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Specification of Sediment Maintenance Flows for a Large Gravel-Bed River

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Specification of Sediment Maintenance Flows for a Large Gravel-Bed River WATER RESOURCES RESEARCH, VOL. 32, NO. 9, PAGES 2911-2921, SEPTEMBER 1996 Specification of sediment maintenance flows for a large gravel-bed river Peter R. Wilcock Departmentof Geographyand EnvironmentalEngineering, The JohnsHopkins University, Baltimore, Maryland G. Mathias Kondolf, and W. V. Graham Matthews Center for EnvironmentalDesign Research, University of California,Berkeley Alan F. Barta Intermountain Research Station, U.S. Forest Service, Boise, Idaho Abstract. Reservoirreleases may be specifiedto flushinterstitial fine sedimentfrom gravelbeds in the river downstream.Choice of an effectiveflow dependson trade-offs amongdischarge, flow duration,and pool dredgingas they determinerates of bed mobilization,sand removal, and gravel loss.A basisfor evaluatingthese trade-offs is developedwith an approximatemethod appropriateto the sparsedata typicallyavailable. Sand and graveltransport are representedwith rating curves.Approximate methods are introducedfor estimatingeffective gravel entrainment,subsurface sand supply,and pool sedimenttrapping. These are combinedin a sandrouting algorithmto evaluateflushing alternativesfor the Trinity River, California. A sedimentmaintenance flow of moderate size,just sufficientto entrainthe bed surfaceover the durationof the release,limits gravel lossand maximizessand trapping by pools.Larger dischargesproduce more finesremoval but at the costof greater gravelloss and reducedselective transport of fines.Dredged poolsincrease sand removal efficiency by providingmultiple exitsfrom the channeland minimizegravel loss if dredgedsediment is screenedand gravel returned to the river. Introduction terms of measurablechanges to the physicalhabitat that may be producedby a flushingrelease, rather than the abundance River channelsimmediately downstream of reservoirstypi- of organisms[Ligon et al., 1995;Kondolf and Wilcock,1996]. cally experiencea decreasein flood magnitudeand sediment Flushingflow goalsinclude removing fine sedimentfrom pools transportcapacity; if flow diversionsare made at the reservoir, usedfor rearing habitat and from graveland cobblesubstrates total dischargeis also reduced.Supply of coarsesediment to used for spawning,juvenile cover, and invertebratefood pro- the downstreamchannel is typicallyeliminated by trappingat duction. A flushing release may also be needed to entrain the reservoir,whereas fine sedimentsmay be introducedto the coarse sediment on the bed surface,permitting removal of downstream channel either from the reservoir or from down- subsurfacefine sedimentand producinga looserstructure that streamtributaries. If the transportcapacity of the downstream facilitates salmonidredd construction[Beschta and Jackson, channel is sufciently reduced,the finer sedimentmay accu- 1979;Milhous, 1990; Diplas and Parker, 1985].Entrainment of mulate on the bed of the river. Controlled releases of reservoir sedimentthroughout the activechannel section may be spec- water can be used to mimic the action of natural floods in ified to preventestablishment of maturevegetation within the removingaccumulated fine sedimentsfrom the channeland activechannel, with a correspondingloss in aquatichabitat and looseningthe gravelbed. Sucha sediment-maintenanceflush- channelcapacity [Kondolf and Wilcock,1996]. Erosion of the ing flow is similar to, but typicallysmaller than, a channel- river banksand floodplainmay be desirableto maintain topo- maintenanceflow intended to maintain erosion and deposi- graphicdiversity and providea supplyof coarsesediment [Li- tionalprocesses throughout the channeland floodplain[Hill et gon et al., 1995]. al., 1991;Ligon et al., 1995;Milhous, 1982; Reiser et al., 1989]. There is a clear need to specifyflushing flows as accurately The two types of flushingflow are broadly complementary, as possible.Released water is typicallynot availablefor stor- althoughtheir specificobjectives may conflict. age, diversion,and power generation,so the financialcost of a Flushingflows are frequentlyspecified to restoreor main- flushingflow can be very large. Becausethe rate and efficiency tain aquatic habitat, especiallyfor salmonids(salmon and of sand removal increasewith dischargeQ, cost constraints trout). Becausethe ecologicalresponse to both reservoirop- suggestthat Q shouldbe aslarge aspossible. The rate of gravel erationsand flushingflows is complex,dependent on external transportincreases with Q, typicallymore rapidlythan that for sand,and a flushingflow can producea net decreaseof gravel factors, and often evident only over periods of years or de- in the channelif gravelsupply is limited by reservoirtrapping. cades,the goalsof flushingflows are most usefullystated in Becausegravel is an important componentof fluvial habitat, Copyright1996 by the American Geophysical Union. gravelloss, or its artificialreplacement, represents an environ- Paper number96WR01627. mental and financial cost of flushingflows that arguesfor a 0043-1397/96/96WR-01627509.00 flushingQ that is a smallas possible. A minimumQ may be set 2911 2912 WILCOCK ET AL.: SEDIMENT MAINTENANCE FLOWS Area of detail with confidence.More importantly,the analogydepends crit- LEWISTONDAM ically on the assumptionthat each channelhas adjustedto a RushCreek steadystate geometryfor the water and sedimentsupplied to it. This assumptionis not likely to hold for channelsdown- streamof existingreservoirs, for whichthe water and sediment supplyhave been altered. Specificationof the magnitude,duration, and timing of a STEELBRIDGE • ]/• • - LEWIST•Nflushingflow requires definingquantifiably achievable objec- tives,developing a set of simple,but representative,functions STUDYx•x:j• /GrassValley Creek N representinggravel and sandtransport and sedimenttrapping ////'--.._ % LEGEND by pools,and combiningthese functions in a sedimentrouting DOUGLASCITY(•c-'5•-x'N X• POKERk•'u•Y •iTEBAR • dredgedpools algorithm, so that the trade-offsamong different flushing op- • WeaverCreek tions can be evaluated.A method for evaluatingflushing flow • ..........• • majortributaries options is developed in this paper for the Trinity River in northernCalifornia. Although developed for a particularriver, I I we believethese methods have applicationto other sites,not Figure 1. Location map. Flushing estimate prepared for for the meritsof anyindividual step, many of whichare obvious reach from Grass Valley Creek to Steelbridge.Sand routing approximationsof more completetreatments, but for the man- usessubreaches bounded by the major pools. ner in which the steps,in combination,permit a quantitative evaluationof the differentflushing options that is appropriate to the level of data typicallyavailable. by the needto entrainthe gravelon the bed surfacein orderto remove fine sediment from the bed subsurface and loosen the gravel bed. History of Channel Change The size of a flushingflow may also be constrainedby the on the Trinity River release capacityat the dam, the financial and legal liability The TrinityRiver drains7640 km 2 of steepterrain in the associatedwith an artificial flood, and the availabilityof water Klamath Mountains of northwesternCalifornia (Figure 1). at the appropriatetime. Moreover, transportobservations in Runofffrom the uppermost1860 km 2 of the basinwas im- both the field and the laboratorysuggest that only a narrow pounded by Trinity Dam (and its reregulating reservoir, rangeof flow producesentrainment of mostof the bed surface LewistonDam) beginningin 1961, as part of the U.S. Bureau (allowinggravel loosening and subsurfacesand removal), while of ReclamationCentral Valley project. From 1963 to 1995, maintainingthe selectivetransport of fine sedimentnecessary about75% of the averagenatural runoff of 47 m3/sfrom the to reduce the fines content of the bed [Wilcock,1995]. The upper basin has been exportedvia a seriesof hydroelectric variousgoals and constraintsfor flushingflows imposeboth plantsto the SacramentoRiver basin,where it is divertedfor minimumand maximumconstraints, suggesting that the range irrigation.Floods have been virtually eliminated on the Trinity of effectiveflushing flows may be quite narrow. River in the reachdirectly below the reservoir.Flow regulation Accuratespecification of flushingflows is hamperedby the hasreduced the meanannual flood Q ma from 525 tO 73 m3/s complexityof the flow and transportsystem and the sparse andthe 2-yearflood Q2 from 484 to 30 m3/s,based on the data typicallyavailable. Both problemsarise from the large continuousdischarge record from 1911 at the U.S. Geological scaleof river reach typicallyconsidered, the spatialand tem- Survey(USGS) gage at Lewiston(Figure 2). In the 35 years poral variabilityin flow and transportwithin the reach,and the followingdam closure,the largestsingle daily mean discharge nonlinear nature of the flow-sediment interaction. At the reach Q hasbeen 391 m3/s, and there have been only 13 dayswith scale,transport estimates are typicallymade with highly sim- Q > 240 m3/s,which is one-halfthe predam 2-year flood. plified models;field calibrationusing spot observations is nec- essaryfor any useful accuracy.The trade-off betweenmodel accuracyand data availabilityis of immediateconcern in this paper, becausethe need existsfor flushingflow estimatesthat !! = MaximumDailyDischarge are both efficient(requiring a minimumof observation)and ;i:! ---o-.-Mean Values:1911-60Annual Discharge &1964-90 ',• i11/60 Tdnity DamCloses i sufficientlyaccurate to permit evaluationof differentflushing ...... - ' ! I ! : •vers•onueg•ns I alternatives. 1,000i _ _'_s27 j• In view of the difficultiesinvolved
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