ML20133F773

Preliminary Estimate of Possible Flood Elevations in Columbia River at Trojan Nuclear Power Plant Due to Large Mudflow in Cowlitz River

ML20133F773
Person / Time
Site:	Trojan File:Portland General Electric icon.png
Issue date:	06/30/1983
From:	Kresch D, Laenen A INTERIOR, DEPT. OF, GEOLOGICAL SURVEY
To:
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ML19264D658	List: ML19264D657 ML20132B237 ML20132B532 ML20132B567 ML20132B600 ML20132B606 ML20132B636 ML20132B645 ML20132B648 ML20132B753 ML20132B773 ML20132B785 ML20132B792 ML20132C141 ML20132C361 ML20132C780 ML20132C836 ML20133F110 ML20133F504 ML20133F510 ML20133F531 ML20133F542 ML20133F569 ML20133F596 ML20133F656 ML20133F674 ML20133F700 ML20133F730 ML20133F743 ML20133F773 ML20133F786 ML20133F847
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7.-....- . s.c. -._m. _ ~c___...,_.. .d.L.r.cinwanim e. A 1 _.___a. ..Lau - ~ I '* U!!!TED STATES DEPARTHEl:T OF THE IldERIOR I j JA '.ES G. ' WATT, Secretary ?- GEOLOGICAL SURVEY 1 ~ U Dallas L. Peck, Director j- ) l ~ 1 For additional information write to: Copies of this report can be purchased free: District Chief Open-File Services Section . U.S. Geological Survey Western Distribution Branch 1201 Pacific Avenue - Suite 600 U.S. Geological Survey Tacoma,' Washington 98402-4384 Box 25425, Federal Center Lakewood, Colorado 80225 (Telephone: (303) 234-5888) l ~ 8 s e 2 ~. a r,n ,1 e ,.e- -w,, ..---aw, -e ,-v -c 4 .-m-

v t..,-.,.x-u s.. +. -- -. ..~u..--..- ... a - -u _. -... -.. . u ,.-a..,: e.- C0!iTE!1T5 e t 'l Page i Abstract------ '.---..'a ---4---------------------------------- d. ~.. t I I n t ro d u c ti o n -- - - * - '- -f- ---- -- --- --- ---- - -- -- -- ---- - -- - --- [ App roa c h, a s s umpti.oris ~r' a nd res ul ts--------------------------- // Scenario 1--Coincident occurrence of mudflow and Columbia R i v e r fl o o d s - - - -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- / Scenario' 2--Columbia River floods subsecuent to audflow ~ s e d i men t d e po s i t------------------------------------------ g P.odel sensitivity tests------------------------------------- d O ~ '- Con:1usions------------------------------------------------- y Sel e c t e d r e fe ren c e s ----------------------------------------- g 1 d i S-1 l l b =#. gp-es.-=%.. =,-e,- 4g o,s..

.2..w m /. ..........x.m. aasr x .2 - v=.u:.- - ..:- - T - ~2=..-- .. = -. ILLUSTRATIONS ~ ~ Page TIGURE 1. Map of study area--------------------------------- A. r TdBLES TABLE 1. Simufated'd5Yc~r~-lurface elevations in the Columbia ... ~ Riv'er a't T'rojan nuclear power plant for the coincid.ent occurrence of a hypothetical mudflow 3 with a. peak discharge of 1.1 million f t /s near the mouth of the Cowlitz River and selected Col umbi a River peak di scharges------------------- ff 2. Simulated water-surface elevations in the Columbia River at Trojan nuclear power plant for selected Columbi,a. River peak discharges subsequent to a deposit of mud' flow sediment in the Columbia River c h a n n e l. - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - -- - - - - -- -- g s i } f e s

- ~... ; : ..,....a..=anaa.aau.a.xx a.,..-. ~ a a -....a - a:A a r METRIC CONVERSIGR FACTORS l Multiply By To obtain 9 f e e t ( f t ) - ---- -- -- - - - - - - ------- ---- 0.3048 meters (m) m i l e s ( mi ) ------------.------------ 1.609 kilometers (km) 3 3 c ub i c ya rd s. (yd )------------------ 0.7646 cubic meters (m ) 3 acre-feet (acre-ft).- '----------.-- 1233. cubic meters (m ) ' " ~~' 3 0.001233 cubic hectometers (hm ) 3 ~~ ~ l cubic feet per 'seco'rld '( f t 1's)------ 0.02832 cubic meters per second 3 (m /s) .- z 28.32 liters per second (L/s) National Geodetic Vertical Datum of 1929 (NGVD of l929): A geodetic datum ] derived from a general adjust:nent of the first-order level nets of both the United States and C,anada, formerly called "mean sea level." NGVD of 1929 is referred to as sea level in this report, s t i 1 ~ p i _c-t

= 4w.. =.:zu w =;a.ww..= ; -... a.a w.~.::..... ..:, a ;ac..a:::.,..:nw.a ~ PRELIMINARY ESTDiATE OF POSSIBLE FLOOD ELEVATIONS IN THE ~ j COLUMBIA RIVER AT TROJAN NUCLEAR POWER PLAllT DUE TO d. A LARGE MUDFLOW IN THE COWLITZ RIVER y. r u ] By Dayid L.Kresch and Antonius Laenen c i. .l s l 3, a } + ~. ABSTRACT i, 3 Failure of the deSiis dam blocking the outflow of Spirit Lake near i j Mount St. Helens could result in a mudflow down the Toutle, Cowlitz, and Columbia Rivers. The U.S. Nuclear Regulatory Commission (!!RC) asked the U.S. Geological Survey (USGS) to determine whether the water-surface elevation in the ' Columbia River at the Trojan nuclear pouer plant, located 5 miles upstream of the Cowlitz River, could exceed 45 feet above sea level if a hypothetical mudflow of the proportions (peak discharge i. 1.1 million cubic feet per second at the mouth of the Cowlitz River) described in the U. S. Geological Survey's Water Resources Investigations ) Report 82-4125 ("Mudflow Hazards Along the Toutle and Cowlitz Rivers from i r ] a Hypothetical Failure of Spirit Lake Blockage," bh C. H. Swift and D. L. a Kresch,1983) were -to enter the Columbia River. A nbmerical 1 ) flood-routing model of the Columbia River indicates th'at the water-surface elevatin at the plant could exceed 45 feet under certain s I ,/ e

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Flood elevations simulated by the model exceed 44 feet for the l coincident occurrence of the mudflow and Columbia River flood flows with recurrence intervals greater than 10 years (640,000 cubic feep per i second) if Manning's roughness coefficients that simulate the hydraulic l properties of mudflows are used for the Co/ ? jfmbia River downstream of the ? Cowlitz River. Simulated flood elevations exceed 45 feet if the mudflow ] deposits 0.50 billi,on ' cubic yards.' of sediment in,the Columbia River ,1 - l upstrecmoftheCow1,{..,._.tz River at a bedslope of -2.5 feet per mile in the upstream directio.n and-t prior,to any appreciable scour or dredging of the i deposit, the Columbia River flow exceeds the 2-year peak discharge (430,000 cubic feet per second). i ( Simulated-flood elevations at Trojan do not exceed 32 feet if Columbia River flood flows of 100-year recurrence interval (850,000 cubic feet per second) or less are coincident with the mudflow and Manning's roughness coefficients that simulate the hydraulic properties of clear-water flows are used for the Columbia River. Tne reliability of the sinulated flood elevations is indeterminate because of uncertainties about the reasonableness of many of the assumptions made for this study. The results presented herein were obtained in accordance with URC guidelines to determine flood elevations using " conservative" (likely to produce the highest possible flood elevations) " hydrologic modeling techniques and assumptions." A = E

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,> a' directive from NRC that the mudflow hydrograph gen'erated in WRI Report 82-4125 (Swift and Kresch,1983) be used as the inflow to the Columbia -i River is of particular concern because in that report no attempt ~was made t' -} to account for the probable deposit of sediment within overflow areas and I J along the Toutle and Cowlitz River floodplains. Simulated flood 1 ] elevations would have been lower if the volume of the mudflow at the mouth of the Cowlitz Piver had been reduced by the volume of estimated j upstream deposits. i j i) i .. ~ j 1 1 i 1 p O \\iA ('l { \\, [6 i l I I L 9 _ -. _ _ i

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~ l;.. IllTROD'JCTIO!1 4. j The explosive May 18, 1980, eruption of Mount St. Helens, in" southwestern Washington, deposited a bulk volume of, nearly 4 billion j cubic yards of debris in the upstream 18 miles of the florth Fork Toutle m I River valley (R. J. Janda, U.S. Geological Survey, oral commun.,1983). L The former outlet channel of Spirit Lake was blocked by debris ranging in n J depth to 500 feet. The contents of Spirit Lake increased from 123,000 acre-feet in, i;he summer of 1980 t'o 275,000 acre-feet in December 1982. l If the lake were to.. fill,p'.the existing t.op of the debris dam, its 't j contents would be. 500,0D0 acre,-feet. 3 J A previous U.5. Teological Survey (USGS) report (Swif t and Kresch, 1983) identified mudflow flood hazards along the Toutle and Cowlitz Rivers associated with a hypothetical breach of the Spirit Lan e debris blockage with the lake surface elevation assumed to be 3,475 feet above sea level and the lake contents to be 314,000 acre-feet. The resulting clear-water outbreak flopd was assumed to entrain 2.4 billion cubic yards ~' of blockage material and produce a mudficu with a sediment concentration of 65 percent by volume that was hydraulically routed through the Toutle F and Cowliti Rivers to the mouth of the Cowlitz River (see figure 1). A mudflow is a flowing water-sediment mixture in s;hich the sediment volume a accounts for between 40 and 80 percent of the total volume of the mixture. When sediment volume is less than 40 percent, the water-sediment mixture has the hydraulic properties of a clear-water is flon. The peak discharge of the mudflow in the Cowlitz River near itis l? .i t f s. ~ l'

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J' 1 3 ~ ~ mouth was 1.1 million ft /s, and the duratio'n of m'udflow exceeded 2 days. !!o attempt was made to account for the probable deposit of sedinent along flood plains or within other overflow areas. The current study of the Columbia River was made in cooperation with the U.S. !!uclear Regulatory Commission (t!RC) to estimate the flood levels at the Trojan nuclear power plant, located 5 miles upstream of the Cowlitz River, that could possibly result from the occurrence of the mudflow described in the previous, USGS report. Specifically, IJRC wanted to know if floob lef,ekdu]'to the mudflow described in that report might be expected to reach. o' exc'eed an elevation of 45 feet above sea level at r the plant. A more extensive USGS analyses of the impact of a mudflow on the Columbia River.is currently in progress. Columbia Piver flood elevations, 2 will be simulated in that study using a sediment transport routing model. That study probably will not be ccmpleted for at least a year because of major no, del revisions that are necessary. s l l s 8 ty w, a f E. y ',1 9 .j, i -p l t ^ $ I i i } l L L

m .~... ._...=..a.~--.:.... z.. w -.... i. APPROACH, ASSU"?TIO!!S, A!JD RESULTS i Analysis of thb hydraulic characteristics of the confluence of the [ hypothetical mudflow at the mouth of the Cowlitz River and Columbia River t flows required the use of an unsteady flow computer model. USGS model K-634 (L. F. Land,1981) was selected for use in this study. Al though the primary purpose of' thaf. model is to simulate and hydraulically route dam-break floods, only the routing portion of the model was utilized in this application. -Hydraulic rotiting in the model is accomplished numerically with, ti e Saint Vgaant flow equations and a nonlinear implicit h finite-difference alg.orithm.-- .- x ~ Model K-634 was sed in this study to simulate water-su'rface elevations throughout a 128-mile-long reach of the Columbia River. The Cowlitz River'mudflow was treated as a point sourci tributary inflow in ' the model. Primary input data requirements for this application of the model were Columbia River cross sections, the discharge hydrograph of the . _. cudflow at the mou'th' of' the Cowli tz River, Columbia River flood-frequency discharges, Manning's roughness coefficients for the Columbia River, and the initial water-surface elevation at the downstream boundary of the s study reach. Twenty-one cross sections were used to define the Columbia River channel geometry for the 128-mile-long study reach extending from rivermile 145.5, 1/2 mile downstream of Bonneville Dam, to Tongue Point at rivermile 17.5. Twenty of these cross-sections were obtained fror ~ 0 1 ~ O . /b

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.. ~. >.b.c :.u :.... La%..w a_ w:;, % nu___ ~.* 's Randy Wortman of the U.S. Army. Corps of Engineers (COE), Portland, Oregon. These cross sections have been used by COE in the Columbia River Dynamic Wave Operational. (DWOPER) model. The underwater sognents of some I of these cross sections were revised on the basis of more recent data (C OE, 1982 ). One additional cross section, located at rivermjle 73.0 at t i Trojan nuclear power plant, was generated using USGS 7.5-and 15-minute i topographic maps and th'e 1982 COE report. The discharge hydrograph for the mudfiow at the mouth of the Cowlitz River was obtained.-from hydraulic simulation computer printouts used in the preparation of WRI.-Report.82-4125 (Swif t and Kresch,1983). Flood-discharge-fregu[ency 'in~ formation for the Columbia River was obtained from Bruce Duffy [U$. Corps of Engineers, Portland, Oregon, oral commun., 1983 ). Peak stages in the Columbia River estuary at Tongue Point result predominantly from high tides rather than f ro-high river discharges. Extreme high tides pYoduce peak elevations of 6 to 9 feet above sea level at Tongue Point. As a conservative approach, an elevation of 9.0 feet was used as the initial downstream boundary condition at Tongue Point for s all simulations. The model was first checked for reliability before using it to ; analyze the impact of the mudflow on Columaia River flood elevations. A t.. Columbia River.100-year flood (850,000 ft /s) was simulated and the computed water-surface elevation at the Tro,ian nuclear power plant, 22 e .. m

.... L c.a.:. u - -...u :: x. . feet above sea level, was 1 foot h'igher than the e evation shown on a COE flood profile (COE,1971) for the same flood. The magnitude of. Columbia River flood elevations at the Trojan nuclear power plant are dependent not only on the shape, duration, and peak discharge of the mudflow hydrograph at the mouth of the dowlitz River, but also to a large degree on the Columbia River discharge during and subsequent to the occurrence of the mudflow and the hydraulic ,j characterist,ics of the' mudflow ma.terial. Maximum flood elevations at Trojan, it was hypo,t,hekidd,',,would most likely result frcm either (1) the

I coincident occurren'ce of.the mudflow and a Columbia River flood discharge or (2) the occurrence of Colu::ibia River flood flows subsequent to the

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mudflow entering and-depositing sediment in the Columbia River during low fl ow. Therefore, flood elevations at Trojan were simulated 'for both of these scenarios. ~/ ii e i -6 p m, .e. .-w..

m.... i. _ a,.l... 2.. = ~ i a. - Scenario 1--Coincident ccurr'ence of Mudflow 7-and Columbia River Floods The hydraulic properties of the mudfica, ccmbined with Columbia River ~ flood flows, could be those for either a clear-water flow or a mudflow a depending on how much mir.ing of the two flows occurs. The Manning's roughness coefficients used for clear-water flows in this study are a i function of discharge and ranged from 0.030 for a discharge of 430,000 3 3 f t /s to 0.027 for a discharge of,- 850,000 ft /s. These roughness coefficients co[rrespo#nd-{i8sely with those used by the COE in the DWOPER i model. Manning s r'o.ug'hness coefficients used to simulate mudflows are a function of flow depth and. ranged from 0.180 for shalloy flow to 0.060 .- := for deep flow. These-mudflow roughness coefficients were computed from a uniform mudflow equation (C. L. Chen, U.S. Geological Survey, oral commun.,1932).and are similar to those used for th; mudfic'.t in Water Resources Investigations Report 82-4125 (Swi f t and kresch,1983). Tne j uniform mudflow equation, which was derived on the basis of the rheological propert,ies of mudflows, is analogous to the uniform flow equation for clear water. Peak flood elevations at the Trojan plant were computed for clear water and mudflow conditions for the coincident occurrence'cf the mudflow and several peak flood discharges in the Columbia River and are presented in table 1. e e 4

y _a.. a,. u ......s=...~ ~ a.~.s.-- . TABLE 1.--Simulated water-surface elevations i~n the Columbia River 5 at Trojan nuclear power plant for the coincident occurrence of a hypothetical mudflow with a peak discharge of ~1.1 3 ~ millio'n ft /s near the mouth of the Cowlitz River and selected Columbia River peak discharges a Water-surface Columbia Riyer el evation .:. _ -- ' di scharge (feet above sea level) - Columbia River, geak, flow condition ', ' ~,-.Ot/.s) -a/ -b/ 2-year peak s P 430,000 25 38 10-year peah 640,000 28 44 is-year peak (,85, oo 2c) 45 50-year peak -- 790,000 30 47 100-year peak 850,000 32 48 3/ ydraulic properties assumed to be those of a clear-water flow. -H L -blHydraulic properties assumed to be those of a nudflew downstream of the Cowlitz River and of a clear-water flow upstream of the Cowlitz River, w y ~ s -s 4 4 ( = i

.:......a.. _. z_c _ >-_ ? An initial assumption that very little, if any, sediment would be deposited in the Columbia River upstream of the Covilitz River fo the coincident occurrence of the mudflow and Columbia River floods was checked for reliability by analyzing computer output from the,model. Comparison of Columbia River discharge hydrographs generated by the model for two cross sections,' one ' located 1 mile upstream and the other 2 miles downstream of the Cowlitz River, indicates that for a Columbia River 3 discharge of 430,000 ft /s approdicately 5 percent of the incoming mudflow would travel -upstream and that there would be no upstream flow 3 for a Columbia River di-schar.ge. of 790,000 ft /s. Considering these comparisons to at least relatively confirm the validity of the initial ..= assumption, no channel adjustments were made to account for potential sediment deposition in the Columbia River upstream of the Cowlitz River. e e o k ~ s e e l s. -_m

.L ...a.. w._.;. a __ _. . a.a. ~ o [' Scenario 2--Columbia Ri' er Floods Subsecuent ~ v to Mudflow Sediment Deposit ~ The magnitude of flood elevations at Trojan produced by Columbia River flood flows subsequent' to mudflow sediment deposition in the Colur.bia River depends to a large degree on the elevation of the top of the deposit upstream of, the Cowlitz Rive'r. An estimate of the maximum deposit elevation upstream of the Cowlitz River was made by assessing (1) the amount of sediment deposited, and (2) the slope and areal distribution of the' depo ~si%, The reasonableness of these deposit characteristics' is of crikkhaT~importance to the accurate simulation of Columbia River flood ile'vations at Trojan. .u-Maximum upstream flow and sediment deposition of the mudflow would be expec.ted to oc, cur during a Columbia River low flow. Simulation of the Columbia River at low flow indicated that 30 percen't'of the mudflow would travel in an upstream direction. R. L. Dinehart (U.S. Geological Survey, oral commun.,1953) s.uggested, on the basis of analysis of sediment data ~ collected for the Toutle and Cowlitz Rivers during the May 18-19, 1980, and March 19-20, 1982, mudflows, that approximately 30 percent of the cudflow material may be finer than sand size (' O.062 millimeters) and ty e s that this fine material will likely remain in suspension and be ba transported downstream. If the entire 2.4 billion cubic yards of the Soirit Lake blockage contained in the mudflow is assumed to be transported to the mouth of the Cowlitz River, the bulk volume of solids in the 30 percent assumed to flow upstream in the Columbia River is 0.72 s e t f -.,-c - - - - - = - -

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s ?.m.. ~.S... L .. i s. a.a * - e s sa --e .. ~. .1: a. billion cubic yards. Assuming 30' percent of these' solids 'are fine materials that remain in suspension, leaves 0.50 billion cubic yards of material to be deposited upstream in the Columbia River. j. The slope and areal distribution of the mudflow deposit was estimated on r the basis of the sediment deposited during the May 18-19, 1980, mudfl ow. That deposit, which is described in a report by Haeni (1983), was .j greatest in thickness at the mouth of the Cowlitz River and had a surface a sloping upst. ream at.an' average ra'te of -2.5 feet per mile. Using that same slope to depos,i,t.t.he,0;.50 billion cubic yards of material up-tream, the deposit in the Colu_mtyia,Ri,ver would reach an elevation of I approximately 30 feet above sea level upstream of the Cowlitz River. -.:= The cross sections in the hydraulic-routing model were altered to t account for anticipated sediment deposition by increasing bed elevations - c as necessary. Selected Columbia P.iver flood flows were then routed i through the altered channel to determine corresponding flood elevations at the Trojan nuclear power plant. For the purpose of these computations, it was assumed that the deposit was not scoured prior to or during the flood flows. The simulated water-surface elevations are shown in table 27 .~

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%sid ~' , s: f TABLE 2.--Simulated water-surface elevations in the Columbia River at Tro.jan nuclehr power plant for selected Columbia River peak 1 discharges subsequent to a deposit of mudflow sediment in the Columbia River channel. The deposit is assumed to have 4 volume,cf 0.50 billion cubic yards upstream from the Cowlitz River with a surface slope of -2.5' feet per mile in the upstream direction. Columbia River Water-surface peak di charge Columbia River- ..,_ 1 elevation ' 3 flow condition ' __ -- ( f.t /s ) (feet above sea level)

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Low flow 250,000 39 2-yr peak 430,000 45 10-yr peak " 640,000 39 50-yr peak 790,000 52 i \\ ~ 6 - e o e .em

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.%u: a=nu .u. u. = 2.a.wa..&:.=.. u. L.w .a. .:.u.x MODEL set:SITIVITY TESTS t Sensitivity of the water-surface elevations simulated by the model to [ (T) the initial tide _ elevation at Tongue Point and (2) the Manning's roughness coefficient for clear-water flows was evaluated. r For an initial tide elevation of 0.0 feet above sea level rather than 9 feet, the simulated elevation at the Trojan plant decreased 3 feet for 3 a mudflow coincident with a Columbia River flood flow of 790,000 ft /s, 3 and 7 feet when (coirichfen}t"with a flood flow of 250,000 f t /s, using Manning's roughn'ess cosfficids for clear-water flow in the Columbia River. = Model sensitivity to Manning's roughness coefficient for clear water was investigated for two cases using coefficients 0.005 greater than those used in this analysis. The simulated water-s0rface elevation at Trojan increased 4 feet for the coincident occurrence of the mudflow and a Columbia River flood discharge of 640,000 f t /s. For a Columbia 3 River flood discharge of 430,000 ft /s, subsequent to deposit of mudflow sediment in the channel, the water-surface elevation increased by 2 feet. i 61 'g \\. s b 9 - 0 ~ = ,7 c

...a.u: ;a- =....a....- 2 -... ' M1..w.mLe--. ........:-.... u i <e C0llCLUSIONS .i .t 4 C Preliminary esti~ mates of possible flood elevations in the Columbia d Ri/er at Trojan nuclear power plant due to the occurrence of the } hypothetical mudflow described by Swift and Kresch (1983) wer9 made for aj two scenarios with a hydraulic routing model. Simulated flood elevations exceed 44 feet above sea level for the coincident occurrence of the mudflow and Columbia River fl'ood flows with recurrence intervals greater than 10 years (640,000 ft 73) $ f htanning's roughness coefficients that 3 simulate the hydrattlic-propylies of mudflows are used for the Columbia River downstream of 'the-Ccwl-itz River. Simulated elevations exceed 45 feet if the mudflow deposits'0.50 billion cubic yards of sediment in the Columbia River upstrIm of the Cowlitz River at a bedslope o.f -2.5 feet ~ per mile. in the upstream direction and prior to any appreciable scour or dredging of th'e deposit, the Columbia River flow exteeds the 2-year peak - 3 discharge (430,000 ft /s). The reliability df the simulated.el_evations depends primarily on the reasonableness of the assumed value of (1) the magnitude of the mudflow entering the Columbia River, (2) the Columbia River lianning's roughness coefficients, (3) the tide level at Tongue Point on the Columbia River, and (4) the volume and distribution of sediment deposited upstream in the Columbia River by the mudflow. The use of the hypothetical mudflow 4 hydrograph described in the 1983 report as the inflor: to the Columbia River, which was a study directive from flRC, is particularly questionable because that report did not include an analyses of the amount of sediment h/ i L j

1..__ .'2.J.a C.a._-1. c:_m - .i a. e-that r.ight be deposited prior to the arrivar of th*e mudflow at the mouth of the Cowlitz River. 4 b '4 ~ O P e@ J . f. t t 1 e e S e 3 k ? e +* 1 ee L, = l s

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..x.~-.w i._:. L1aQL_ .a..u;-...:. b :.AL.:.l.u SELECTED P.EFERENCES - Dinehart, R. L., ititt.er, J. R., and Knott, J. M.,1981, Sedicent data for. streams near Mount St. Helens, Washington, vol.1,1980 water-year data: U.S. Geological Survey Open File Report 81 -822, 82 p. l r j Haeni, F. P.,1983, Sedipent deposition in the Columbia and Lcwer Cowlitz ] Rivers, Washington-Oregon, caused by the May 18, 1980, eruption of Mount J St. Helens: U.S. Geological Survey Circular 850-K, 20 p. o .. ~ Land, L. F.,1981, Gene'ral purpose dam-break flood simulation model (r.-634): U.S. Geological, Survey Water-Resources Investigations 80-116,101 p. Lombard, R. E., Miles, M. B., Nelson, L. M., Kresch, D. L., and Carpenter,.P. J.,1981, Channel conditions in the Lower Toutle and Cowlitz Rivers resulting from the mudflows of May 18, 19'80: U.S. Geological Survey Circular 850-C,16 p. Swift III, C. H., and Kresch, D. L.,1983, Mudflow hazards along the Toutle and Cowlitz Rivers from a hypothetical failure of Spirit Lake i l bl ockage: U.S. Geological Survey Water-Resources Investigations 82-4125, [ 10 p. j U. S. Army Corps of Engineers,1971, Columbia River and tributaries, Wash,ington and Oregon below Bonneville dam--flood profiles: Portland District, Map C L-03-l i c. ~ ~ s N 9 h

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.m. -.Lt -.:w ..~.-.--...~.a..a.2..a..-. t e e 't. St.* Helens creption, the challenSa to' restore and protect: istrict, 84 p. f 4 a snel status Columbia F.fver,Portlar.d District, 29 p. S O g A 3 1 c i f ." m e t 9 ) i 6 h 6 6 ") 5 r- ^ 6 g en e m % m,

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