.in which "M"is the order number of flood peakS arranged in descertd~ ing order of magnitude and "N"is the number of years of re¢o1:d,was used to determine the plotting positions of observed data.Exhibit 42 shows the resulting flood frequencY curve. t1 ) I " -----------_..~". .'..''.'.,.'.---.-.'---Tt--~--.------.--.'~-0 --' l ••,.-."•".•."""••-.• •'.• • • '..,•'"~~'..•p ,~ ••..1 n!' l:) £1 11.0 COMl'.t\RISION WITH PREVIOUS Pm'STUDIES I'M}'studies have been made by the caE:and ACRE:S for the Watana and Devil canyon sites (1,2,3).Ro~ver,these studies give details on derivation of PMF for Watana site only.PMF peaks and volUllles from these studies and those from the present study are listed in Table 33.A brief discussion on previous studies relevant to the current study is provided below. 11.1 PI-iF A comparison of Watana Msin average PMI'is shoom ot!"""ibit 43. The CaE PMI'is based on tentative e",timates of PMI'provided by the NWS (1 ,3)•The spring l'Ml'is es timated to be ab out 7a pe rcent of ",ullllller PMI'. Thus,PMP used with snowmelt is about 6.3 inc.hes. ACRES estimated PM]'based on six storms recorded Wi thin the 'llasins during 1955-1980.The isohyetal pattern of the July 1980 storm was considered to be ~ll defined.The isohyetal patterns for the other storms were derivec\ thro\1gh isopercenta1 techniq\1E'using isohyetal pattern of the 1980 storm as the base map.Of the six storms,the August 1967 was most critical with a =imization factor of 2.0 in August.Thi'"storm wa",assumed to occur in June with a maxiJJl.izatio n factor of 1.4.This resulted in the estimated 10- day l'Ml'of 8.7 inches for the Watana Basin.The weaknesses in the ACRES PMI' estimate include,tl1e development of isopercent a1 and hence isohyetal pat- tern for the six stormS analyzed using the isohyetal map of a single ",torm as tl1e base map,the assumption of the Augu"'t 1967 storm occurring in June and the conibination of a long durat;l.on ",evere storm with extremely critical snowmelt.This ha",resulted in an overly conservative high flood volume. The narza estimate i",based on direct transposition and maximization of ",um- mer ",torms.Seasonal variation of l'MP wa",assumed to be similar to that of histo rical extr eme rainfall"'.The es tima ted all-",e aso n Pl·!l'f or \'Ill.tana is 11-1 .. •to D- r • REFERENCES I Table 2 AVERAG~PRECIPITATION AND TEMPERATURES AT SELECTED CLIlviATOLOGICAL STATIONS Station Period of Record Jan .1 Feb.I Mar.I~r.I May 1Jun ·1 Jul.IAug ·.1 Sep.IOct.i Nov.,pee.IAnnual • A.Precipitation (inch) Fc:rirbank~,1949-79 0.59 0.42 0.43 0.29 0.57 1.31 1.81 1.84\1.04 0.73 0.69 0076 10.48 B1g Delt~1942-79 0.37 0.27 0.29 0.27 0.87 2.38'2.59 1.97 1.12 0.54 0.38 0.39 11.44 Gulkana 1942--79 0.54 0.49 0.36 0.21 0.61 1.40 1.86 1.55 1.57 0.87 0.74 0.90 11.10 Rika's Landing 1969-79 0.43 0.25 0.26 0.40 0.76'1 ..83 2.04 1.79 0.98 0.81 0.91 0.50 10.96 Paxson Lake 1968-79 0.72 0.61 0.86 0.55 0.83 1.96 2.79 2.29 2.20 1.84 0.88 1.10 16.63 Talkeetna 1922-79 1.66 1.60 1.70 1.22-1.33 2.03 3.44 4.74 4.46 2.93 1.85 1.59 28.55 Summit 1951-75 .89 1 ..19 .86 .072 ..60 2.18 2 ..97 3.09 2 .•56 11157 1.29 1.11 19 ..03 B.Temperature (OF) Fairbanks 1949--79 -12.5 -5.3 8.4 29.9 48.1 59.4 61.5 56.6 45 ..1 24.9 3.5 -9.4 25.9 Big Delta 1942-79 -5.6 1.7 11.6 29.7 46.6 56.9 59.8 55.2 44.1 25.2 6.8 -4.2 27.3 Gulkana 1942-79 -7.7 2.3 14.0 30.0 43.5 53.7 57.1 53.3 43.6 27 ..4 6.6 -5.1 26.6 Rikaes Landing 1969-79 -10.1 --2.1 11.6 30.5 46.8 56.5 59.4 54 ..8 44.0 25.1 4.2 -5.0 26.3 Paxson Lake 1968-79 ....9.0 0.1 9.6 23.7 39.1 48 ..7 53.2 50.6 41.5 24.9 5.4 -3.2 23.7 Talkeetna 1922-79 8 ..2 15.1 20.0 33.0 44.7,54.7 57.9 54.9 46.1 32.8 18.4 9.2 32.9 Summit 1951-75 -0.6 5.5 9.7 23.5 37.5 48.7 52.1 48.7 39.6 23.0 9.8 3 ..0 25.0 • •-:.rTr·-J;;;;J"ft7IA---'"'.,,----~---..,,--Ext-nefrs )) Nr~ 1> Table 3 a:::::J r.;;:;;ao;....... No.Name SELECTED STREA~-i GAGING STATIONS Lat.Long. Drainage ·2Area,m1.Period of Record I I I (1)(2)(3)(4)(5)(6) 15277100 15277410 15281000 15282000 15284000 15290000 15291000 15291200 15291500 15292000 15292400 15292700 15292780 15294005 15294010 15294100 15294300 15294345 15294350 15294450 15294500 Eagle River at Eagle River Peters Creek near Birchwood 1<nik River near Palmer Caribou Creek near Sutton Matanuska River at Palmer Little Susitna River near Palmer Snsitna River near Denali Maclaren River near Paxson Susi-tna River near Cantwell Susitna River at Gold Creek Chulitna River near Tal :etna Talkeetna River near Talkeetna Susitna River at Sunshine willow Creek near Willow Deception Creek neaL'Willow Deshka River near Willow Skwentna River near Skwentna Yentna River near Susitna Station Susitna River at Susitna Station Chuitna River near Tyonek Chakacbatna River near Tyonek 61°18'149°34'192*Oct.1965-Jun.1981 61°25'149°29'87.8 Aug.1973--Sep.1981 61°30 149°02'1,180*Oct.1959-Sep.1981 61°48'147°41'289 r~ay 1955-Sep.1981 61°35'149°04'2,070*Apr.1949-Sep.1981 61°43'149°14'61.9 Jul.1948-Sep.1981 63°06'147°31'950 May 1957-Sep.1966 Jul.1968-Sep.1981 63°07~146°32'280 IJun.1958-Sep.1981 62°42'147°33'4,140*r~ay 1961-Sep.1972 Jun.1980-Sep.1981 62°46'149°41'6,160*IAU 9 •1949-Sep.1981 62°34 w 150°14'2,570*Feb.1958-Sep.1972 May 1980-Sep ..1981 62°21'150°01'.2,006 ~Jun.1964-Sep.1981 62°11'150°11'11,100*May 1981-Sep.1981 611)47"1491.153'166 Jun.1978-Sep.1981 61°45'149°56'48 May 1978-Sep.1981 61°46'150°20'592 Oct.1978-Sep.1981 61°52'150°22'2,250*Oct.1959-Sep.1981 61°42'150°39'6,180*Oct.1980-Sep.1981 61°32 1 150°31'19,400*Oct.1974-Sep.1981 61°07'151°15'131 Oct.1975-Sep.1981 61°13'152°22'1,120*Jun.1959-Sep.1972 • .- *Approximate ~~"'-'~""""-~A---~-~""'---:---.,,--'"EXHilB1TS-.c=~---I!~~. • • Table 4 SNOW SURVEY STATIONS • Period of Record No.of No.Station Lat.Loney.E1ev.From To Years (N)(W -(ft) 1 Monahan 63°18 ,147 °39 ,2,710 1964 1982 19 2 Clearwater Lake 62°59 ,146 °58 ,3,100 1964 1981 18 3 Lake Louise 62°17 ,146°30 ,2,400 1964 1982 19 4 Little Ne1china 62°07 ,147 °36 ,4,160 1968 1981 14 5 Oshetna 62°23'147°29 ,2,950 1964 1981 18 6 Fog Lakes No.1 62°47 ,148°30 ,2,270 1964 1982 19 7 Devil Canyon 62°39 ,149 018'1 350 1977 1982 6, 8 Buttle Creek 63°01 ,147 °54~3,000 1981 1982 2 9 Cirque 63°28'147 °27 ,4,700 1981 1982 2 10 Ice Cave 63 °30'147 °25 ,4,000 1981 1982 2 11 w.Fork Glacier 63°33'147 °10·5,050 1981 1982 2 12 ~It •Hayes 63°31 ,146 Cl54 ~4,150 1981 1982 2 13 Caribou 63°25 •147°05 ,4,100 1981 1982 2 14 Male Mute 63°23 ,147 °Il ,2,600 1981 1982 2 ( t 15 Jatu Pass 63°27 ,146°44'4,500 1981 1982 2 \fJ) 16 Pyramid 63°25 I 146°53 I ;~ 4,800 1981 1982 2 'ia- 17 East.Fork 63°24'146°51'2,850 1981 1982 2 ~18 Watana 62°50 I 148 °24 I 2,200 1981 1982 2 19 Kosina Creek 62°42 I 147°59 ,2,600 1981 1982 2 20 'l'yone River 62°40 ,147°06 •2,500 1981 1982 2 21 Denali 63°06 •147 °27 •2,700 1981 1982 2 Note:East Fork at elevation 5,.200 ft l Valdez Creek and BoUlder North were installed in 1982. •• .. .43 in. .16 in .. 1.45 in. -0.79 1.45 -.43 1.45 -.16 Table 5 COMPUTATION OF BARRIER ADJUSTMENT FOR TRANSPOSING THE AUGUST 12-14,1967 STORM TO THE SUSITNA BASIN Total precipitable water: Precipitable water in place,1000 ft.barrier Precipitable water blocked by 3000 ft.barrier Adjustment - Maximum 12-hour Dewpoint: I Accumulated Percent of 72-hour Value Incremental 6-hour Percentages Above Watana Accumulated PHP (in) Incremental PMP (in) Above Devil Canyon Accumulated P~1P {in) Incremental PMP (in) 1-. I:. .. ' Table 6 6-HOUR ACCUMULATED AND INCREMENTAL PMP Hour 6 12 18 24 30 36 42 48 54 60 66 72 30 43 53 61 68 74 80 85 89 93 97 100 30 13 10 8 7 6 6 5 4 4 4 3 2.0 2.9 3.6 4.1 4.6 5.0 5.4 5 . .7 6.0 6.3 6.6 6.8 2.0 0.9 0.7 Oo5 0.5 0.4 0.4 0.3 Oo3 0.3 0.3 0.2 2.2 3.1 3.8 4.4 ?.3 5.7 6.1 6.4 1).7 7.0 7.2 2.2 0.9 0.7 0.6 0.5 0.4 0.4 0.4 0.3 0.3 0.3 0.2 J 1 r r r •t}/J. 1---. '....... ·to-..:1: JC;jI... • .. Weight.ed Depth (in ..) (6) Percent of Area in Band -It:.)\.... PMP for Base Value of 4 ..5 in ..at 20-inch MAP (4) PMP in Percent of Base value1../ (3) ADJUSTMF.NT OF BASE NON-OROGRAPHIC PMP FOR OROGRAPHY IN WATANA BASIN P~1P from Graph (Exhibit J.l) (2) Base value is PMP a.t <20 inches MAP .. Table 7 MAP (in ..) Band (1) 1.1 <20 6.5 100 4 .5 36 1 ..6 I 20-30 7.3 112 5 .0 27 1 • 4 ,T 1.fI 30-40 8.6 132 5 ..9 15 ..9 40-50 9.5 146 6 ..0 10 ..7 50-60 10.3 158 7 .1 5 ..4 60-70 II ..0 169 7 .6 3 ..2 >70 11 ..3 174 7 ..8 3 ..2 -5 .4 It I I~rl~~-~------~-_._"_._---_._. 4i..! Tablp 8 • ~ I ~•I•I I I(JJ11-1m :fxw .. .. May June JUly August .61 1.10 1,,29 1 .851./ 31 55 65 931..1 .83 1 .05 1 .20 .701.1 69 87 100 58.&./ 1.25 1.29 2 ..04 1 .821/ fil 63 100 89'&.1 .94 .97 1.40 .951/ 07 69 100 681.1 .69 1.30 1.22 1.111.1 53 100 9L1 85.&.1 .78 2.67 1.90 1.311/ 29 100 71 4 gg,,1 .51 .90 1 .78 1.001/ 28 51 100 56£.1 48 75 90 71 53 83 100 78 VARIATION IN GRFATEST OBSERVED ONE-DAY PRECIPITATION OF RECORD ?OR STATIONS NEAR THE snSITNA BASIN WIT3 10 OR MORE YEARS OF RECORD Station No.March April Alpine 1 1.98 ,.80 100 40 Gunsight 2 #33 .61 27 51 Gulkana 3 .81 .28 40 13 Snow Shop Lak~4 ..60 .75 43 53 Rika 's Landing 5 .30 .37 23 28 Rapids 6 1 .07 1.28 40 48 Paxon Lake 7 .95 .73 53 41 Avg.Percentage 46 39 Percent of July 51 43 l/Depth in inches ~I Percent of highest value u···.··. ~.. I _~____,_..~:o"-;-_V_"_--·_..._~_:-__q •.••••~._.__.._---.......~ Station No.March ~ril _M~June JUly ~usust~(see Table 8) 1 2.85 1 .33 1.70 2.80 3 ..21 6 ..03 2 .56 .85 1.83 4.35 3 ..26 3.19 3 1.32 .82 1.51 4.07 3.32 4.19 4 .82 .83 2.29 3.54 4.26 3 .13 5 .56 .78 1 .44 3.52 4.59 2.94 6 1 .55 1 .29 2.02 6.73 5.91 4.318 7 2.65 1.60 1 ..52 3.20 5.74 4.44 Percent of JUly 34 25 40 93 100 93 VARIATION OF GREATEST MONTHLY PRECIPITATION (inches) .. 62 61 100 95 July August 1 ..53 1.45 57 78 June 1.19 48 49 May .76 38 .59 ~ril 43 Table 10 Table 9 40 33 ..50 March VARIATION IN PRECIPITABLE ~vATER ASSOCIATED WITH MAXIMUM l2-HOUR PERSI.8TING DE~TPOINTS IN THE SUSITNA BASIN Mid-month Dewpoint (OF) Precipitable water (in ..) Percent of July 1L Table 11 SUB-BASIN RAINFALL (inches) Arranged Sub-basins Time seruence(hr)%)1 2 3 4 5 6 7 8 9 10 0-3 1 .10 .08 .0P,.06 .04 .08 .09 .09 .05 .08 3-6 2 .20 .17 .16 .12 .07 .16 .19 .18 .09 .15 6-9 2 .20 .17 .16 .12 .07 .16 .19 .18 .09 .15 9-12 2 .20 .17 .16 .12 .07 ,,16 .19 1118 .09 .15 12-15 2 .20 .17 .16 ..12 .07 .16 ,,19 .18 .09 ..15 15-18 2 .20 .17 .16 .12 1107 .16 .19 .18 .09 .15 18-21 3 .29 .25 .23 .18 .11 .24 .28 .27 .14 .23 21-24 3 .29 .25 .23 .18 .11 .24 .28 .27 .14 .23 24-27 3 ..29 .25 .23 .18 ;11 .24 ..28 .27 .14 ..23 27 ....30 4 .39 .33 .31 .24 .15 ~33 1136 .35 .19 .32 30-33 5 .49 .43 .39 .32 .19 .42 .46 .45 .23 ..39 33-36 5 .49 ..43 .39 .32 1119 .42 .46 ..45 .23 .39 36-39 19 1.86 1.59 1.49 1.16 .71 1.56 1 ..76 1.70 .88 1.47 39-42 11 1.08 .92 .86 .68 .42 .91 1.02 .98 .52 .86 42-45 7 .69 .59 .56 .44 .27 .58 .64 .63 .33 .54 45-48 6 .59 .50 .47 .38 .22 .49 .56 .54 .28 ..46 48-51 4 .39 .33 .31 .24 .1.5 .33 .36 .35 .19 .32 51-54 4 .39 .33 .31 .24 .15 .33 .36 ..35 .19 .32 54-57 3 .29 .25 .23 .18 .11 .24 .28 .27 ..14 ..23 57-60 3 .29 .25 .23 .18 .11 .24 .28 .27 .14 .23 60-63 3 .29 .25 .23 ..18 .11 .24 .28 .27 .14 .23 63-66 2 .20 .17 .16 .12 .07 .16 .19 .18 .09 .15 66-69 2 .20 ..17 .16 .12 .07 .16 .19 .18 .09 .15 69-72 2 .20 .17 ..16 .12 .07 .16 .19 .18 .09 .15 Total 9 ..81 8.39 7.83 6.12 3.71 8.16 9.29 8.95 4.65 7.73 • Table 12 ·.. ... 12.6 13.0 23.3 14.01./ 15.6 18 ..5 29.5 34.9 38.6 Snowpack Water Equivalent (in.) 100 III 147 124 185 234 277 306 Percent of a-I ,000 ft Elevationl l VARIATION OF SNOv.lPACK WITH ELEVATION 4,000 Taken from Table 2-5,HMR 42 (12). laO-year snow water equivalent with 95 percent one sided upper confidence limit,based on frequency analyses of annual maximum.water equivalents observed at six snow courses in and around Susitna Rasin with 14 to 19 years of record~elevations of snow courses ranged between 2,270 to 3,100 ft,msl with the exception of one station at 4160 ft,msl. Elevation Zone (ft) o -1,000 2,000 -3.000 5,000 -6,000 1,000 -2,oon 3,000 4,000 -5,000 7,000 -8,000 6,000 -7,000 8,000 -9,000 1.1 1.1 ..• .. Elevation Band (ft.) MAXIMUM DAILY SNOWMELT WINDS (MPH)AT ANEMOMETER LEVEL Table 13 500-1000-2000-3000-4000-5000-6000-7000-8000-9000- Day 1000 2000 3000 4000 5000 6000 7000 8000 9000 10,000 Source:BMR 42 (14). 1 30 30 33 36 38 41 42 44 45 46 2 21 21 24 26 28 29 30 32 32 33 '~3 17 17 19 21 22 24 24 26 26 27 4 15 15 17 18 20 21 21 23 23 24 5 14 14 15 17 18 19 20 21 21 22 6 13 13 15 16 17 18 19 20 20 21 7 13 13 14 15 16 17 18 19 19 19 8.12 12 13 14 15 16 17 18 18 18 9 11 11 13 14 15 16 16 17 17 18 ~•..10 II 11 12 13 14 15 15 16 16 17 11 10 10 11 12 13 14 14 15 15 16 fIt1210101112131414151515! 13 10 10 11 12 12 13 13 14 14 15 I I14991011121313141414 CIJ.J-o--15 9 9 10 11 12 12 13 13 14 14 «J-'I ':I:,>< 20 8 8 9 10 10 11 11 12 12 12 !'UJ I Table 14 ARRANGED nAILY SNOWMELT L~IF TEMPERATURES f DEw""POINTS AND WIND FOR MAY PMP Air Temperatures.!...1 Dewpointsl/Wind2./-of of mph Elev.-----------------------'..,,_._-._------ Zone,ft Up to 2,000-3 ,000-Up to 2 ,oon-3 ,000-2,.000-3 ,000-4 ,000- Date 2,.000 3,000 4 ..000 2 ,000 3,000 4,.000 3 ,000 4 ..000 5 ,000 May 12 59 .6 58.1 55 .1 39 .6 38 .1 35.1 15 17 1813-11 65.4 63.9 60.9 40.4 38.9 35.9 17 18 20 14 62 .2 60.7 57.7 42 .0 40 .5 37 .5 10 11 12 15 58 •a 56.5 53.5 44.0 42.5 39 ..5 10 11 1216~.1 46.0 44.5 41 ..5 44.0 42 ..5 39.5 24 26 2817.!±.1 48 .°46.5 43.5 46.0 44.5 41 ..5 33 36 3818:t1 44.0 42.2 39.5 42.0 40 ..5 37 ..5 19 21 221965.4 63.9 60.9 40 .4 38.9 35 .9 10 10 122064.9 03.4 60.4 39 ..9 38 ..4 35 ..4 10 10 122164.4 62.9 59 .9 39 .4 37 .9 34 •9 10 10 122263.9 62.4 59.4 38 .9 37.4 34.4 10 10 122363.4 61.9 58.9 38 .4 36 .9 33.9 10 10 122462.9 61.4 58 .4 37.9 36.4 33 ..4 10 10 122562.4 60.9 57.9 37.4 35 .9 32.9 10 10 122661.9 60.4 57I~35.4 32.4 10 10 12 -- 1.1 For lowest zone of a sub-basin: Zone 1,000 -2,000,sub-basins 1,2,3,4 Zone 2,000 -3,000:suh-basins 5,~,7,8,q Zone 3,000 -4,000;sub-hasin 10 ~I For average elevation of a sub-basin: Zone 2,000 -3,OnO;sub-basins 5 Zone 3,Ono -4,000;sub-basins 1,2,3,6,7,9 Zone 4,000 -5,000,sub-basins 4,8,10 1.1 Highest temperature tv/a days before PMP. ~I Three days of PMP. .. .. • ARRANGED DAILY SNOv,IMELT AIR TEMPERATURES. DE~~OINTS AND WIND FOR JUNE PMP • J I.. I • I•I It• .. --- slJ Dewpointsl/Wind2../ OF mph -.-'----.'---f--------....--j-----_.--- 00-Up to 2 ,000-3 ,000-2.000-3,000-4,000- 00 2 ,000 3 ,000 4 ,000 3,000 4,000 5,000 .5 25 .0 23 ..5 20 ..5 9 9 10 .5 27.0 25.5 22.5 9 9 10 .5 23 ..0 21 .5 18 .5 9 9 10 ·5 23.0 21.5 18 .5 9 9 10 .5 22 ..0 20 .5 17 ..5 9 9 10 .5 25.0 23.5 20.5 9 9 10 •5 25 .0 23.5 20.5 9 9 10 • 5 25.0 23.5 20.5 9 9 10 .5 26 .0 24 .5 21 .5 9 9 10 .5 26 ..0 24.5 22 ..5 9 9 10 .5 27 .0 25 .5 19 .5 9 9 10 ·5 24 ..0 22.5 24.5 9 9 10 ..5 29 ..0 27 .5 21 .5 9 10 11 . 5 26.0 24.5 21 .5 9 10 1] 35 26 .0 24 .5 21 .5 9 10 11 .5 27.0 25.5 22.5 9 10 11 .5 24.0 22.5 19 ..5 10 11 11 ,----- .5 30.0 28.5 25.5 10 11 12 .5 31.0 29 .5 26.5 10 11 12 ..5 29 .0 27 .5 24.5 10 11 12 •5 31 .0 29 ..5 20.5 ]1 12 12 ..5 30.0 28 ..5 25 ..5 11 12 13 .5 30 ..0 28 .5 25 ..5 II 12 13 ·5 32.0 30.5 27.5 12 13 14 I.5 31.0 29.5 26.5 13 14 15 ·5 31.0 29.5 26 ..5 13 14 15 .5 31.0 29 .5 26 ..5 14 15 16..5 31 ..0 29.5 26.5 15 16 17 .4 47 .9 46 .4 43 .3 15 17 18 .5 47.0 45 ..5 42.5 17 18 20 .3 49.8 48 .3 45 .3 10 11 12--_-..- '--"'----"""'" Tahle 15 34 36 32 32 31 34 34 34 35 35 36 33 38 35 35 36 33 39 40 3R 40 39 39 41 40 40 40 40 63 62 65 -----,--- Air Temperature of ~~~:.;ft~~OI2.0-0n-3.~ Date 2,000 3,000 4,0 May 15 39 .0 37 .5 16 41 .0 39 .5 17 37.0 35 .5 18 37.0 35 ..5 19 36 .0 34.5 20 39 ..0 37 ..5 21 39.0 37 ..5 22 39 .0 37.5 23 40 ..0 38 .5 24 40 ,,0 38.5 25 41 ..0 39 .5 26 38 ..0 36.5 27 43 .0 41 .5 28 40.0 38.5 29 40 .0 38 .5 30 41.0 39.5 31 38.0 36 .5 -,-- June 1 44.0 42.5 2 45 .0 43 .5 3 43 ..0 41.5 4 45 .0 43 ..5 5 44.0 42.5 0 44 ..0 42.5 7 46 ..0 44.5 8 45 .0 43 ..5 9 45.0 43.5 10 45 .0 43.5 II 45 ..0 43 ..5 12 67 ..9 66.4 13.2./72.0 70 ..5 14 69 ..R 6R .3 I---___" ..• .. 2 ,3 ,4 6 ,7 ,8 ,9 Table 15 (Cont'd) 1./For lowest zone of a sub-basin: Zone 1,000 -2,000;sub-basins 1, Zone 2,000 -3,OOO~sub-basins 5, Zone 3,000 -4.000;sub-basin 10 ZI For average elevation of a sub-basin~ Zone 2,000 -3,000:sub-basins 5 Zone 3,000 -4,000;sub-basins 1.2,3,6,7,9 Zone 4,000 -5,OOO~sub-basins 4,8,10 1.1 Highest temperature two days before PMP .. 41 Three days of PMP. - Air Tern per a t ure sl/Dewpointsl..l Windl.l of QP mph IElev.------------.._-~--....._----..- Zone,ft up to 2,000-3,000-Up to 2,000-3,000-2 ,000 ....3 ,000-4 ,000- Date 2,000 3 pOOO 4 ,000 2 ,000 3 ,,000 4 ,000 3 1'000 4 ,000 5 ,000 June 15 66.3 64 .8 61.8 50 .3 48 .8 45 .8 10 11 12 16Jil 55 .5 54.0 51.0 53.5 52.0 ~9 .0 24 26 28 17J±/57,,5 56.0 53.0 55.5 54.0 51.0 33 36 38 l8~1 53.2 51.7 48.7 51.2 49.7 46.7 19 21 22 19 72.0 70 .5 67 .5 47 .0 45.5 42 .5 10 10 12 20 71.5 70.0 67.0 46 .5 45.0 42.0 10 10 12 21 71 ..0 69.5 66.5 46.0 44 .5 41 •5 10 10 12 22 70.5 69.0 66.0 45 ..5 44.0 41.0 10 10 12 23 70.0 fiR .5 65.5 45 ..0 43 .5 40 .5 10 10 12 24 69.5 68 .0 65.0 44.5 43.0 40.0 10 10 12 25 69 .0 67.5 64 .5 44.0 42.5 39 ..5 10 10 12 26 68.5 67.0 64.0 43.5 42.0 39 ..0 10 10 12 _.......-------IIIIIIJII----....---..------------....------~~....------.......~ Month Day April May June 1 52 .0 65 .4 7 2.0 2 49.4 62 •2 69 •8 3 46 .8 59 .6 67.9 4 44 .9 58 .0 66 .3 5 43 .4 56 .3 65 •0 6 42.3 54 '"9 63 •8 7 42 •0 53.8 62 •7 8 41 •9 52 •8 61 .8 9 4 1 .8 51 08 61 •1 10 41 • 7 50 •9 60 •5 ~.1 1 41 ~6 50 .3 60 ~1 12 41 .5 49 • 7 59.6 I1341.4 49 .2 59 •1 I1441.4 48 9 58 6•• 15 41 .3 48 .6 58 .2 (,fJ, 1--16 41 •3 48 •5 57 .8 1-'to1741•2 48 .4 57 .5 r'i:IX1841•2 48 •2 57 .3 W 19 4 1 .1 48 ..1 57 •1 20 41 •1 48 .0 57 •0 .'•-.1 •.. '" .. • .. Table 16 MAXIMUM DAILY TEMPERATURE PRIOR TO SPRING PMP (OF)l/ 11 Based on IOO-year return period temperatures at Anchorage. Note:These temperatures ar0 applicable to elevation up to 2000 ft.For higher elevations subtract 3°for each 1000 ft above 2000 ft. ......'.I ....•-.'. II These dewpoints are applicable for elevations up to 2000 ft. Above this elevation decrease at the rate of 3°P per 1000 ft. Table 17 .. I I l'~ I.•.Y.i~....,~,,£9 .J: .~ W • June 54.0 52 ..0 49.7 May Month 44.5 42.5 40.5 43.0 47.6 57 ..0 41.5 46.0 55 ,... .:::> 40.0 44 ..0 53.5 37 .0 42 ..0 51 •2 April 40.0 38.5 35.5 Maximum l2-Hour Persisting Dewpoint Sea Levelll DEWPOINTS (Op)FOR 3-DAY MID-~ONTH PMP STORM For 2000-3000 Elevation Band Greatest 1st day Greatest 2nd day Greatest 3rd day Grea test 1 st day Greatest 2nd day Greatest 3rd day L ~,, Table 19 DAILY NET RADIATION (Langleys) FOR JUNE PMP Elevation Zones ,it Elevation Zones,ft 2 ,000 3 ,000-4 ,000-2 ,000 3 ,000-4,000- Date 3 ,,000.1.I 4,aooU 5 ,00 O.a.l Date 3 ,QOO~I 4 ,000 2..1 5 ,OOOa.; - May 15 438 462 487 June 1 548 577 605 16 443 468 493 2 554 583 611 17 450 475 500 3 565 594 623 18 456 481 506 4 577 607 636 19 460 485 511 5 579 609 639 20 463 489 515 6 590 621 651 21 470 496 522 7 603 634 665 22 476 502 528 8 622 653 685 23 480 506 532 9 641 674 707 24 483 509 536 10 673 696 730 25 487 513 539 11 697 732 767 26 494 520 547 12 863 902 940 27 500 527 553 13 884 '323 961 28 506 533 560 14 846 883 920 29 509 536 563 15~I 751 785 819 30 523 551 578 16.!±.!705 732 759 31 535 563 591 17JiJ 711 738 765 18 674 701 727 19 20 966 1005 1043 21 961 1000 1038 22 913 950 986 23 883 918 954 24 856 891 925 25 817 850 883 26 799 831 862 ..-.-----...i 1.1 Sun-bas inS ~I Sub-basins 1,2,3,6,7,9 ~I Sub-hsins 4,8,10 ~I Three days of PMP .. • L L I ':~ I t II, 1 I I, ! Elevation Zones,ft 1000-2000-3000-4000-5000-6000-7000-8000- 2000 3000 4000 5000 6000 7000 8000 9000 Sub-basin 1 Area (mi 2 )57 223 206 136 6 2 Water Equivalent (in)13.9 14.9 15 ..6 18 .5 23 .3 29 .5 MAP (in)30 30 32 32 35 40 Sub-basin 2 Area (mi 2 )21 163 148 125 3 Water Equivalent (in)13 .0 14 .0 15 .6 18.5 23 •3 MAP (in)30 30 30 35 40ISub-basin 3 Area (mi 2 )11 91 246 165 59 8 Water Equivalent (in)13 .0 14.0 15.6 18.5 23 .3 29 .5,r.1AP (in)30 30 30 35 35 40 "Sub--basin 4 Area (mi 2 )3 110 223 237 128 24 IJWaterEquivalent(in)13 .0 14.0 15 .6 18 .5 23 ..3 29 ..5 !- MAP (in)20 20 25 30 35 40 1'j ISub-basin 5 Area (mi2 )776 244 23 17 jWaterEquivalent(in)14.0 15.6 18.5 23 ..3 , MAP (in)20 20 20 25 l<nSUb-basin 6 Area (mi 2 )295 342 142 8 3 J ....... 1-Water Equivalent (in)14.0 15 •6 18 .5 23 .3 29 ~.1£0....\1-MAP (in)25 35 50 55 55 1::I:IX,Sub-basin 7 I .,lWa(mi 2 )A Area 55 49 61 23 IWaterEquivalent(in)14.0 15 .6 18 .5 23.3romp(in)20 30 45 45 I,. SUb-basin 8 , --=M (mi 2 )IArea151160232121413522! Water Equivalent (in)14.0 15.6 18 ..5 23 .3 29 •5 34.9 38.6 r~IAP (in)30 40 45 50 55 60 70SUb-ba$in 9 Area (mi 2 )153 177 5 Water Equivalent (in)14.0 15 .6 18 •5 HAP (in)20 30 30 SUb-basin 10 Area (mi 2 )98 106 44 29 3WaterEquivalent(in)15 •6 18 ..5 23 .3 29 .5 34.9MAP(in)40 50 60 65 65 .. • .. VARIATION OF SNOWPACK AND ~1EAN ANNUAL PRECIPITATION WITH ELEVATION ZONES IN THE SUB-BASINS Table 20 SELECTED FLOOD EVENTS IJi ttle Susitna River nr.Palmer • •• 5,890 2,680 2,630 3,420 773 236,000 144,000 49,600 35,900 45,600 57,700 1,800 3,650 4,330 8,600 2,400 39,700 21,500 82,100 31,700 1,790 5,160 2,550 1,890 2,560 2,340 2,530 2,030 (cfs) (5) Flood Peak Paoe 1 of 4 oJ r'· ~Jl. (4) Flood Event-. ~; Sep.15-25,1967 Sep.11-15,1972* Jul.05-18,1975 Sep.10-20,1978* Jul.25-31,1980 Jul.09-16,1965 Jun.22-28,1966 Aug.08-13,1966 Sep.05-25,1967 Aug.07-18,1971 Aug.06-19,1981 Jul.26-28,1967* Aug.09-12,1971* Jun.13-15,1973* Jul.13-15,1975 Jun.05-15,1978 Jun.04-14,1964 Aug.02--17,1967 Aug.06-12,1971 Jun.06-20,1972 Aug.22-28,1955 Aug..19-25,1959 Jul.18-22,1967* Aug.13-16,1967* Sep8 11-14,1972* Aug.20-23,1913* Ju1a 21-29,1980* Jul.09-12,1981* ~ 289 192 61.9 2,070 87.8 1,180 fl:. (mi2 ) (3) Drainage Area """!fi;.1. .--~-:,1. Table 21 .~"r<L" EX HilB1TS 11""~ Station Name (2) Matanuska R1.ver at Palmer caribou Creek nr.Sutton Eagle River at Eagle River Peters ('reek nr.Birchwood Knick River nr.Palmer ~- (1) 15290000 15284000 15282000 15277410 15281000 15277100 Station No. .. • SELECTED FLOOD EVENTS • If 76,200 59,000 62,700 62,000 14,800 38,200 16 6 400 23,900 21,300 5,700 9,260 5,700 5,950 38,800 28,500 18,000 90,700 87,400 82,600 31,800 43,500 51,700 65,300 39,200 33,200 61,800 (cfs) (5) Flood Peak Page 2 of 4 AL:~.. Aug.19-25,1959 Aug.04-20,1971 Jun.10-25,1972 Jul.26-31,1980* Jul.09-13,1981* Jul.27-31,1958 Aug.09-13,1971* S8p.10-14,1975* Jul.27-31,1980* Aug.02-17,1967 Jul.25·-31,1980 Sep~13-21,1980 Jun.04-10,1964 Auq.04-20,1971 Jun.10-22,1972 Sep.10-17,1975 Jun.17-27,1980 Jul.25-31,1980 Jul.09-15,1981 Jul.10-15,1959 Aug.08-22,1959 Jun.28- Jul.03,1967 Jul.15-24,1967 Jul.25-31,1980 Jul.31- Aug.06,1981 A.ug.12-19,1981 Flood Event k , (4) 2,570 950 280 6,160 4,140 ~:, (mi 2 ) (3) brainage Area lIT Table 21 ~ EXHil'BITS -----~..--.. ~~" Station Name (2) Chulitna River nr.Talkeetna Susitna River at Gold Creek susitna River nr.Canic.wel1 Maclaren River nr.Paxson susitna River nr.Denali 15292400 (1) 15292000 15291500 15291200 15291000 station No. # • 15294005 Willow Creek nr.Willow 166 15294100 Deshka River nr.Willow 592 • •.. .~~,;;:.;,...i.....-J 4,550 2,040 9,900 3,470 45,700 40,900 38,100 16,800 32,000 23,500 36,500 4,890 1,990 2,620 4,380 8,140 31,000 32,000 46,000 33,500 100,000 85,000 167,000 230,000 (cfs) (5) F.lood Peak Page 3 of 4 Flood Event ". t> (4) Aug.12-15,1967* May 19-29,1969 Jul.14-17,1979 Jul.23-29,1979 Jul.27- Aug.01,1980* Jul.09-14,1981* Ju1 ..31- .1-\.u 9 • 05, 1 981 Jul.27-29,1980* Jul.09~12.1981* Nov.07-15,1979 Jun.30- Jul.OS,1980 Jul.10-16,1980 Jul.25-31,1980 Sep.13-20,1980* Aug.01-07,1981* Aug.10-22,198J. Aug.13-20,1967 Aug.05-17,1971 Jul.10-18,1980 Aug.06-11,1981 Jul.09-20,1981 Aug.01-09,1981 Jun.23-30,1975 Jul.25-31,1980 2,250 6,180 2,006 19,400 (mi 2 ) (3) Drainage Area J""" Table 21 SELECTED FLOOD EVENTS . ~ (2) eXHillBITS ;r h. Sta.tion Name '1(__ "7'""'....--;---..............-.------~,.......--. y.- U,~ Skwentna River nr.Skwentna Yentna River nr.Susitna Station Susitna River at Susitna Station Talkeetna River nr.Talkeetna 1'J"f'"'~.............~,",.....·.4%"":1 (1) 15292700 15294300 15294350 15294345 Station No •. ~~ # • *Flood events analysed for dimensionless graphs. SELECTED FLOOD EVENTS • It T 6,630 7,620 2,340 4,660 1,760 22,200 (cfs) (5 ) Flood Peak Page 4 of 4 (4) 1 Flood Event r~-~-. Sep.18-27,1977 Nov.04-20,1979 Jul.09-15,1981 Aug.11-14,1981 Sep.19-22,1981 Aug.04-20,1967 t --- 1,120 131 (mi 2 ) (3) " Drainage Area r. Table 21 () tt _.:::::::;;.t.~ EXHiI:~nrs~-~-·_·......•...-.,jIr: ';~\ v (2) Station Name Chuitna River nr.Tyonek Chakachatna River nr.Tyonek -*-~C~.1(:~-"""t'tCr4llllll!tt -"1'!~.~"t:<~ (1) 15294450 15294500 Station No .• -_.~---_.~--- .."'~~ • I 1/assumed equal to time to peak • Little Susitna River nr.Palmer " 10 r .--,I....•lc.''::7)t.r;.;;;,.r::~~ Table 22 l::~""""'" Length of Main Length to Overall Area Stream Centroid Sl07e 1'-La 01./ A,mi 2 L,mi LL c -:-I S ~L e ,mi S ft~TI1i 289 31.2 13.5 151 34 12 62 16.2 7.3 366 6 15 192 29.0 16.6 131 42 33 2006 88.5 48.7 89 457 27 166 19.8 9.3 280 11 16 950 58.8 24.5 109 138 39 280 31,.7 17 ..9 155 46 27 592 69 ..3 33.9 288 533 57 ,~.- WATERSHED PARAMETERS OF BASINS ABOVE SELECTED STREAM GAGING STATIONS -'''~EXHil'SrrS -~~~.~",,-' 'I;t~~s.:.~."1C:~'~~ Gag:iT!9 Station Caribou Creek nr.Sutton Eagle River nr.Eagle Maclaren River nr.Paxson Talkeetna River nr.Talkeetna Susitna River nr.Denali Deshka River nr.Willow Willow Creek nr.Willow ?...__~~~~·~t~ ~:::::::-.::::t~~4' • • .. 3,170 3,310 3,760 4,110 2,710 3,290 3,800 4,300 3,080 4,400 .1:\'-»-J 42 28 30 48 79 46 18 36 42 28 33 Average Lag l !Elevation (hr)(ft) 169 49 58 244 1174 213 12 98 163 46 81 LLc )1"I/s J IA"- 86 135 123 83 28 29 115 125 35 155 101 Overall Slope of of Longest Watercourse -(S,ft/mi) L "'" Table 23 L _Q ----~_. 26.8 17.2 15.3 36.8 58.0 18.1 3.8 21.5 22.7 17.9 19.4 t_ IJength to Centroid of Sub-basin (Ln ,mi) ~... [--. EXHIB·JTs 58.5 33.1 42.0 60.2 106.7 63.3 19.8 50.9 !·2.7 31.7 41 ..9 Length of Longest Watercourse (L,mi)- 't._ WATERSHED PARA~1ETERS OF SUB-BASINS UPSTREAM FROM GOLD CREEK -:-/lslo ·32 J!¢:":"" 630 460 580 725 1,060 790 188 762 335 280 350 (A,miZf Drainage Area .~ \<<e: -8.2 (LIJn"-'" .~,.,,.C' Lag 1 2 3 4 5 6 7 B 9 10 11 I.-------..•......,...&...••••r Sub-basin tU' 1/ • • Tahle 24 SOILS DESCRIPTION • u Soil Number EA2 101 102 IU2 Descriptionl.! Sandy,nearly level,major components poorly to to very poorly drained,water table near surface Clayey,nearly level to rolling,~ajor components ~orly drained over shallow permafrost Loamy,nearly level to rolling,major components poorly drained,shallow to moderately deep perma- frost Very gra'Telly,hilly to steep,major components well drained soils,permafrost many feet deep d 1 .2/Hy ro.oglc- Group n D D B IU3 it ... RMI ~ ki 5010 i 5015iJ I 50164 5017 Very gravelly,hilly to steep -rough mountainous land,major components (55%)well-drained soils, permafrost deep,other components contain areas of rock outcrop and poorly drained soils with shallow permafrost Rough mountainous land,steep rocky slopes,ice- fields and glaciers. Very gravelly,hilly to steep,well drained Very gravelly,nearly level to rolling,\",ell to very poorly drained,shallow to no permafrost Very gravelly,hilly to steep also loamy nearly level to rolling,well to poorly drained,shallow to deep permafrost Very gravelly,hilly to steep,rough mountainous, well to poorly drained,shallow to no permafrost C n B C C C I 1.!Source :Exploratory Soil Survey of Alaska,U.S.Department of Agriculture. ~/Estimated from soil description. .. Percent Hydrologic Soil Groupsll Initial1.1 Adopted Suh-basin A B C D Estimate Rate1-/---- ---- I 6 74 20 ..07 .03 2 94 6 .08 ..03 3 22 59 19 .08 .03 4 36 19 45 .08 .04 5 13 23 64 .05 .04 6 82 18 ..07 .04 7 98 2 .08 .04 8 27 73 .04 ..03 9 34 66 .04 .04 10 56 44 .05 .04 11 5 58 37 .06 ..03 1:../Based on lower limit of recommendeo rate due to distribution of permafrost in the Rasins (20}. .. • li ---,;.--·~l:::1r=-::1t;:,-)1;''::":'1:1::'',:-~It:':',~_.!lI:t•.t.>~j-~~ Table 25 ADOPTED RETENTION RATES FOR SUB-BASINS EXHll:SJTS~~·~~'-'-~'-.~,. l/See Exhibit 27. :1./Rasecl on reconstitution of historic flood events. ~::::~~- /I ~ I • • 3.57 2.80 2.93 5.30 2.34 5.13 1 ..60 0.80 1.80 1.20 6.10 September 11-22,1982 2.64 3.08 1.38 2.09 7.58 1.23 4.07 8.27 4.04 4.00 July 4-15,1981 3 ..99 Frecipitation (inch) Table 26 3.26 1.75 2.21 1.82 3.34 3.10 3.43 1.88 1.78 SUB-BASIN PRECIPITATION FOR RECONSTITUTION OF HISTORIC FLOODS July 25-31,1980 2 ..61 9 7 6 2 8 3 4 5 1 11 10 ~.Sub-basin j 1 r~ !1 :I +.1 RECONSTITUTION OF JULY 25-31,1980 FLOOD USING THE HEC-1 COMPUTER PROGRAM dOFloodVolumeFloodPeakTimeto Peak-Obs.Camp.Dif£.Percent Obs.Camp.Oi£f.Percent Obs.Camp.Di£f.(c£s-3 hr)(c£s-3 hr)(cfs-3 hr)Diff.(cfs)(c£s)(cfs)Di££.(hr)(hrl (hr)-:596.,748 626,956 30,208 5.1 11,300 15,369 4.069 36.0 120 13S 15----,_.------1,244,230 1,217,764·-26,466 -2.1 20,700 23,159 2,459 11.9 138 144 62,214,400 2,037,160 -177,240 -8.0 33,300 34,746 1,446 4.3 132 153 21 • •.. 9 -15 Diff. (hr) 'rime to Peak 204 189 Obs.Camp. (hr)(hr) 183 192 Flood Peak Time to Peak Obs.Camp.Dif£.Percent Obs.Camp.Di££.(crs)(c£s)(cfs)Di££.(hr){hrl (hr) 24,300 23,812 -488 -2~0 87 90 35,950 6,791 841 14.1 90 93 328,500 33,156 4,656 16.3 102 105 350,600 58,700 8,109 16.0 108 114 6 Flood Peak Obs.Camp.Diff ..Percent(cfs)(cfsL (cfs)Diff. 21,300 20,709 -591 -2.78--------65,000 52,913 -12,087 -18.6 9.1 -2.9 1 ..1 -2.8 Percent Di££. -~-~~"-r--:r- 71,490 -6,919 14,451 -57,871 Table 29 -450,316 -13.9 Difft' (c£s-3 hr) 01££.Percent (c£s-3 hr)Diff~ -181,945 -17 o Table 2B RECONSTITUTION OF JULY 4-15,1981 FLOOD USING THE HEC-1 COMPUTER PROGRAM Flood Volume Flood Volume 854,870 235,701 1,279,851 2,037,564 Camp .. (cfs-3hr) RECONS'rITUTION OF SEPTEMBER 11-22,1982 FLOOD USING THE HEC-l COMPUTER PROGRAM 783,380 242,620 1,265,400 2,095,435 Obs. (cfs"'3 hr) 3,237.150 2,786,834 Obs..Comp. Jcfs-3 hr)(0£s-3 hr) 1,068.132 886,187 .~.~..-,._~-_..~._~'..··"·':-~-"-·--~··-·-EXI-I!I:BftS Table 27 Station Name Station Name Station Name Susitna River near Denali Maclaren River near Paxson Susitna River at Cantwell Susitna River at Gold Creek Susitna River near Denali Maclaren River near Paxson Susitna River at Cantwell SusitnaRiver at Gold Creek SusitnaRiver near Denali Maclaren River near Paxson Susitna River at Cantwell Susitna River at Gold Creek It 1 0 0 0 .03 2 0 0 0 •03 3 0 0 0 •03 4 3 .2 0 0 .4 $04 5 2 •1 0 1.1 .04 6 1 •5 a 1 .5 .04 7 0 0 0 •04 ~, 8 a 0 .1 ..3 .03 1 1 9 1 •7 0 1 ..2 04 I•I10O.9 0 O.9 04•I 11 0 0 0 .03 l~l.~.1mI.···j:t; I >< 1 OJ I I j I I I I t l ! I u Sub- basin Table 30 INITIAL LOSS AND INFILTRATION RATES BASED ON RECONSTITUTION OF HIsrrORIC FLOODS Initial Loss,(inch) Infiltration July ,1980 Flood July 1981 Flood September 1982 Flood _~_R_a~t..,.e~...-- (inch!hr) .. • • .. 231.,<.XX>2.12xlcP 20 23 500 0.93xlcP 20 254,<XlO 3.05xlcP 20 342.47x10 6 , 1.04x106 3..51xlO6273,000 20 26,300 20 299,000 20 39 297,000 3.8Ox1cP 20 11,200 0..44x106 20 309,cxx>4.24xl06 20 42350,000 4.36xl06 20 12,600 0.5OKI06 20 362,000 4.86x10 6 20 47 ,;. " .. furation Creager's (day)c Total Flood Total Flood Volurna (af~ Peak (cfs) 1_ Ihration (day) BaseflG1 Ip- l...~ Table 31 BaseflCM and Sl1OWi1Elt Dischargall Volune furation Peak Volu.tre lliration Creager's(cfs)(af)(day)(cfs)(af)(day)roJ 46,000 1.35x1cP 20 261',00)2.57x1r.P 20 3647,000 1..43&:106 20 311,000 2.9Oxl0 6 20 41 Dlschargell Volum~ (cfs)(af) L_ Si.JM.1ARY or FROMBIE MAXJM.M FlOOD 20 20 lltratinn (day) lliration (day) .~ Volune (af) Volme (af) i.22K106 1.47x10 6 ~ DirectRm10ff Direct Runoff Peak (cfS',) Peak (cfs) 221,000 26.:f,000 R.=---L :;:-.l'..~ .Je.::tJ:.IJU~!""_"!".i'~ Watana fuvil Cam.yon lVatana Devil !CarlYon lVatana fuvil Canyon 1.Mid-May 2"Mid-June lJ Discharge Coincident with PNF .. t..;. A.FlO<X1 based en all season PMP B.li'loOO.based on May r:rrl June PMPs with snOWlIElt -"'e>'ir-...j..,..··.....'''-.......T ....,--..;;,rff'~~''~''~~-_··_~.."_!, ~~~.,. .. &•f • .....J.., e ...."•'..-..It I ... ' Table 32 MAXIMUM INSTANTANEOUS HISTORICAL FLOOD PEAKS South Central Alaska Drain- age No. Stream Gaging Sta~,ion Area -------------~~~~~~~~~~-----------~mrzy 1 15195000 Dick Creek near Cordova 2 15208000 Tonsina River at Tonsina 3 15212000 Copper River near Chitina 4 1521:0000 Power Cr.,.~'k near Cordova 5 15219000 w. Fork Olsen Bay Creek nr. Cordova 6 15236900 Wolverine Creek near Lawing 7 15238820 Barbara Creek pear Seldovia 8 15238990 Upper Bradley River near Homer 9 15239000 Bradley River near Homer 10 15239900 Anchor River near Anchor Point 11 15241600 Ninilchik River at Ninilchik 12 15258000 Kenai River at Cooper Landing 13 15266300 Kenai River at Soldotna 14 15267900 Resurrection Creek near Hope 15 15271000 Six Mile Creek near H~pe 16 15273900 S .F. Carnpbell Cr~ek near Anchorage 17 15274300 N.F. Campbell Creek near Anchorage 18 15274600 Campbell Creek near Spenard 19 15275100 Chester Cr. at Artie Bld. at Anchorage 20 15276000 Ship Creek near Anchorage 21 15276570 Ship Creek below Powe=plant at Elm~ndorf 22 15277100 Eagle River at Eagle River 23 15277410 Ptaters Creek naar Birchwood 24 15281000 Knik River near Palmer 25 15290000 Little Susitna River near Palmer 26 15291000 Susitna River near Denali ' 27 15291200 Maclar~n River ~ear Paxson 28 15291500 Susitna River near Cantwell 29 15292000 Susitna River at Gold Creek 30 15292400 Chulitna River near THlkeetna 31 ·15292700 Talkeetna River near ~alkeetna 32 1529400S Willow Creek near Willow 33 15294010 Deception Creek near Willow 34 15294100 Deshka River near Willow 35 15294300 Skwentna River near Skw~ntna 36 15294345 Yentna River near Suditna Station 37 15294350 Susitna Rive1· at Susitna Station 38 152944:\:Q Capps Creek near Tyonek 39 15294450 Chuit.na River near Tyonek 40 15295600 Terror River near Kodiak 41 15296550 Upper ~numb River near Larsen Bay 42 15296600 F·ar1uk l1iver at Outlet Near Larsc:n Bay 43 15297200 My~tle Creek near Kodiak 7.95 420 20,600 20.5 4.78 9.51 20.7 10.0 54.0 137 131 643 2~010 149 234 25.2 13.4 69.7 27.2 90.5 115 192 87.8 1,180 61.9 950 280 4,140 6,160 2,570 2,006 166 48.0 592 2,250 6,180 19,400 10.5 131 15.0 18.8 100 4.74 ll Caused by release of stored water behind Knik Glacier Period of Record 06/70-9/81 10/55-9/81 10/55-9/81 08/47-9/81 09/64-1/81 10/66-9/81 06/72-9/81 10/79-9/81 10/57-9/81 06/64-9/81 04/63-9/81 05/47-9/81 05/65-9/81 10/67-9/81 06/79-9/81 10/66-9/81 10/67-9/81 06/66-9/81 06/66-9/81 10/46-9/81 10/70-9/81 10/65-6/81 08/73-9/81 10/59-9/81 07/48-9/81 OS/57-9/81 06/58-9/81 05/61-9/72 08/49-9/81 02/58-9/81 06/64-9/81 06/78-9/81 05/78-9/81 10/78-9/81 10/59-9/81 10/80-9/81 10/74-9/81 07/79-9/81 10/75 9/81 06/62-9/81 07/74-9/81 08/75-9/81 05/63-9/81 Date Max. Q (cfs) 08-07-81 2,600 06-17-62 8,490 08-08-81 380,000 09-25-49 5#540 09-12-72 1,030 08-21-81 1,810 10-22-80 1,310 08-10-80 2,180 08-10-79 6,020 10-22-80 4,680 04-24-·/4 1, 240 09-21~74 23,100 09-09-77 33,700 07-12-80 3,380 07-12-80 8,070 08-12-81 476 08-09-71 107 08-13-81 451 09-24-81 198 06-21-81 1,860 09-09-71 1:600 09-18--67 61240 09-16~80 1,200 07-26-61 355,000 08-10-71 7,840 08-10-71 38,200 08-11-71 9,260 08-10-71 55,000 07-07-81 90,700 07-20-61 75,900 08-10-71 67,4001 07-28-80 4,450 06-21-80 751. 11-13-79 9,920 06-09-77 51,600 08-13-81 116,000 07-29-80 230,000. 06-25-80 710 11-11-79 7,620 10-21-80 3,400 10-21-80 988 11-11-79 1,760 Ol-03-77L . 1,350 cfs/ mi2 - 327 20 18 270 215 190 63 218 111 34 9 36 16 23 34 19 8 6 -I 21 14 33 14 301.11 12ii 40 33 13 15 30 34 27 16 17 23 19 12 68 58 227 53 18 285 .. • Table 33 • 42 34 36 20 20 20 4.24xl06 3.05xl0 6 2.57xl0 6 Volume Duration Creager's (at)(day) probable Maximum Flood Peak (cfs) 309,000 254,000 267,000 234,000 3.67xlO6 20 32 326,000 7.59xl06 20 44 COMPARISON 0:&'(PMF STUDIES FOR WATANA DAM SITE May Flood June Flood July-Augus~Flood Acres American,Inc. Harza Engineering Company Corps of Engineers Agency Responsible for the study .. •• (J) I--to-:r::><w •• EXHIBIT 1 PMF STUDY SUSITNA RIVER BASIN " .'.'~ --..!-- I LEGEND + SUSITNA RIVER 8ASIN RIVER CITY GLACIER SCALE () -1-...-:c--;r-'--"."M.~_••• -.-'lo.... ",'"'"1t I... "I~"",..r' 5() C='"E •.1 ,, • I "., , t{FI~ED l:,~"'."..'••"".•••'.'."".'.' ...> , '-:< i I I j \ I, i ! i 1 i I I i I I I i 1 i i I \ ';"4~:~~_-- PMF STUDY SUSITNA RIVER BASIN _~_....::.:j_•.."2.'~"h!i"I;·2~1 I /l.~).... ---~......-~..,-~:,"",,"-; ; i .l i l:, ! t I I l ~ '~' ! j \ I ,~ .~ '..........-~......._._........._,w.I:...........__.....;_......__..._.-~_.._..."'........"....._-------~_.......-•.J Cy •..0''"-'~..~"-.,'/.-:--..,'''....-<,",;.,,~(. "....~_<~ .- l. '0 ". ~" I ",.... +,;./2° ·1 !\;) \ '....'\ fL\'OJ ';:I/uc.........~~..,....."'......'-J.,! ,....,t'0 j 'i'-,'".;i •."......I.'"'I (.... ~. ~/'. .-",*,~,.... \'--0.7;:'-_//,/' I'')/''-/"'....."'}.'.."'1 ,- r \ I I ~'fo"..?.;"-\:--'.'"'//-.'-------,._...~ r ..../.".,..---... ':'/"-.:...(/i \ """,.('E"\f'-':'.//'.~' "v:"':.~....",.I,1...../-'~r .......---/ + \. c ,-L p.. .,p.- J;'~,:r'-/ /7 '-.........Jc.'ANClIOt!/!G}/.•• ..,.J' J,~,~.H-.. ~" ~t'j / .,",:, \.\.. /"\/-..-....::::~/'........ J'.............'''','\J ) ~-:.,-'("~ ./;,.'1 1,,//1;"/-,.. \:"V ?1: LV.....~..'.{''',Net''",,'-"AM """"/IC/•S-C<.rNO '-""'\ \ \II-" l-~'60\~~\+ ~'v N _.....__./1/J..~.•__....,__._-_.....-..•._•.•..-.....-._•._~.•.-_.._•../-.'-"'0""_..._~__J .....'".... .' / + ,............ ~/,,-\ VV ~ ...J ;' .' .... ~\L. Q..oJ'" ,@ I";'-~:,.//"','.., N";'" ~CAL£~~ ,.1.':"';::;Z'''::::'''::'::':''''='-OJ + LEGEND SUSITNA RIVER BASIN RIVER CITY GLACiER I,' }~; ~. ,,,,0"'-- ""~,,,,".- ~ ,..." r".·! --~_-,-_~.--------_••-~._,.--EXHIBIT 1 I'-·-'·~·'-·---····-'-A··f.-Cf/C-OCEAN.._~~"'-''''-'''''-'''~'''-''''-'.y-,__,""'_b"",,",'_,~Z...-)~ ---.''.'.··l...-------.-If ~,..."':_.-.-...---,.~"j..•.:----------_...._...---".~\,....~.;._.._.,._------"_......-?(j"_\~"':i'-_..~, r-_.,~'-.,..'. I h.,..;,/''''~'I'\'.'~'<',.r"-r'A'VA v'.''''']55'''''....r/-Alt<!ilN'!..•.,·2 \\/c;."'../"~'1\(/,\..clllR.BAA/K:5 ..:t._..'--.._-,-"'l..A..~.'..l't I .l .--l •r'•',;/~'~·/"""\"ON .,/.......'~",y .~";7::;-.•~.."- ' /.......,yv.....'-'"_,-',~ 'j'~'--,-(•.•",.1.~,'~ :",..'.i I .I""'\1;\.~'",•-:;u.r.1"/,,1Ar-~;"""'",/'J·.....,'.,.~...'~ i I 'j-./"11-''0 ;,...,"'::'"::'.";;,;;;~1 '•~•'...,-'"'"-<I·.,,.-'..-,-,-.I ...,'_~".:,/,.'J ,;:...,. '.'".,"..,,'-"--.,.-•".'\.4 (.e'••~"'--''."'........,.(····1;/··//"'...'.~A.L.A::"'"..'";.~.-'='"'"":,.-;;""o·".,',;~:++G'I 'I.,/'•'r',':'~"6'~',-.'.,..../c , :I ,+'."'u /.:r.'0 eE/"'/,:-T-'-+--~/-./"C r~..,_"c , I I /;;.__.._..":.1 .."'"".,.,'_~f" r r ~ I ( l 1 c-1() ,It\'; ..<,....•r,,;.\1t.~~,i~...~~J~...."..~:,::~\'"..~ I. f1..;- l· EXHIBIT 2 PMFSTUDY SUSITNA RIVER BASIN STREAM GAGING STATIONS {'? .>(~.\ f<U'~ o ,0 \"" >< ;~:I o ~:::>\1.,\_0 X 5040 ,1.'0'~J':'~ 20 30 LEGEND USGS STREAM GAGING STATION WATERSHED BOUNDARY SUB-BASiN BOUNDARY SUB-BASIN RIVER GL~CIER '0 -1-v.c",c.... /-t.- <-$')-. SCALE ()5 10 ,,---...o,....- r:$Y ; HARZA·EBASCO SUSITNAJOINT VENTURE AUGUST,1983 /II!;'2"lZ,/OO ~ " iJ & t, I I l:f ,""- r ! d:cJil ( EXHIBIT 3 ...,:--1 ..•.1"(;'>';;~"::"f'~:•.,.,.• -,,,~~.•~..,,~~".':"..\.~~.".~'-l PLOt,;~xC£E'.DP.IG Ie%>Or 'T/HE ~L(/W F:'-C£cJ)/"'/c,sa%t:J~"Tlr-IE ..cLOW Ex.ClEe,D/~C;qa%o~77....../c.~_Jl_ LE'~EN.D PMF STUDY SUSITNA RIVER BASIN. STREAI\1FLOW CHARACTERISTICS OF SUSITNA Fuve'R ABOVE GOLD CRF.EK STATION i'l~.....;.~ prA=:"·,U"/1·~;.."~ ", ! 5".1 ! 1 I J r",t-"I/'rJ .,\,\ I \~ , J /,1 t,,\rl>"".\.l'J .K.II1fl.'\I I :J 1 \ i "j 1\-..\'\ 1-j'I.~\II'. i /i ~~, !t ' ,;l J.\':/.'.A"",~_~.,_.'.'.-'Ii.'....~,~~·:'F"':C.,:..;;=::,;,~{:~/.'\.:.'y~V'- -..J J~./....1 A I I"J -•.,*t .~t t •~;'"-=---r so·,v'.l:I ~~- 3 "\/1\/ M I ",'CAr'A I ""\ l f\,'\ t I..'I \ J :\ f J ~ I,'I /;J"'~~\ ,\ I I ('.... J "\\ /1/', \ J ..........\ I J t '\,\ );1/.-:\. ,)~'..".~-----.....---..L"--:-..Z.:~.:£.E.=ff ,I"~~.-A -..•.°r-,.I'I 1,..1 I .,.I .,..j .1'1 ..• 0 tv I 1:;t,j 1=,..,A ~.J _ 40~ 2°7 1''',')!CL;:''PE.'j P:,/!.:'/;;.I'IR ?/.XSo t/ S(}Sl7iV.A I<":/F,~:"'17 c;C)~:.....(,!J:?t:£k S'J - 10 - . 30_ I 1':( \J "I ~Q<: " ..t,i £: PE/?,/tl.D :11S7-'Cf " tl,;',,::~.///"I t ,.,-~~...~.::..~--;. !....,J t ., '-. .~ .l .,......"'<'11 :\ 1\, ,"V i\\\' i /v e:'. ~:.or:t -1\\ f', '.... -'"\."\ +• '\. l~ lllt '\" ;t \J -.....~, 10..'" I~ ."'";- t .s 'I::;,t ·/I~ , .~ J "I I,,"' ) 1 f 'f'·,.., I.• 'J i"A', ...I 1'.;",~ ,t • !I -../If I '. .j r"\ 1 :r ..'.".."".... ,i ,.~.'I.~'""'j,J '"-").V-4''''~~r.;"~"·~"~I ..;,.!r.I ..I :'0~~F~~.,-·r'__r-_.,..,;..I . "',...._.•.~_~4 t !/'•l',-t·"I ..."01 ....\;.j !II'"" ,,,,ca.:.~ ~t ... .1:* I;'~ ....... -4 .--1: S':"_"tT:\'/R/i..,'ck:/V&,C///TI-I/E'I.r. !L!• /5· ....... i,;- ~-' \'} l( fJ I;) i "l~ .'.8' I HARZA-EBASCO 5USlTNA JOINT VENTURE AUGUST.1933 ~---::=::n. I I EXHIBIT4 j t,~;~,,,r~~.ii -,---...,_.....~,~-<>-'"'.~"'.~--_._-- .,,'~,.,.'_~i:~i,.l,. i',.".Q'.o··Y~:_,:..,.;.',"--.....,'.;..".",~~,,~:...c.~'Ir, I ~""__~.l,",*"""""c__,tOJt-·,',..,;io'........~......~"'"""'!·~·se ~·d PMFSTUDY .."..".•_·.•·••• SUSITNA RIVER BASIN ANNUAL FLOOD PEAK DISCHARGES:'" SLISITNA RIVER AT GOLD CREEK .. ~-_."'.,...,_.--._,.....~..,'"".......;..."'.-.....--_..~ SEPTEMBER ""........._-~~-....~-"--""---_.~'",..'...;.,..<.~ ,'__5 .·.··-__·.·_·_""',· Sq ..,. 55 • -~.-~..~..,,~..,--~ •.~4'--"'·-~...,,_.~'_""'"--.>'-.......'.•.".. _..:.._..:..--~.._-~...:.-....."..-""-~,."'..,, •"."...-0-'-' ;..'JG.usr G.7. ,t.\" S4.- , 7'1 • 80 •. '63';'"-·58.. Of 70. 6~. ;.t t;z. 72.. 73.77·61. 56. liG • '7$-,_. ·,,"-·--~--"-sz·;, f:8!,. .~'f ~-._~.~~·o~_...":..-..~,,........,:...,~~,....,....~-",--;i _,-,-,~""_,_,_,-,-~_"_:""",,,,_~,,--~_,_:_I1L.-...-. 64 •.' •.;0.>"...""._.,,,,.,._,.~,.;~,...~.•.,._,....'.-.~__..~~"".-...~"...-_.,..__:_"._.-._.-....-..-..~.-•.-~......;,_.-.r.-;;.,·o ._.,~".~."~........,,,"'",""."....,.._...........,...,,'...,",""".="~""''''''i.__..,.",..... 7(. ,-.'.-~,~,<~---:;------;~·r'·----·.---·~""'""'-~" OF RECtJ!?.!):11s/-8/ (,/97~76 N/.sJ/N~) 'It ~"'''',,~---'.."-..:.._--_.."".-'-...;..~"..~~--,.__.."":-=..............- PEA?IO.D 2Q "$.~_...........n-7Tj-"-T~TtiTIT;·'r~--·~~'i ---~~'HAiiii\:'EBAScii'susiTiJA'joi~iVENtue<c,A:UGUhT"1~~'"-""...C,..;,.,.;..+J..0-..c_+..,..,",-"-,+.,,,-"1,,~..,-.i.,".-'-..:~.:._!':i },L'"iLA .,.',.'..~;,>0',;',.,'";,;;;,! ".,.,.."i'AI'Il,L ,I',/.·)ve"aER DlCEMBER 30 35'" 90 ~ S0't 4.tQ, 4.f (35 40 80 7S~ '4, \.I 70 ~~.... 1":5 IoU' C>\3 ~ ~O~.... \J ~«,..... j,.)~ -, 1 l I I J f""""1ij = t ( ~ t.... ~ ~ 1t iII '\~";,~-:;i$?~.t";:~n~~·~~'"7:~~;:,·~·*t~~~~~.w:w,~~.;~~..:·_1j,J.:;:-........Ji!LidI.r'__*'3 AfHlZttlHISldZEiliidiU _f 1!iiAiI!~__iii .,tibI:~.......ini.......IiMif I zi:i:llSiai*dlLi*,'._11IIiilIt".1.-..d tii...wa&:i::_".~ • .. .) I I, I i 1 j m; X ::t:-OJ--i 01 .~ """""',- J t .ti"."';"t !t(u')'I I I ! ! "(/n'C.,{(.J) ~Ill::;=-> 1".J'0 ,('/(:.;. R,'I.UV bJ6"!;;oYS'/';'.J G:,lac.,·e r boo nels t"J fC1rpr~-,,"..-Ic..) :::::::::.--:.:t --...~,- ~ +0 '~2 30 ........,...4~"~ LEGEN.f) o \!) ~ +-63°301 ~, r 3°0 : PMF STUDY SUSITNA RIVER BASIN. MEAN ANNUAL PRECIPITATION ~ 30 '- 1 ~_..> :c.,...,'-;::J ~ ~ \, ), ) ./,-......./' I '-~V / \ " 1 t t) ~.....+ 1\\ o \T- 'r....+ -"~"-:r"n-rE:,NleJli]t----- ....,-_.-"..' i I .1 ..,...~""...~/../'""-.':y..?.. 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",,11'5/L....A ,,-AX}!€ /J-t?,S.dN '/if 2AJh.AN .,SUTT~ "F"'IR"'1'",d~J r X A MAR,/<;(PA<M'~W . W4,,.<A ~'TA"'ti$"'" •,~O~~ ~~_.i 1.-'i o 5 '10 )( ....kll'lITe LEGEND WATERSHED BOUNDARY SUB·aAS1N BOUNDARY SUB·BASIN RIVER CLIMATIC STATION RECORDING.SJ'ATION SNOW COURSE <;'-f J:.' SCALE @ ANGH()/Ut6£ ..._:ilIL~,-~~~~,:~~:~1 1. -..,,--......_,o----... @ x I HARZA·'BASCO susrtNAJOINT VENTORE AOG!JST•.1~B3 ~.----~'WA""/ol.:1#16-ULWA "'A ---i:j ___,_._-__r-_.-__._~-----~~~~-~---------"-'-'-'-----._-.:--~--~.~~-....---;~~---r~0""..","" ""'-! ~ 1 J t } ~ f f I, 1 I • 11 , -P Ii 10 zo toc:E~....,:oj ~ 5td'tue.Mdes ..... -1-""'~It.'J, \"\(p -r <.~~o ..1l4tCa .I(,Z.~ PMF STUDY SUSITNA RIVER BASIN DEVIL CANYON AND WATANA BASINS .AIl £.levat/o06 j Feet Abo(e Nean ..)?d Le-vel ~ :c Scale...1'''"1-Of t.z ..J. II~T~~ 1I""'l+:to~~ ---,-,-,-,-.-C->~__,_______9_......, - 151:) +c.b l~ +<.a-30 - IiAflZA -EBASCO SUSITNA JOINT VENTUflE AUGUST,1983 "--·"M-rrt=:I~~-~-------~_.--...... .~j'"·"1_Y }, ,. .. .. i Ij. I I I i i ...--'! 11.&',HI· .r.---r .I~" '""'{il\Z,' -'.~ -mz" f\J Statue M.te5 ~ m X 05/0 SO 10 IO:I:is Ella;--f 00 AJl PreClpttatxxt Ifl Iflchf.S nL- --t Scale: PMF STUDY SUSITNA RIVER BASIN ISOHYETAl PATTERN OF AUGUST 12-14,1967 STORM r .~\.~ r--~ ~\. " +~ + ~• 0.1 l;J,l'o~ ...M~ ,.. -h:i~o .,.- ~.I-.ot I.o...~ .Pi "'.....G.~t + -k4tI O v/>l...t......1<. 2.e.. ~.'.'"~.'...'..~r- \-'\II<.t.r.\f.,\~~ .11,,"L.ke,..-.la' .~~ '~i:::Y /~ !-h•. tI'-- ~. 1150· .;..."",'" is".,.~c~!~,'-~~ ~ '.'"'t.~kr.l<.c. HARZA -EBASCC SUSITNA JOINT VEr\JTURE AUGUST,1983 3' / A.... It,:nlt.1!"'-5;'..' iJ-~~ -~'~'~"--A~flEND~-C:V-~'~~-~~-l ~ • • .~. \ \ \ \ I \ 11 II i •.. .. " EXH1SIT91 1 I I~"ojt!e.fAy ~e-( l=icvc;(,li O';/ot>O fl: ~.,'.'." PMFSTUOY SUSiTNA RIVER BASIN ELEVATSONS OF MOI!)TURE INFLOW BARR1EHS l.•t •.:.rJ...r ..'.l...'fr .'•#','".}'.....• •,..~~Q ./"tI .. HARZ.Go .EBASCO SUStTNAJOlNTVENTURE AUGUST,1983.'....,.•.,.-.._,........,.._____'_,',........,~_.,.,_•.i""".".,~_.."..'11 ."....._..•."~_.,.,,."'_·~iooI..'..,,'. . i sovl<ce:I/S w/3 T/::J Alo.47 Jt163 ,I #' \ I \•I ! i \ I! I.,. r r . -+ NOT~ PMP= PRfiCIPITATION REPRESENTED BY THE ABOVE STORM ISOHVETS MULTIPLIED BY THE COMBINED MOISTURE MAXIMIZATION AND BARRIER FtEDUCTION F'CTOR OF 1.07 ~ PMF STUDY SUSITNA RIVER BASIN \ t \ \ LOCATION AND ORIENTATION OF A~GUST 12-14, 1967 CHENA STORM OVER SUSITNA BASIN : HARZA. EBASCO SUSITNA JOINT VENTURE AUGUST, 1983 EXHIBIT 101 I . I J . . l -.:...:.;...:.;..:.:=;::...:;,_-=~-oJ~-,q , .. '""' .-_,,. • ••>o< ~:<;··---,._!It" "'" ,,,. .. .,.,,.,_ .. ,~ ... ~,.,..,..._._._,_~ ..... ......__ .. ~"'''-""'"' .. ' ~U' ..,. •• , ,_.,..~10 ._,.,..,. ... ,. .. -.. ............ _..,.._..,,..,._~,_ .. --... """"',...,......,_-~; FEE~ AUT< I.aJ i.MANU! !.- EXHIBIT 12 ''£''.....t .....·.L, ~-~~- ></~=-"Gt "'-'''---''--'--I .!,J1~.r ;/1 -t ._" ~./",....,/ /.,///,: :..'::"......-.:~:~:/:""'"',"t.~ <)-c..•/;'-'..../:- .:.;,...t../.1,11,.. ,::.".,,. tI J'...." ..., .L ~,/')'>:7: .Z'SC/UYC('"vi......t :;6.:-/:;:;'("t-,'/~ t,/~-f,..y.s/'~d 80,""""",./"".....>' I"'~. -. t).'. //.,"1 Fi ..--..........-"..... PMF STUDY SUSITNA RIVER BASIN ANALY:;ISOF AUGUST 11·14,1967 STORM BY iSOPERCENTAL METHOD 9----1 l! .! -',, / GGlLI<ANA / I 0,(. /1·/ a 8/Ci ..l)E'L ,Ii 'J. o L.EGEND 5' O.(j //·4 .., +I \"':r......+.f.. ....- ------..•__.__.~----_.__..-'--,,_..'".,.•.."_-..-_._.----~- ('" .,.> t-"1 ! .I /. X..-I ( .\ J \...."\ (....v';"J \ \ ,__._,....,:,_.-_""'0.-'•..,.-,.-••~.;.".., ,.._..'""_.__-"~_;,••_",.-.'"'."'.'" .... ........ 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I ,-"~,. ___4- .•,".,~E~£~~.._.--·....-~HIBIT13 1 __ _Wr-<.l-nS/'...d 8ov r>dt7l'Y 'I 10-__-10 ZSO/u('c('"It1',!L,'t·"./()/;.s./lYe~y;,I_-:-,~---"..x Ja 0 'I ,;v//IP I .I.tQ/'y<'!:(p~<:B..;): PMFSTUDY SUSITNA RIVER BASiN ANALYSIS OF AUGUST 23·25,1955 STORM BY ISOPERCENTAL METHOD '-70 LJ"::'/ltJA 1:1 f •I~ IF; --\- + ""Clo~ !'\+''':3 I ~.~ 3"", "" ._."...•,••••.•.--_"'Il!-'". 8/4 ..DEL r/1 + :) ("\-...., ~\xJ '\f J /0 ................... -t \~,).,I X J-\l"fl\I \....~ '-, ;.,'"\.•t ~\. ..\\ .\ 'l \ d ~\ \1 . I \ ):\ S \ l:/\ LyJ "" ''1 '. " \.r-~r...,I lJ p" ...__-"••.".•.0 ,-.._.,__""-,."-.....~~•...,-...,"--'• -• ~'~lL ~...c-.. o '+62 3·~• "'""X('~-_~~'-r,-5 ,,\.. f '"L-",,7,\' _""J 'I ,-.""\"_/-'0'"___",{'"_<'0...:://£1 - ->,''-' \ ......\.....:•'"'"I"".~("-J~l"'j "....,,•?':,r v~("""':'-',"k l..:'"",,,'-...'..) ...~='•I '')',.',(_/' -=,_C'l~,-r:::-""""r I .+"-"_"_.\".'.~L1I,.~-,,',,,J'-.'' /"_.fIr;'..').,+\1-'',\",">').) 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EXHIBIT 26 ~r)l"~ /'/ I PMF STUDY SUSITNA RIVER BASIN SOIL CLASSIFICATION 50 J 4030 #!~~O 20 LEGEND WATERSHED BOUNDARY SUB-BASIN BOUNDARY SUB-BASIN SOIL NUMBER,SEE TABLE • SCALE 0 5 10 HARZA -EBASCO SUSfTNA JOINT VENTURE AUGUST,1983 1 Jt tl J J x .._', • .. EXHIBIT 27 50 PMF STUDY SUSITNA RIVER BASIN HYDROLOGIC SOIL GROUPS 4030 «~~O 20 LEGEND HYDROLOGIC SOIL GROUP WATERSHED BOUNDARY SUB-BASIN BOUNDARY SUB-BASIN SOIL CLASSIFICATION BOUNDARY RiVER SCALE 0 5 10 i HARZA'"EBASCO SUSITNA JOINT VENTURE AUGUST,1983 ........ ., h . t HARZA.-EBASCO SUSITNA JOINT VENTURE AUGU$T,1983i-.."__._....,._....__ LEGEND • »r .. i r m X :I:-0:;--i l'.) to -')~ /j?./~~'n;. '2:,04'• '2.3;:; •,~'(t.karze:< Lf PMF STUDY SUSITNA RIVER BASlN ISOHYETAL MAP OF JULY 25-31,1980 STORM 5" ~- .....~..-'3 5" ..::=.=:..~--- .Lj ---_.._._.__.._.._-_...._-_...".""--'--''...._-_..----_._--------- -=::.:....= • ~~·T·.-.-..······__·__·~_ '.,C J J ;..~:::;.I "~~-::~10/.........]/';"-1 "...t--C::,.;-Ii r I I I j I I I I I .".-- / "",I ¥~ "\ 3 1,98•~IK~.'-/J;.( SCALE (ij PRECIPITATION STATION .----DRAINAGE BASiN BOUNDARIES leI ...../0 ISOHYETAL,INCHES .z-------.!---~ I "'4tS!A u~~:~>--'~·-----=------------ - • ~-:-----'-.t.=::=:t=::::::::=t:::::=::== ====:::::::::.=:::::::::======~::---=il:±"~e--_......~~ofJ r1-._-~-~~'-""'--'~.....I • •.. f\ I m X :J:-OJ -I w... '2'6(/=: •4,:x:_;);," ?J,Ob•Gt,..//kcu,l PMFSTUDV SUSITNA RIVER BASIN ISOH\;'ETAL MAP OF SEPTEMBER 11-22,1982 STORM 01' 6' , \--?,O....-7 _ I,Lf3•McKIY1/ey /Vl,'j;.Ef'S ,,""'" /' /---/ ( // / \ ") / 7C /0 (.)It'.,Ie.' ''''Ililli',=-==' LEGEND =.... t SCALE 6 ..0•'5J.;e r»1t::J ~ S- .,--- •PRECIPITATION STATION _____/DRAINAGE BASIN BOUNDARIES 10_//0 ISOHYETAL,INCHES HAHZA -EBASCO SUSITNA JO:NT VENTURE AUGUST,1983.._---.•.",--------.. « . • ..r m X:r-03 -f ww Ii 2/,2 • 'pAkS,,""· 3.~ il GG(lka .../~l.. /~ "", PMF STUDY SUSITNA RIVER BASIN ISOPERCENTAL MAP OF JULY 4-15,1981 ST~RM / t:? ~',,--Lj 7'/0,..:...£5~..o/e>c:> /B./•18 ../''''1c 1<'1 Pliey LEGEND 10 j~b• 7;;Il::e e '/~I ~<. SCALE /0 I '">'c_.'_---:.._,,,_c -----....~,_• ra PRECIPITATION STATION ........--DRAINAGE BASIN BOUNDARIES /f \.",/v ISOPERCENTAl,PERCENT HARZA -EBASCO SUSITNAJOINT VENTURE AUGUST,1983 -~~~""'-......---.--',----.-~-----~-----, , • If r m X:r.:-l»--i w ~ /0 //'$• P4.XSD.J/ ---/-,2 '2,6•r:.>tA-lkC<·N.< PMF STUDY SUSITNA RIVER BASIN ISOPERCENTAL MAP OF SEPTEMBER 11-22,1982 STORM Lj /? ft//L..cS 9JS•Me K/r//ey -=zseuo.......... /0 LEGEND 10 0 /".::~ WAil.!!.•• I ._-----~--------------.............._---------------'-------~-,-.. SCALE &PRECIPITATION STATION ----.DRAINAGE BASIN BOUNDARIES It)....-/.(..'ISOPERCENTAL,PERCEN1" HARZA -EBASCO SUSITNA JOINT VENTURE AUGUST,1983 ,.------',""'••,-~--.---,--.-_-.""".....I• • .. If m X J:-D:J =i w U1 ---..:--.. • I '. •" .,. ., .'I ' • PMFSTUDV SUSITNA RIVEABASIN SUSITNA RIVER AT GOLD CREEK RECONSTITUTION OF JULY 1980 FLOOD u '.,to ! 1. I I . . . .!. ··I I·..·i-;~. ,··· /q81 ~. .', '"I .I, i , ,J"}; i ~.'to ",.i ..I i .t··..~.. I " I; t. .-..... • '! f·•f•I •I I I I. I I' I , i ! ,1 '" • i ' j T~ . .L :- .. T ,t 2fi 'Z6 £7.Z f3 29 .30 S"/ -------------.. S-o ~~o \.) ~. ~......3G> o /0 '\. k ~ It 20 HARZA-EBASCOSUSITNA JOINT VENTURE AUGUST,1983• • $ .. [J m><:I:-tXJ =i wen I, I = •~, If) ••-'t '" !..-; ., I·• IY •I ,I ..•I !. " o oSer'v-i:r:l1 :F/o/tv' {3 I I"PMF STUDY': sustTNA RIVER BASIN SUSITNA RIVER AT GOLD CREEK RECONSTITUTION OF JULY 1981 FLOOD •t.. + I IZ, .,'" i" • i . I,t·.'it,'·~'t~ull~t~~Q j,z::i~w II_..,.i I ,.. II. i I ! 'i ~j I 'j I. I I •t " ,i ! ,;'h .~•••~....t I /~I "I t ~., '\L'l--:.:!j:!••'.,--,~..~ , t,. /0 l i•• ...j'- i . "j I ' L 'I I ]1 'I '.....,.,.•",1,1"-,I.I i I '( L 1981 ·1 .. , I.. " 1 1 r' ~. ,, 1, ,t T I J. ! .,, . ------'-----------------------«:.,.~......... <:3 'Tu.,y , I. t ... I I' I .i.I'1 I ,' I ,jII I 1·•~_.........'".....'t ."-t-<.. ,i I ! .•.-"...,i·••I', :••t • '0 I .~ i ! f,I I I "' 'S' f ~•~•-~•••• !J'-tw_-*,...........!"'t"••- -'j/o '\. 70 /0 o so ZtJ bO ~30 ~l( \'?'r- U \Jt ~'(/ ~ ~ " HARZA.EBASCO SUSITNA JOINT VENTUR~AUGUST,1983 ."';"'1"-"~.31Jllt.aaY:~Uh.~ # • • • m X ::I:-OJ =i w 'to.! " 22. I ! I'. ••~.•*•;•~ •••t i, I ZI., I I I 1 !. I ;..,.l 1 i 1, .,..,..~ zo . .-r"'; II \. 'i:1-.• ~ ~_.'•••~.t -#.~ •.i.., •! /9, ,... t ·1 -·t·•'. PMF STUDY SUSITNA RIVEfi BASIN SUSTINA RIVER AT GOLD CREEK RECONSTITUTION OF Sl:PTEMBER 1982 FLOOD /8 i.. ! .1 /9 £3 Z; ..."•••cI" r .. I 1. ./7 i . ~ "'I , .."·"t. /6 ": t I. 'I '! I" .f ..j,-j''-r I S,~fi te,)-J beyi t"<... I~- t~ r~'",:t.... i dh:S.eJ-I;'~:;; Flow; i' I, !_. i-'·t ~ I ! ! ~" ........' i·: I.';. l. i i, i.1 ,\ l'.I ,I,'"1 I.. to' 1 1 ,.•j_....'J I ..'t·..........:t I •:. I 'j I I'•,tI'. .I'I,•·1 ••',...t·,.' :....I'!:;' I .."1"I ....•••.,...'t ''",.'• •1 I ..'.I."..:I ','!.... .j •.'.",i ,.•l'.'I'.'\.I j'1,\". .,I I'. ':.,':'".., ';"'1':'+..t_~I .',I :~I"•;".'1.1 '.• .\.,!",.'"1 ,..'l'.'."1 .;.',',.,.,..."I'".,.,'.,..." '.11 11 .14,~.,~!,:~~"'i•-".'t~:'1·.~~/~.'·H1.:.~1.f~d l'"."•.....',E/":I..",;"....'.',.,'",.,,:',...OW .t'I •I I .l'l'~j...',..."I : I .', ,...f ,,:''",.•: ,.t' ."4- •i " "."-..' I .• I' 1.1./ ,,,....,; .....t" . :" ... ~~t i", t • ~. 13/z ~•. a.•i ...".., !#.,i ..~",,~""4._ , I II I ,•.t •Ii ,!I I I ,••I ,,, I I , I 1 I • • • •,•".•!'.1'J'I ,f I I •I.I ,,I ,:I '.I 4 ,I I •l..-..-t........"i ...-14-+-.............._,-.-.........~......l-t ,\ ,l •f'1----11---- o o '9() 10 "- ~20 ~, \t ~\l.~~ ~. ~" ~ ....... I-IARZA _Et.\SCO SUSITNA JOINT VEN1URE AUGUST.1983 .~~U/A.Y3liU:A·:."~...~ .. • •0 1 -<l'\..r-.____ . ,~~I <\>,'~I ,f'/~ \~. • r I • .. EXHIBIT 38 PMF STUDY SUSITNA RIVER BASIN CHANNEL ROUTING SCHEME WITH MUSKINGUM ROUTING COEFFICIENTS #0 Ie.:/VIf./S'k.l~~"~X e.r 14a /rQ C).'2 +u_14.//6~-:"I#1S. /"1.t<S k ~""ff ~#'1 Ks-h(;tN'~ on dn::z1 .....e;n1 • •(}6servecl h}/4f"O" J"ap6 .::i J/~I /4fi !:lIe D I H--- K=3 hI- . HARZA~EBASCO SUSITNA JOINT VENtURE AUGUST,1983 I I I I 'I :1 ! I I •.. ·w EXHIBIT 39 1 2 3 4 5 6 7 8 9 10 11 12 13 .4 5 6 7 8 910 1112 13 1415 16 17 18 19 20 T4ME,DAYS TIME,DAYS I", \ j .~PEAK =3~.O("• 20·DAY VOU \PEAK =267,000 CFS ~4 ,20-DAY VOLUME =2.34 x 106 AF~ \ J \ 1/.< I'1\ I \, I I "'I \I ~ \), \IF ~ J \[)I \ --II ~".........~ ~I MAY JULY --AUGUST PMF STUDY SUSITNA RIVER BASIN PMFINFLOW HYDROGRAPHS AT WATANA HARZA ..EBASCO SUSrrNA JOiNT VENTURE AUGl. .. HARZA.EBASCO SUSITNt.JOINT VEN:rURE AUGUST:1283 ~ ~ ..~~~.&;;:'X-j ..·"!:;.,'-_.~! EXHIBIT 39 .~ i;; $ PMFSTUDY SUSITNA RIVER BASIN -~-..... PMF IN/FLOW HYDROGRAPHS AT WATANA 1 23 4 5 6 7 8 9 10 11 12 13 14 15 1617 18 19 20 TIME,DAYS JULY -AUGUST ,., I 1\'I '1'1 I I I I I I PEAK'"267,000 CFS '20-DAY VOLUME =2.34 X 106 AF \ \ .\ )\ f \ /\ ",.V "-"-l.-.-- 80 60 40 20 o 280 260 240 220 200 ~180u g 160 or- •140w ~120 <! :x:100uen o r.~ 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 1617 18 19 20 TIME,OAYS JUNE PEAK'"254.000 CFS 11\- 20-DAY '"3.05 X 106 AF I \ j , I \, \ II ~I\.,II ~ J .....\ V f\ L,.....1'00 ,..,... 300 280 260 240 220 200 ~180 u o 160 00 r-140 ui ~120 ;2 100 uen 800 60 40 20 0 .- !.<-,-~-" 1 -.--..~-~'-..f .~- PEAK =309.000 CFS I-.~_l- I 2o-DAY VOLUME =4-24 X 106 AF l\ 1\ \ I V -1\ I \ I 1 i\ \ ~ I \ II .....~i ~ I-' 320 300 280 260 240 220 200 .. .~180 o o.160oo r-140 W t!:I 120a: ;2100 u ~80o 60 40 20 o 1 2 3 4 5 6 7 8$10 11 1213 1415 16 11 18 19 20 TIME,DAYS MAY "f, f I I } ~ifl1. 1I;:: l I J, FEED FOL LENG ... -12 -11 ---8.5 --8 I 1 j @I •,I ,OJ ~.c, EXHl61T 40 ..ro'_~>;;lO ::~r ""'<Mi'~"\'~l ,\Sf ''.y;t,l, 'l' PMF INFLOW HYDROGRAPHS AT DEVIL CANYON 1 2 3 4 5 6 7 8 9 10 11 12131415 16 17 1P.j·19 20 TIME,DAYS JULY -AUGUST PMF STUDY SUSTINA RIVER BASIN ~L-~--..I r;_~E1K ~3~l,O~fI ~F~ i I \20-DAYVOI.UME=2.90x 106 AF 1---+-+-+--,-<'-f-.;.-=.-1:1-"'4"---+--+-+--1---1--1--1--1 I \I -1-1-+-+-1---+ 1 -+-1--+-+-+--1 ,'1-t--~-+--"'--+-HI-+-'--~'-'-~+-+,-+---+--+-'-+--1 :I I ,I .....---,..-,f----+H--+--l-\--r--t--f--t---t--+-+--l---l---I,:i 'Il--'-+'-~---1-.1_'\i ----1---11-1--4,!i \I I 1---:-:+--t-[-+--j;\..-t-+-t :-V .\.. i /' .I "d--t--:""'-+-+-+-+--l---IJ..--V !~,J.........--__ 1 -: I--l.o.-I--l.--I.--l-,...I-...l..-4-.......l.-.L..-J..-l..-J...-..--...-..-l.........-I w'140 ~ ~120 :I:g 100 o 80 60 40 20 o 320 I 300 I 280 260 240 \I 220 I 200 en ~ tJ 180 08 160...l-'---~''I~"-+-\I 1 2 320 I I i I I I I 'I 300 ,"I I I I I Jr\ 280 I I ,'I I \ 260 i:+-t'j ···t "f"+-- 240 I."I r,:f<1·;ll!.,-++-. 220 1---....4,~--t-+-~~l,-...,-fl I'i I 200 t-PEAK =299,000 CFS ,..•_~.I'.'i I ~180 20 DAY VOLUME =3.51 x 106 AF :.1 ! U I j -1------.----1-\I ~160 .t--- w"40 f---.-.+----',.--,.-..-----t-1 -t--4-....-j........,j~)-.1,I ;2 120 .~I!.'\..J I ':I~100 1'---'i I t-..'-'\TJ \1-' 80 .-1-...,--I ,J ,i ~ 60 I----+,-+--+-+-,-+-ljJH-!-+\,,-'"I ~~i.=-.~I r--.~ o I 1 I i I 3 .4 5 6 7 IS 9 10 11 121314151617 1819 20 TIME.DAYS JUNE ~. --.11'~.'.,C':-__,:'_'"..........-_~.-_____"~__.',._'"....,...--~~...•..'.'~',".':',---'"-'..-.....,........,.....,..-~,--.=--u MAY 380 360 rFtii~u.1 340 ~+--t--'l--I\-+-+-+-+-+ 320 _.i I D-~.~,j !U'i 300 --~"-..PEAK =362000 C.FS -: 280 ;20 DAY VO~~ME i 4.86X1J~~~_ ;i I !I 260 -_...---.j ,l 240 .~...l.- 220 ~--~,--~-t-+-,-_.._.-.-,··t·'--- '!,I en 200 -_·....·---·_·r,'1"",- ~'....u 180 '.,..-..- o .8 160 I.... w~140 --,--II I ~,120 \1« :J:g 100 o 80 1----1.-+---..-,.,+.+ 60 I--I-i---'+'+' 401-.....-1,,--+--_·++_"-'.',.1- 20 o 1 2 3 4 5 6 7 8 9 10 1112 13 141516 171819 20 TIME,DAYS HARZA-EI3ASCO SUSITNAJQINT VENTURE AUGUST,1983 r1":- i"':1+J; k~ r L r J-f. fi- l'l~ h K~ ~, '~ ~ ~"'~,t""'~~''''1'-'"'~o'r~·''''';t'''·''··:·''"":':i ,·ft;·~~~%~-.:..··;··'~·_·:;·:::..·";'-t·"r:::...····_,,,,-,."..-..-.---_. l~ m><:r:-\XI--f +:a. N ,r • !' I,Iq -" ','',f / IOeJO 't •• I I I 100 t SO,+. rv '0:.~ -, ~J • 1,,:,:~I ...... .\t ~~.f'j ". "1"- l,IJh: y 1- ".J • j t'· t• ;I ~~~ .'. ,., I I .i ! I I ~. ,.. ".:'"I j','"rr '""',!;. I '.!I"",I··",...I':...'I "••1 •I j.f'•I J•'~.,1 I '••'"I • , .I''1"•I,:.;1.,it .'.:!~:;'I ,::,',;:1','i.,~:I"I ',I'j 1 ,I,,'.1 "I,••,I.,.•,..,I,·'.,.,•,.....'"., i· 1 :'II,"l::.jI,""j'.1 1 ',1::1;'1'1 'I,11 :",,1,:, t •f t 1 ~~I I I .,t,·.J f.•.':f:'I.;i ..':.!I '1.1 I ,~.;!',I!f 'I ,.,I:''..j.~" .'.",,, r t 'ff'I ••I '/f'"I'• ,I.'/•I ",, I ,.,: ,"!...'I 1 ".'I j I :,;!• ,I .:,~,.,I},,-'f , • ' • • , ' •I "l''I ',I,I,.,()F ,REC;O/?D I f1 ~11 (i!.,/J.3 Y'4,A If.~1J ..1 i;.I:/,1,1.It,.!,'1 "',,'., . I .I I t:I,'I /".,I I ••,I,I '.." '.:.'t '/I I'"/'I,:.fll I"."'J"",t'. ''.'"·,..;I..I • I • • I I ••I "I •; I I IJ.:.I;I -II 'i "J.I:·,.it ,I i 'j'll:I,I,H L'••1 ..•!..,.,...'1 ...t,'I'Ii'1 f..,..t I I ..,...,..~J I ...:'".I..;I',_I',,'•.i '.':'!'',.'I::!:,1 ';"lii,'lj:i 1 il.';:/'l.::.I:' 'll'':'j',,i,l;'11'~1'1 ";'il!ilf';;':ir,j :1/'';';.:i:'/"I ' " ;.'It.:,I I I J.1./',I .iii 1 j::1 .:•'I :' ;, ;•.,!I " " I I I..',I .'.'.I ,'", ,' ' ••+,..,I ..II ,f ••'•'j'I ..!I 'f •I ',I'~,j J "1.[.I'".f ""11.. J .I ..,.I'"I .I',"." •I I,ttl I ~,.;f t t ~t •t I I .,I I''", t -t 1-..I •f ~:I I f I \..'..~••.1 i ••',.I t I.J "'"I ..,I,'..I I I .. ..''I'I ""',-t;j':I'jr!'•...!d'" .,:~il ;'1.';.i,,t.:' ./,.':'1'1 1 I';',;',".11/,"/1 1 ",L"t,'I'I .i/'" ,'i':I'J!':11',:1,1:,:,;,].II,'",:,.",1 ':1'1 ~" i :,I 'I •'j I 'I:I''.,I.I ' " /"I'"J I "I I •"I ,.:.«:.:I '/" , ";'.I I 1'1 •'I 1 ,", I I I ..t 1 f 9:'I:"!"I I . 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'r',:.It >!.fA 1.<',olff'A,1,(' j;l'j L:"j"•''''I:'I,·l ..),'j"f',I.,1.:,/,.~ao: ! • ''.1 ;...',.I ..,,.:I I I •",.,.....""I "' •F"O,it>o~'ct=sI.'&•,~,.J ".~I ..+j ....'":.1"I •r '-I ••,I .............'I".,.-to 1._.&1''1 I 4i;,• ;t f·....'..I'r t,....I ,...,....::..•,_ t ',;•j • •I I',I:I.1 •I;!!:'r ,',:;, ;; :',,•I.'I I ',.I'• ,f"f , , •I ,'".".,';"j•.'"'r /"1"'J',.~..'"I''W'"j",1./r I I I •I,.,1''''.,/..,.~.:.~..•~','. I I I • ,l I',I , .••I !" j.••','I ..•fI j I., , 1 .':I 1'.•I '.''".:I 'I .J PMF STUDYj'.i"•"• ,".•,'., '':.J.i':'·i'.'.t ::i ,;:I :I '.!:I""SUSITNA !'I!VER !lA-SiN I;,'.;~,~·1'"ti :,,;I.o~.f i I ~'I',:J..Fi.OOD FREQ~ENC.Y CURV~ i ·~F.'j,..;i;·!I.:.;'-:SUSITNA RIVER AT GOLD CREEK • • . I 0 ,I , , ,I .I '..,'.I I.!'":I 1 I I I I . :/'. •f I ' •,',;I""I ", .I ,I : : 'I !j I I 1 ,i 'f •.'I ' .I'.I'"jI,"I I':..:J',I,..I ...•t -'. ! "2 ;;'I I(:J. ,1i".q..t,•" ,:,'I . W£'TVRN P'~Rh?2>j YE,-I'~:; ../I i,",, . i 'I PE'R/O.LJ ..., HARZA·EBASCOSUSITNA JOINT VENTURE AUGUST,1983 4! \J ... S '-u '0,\l~~u ~ "J 4 10 ..-..""-'"HAHZA-EBASCO SUSITNA JOINT VENTURE AUGUST,1983 .... • APPENDIX ......."..'..'....' /•Z ,3 t ~.I .Dt Y.5, HARZA -EBASCOSUSITNA JOINT VENTURE AUGUST,1983 • • [] m X :I:-I:XJ-..... .1::0 W cnGl,::0 /0 "'''''' I ~ ..,.,..-.-8·~.,............' / PMfSTUDY SUSITNA RIVER BASIN PMP FOR \\lATANA BASIN I.....,., ./. ? ~·8 /C;.~II ~'.!~caE 3-P~Y'yt.JN PM P I-/At.?2A3-j)~yo ..jUL-AUG P/VIP 9 :/ / t' I . I {{ I I i ..;' /1 76 ~','',11"/ ../.. I'..",..... I' I /' 1..,. I. ., i ,t-'/ / ~.../'-........... /' A cRF.:3 /o~.,DA y JUNE PMP L.c __..~L ..._,.'c l: .---.-'-'/:-'--'....-------- r t .......---.--. r~ 8 \I) ~ ~\j ,~ '2 "'-» G ~ k ~'~ "~ /l ~ 4.t t\: ~ o Jo ~ • .-•...".•!-"F • f I I J .. •• ><-czwa.a. <C # • ---A3 WI:ATHER FEATURES CONTRIBUTINd TO THE FLOODS By E. D. DIEMER, U.S. WEATHER BUREAU INTRODUCTION To completely describe the meteorological conditions involved in a: rainstorm, it is necessary to describe weather features which occur on various scales. Large-scale phenomena controlling the east-central Alaska flood weather had an areal coverage ranging from the size of Alaska to that of most of the Northern Hemi- sphere. Such features can be described by using data obtained from the meteorological observation network. Small-scale features are just as significant and may range in size from a few hundred square miles to tens of thousands of square miles. A detailed anal- ysis of rainfall from numerous surface observations combined with radar observations is one approach to describe the small-scale fea- tures of rainfall distribution. In general, owing to the lack of ob- servations in the Alaska interior, discussion of small-scale weather phenomena will be limited to areas where small-scale features can be identified~ Two coincident factors are required for precipitation: moisture and upward vertical ~~._,tion. Exh·emes of these factors occur as small-scale weather phenomena. The general synoptic patterns of large-scale weather systems, such as extensive high-pressure areas, low-pressure areas, and frontal zones, form an envir~mment which is conducive to the development of the smaller scale !features. The - CLIMATOLOGICAL BACKGUOUND A pronounced continental climate covers the area north of the Alaska Range.The heaviest precipitation at Fairbanks is generally from June through September:the average monthly totals are June, 1.39 inches;July.,1.84 inches;August,2.20 inches;and Septemberll 1.10 inches.In other months,less than 1 inch of precipitation (wa- terequivalent)is received.The normal yearly total is 11.29 inches. l\fuch summer precipitation is of the shower type.Fairbanks has an average of five.thunderstorms from June through August,and showers and'thunderstorms are quite common throughout thle Alaska interior during these months. Summer precipitation inthe interior of Alaska is associiated witlI1 a marked hlcl:ease in both moisture and solar radiation.Much of the moisture originates froIl1l the Bel'ing Sea and Northwest Pacji- fie Ocean,whieh warm to 47i°F and 51°F,respectivelY,by August. Surface dewpoints over tbeBering Sea in August are in the mirll- forties and in !the NorthweSit Pacific reach the upper forties.SUJr- f<lce dewpoints iln the fotties are not uncommon in the Fairbanks area during .TulJr and August.l\.faximum dewpoints during August 1967 ',vere in the low fifties.Evaporation supplies moisture that iis necessary for shower formation,but the amount from this source is too small for consideration in the precipitation amounts recorded during the August flood.Fairbanks has about 22 hours of daylight per day in the laUer part of June and about 16 hours dUring August. Normal daily maximum temperatures for June,July,and August are 71.1°F,'/1.7 of and 65.3°Ji"',respectively,with record highs nealr gO°F•.The pronounced surface heating leads to upward vertical motion of the air by decreasing its stability,and thus summer showeracti vity increases. Another feature which has a strong influence on summer pre- cipitation is the annual migration of the Arctic Front,which sepa- l'tltes the cold dry polar air from the warm moist maritime (til'of the Gulf of Alaska lmd Bering Sea.The mean position of the Arctic Jt"ront in Jtmual'Y is along the North Gulf coast and Aleutian Chain. By July the Al'Ctic Front has moved nOl·thward to a mean east-west l)osition ncar 65°N.latitude,which is about the latitude of Fair- banks and the Chena River basin.A weU-developed Arctic Front smaller features usually play the crucial!role in extraordina~y events and al'eactually responsible for mUlch of the weather.This is particularly true.for precipitation that Jrnayoccur in a narrow band of cUlllulus clouds or that may be assorciated with aparticuJar orographic feature. }<'1.0008 OF 19li7 IN THE UNITED STATES .. It r:trl A5 LI'Jr::::JCj[:jt::::l EAS'l'·CEN'l'HA l..ALA SKA r,-',. '.."::;;;;.clc:J can increase precipitation along the frontal zone b){increasing the upward vertical motion of Warm moist air moving northeastward from the Bering Sea.Moist air moving northward from the Gulf of Alaska produces little precipitation at Fairb<mks because the moisture is dropped HS precipitation during the fOliced rise over the Alaska Range.Thus a large and continuous moisture supply for F.ail'bankr,must occur as a strong northeastwal~d flow from the Bering Sea area.Moisture from the Bering Sea mnd the position of the Arctic Front were of major importance among the synoptic features producing the rainfall leading to the F'aiJ!'banks flood. The principal tracks of low-pressure centers are normally from the Vicinity of Japan and the Kamchatka Pel1linsula (U.S.S.R.) northeastward into the Bering Sea.Fl'om the Bering Sea the storm tracks turn either eastward north of the Brooks Range bto the Beaufort Sea or eastward across central Alaska between the Alaska Range and the Brooks Range.The low-preSSUli'e systems affecting Fairbanks during August followed the latter path. NOl'mal sea level pressures for August bldicate that a low- pressure area in the North Bering Sea with northeastward flow across the Bering Sea into central Alaska is tlhe average situation. The low-pressure area and northeastward tfO\1V'are reflected in the upper airflow:a north-south layer occurs at 10,000 feet near 180 0 longitude,and southwesterly winds move from the central Bering Sea into central Alaska.The position of the trough and winds coincides with the principal storm tracks. SYNOPTIC FEATURES Rainfall stal'ted about August 8 and continued until about Au- gust 20 over a wide area in east-central Alaska.Figure 1 is an iso- hyetal map showing the distribution of rainfall during August.The area covered by heavy rainfall is enclosed by the 3-inch isohyetal and includes the lower Tanana River basin and adjacent basins to the north draining into the Yukon River. August precipitation data for locations in east-central Alaska are given in table 1.Figure 1 shows the locations where the precipita- tion data were collected.U.S.Weather Bureau gages are in valleys and thus did not l'ecord the greater amounts of rain that fell in the mountains.Rainfall as high as 10 inches was unofficially reported in the Chatanika River basin 60 miles northeast of Fairbanks.For most of August the rainfall occurred during the same period over widely scattered locations.Figure 2 shows cumulative daily rain- fall at Clear Ail'port,Nenana,and Chena Hot Springs from August 8-19.Clear Airport and Chena Hot Springs al'e about 100 miles L'..:Jr---,""-,L ..->-.....tt::.:::Jr::;::J........~, iL..,,--.o......I~Ait:::..:...:.~-......", A4 "---: ..• • • .. -- A7 ---1eI!IIIlll..-.- ----J----- 18()hyet Number ill rail/lall,iTlillChell - EAST-CENTRAL ALASKA EXPLANATION - 018 U.S.Weather Bureau precipitation gage Number rlj"erlltl!table 1 - Bureau stations reported an excess ofS inches for the da~r of heaviest rainfall. At the Fairbanks Airport,6.15 inches of rain fell during the pe- riod August 8-15.The severity of the storms can be illustrated by for August 1967,east-central Alaska. - / J 100 MILES IL ~oo l-. ~~~#' \'(~~\)";oj";./ 1h'l· TT+1 I k'":1 I",Uf1 ..\ ,'.....~~../J '("1.I;'-1'\I~~'t J"'-104,'.1\"",e·J Lc.·:I'''Ll~i''~,J1 •,,'J~.lC),-••...~-.,~<\,<f,•g ~"" •"'~"•.fU < ''pI .)M ,\'~ "b'l:t '-"",., '" ...~ FIGURE 1.-Total rainfall dil:;tribution li'LOOOS OF 1967 IN TIlE UNITED STATES .,,~ ,) '.,h~I f~ 6!)' apart.The cumulative daily rainfall curves at the three stations are vel'ysimil~r and indicate the general nature of this storm.Maxi- mum daily r2,inf&1l·of the stol'm period occurl'ed on either August 12 or 13 itt half of the locations given in table 1.Eight of the Weather A6 .. • ~__d'~"""..-'rx't d .. "' o!f 11!11 ,. ;>11 .. ,_, f1 u 0 ' fl L.J. c r: L..: f' ' L, f ' I.... c L L ( ~ ~ r ·~ TABLE !.-Precipitation, in inches, du.ring August 1967 in east-central Alaska [Adapted from U.S. Weather Bureau (1952}] No. (fig. 1) U.S. Weather Bureau station Monthly total 1 3 4 5 7 8 9 10 11 12 13 H 15 1 Bettles FAA Airport.--------------- 2 Big Delta FAA---------------------3 Canyon Village ________ . ____________ _ 4 Central No.2----------------------5 Chalkyitsik.-----------------_____ _ 0 Chena Hot Springs ________________ _ 7 Circle Hot Springs _________________ _ 8 Clear Airport----·----------------- 9 Clearwater •• ---------------------- 10 College Ma~t ObsY------------------ 11 Delta Junction.-------------------- 12 Eielson •••• ------------------------13 Fairbanks WB Airport _____________ _ H Fort Yukon •••• -------------------- 15 Galena ________ -------------------- 16 Gilmore Creek •• ------------------- 17 Indian Mountain------------------- 18 Manley Hot Springs.--------------- 19 McGarth WB Airport--------------- 20 McKinle:v Park·------------------- 21 Nenana FAA---------------------- 22 Northway FAA AP ----------------- 23 Rampart No.2------------·-·------2-l Richardson _____________ ----------- 25 Summit FAA.---------------------26 Tanana. FAA __________________ ----- 27 The Gracious House _______________ _ 28 Trims CamP---------------------·--29 University Exp. Sta ________________ _ 30 lh"est Fork.------------------------ 31 Vt'ild Lake No, 2------------------- 32 ~·o~der Lake·--------------------- No. (fig. 1) 1 2 3 4 5 (j 7 8 9 10 l1 12 13 14 15 16 17 18 19 20 21 22 23 24 25 20 .17 28 29 3G 31 32 U.S. Weather Bureau station Bettles FAA -~irport ••• ·----- Big Delta FAA ................ . CanyonVUlage~------------ Central No, 2 ••••••••••••••• Chalkyitsik ... -·-------------Chena Hot Springs ........... . Circle Hot Springs •••.••••••• Clear Airport ................ . Cle:i,<water •• ---•••• __ -----•• College Mag Obsy ••••••••••• Delta Junction ••••••••. -----Eiel~on •• _ -----·-••••. ~-----Fairbanks WB Airport •••••••• Fort Yukon ••••••••••••••••• Galena ................ -----· Gilmore Creek ••••••••••••••• lndir.n .:\lountai!l •• ---·-·-· •• ::\lanley Hot Sprmgs •••••••••• ~!!.'Grath WB Airrort ........ . ~~cKinl"l Park ••.••••••••••• Nenanl!. FA.-\ •••••• , ••••••••• Northws.J FAA AP .......... . Rampart No.2 ............... . :Richardc{)n ................ _ .. .. Summit FA.A ••••••••• ~·--·-- Tanana FA;\ ............... . The Gracious House .......... . Trims CamP.-_ •••••••••••••• t:niversit~ t:>cp, Sta ........... . \Yest ForM; ••• ···-·--·-··-··· W!lri Lake No, 2 ....... """"""" ,,. an~er Lah:J ••••••••••••• --· ------------- Monthly total 2.17 1.56 1.12 5.70 1.37 7.74 5.75 10.06 2.37 7,36 2.48 7.47 6.20 .35 4.70 {;.49 3.-14 6.89 3.41 3.45 8.21i 1.85 6.08 2,87 [t52 5.06 7.54 9,10 6.57 4.52 2.00 3.81 2.17 1.50 1.72 5.70 1.37 7.74 5.75 10.06 2.37 7.36 2.48 7.47 6.20 .35 4.70 9.49 3.44 6.89 3.41 3.45 8.26 1.85 6.08 2.87 3.52 5.06 7.54 9.16 6.57 4.52 2.00 3.81 Trace------------------------------------------Trace 0.18 0.24 0.22 0.12_______ 0.14 0.01 0.24--------------Trace .• -------------------0.12 .09 .09 .04 .42 0.03 .02 Trace .69------------------------------------------.01 .o3 .21 .o4 .49 .oa _______ ------- .23 0.09------------------------------------------.60______________ 1.17 1.32 .73 ,33 .Q(l__________________________________________ .09 .08 .33 .01 .59_______ .02 .03 .06 .02______________ 0.05______________ .23 .50 .42 .18 3.13 1.50 ,63 .34 .06 Trace •• ----------------------------------------.34 .02 .01 1.66 1.62 ,84 .1~ Trace----------------------------Trace---------------------1.00 .70 1.70 4.58 1.20 .3 .02 .15.-----------------------------------.05 .10 .05_______ .65 .15 .15 ______ _ .03 .0~------------------------------------------.50 .21 .05 2.25 2.51 .65 .49 .17 .05_______ Trace---------------------.28 .09 .06 Trace .48 .11 .10 Trace .09--------------0.02 •.••••• --------------Trace .68 .29 ,65 3.61 1.42 .38 ,05 .02------------------------~-----------------.02 .56 ,05 .87 3.42 .69 .47 ,07 .01--------------------------------------------------------.10 ,10--------------.05 .02 --------------0.01---------------------0.04______________ .01 .83 1.40 .49 ,32 .18 .09 .11------------------------------------------.67 .32 .04 3.38 2,80 .75 .70 --------·------------Trace---------------------.11 .16 .04 .41 .86 .02 .25 .11 .19_________________________________________________ .08 .02 .14 3.33 .48 .54 .58 .07-------Trace _______ Trace ______________ Trace_______ .05 .07 .. 33 .22 .70 .41 .2r._________________________________________________ ,41 .25_______ 1.80 .02 .32 .05 .os •••. --------------------------------------.01 1.01 .o6 .52 3.04 2.22 .8o .10 .03 .. ____________ Trace ••.•••. Trace....... .28........ .01 .07 Trace .09 Trace ••.•.•• .02 _______________________ -------------------------.28 .11 .98 2.00 .39 .32 .2f\ .1o__________________________________________ .15 .o8 .14 1.01 .28 .3o .os .34 .12 .01 Tr&ce ••••• -------·--------.30 .30 .48 .16 .18 .47 .28 .03 .02 ••••• ---------Trace •.. -------------------------.15 .10 .44 1.96 .13 ,36 .07 1.63______________ .05_______ 0.02 .05 .02 .53 .75 .34 .50 1.08 .77 .77 .85 .67 ••••••.••••••. Trace ••••• ----------------.37 .35 .40 .73 2.44 .91 .29 .06-------------------------------------------------.50 .19 .17 3.28 1.15 .63 .31 .54 Trace •• ·--·------------·-··-·----.05....... .28 .11 .21 1.78 .44 .35 .27 • -.29 .17 .01 ___________________________ _ ··-·:os::::::::::::::::::::::::.::::::::::::::::::::.::: .37 .36 .os .49 .87 .3o .31 16 17 18 19 20 21 22 23 24 25 20 27 28 29 30 31 0,40 0.48 0,03 Trace Trace ••••••• Trace 0.13 0.08....... 0.08 0,04 0.01 ····---·-····· 0.01 Trace Trace 0.25 0.12 Trace-------------· Trace .12 .02 .................... . --T;i~~::::::: ::::::: Trace 0.06 .04 ••••• ---------------. 0.00 ••••••••••••• -·---··-·--·--. Trace ..... . .02 .04 .04 0.10....... ,0-l .02______________ .04 .01 .03._____________ 0.02 .2'1" Trace .08 Trace Trace ••••••••• ---------·------·-........ .08 ••••• -----··------·-------· ••.••• ---··-··. Trace •••••• :....... ..27 Trace Trace ,17 Trace .03 ••.•••• -------.13 Trace ....... T.rac:e .08 .01 Trace........ .19 Trace...................... .27 Trace .03 Trare .01....... .25 :n .05 ••••••• ---·--. ,04 ••••••• ••• • • • • .30 •.••• _ .•• ------·--·. ------.. .16 Trace •• ___ •• ____ •••••••••• -·-·--·------··-------------.15 .05 .25 ,4Q_____________________ .20. __________________________ _ .03 Trace ••••••• Trace.---------·--·-------·------.00.............. .44 .04 .................... . --·----------·-·········-·-· .07 Trace .79 Trace ............... Trace .09 .03 •••.•.• Trace Hi .02 Trace ,02 .02....... .22 Trace Trace ••••••••••.••• Trace Trace Trace Trace Trace Trnl'e Trace ............... Trace ••••••• Trace .01....... .OL...... .01 Trace ••••••• Trace ....... Trace ·--·~ai ----~.to::::::: ----:is:::::::::::::: ---r:f;.c~-· --~io ----:.ts :Yg :?A --1-;.;.~;:::::: :::::::: ---r~are · ·1-;.;e:; -···:sii·--·~a7--·-:22::::::: ..... :~:--··:a.t :at.--~~~ :8~:::::::::::::: :a~ :8g:::::::::::::: ····:o:t .63 .o4 .15....... .o3______________ ,3o .o3 ....... _______ .on.............. .2-1 .05 .18 .41 .10 .18 ,07 .49 .01 .OL •••••• Trace .08 .............. Trace .03 .•••••• .01 .01. ----.05 .04___________________________________ .05 .12------------·-··-·····-··-·· ............. Trac-e .0-1....... .01 .02 ,03.............. ,21i Trace ........................... . ............... Trace Trace .90 .10 ,09 .03.............. .06 Tr!lce.............. .01 .12 .38 ,02 .02 Trace..................... . .:?:? .08.......... ........................ .80 .20 Trace.............. .07 ....... T1=ace .54 T.rare ..................... Tfnce Trace ....... Trace Trace ,23 .04 Trace .Oil Trace .10 .13 .22 Trace Trare .04 Trat'e .•••..•• '"rrace Trace •••••.• -~! .24 .OZ Tra 1 c 5 e Tra0c~·-····· .-t9 TTrace .03 ••••.• 0 • ,04 Trace.............. ,31. ..••.• ·-"·····-··--··-· . • ----·-·-··· .. •• race....... .3 .05....... .•••••• 0.08 .18 ..•••.• .15 .31 .04 .o3 .10 ,13 .o6 .09 .ts .o9 .os .on .04 .oa .iii .•••••• Trace Trace ............................ Trace Trece T.race....... .09 .19 Trace Trace •.••••••••.••• .29........ Trace ........................... -·····-·-··· .......... .05 .01...... ........ .14 ...... . .25........ .81............................ .20 .22 .02....... . .03 •••••••••••••••••••••••••••• Tr11<:e ••••••• Trace .o;r Trace .03 .U .03....... .11 .05 Trare .............. Trace ...... . ..... z > (!) ,. AlO (/) w :I: 8 () 7 z z .J 6 ..1 ~ z < 5 a:: w > ~ 4 ::l ::E ::l 3 -() 9 l<"LOODS OF 1967 IN THE UNITED STATES ,,--------------------- / / ,"' --/ _,.-- ! // ,/ / J--,.__Chena Hot Springs I~ /J ,y I Nenana-Jf ,z I I I 10 11 12 13 14 15 AUGUST 1967 16 17 18 19 FlGUltE 2.-Cumulati'rl:e daily rainfall ,at Clear Airport, Nenana, and Chena Hot Springs, Autrust S-19. several comparisons. The August 1967' rainstorm was the heaviest since 1·ecords began in 192t.; arid exceeded the previous maximum amount by 31 percent. The stor1n precipitation of 6.15 inches is ovet• half the normal annual p1·ecipit.ation at the airport .. Compar- ison with the previous record storm, that of August 1930, is as follows: Augu•t JBSO August J/181 Total storm precipittttion ·-··" . ... 4.69 6~15 1\faximum daily precipitation .. .. 2.33 3.42 A series of storm systems accounted for such an extended period of rain. Thus .. it is convenient to consider the rain period in fm1r sections: the }Jeriod prior to August 8, the period o.f August 8-11, the heavy rain of .August 12, and the period of August 13-20. CONDITIONS PRIOR 'fO AUGUST 8 The Weather Bureau airport station at Fairbanks rec0t"ded a total of 1.13 inches of precipitation during the month of June, 0.26. EAST-CENTHAL ALASKA All inches below normal. During July, precipitation was 1.50 inches above normal, and the total precipitation 1·ecorded was 3.34 inches. Of this total, 1.27 inches fell on July 24; this was the last heavy :rain of the month. Fm· the 14-day period July 25 through August 7, 0.39 inches of rain fell at Fairbanks, but of this amount only 0.02 inches fell during the first week of August. 'Similar conditions prevailed upstream in the Chena River basin, judging from the precipitation record at Chena Hot Springs, the only observation station near the headwater.11 of the Chena River . Both the 1.46 inches of p1·ecipitation recorded during June and the 3.68 inches recorded during July are about one-third of an inch higher than the amounts t·ecorded at Fairbanks. These are not signi- ficant differences considering that the usual amounts of precipita- tion differ and that Chena Hot Springs is at a higher elevation where the effects of noarby mountains are greater. On July 24, Chena Hot Springs received 0.54 inches of rain, compared with 1.27 inches at Fairbanks. During the 14-day period July 25 through August 7, 0.83 inches of rain fell at Chena Hot Springs, but only 0.13 inches of this amount fell during the first week in August. In general, conditions prior to August 8 were about normal. CONDlTIONS AUGUST B-11 Rainfall of direct consequence to the flood began on August 8. Figure 3 presents the isohyets of total p1·ecipitation August 8 to 11, inclusive. The isohyets are a gross depiction of the 1·ainfa.ll distri- bution because there are only 32 precipitation 1·ecords in about 80,000 square miles and these are in valleys or at n1ountain passes. During August 9, 10, and 11, the Chena River basin received about 1.5 inches of rain. Two features of pat·ticu1ar interest affected the rainstorms. The first feature was a tropical storm named Typhoon Hope, which had moved into midlatitudes and had become an extratropica1 low- pressure center near 40° N. latitude, 175° E. longitude, about 700 miles south of the Aleutian Islands, at 1800Z on August 9. (1800Z is 1800 hours G1•eenwich time, which is equivalent to 0800 hours Alaska standard time.) Typhoon Hope continued northward and by 1800Z on August 11 had formed the deep low-pressure system northwest of Shemya. Figure 4 shows the synoptic patterns at 1800Z on August 11. The second feature was the Arctic Front in central Alaska near 65° N. latitude. A low-pregsure center just west of Fairbanks at 1800Z on August 9 influenced the Fairbanks weather until about ooooz August 11.By this time it had moved eastward into the Northwest Ten'itories (Canada)near the mouth of the Mackenzie River,and the Arctic Front had moved to the south of Fairbanks.At about 0600Z on August 11 an open wave,or small low-pressure center,had formed on the Arctic Front south of Fairbanks causing the front to again move northward.Referring to figure 4,the effect on the front can be seen just nOl'th of Fairbanks.Another low- pressul'ecenter llad moved el~stward along the Arctic Front from 178 0 E.longitude to the North Bel'ing Sea west of Nome,Note that the deep low pressure northwest of Shemya had set up a long south- west fetch for the Fairbanks area,Strong'low-level winds began a renewed influx of moisture to central Alaska.The low pt'essure • " e:'.~ A13 c::.:...:..~~ L Ldw~pressure system H High-pressure sY!ltem 1 ....,?o· ",1\0<._~ r=:nE~ W urlll fl'llnt y----., •••A .A.__.... Occhuletl front ........._~ l'tatillliary front C.:.J EXPLANATION EAS'l'-CgNrrRAL ALASKA 't:"_.,] FIGUItE 4.~Surfacc weather chart,1800Z,August 11. C_..-, •..A • Cold hont --~1020--- Line of equal !;!l!ll-Ievd pre!;!sure.in millibar!;! west of Nome continued rapidly eastward providing the vertical motion necessary for precipitation.About this time the low pres- SUl'e in the Bering Sea,formed by the extratropical storm,began to play an important role in the Fairbanks and Chena River basin rainfall. C~:J,....-.~, J·1 ,). " .,-J !.•14 ~/•t'hllll')'lllak \, <~.\.\., .~, " """,,,,.~~'::.~:~(~~~ ,..i> V"H,·lI,·• 1 :'0· II -(J o 20 4U tiO 80 100 MILES , J I I 1 I I"\t\\\I'1 /';11'~'111 _.1/ t ....~.,,~,.ysr)' ,F~/~Y ltiullilart !L.'~ FLOODS OF 1967 IN THE UNITED STATES J .1 ''-'-.....- t I. I ';\II.,I.<lI,I-t b',' F1GUU~3.~Isohyets of total rainfall dist1'ibution,August 8-11.Numbered points are U.S.Weather Bureau precipitation gages identified in table 1, A12 l{J 9~ • 'r~·-··-r c....-·"'""1 c::~~~.:::J c~:-::::::.c:::::o t::::.::.~c:::~=~-=--- AVl l"LOODS OF 1967 IN 'l'HE UNITED S'!'A'l'ES EAST-CENTRAL ALASKA A15 " b~lQ FIGURE 5.-lsohyets of total rainfall distribution,August 12.Numbered points are U.S.Weather Bureau precipitation gages identified in table 1. UONDITIONS AUGUST I:! Haill hegan falling again at Fairbanks about midnfternoon on August 11 and continued until the early morning of August 15.On August 12 the heaviest l'ain fell:3.42 inches at the Weather Bureau airport station at Fairbanks and 3.13 inches .at ClIena Hot Springs. Figure 5 depicts the isohyets for Aug'ust 12.Tl'he 3-inch isohyet en- CONDITIONS AUGUST 13-20 At OOOOZ Algust 14 the Shemya low had moved to a position southwest of Norton Sound,and another low had formed in the Bl'istol Bay area '-northeast of Cold Bay).By 1200Z August 14 (fig.7)a series of low-pressure centers was e·vident.The Bristol Bay low extended northward from Cook Inlet (sou~h of Anchor- ige)as a trough line,and the low to the north had moved into Nor~ ton Sound.An extensive area of low pressure in the South Bering Sea just to the north of the Aleutian Chain had become well formed. At OOOOZ August 15 several waves were still active along the Arctic Front.The last of these was located in eastern Norton Sound. The low-pressure area moving into Bristol Bay continued east- ward into southwestern Alaska.This was the last low-pressure sys- tem of this series of storms.Figure 8 shows the eastward move- ment of this low-pressure system.By OOOOZ August 15 the winds at Fairbanks became southerly,a direction which is not condul1ive winds existed across the Arctic Front.V?eak low!:!or open waves were moving along the frontal system.The degree of instability characteristic ofa Great Plains squall line was lacking;however. the cumulus development must have been considerable to produce the rainfall amounts obsel'ved. The low west of Nome moved rapidly eastward and decreased in intensity.By 1200Z on August 12 the remainder of this low appear- ed as just an open wave on the Arctic Front northeast of Fairbanks. The frontal system in the Bering Sea began rapid eastward movement forming another low on the Arctic Front in the vicinity of Norton Sound south of Nome.The parent low remained north of Shemya. By 1800Z on August 12 (fig.6)this low had crossed Norton Sound and was about 250 miles west of Fairbanks.Moisture influx l'emained strong at low levels with the long southwest fetch €xtend- ing from Fairbanks to the North Pacific south of Shemya,a dis~ tance of about 2,000 miles.At 0600Z August 13 (2000 hours Au~ gust 12 local time)the low had passed Fairbanks and merged with the general trough of low pressure extending from Fairbanks east~ ward into Canada.The strong southwest flow of moist air continued into the Fairbanks area.The Arctic Front remained in almost the same position just to the north of Fairbanks.The parent low north of Shemya had decreased in intensity and began moving along the Arcti.c Front toward central Alaska. 2~) .25 o 20 40 6U 80 100 MILES I I I t I I 150· ~~r ---_.-.,1.1 (~ '" $ AlIal..tkl'l 14'>" Vl't"'lil'•---'-r~----r-.~--~-./-'_-- @'~:C !11,/::~).'l\l\,I!I~<-'~...(hlllk;.il.:iik_-----------7::~,...:.~.1'~'~J~1~~r .I"'\.sr ti • J/HaUlllurl closes an al'ea about 180 miles long and 40 miles wide.The eastern extent of the 2-inch and the 3-inch isohyets was not defined,but was positioned to coincide with obsel'ved high water or flood condi- tions of the streams in that area.. There is a strong sim~larity between the weather systems that affected east-central Alaska,especially that of August 12,and a synoptic situation which leads to squall-line formation in the Great Plains of the United States.Strong low-level winds provided an in- trusion of warm moist air,and a crosscurrent of strong upper • .., .. • .. ( A18 I r f i t; ( [ rt ~. r f r f r FLOODS OF 1967 IN THE UNITED STATES 0 --Jo2o--- Line of equal sea-level preuure, in mllliban • • A-• Cold.front 300 EXPLANATION ... ·- WGrm front ,. . . . Occll~d&d front • • ' . Staticru:.ry front 600 MILES H Hiah-pressurtl 1yatem L Low-presaure ayatem FIGURE 8.-. Surface weather chart, 0600Z, August 16. ··-~ All the meteorological cnnditions required for rain were present over east.central Alaska during each storm passage; therefore, each storm dropped ita maximum precipitation in the same general area. A small change in the development and movement of the low- ]lt'e&sure centers could have placed the vertical motion out of phase with the main moisture supply. For example, a slight shift in the low-level winds from the Bering Sea could have moved the moisture several hundred miles from east·central Alaska, or a slight fluctua- tion in the Arctic Front could have moved the area of maximum precipitation north or south. Many floods, especially on small streams and rivers, result from exceptionally heavy rainfall from one storm. The August 1967 floods in east-central Alaska, however, resulted from al). accumula- rr " li ~ r [" ,. .. r L ~ ·--~ c-l c· EAS'f-CEN'fltAL ALASKA [.·.··'1 " -:J tion of rainfall events from a rapid sequence of storms over about a week's period. Only one of these rain periods, August 12, pro- duced exceptionally heavy precipitation. 6~· 145' 150' IJ Lake 0 20 40 60 80 100 MILES L I I I I J FIGURE 9.-Ieohyets of total rainfall distribution, August 13-20. Numbered points are U.S. Weather Bureau precipitation gages identified in table 1. l •