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HomeMy WebLinkAboutReservoir bank erosion caused and influenced by ice cover 1982Special Report 82-31 December 1982 US Army Corps of Engineers Cold Regions Research & Engineering Laboratory Reservoir bank erosion caused and influenced by ice cover Lawrence W. Gatto Prepared for OFFICE OF THE CHIEF OF ENGINEERS Approved for public release; distribution unlimited Special Report 82-31 December 1982. US Army Corps of Engineers Cold Regions Research & Engineering Laboratory Reservoir bank erosion caused and influenced by ice cover Lawrence W. Gatto []:{]&OO~&c§OO&®©@ Susitna Joint Venture Document Number Please Return To DOCUMENT CONTROL Prepared for OFFICE OF THE CHIEF OF ENGINEERS Approved for public release: distribution unlimited For conversion of Sf metric units to U.S./British customary units of measurements consult ASTM Standard E380, Metric Practice Guide, published by the American Society for Testing and Materi- als, 1916 Race St., Philidelphia, Pa. 19103. [ c r L c c c c n L c r ~ c p L [ c p L u c ·c~ . _:. f' 1~ Unclassified SECURITY CLASSIFICATION OF THIS PAGE (When Data Entarad) REPORT DOCUMENTATION PAGE READ INSTRUCTIONS BEFORE COMPLETING FORM I. REPORT NUMBER 2. GOVT ACCESSION NO. 3. RECIPIENT'S CATALOG NUMBER .Special Report 82-31 4. Tl TL E (and Subtitle) 5. TYPE OF REPORT 6 PERIOD COVERED RESERVOIR BANK EROSION CAUSED AND INFLUENCED BY ICE COVER 6. PERFORMING ORG. REPORT NUMBER 7. AUTHOR(e) 8. CONTRACT OR GRANT NUMSER(a) Lawrence W. Gatto 9. PERFORMING ORGANIZATION NAME AND ADDRESS 10. PROGRAM ELEMENT, PROJECT, TASK Army Cold Regions Research and AREA & WORK UNIT NUMBERS U.So Engineering Laboratory CWIS 31568 Hanover, New Hampshire 03755 II. CONTROLLING OFFICE NAME AND ADDRESS 12. REPORT DATE Office of the Chief of Engineers December 1982 Washington, D.C. 20314 13. NUMBER OF PAGES 30 14. MONITORING AGENCY NAME & ADDRESS(ll dlllerant from Controlllnf Olllce) IS. SECURITY CLASS. (ol thla report) Unclassified -~ ISa. DECLASSIFICATION/DOWNGRADING SCHEDUL.E 16. DISTRIBUTION STATEMENT (of thla Report) Approved for public release; distribution unlimited. I 17. DISTRIBUTION STATEMENT (ol the abatract entered In Block 20, II dlllarent from Report) 18. SUPPLEMENTARY NOTES 19. KEY WORDS (Continua on reverae aide II naceaaaty and Identity by block number) Banks (Reservoirs) Erosion Ice Reservoirs 20. ABSTRAC::"r (Caati:aue .. ,......,... aifl» II .._...._,. ..t. ldenJI{T by block nwnbar} The purpose of this study was to evaluate the importance of reservoir h~nk erosion caused by an ice cover. The evaluation is based on a lite~ature :review · and on inferences made from field observations and experience. Very little is known about the amount of reservoir bank erosion caused by the actions of an ice cover, although considerable information is available on the processes of ice-related erosion along the shorelines or beaches of oceans, rivers or lakeso The importance of ice-related erosion along a reservoir bank seems to be deter- mined primarily by water level. If the reservoir water level is high enough -- DD FOAII I JAil 73 Unclassified 1473 EDfnON OF J NOV fiS IS OBSOLETE SECURITY CL.A.SSJFICATION OF THIS PAGE (lnten Data Entered) [ c [ c c E c f'l L c r c E c c p L [ c [ [ Unclassified SECURITY CL-ASSIFICATION OF THIS PAGE(WJI-Data Entered) 20. Abstract(cont'd) for ice to act directly on the bank face, the amount o~ erosion caused by ice could be substantial. If the water level is below the bank, ice would have no direct effect on it. However, ice could indirectly increase bank instability by disrupting and eroding nearshore and beach zones, which could lead to bank erosion. .... ii t'n::: las . ..; { f lL'd SII!:CUIUTY CI.AUI,ICATIO-. or TM!!> PAGE(lt?len Dete Entered) [ [ [ [ [ c c n l [ r [ E [ [ h u [ c [ [ PREFACE This report was prepared by Lawrence W. Gatto, Geologist, Earth Sciences Branch, Research Division, u.s. Army Cold Regions Research and Engineering Laboratory. The work was funded by the Office of the Chief of Engineers under Civil Works Work Unit 31568, Erosion Potential of Inland Shorelines and Embankments in Regions Subjected to Freezing and Thawinge The author thanks James Wuebben, Ice Engineering Research Branch, CRREL, and Bruce Brockett and Dr. Daniel Lawson, Earth Sciences Branch, CRREL, for sharing their field observations. Most of the photographs in this report are from their collections. Mr. Wuebben, Dr. Lawson, Dr. John Reid of the University of North Dakota, and Dr. Tsong Wei of the U.S. Army Corps of Engineers, Missouri River Division, technically reviewed the manuscript. iii .... [ [ [ [ c E [ r, l [ c c B [· [ JJ L c L [ [ CONTENTS Page Abstract--------------------------------------------------i Preface---------------------------------------------------iii Introduction---------------------------------------------1 Ice as bank protection------------------------------------2 Ice-cover erosion processes-------------------------------5 Direct erpsion----------------------------------------5 Indirect erosion-------------------------------------7 Ice rafting-------------------------------------------8 Factors influencing ice erosion-----------------------11 Erosion features caused by ice covers---------------------13 Ice-push features-------------------------------------16 Ice-scour features-----------------------------------19 Summary and conclusions----------------------------------19 Literature cited------------------------------------------20 ILLUSTRATIONS Figure 1. Narrow zone of shorefast ice------------------------2 2. Attached ice shelf----------------------------------3 3. Protective shorefast ice along Wilder Lake, Norwich, Vermont, 4 March 1981---------------------------4 4. Icefoot along Grand Traverse Bay, Michigan, spring 1975--------------------------------------------4 5. Erosion during spring breakup-----------------------6 6. Reservoir ice cover laid down onto the exposed res- ervoir bottom below the banks-------------------7 7. Mobile ice------------------------------------------8 8. Bank sediment deposited on shorefast ice------------9 9. Sediment frozen to the bottom of ice laid down onto the reservoir bed-----------------------------10 10. Dirty ice after shoving and grinding along the shore-10 11. Wind-driven ice-------------------------------------14 12. Ice pile-up on Belle Isle, Detroit River, Michigan, during an ice run----------~-----~--------------15 13. Bank damage due to ice ride-up----------------------15 14. Trees broken by ice being let down as the reservoir water level dropped after a winter flood--------16 15. Ice-push ridge formed at the end of November 1979 during a storm with 45-mph northwesterly winds--17 16. Ice and shore sediment uplifted where an ice pres- sure ridge intersects the shoreline-------------18 iv ... [ [ r LJ c c [ [ r' L [ [ [] E [ [ JJ u c [ [ L INTRODUCTION RESERVOIR BANK EROSION CAUSED AND INFLUENCED BY ICE COVERS by Lawrence W. Gatto This report was prepared as part of an investigation of reservoir bank erosion processes and impacts in cold regions. The specific objectives of the investigation are 1) to measure and analyze the causes of and factors contributing to bank erosion, 2) to determine the relative magnitudes of the causes and factors, and 3) to assess the environmental impacts result- ing from the erosion. This report was done as part of the first two objectives. The data acquired and the process-response relationships analyzed during this investigation will aid in developing improved methods for predicting the erosion potential of a slope, the rate and areal extent of that erosion, and the time required for a slope to reach a noneroding dynamic equilibrium. -- Simons et al. (1978, 1979) and Pincus (1962) described some of the causes of and factors contributing to bank erosion and recession, and Seibel (1972) and Sterrett (1980) describe the complexity of their inter- relationships. This report, however, focuses on bank erosion caused just by reservoir ice. It is based on a literature review and on field observa- tions made by colleagues or myself. The literature review showed that the causes and impacts of erosion have been identified; however, the magnitude of erosion caused by a given process is only superficially known if at all. In fact, Ouellet and Baird (1978) feel that it may be impossible to quantify the amount of erosion that any one process contributes to total bank erosion since there are many contributing processes, all of which are interdependent. I did not find any references that reported measurements of ice-caused bank erosion. Most of the literature on ice-related erosion dealt with ice erosion processes along oceans or rivers and did not address ice erosion of reservoir banks. Since the processes that cause ice erosion along oceans, rivers or lakes can be similar, I reviewed references that dealt with all / threeenvironmentsandusedfieldobservationstoinferhowice—erosionprocessesmayactalongareservoirbank.Bynecessitythereviewwaslimitedtopapersintheopenliterature.Theses,dissertationsandconsultantreportsnotreferencedindatabasesorintheopenliteraturecouldnotbeobtainedandreviewed.Inaddition,foreignlanguagepaperswerenotreviewed.ICEASBANKPROTECTIONBeforedescribingsomeofthewaysanicecovercanerodereservoirbanks,itisimportanttomentionthaticecanalsoprotectthebankfromerosionbywavesandnearshorecurrents.Erosioncausedbyiceactionisfrequentlyreducedbyanearshoreicebarrier(Outhet1974).Avakyn(1975)reportedthatreservoirshorechangesusuallyoccurwhenthereservoirisice—free.Whenicecoversariver,lakeorreservoirfromshoretoshore,itdampenswavesandprotectsthebanksfromnormalwaveerosionprocesses(MacCarthy1953,Varjo1960,Brochu1961,CoakleyandRust1968,McCann1972,Cohn1975,JohnandSugden1975,Hadley1976,OuelletandBaird1978).Erosionrestartsatbreakupwhentheicebecomesmobile;theicescrapes,shovesandscourstheshoreorbank,andtransportssedimentaway.Figure1.Narrowzoneofshorefastice(5December1980,WilderLake,southofHanover,NewHampshire).2 Theicecoverusuallybeginstoformalongtheshorelineofareservoir.Itcanbeattachedtothesedimentattheshorelinearidcontinuetogrowoffshore(Fig.1).Aftertheicecompletelycoversthereservoirandthewaterleveldropsduringthewinter,ashelfoficecanremainattachedtotheshoreastheicefartheroffshorecracksandcollapses(Fig.2).a-vc4BAa.Theshelf(A)isbelowthebanktoe(B)andisseparatedfromandhigherthantheicecover(C),whichisbeingloweredasthewaterleveldrops.b.Acrackhasformedbetweentheiceshelf(A)andtheloweredicecover(B);theglove(C)isforscale.Figure2.Attachediceshelf(30January1980,OrwellRes—ervoir,Minnesota).AB3 Figure3.ProtectiveshorefasticeNorwich,Vermont,4March1981.IIalongWilderLake,Figure4.IcefootalongGrandTraverseBay,Michigan,spring1975.Notethesedimentdepositedwithintheiceandontheice—footsurfacebywavesovertoppingtheicefoot.4 [ [. r 6 c c [ [ n l [ r c E c c p u c [ _[ _[ This shelf can remain intact through the winter and protect the shoreline from direct erosion by mobile ice farther out in the reservoir (Fig. 3). The ice foot (Fig. 4) that often forms along portions of the Great Lakes also stops shore erosion in the winter (O'Hara and Ayers 1972). Descriptions of an ice foot as a protective structure along lakes are common in the literature (Zumberge and Wilson 1954, Dionne 1974b, Dozier et al. 1976, Evenson and Cohn 1979). An ice foot can also erode a shoreline by freezing to the shoreline material during the winter (Nielsen 1979). In the spring the ice foot can fall from the shore, removi~g attached sediment and bedrock (Hamelin 1972). The ice and attached material then float away and the sediment is deposited in the lake as the ice melts. I have not observed nor found documentation of an ice foot forming along reservoirs. It may be that small ice-foot features form but do not reach the size of those reported along the Great Lakes. Judging from the time required for an ice foot to form as described by O'Hara and Ayers (1972), I suspect that most reservoirs do not remain ice-free long enough ... into the winter for winter storms to cause the repeated wave spray necessary for large-scale ice-foot-formation. Lawson 1 suggested that an ice foot may form in the large reservoirs of the western U.S. ICE-COVER EROSION PROCESSES Direct erosion Ice can erode reservoir banks when the reservoir water level is high enough for the ice to directly loosen and remove soil particles by pushing or grinding along the face or toe of a bank (Van Everdingen 1967, Jahn 1975). This some~imes happens along rivers, especially during spring breakup (Fig. 5). Typically during a river ice run, mobile ice will slide along shorefast ice at a shear wall (Outhet 1974). Occasionally ice will slide directly against the bank and erode the soil (Martinson 1980). This erosion can be significant enough to cause local bank failures (Dionne 1974b), although Eardley (1938) reported that the amount of bank erosion by ice along the Yukon River appears to be inconsequential. These ice actions can also disrupt the bank soil structure sufficiently to reduce soil strength, so that the banks are more easily eroded by other processes. 1Personal communication, CRREL, 1982. 5 ( / ~- Rose(1946)describedapredictiveanalysisoficepressuresbasedontemperatureandicethickness.Hereportedmaximumprobableicethrustsof75,000—300,000N/rnofshorelineinthecontinentalUnitedStatesaridabout440,000N/rnincolderareas.Healsopointedoutthatloweringawaterlevelreducestheicepressureagainsttheshorelinebecausetheloweringwouldlikelycausetheicetobendandcrack,rupturingthecontinuityoftheicesheet.Thisreducestheicethrustatthewaterline,exceptwhenwaterfillsthecracksintheice,freezesandexpandstheicecover;iftheicecoverwascomplete,thisexpansionwouldthenincreasethethrustattheedgeoftheicecover.Fieldobservationsshowthatwaterlevelsinmanyreservoirsareloweredbelowthetoesofthereservoirs’banksduringthewinter,andtheiceislaiddownontothereservoirbottom.Theiceisthenineffectiveincausingerosion(Fig.6).Thewaterlevelnormallyrisesinthespring,andiftheicepersists,itcanbeliftedandagainmovedbywinds.Theicemayerodebottomorbeachsedimentduringthistime.Ifthelevelisraisedhighenough,themobileicemaystrikethebankdirectly.ReservoirswithcompletecoversoficeduringwinteraremodifiedbyFigure5.Erosionduringspringbreakup.Thewaterlevelishighenoughformobileicetogrindandshoveagainstthebeach(A)andthebank(B)directly(spring1979,southwestofMirrePoint,St.MarysRiver,Michigan).6 Figure6.Reservoiricecover(A)laiddownontotheexposedreservoirbottombelowthebanks(B).icemainlyduringspringbreakup.Typically,asspringprogressesandtemperaturesrise,theiceinitiallymeltsalongtheshore,leavingapartialicecoverthatmoveswiththewind.Also,thewaterlevelsofreservoirs,especiallyflood—controlreservoirs,usuallyriseinthespring.Themoremobileicecanthengrindandshovealongthebanks.Theseiceactionsmaynotbeasstrongalongreservoirsasalongrivers,however,sincerivercurrentsareusuallymorerapidthanwind—orthermallyinducedcurrentsalongreservoirs.Wuebben2pointedout,however,thatrivercurrentsarenotalwaysfasterthanreservoiricemovedbywindshear.Also,wind—drivenicecanbepushedperpendicularlyintoareservoirshoreorbank;thismorenormalforcemaycausemoreerosionthantheriverice,whichusuallymovesmorenearlyparalleltotheshoreline.IndirecterosionIfthewaterlevelofanice—coveredreservoirisnothighenoughfortheicetoactdirectlyonabank,theicemaycontributetobankerosionindirectly.Whiletheicecoverismobile,itcanpush,shoveandremovebeachandnearshoresediment(Fig.7).Thiscancontributetounderminingofthebank,whichincreasesbankinstability;thebankmayeventually2Personalcommunication,CRREL,1982.A—*7 BFigure7.Mobileicc(A)cangrindandshovethereservoirbottom(B)asthewaterlevelrisestoahigherlevelatthebaseofthereservoirbanks(C)(February1974,LakeKoocanusa,Montana).collapseandthebanklinerecede.Icecanalsoremovesedimentthathaserodedfromabankandaccumulatedalongthebanktoe.Byremovingthisaccumulation,waterwavesandcurrentscanerodethebanktoedirectly.IceraftingIceraftingcanalsoremovebankorbeachsedimentthatbecomesattachedtoorincorporatedintotheicecover.Dionne(1974b)observedclay—toboulder—sizesedimentatthebase,onthesurfaceandwithintheicecoverontheSt.LawrenceRiver.Atspringbreakupthesedimentistransportedandredeposited.IntertidalsedimentsarealsofrequentlyraftedinLabradorfjords(Rosen1979).ReimnitzandBruder(1972)reportedthatonlyaninsignificantamountofsedimentisraftedawayfromarcticAlaska’sdeltasduringseaicebreakup.Muchoftheshorefasticemeltsinplace,andmostoftheincorporatedsedimentisdroppedbeforefinalbreakup.Raftingoflargequantitiesofbeachmaterialattachedtothebottomofanicefootoccurs,however,intheAntarctic(Joyce1950).Theimportanceoficeraftinginlakesorreservoirsispoorlydocumented.Ifoundnoreferencesthatdescribedtheprocessesoficeraftingorthatreportedmeasurementsofthequantitiesofsedimentthatareice—raftedinlakesorreservoirs.Fieldobservationsshow,however,thatsedimentfallenfromreservoirbanksisfrequentlydepositedonthesurfaceofshorefastice(Fig.8),especiallyinthespringasthebanksthaw.-..-8 b.Banksediment(A)sliding(B)orflowing(C)ontosnow—coverednearshoreice(D)(4March1981).Figure8.Banksedimentdepositedonshorefastice,WilderLake,Norwich,Vermont.ra.Banksediment(A)sloughing(B)ontonearshoreice(C)(15December1981).D9 Figure9.Sedimentfrozentothebottomoficelaiddownontothereservoirbed(February1974,LakeKoocanusa,Montana)—Figure10.Dirtyice(A)aftershovingandgrindingalongtheshore(20February1981,ConnecticutRiver,Haverhill,NewHampshire).10 [ [ r L [ [ E [ r L [ [ c [ I' 1 .._. C. ~; L c c [ . [ When ice is attached to the shore or bank or laid down onto the reservoir bottom, sediment can also freeze to the ice (Fig. 9). On many lakes, though, the shorefast ice melts in place and drops its sediment without transport. It seems reasonable to assume that some sediment would be removed if the shorefast ice becomes mobile. When ice grinds and shoves along the shoreline and banks of rivers, it can accumulate sediment on its surface. The sediment becomes frozen in and on the ice (Fig. 10) and can be transported away. Bank or beach sediment can be removed from along a reservoir in a similar fashion, but the water level would have to be high enough for the ice to act directly on the beaches or banks. Factors influencing ice erosion The amount of bank erosion caused by ice processes depends on the interplay of many factors: water levels and their fluctuations; ice strength and characteristics; mobility of the ice cover; degree of ice at-.- tachment to the beach, nearshore or bank sediments; the extent to which the bank, beach and nearshore sediments are frozen; and shore configuration. The importance of water levels, ice attachment to sediment, and mobility of the ice has been discussed briefly. ·Additional information on the strength, properties, mobility and duration of and pressures exerted by an ice cover is available in the proceedings of the following conferences: Conference on Ice Pressures Against Structures, Laval University, Quebec, Canada, 1966; The Role of Snow and Ice in Hydrology, Banff, British Colum- bia, Canada, 1972; International Conferences on Port and Ocean Engineering under Arctic Conditions; Symposium on Applied Glaciology, Cambridge, En- gland, 1976; International Association of Hydrologic Research International Symposia on Ice Problems; and Third National Hydrotechnical Conference, Canadian Society for Civil Engineering, Quebec, Canada, 1977. The following individual reports also address ice formation, ice structure, ice properties, ice-cover characteristics and duration, and ice effects: Korzhavin 1962, Weeks and Assur 1969, Michel 1971, 1978, Donchen- ko 1972, Marshall 1977, Ashton 1979 and Haugen et al. 1979. Ficke and Ficke (1977) summarized the literature on ice formation, characteristics and effects on navigation, currents, shoreline structures and water quali- ty. Drouin and Michel (1974) reviewed the literature from 1922 to 1968 and described the thermal properties of ice and ice-cover expansion or contrac- tion • 11 [ [ r L r u c E c n u c c c [ c [ c u c [ I~ ·r· ~ .:__j Several papers address the pressures ex~rted by. ~n ice cover on shores, dams and other structures and discuss the factors that affect the magnitude of the pressures (Rose 1946, Monfore 1952, Zumberge and Wilson 1952, 1993, Montagne 1963, Sommerville and Burns 1968, Pessl 1969, Bergdahl and Wernerson 1978). Some of these factors are air temperature, solar energy absorption, ice temperature, rate of temperature change in the ice, coefficient of thermal expansion, rheology of ice, ice thickness, amount of water-filled cracks, and shore restrictions. If the shoreline or bank sediment is frozen, ice effects can be mini- mal (Harper et al. 1978). Freezing along Monomoy Island, Massachusetts, creates a stabilized beach of rocklike coarse sand, which slows or stops normal beach processes (Davis 1973, Davis et al. 1976). Along the Great Lakes shores, after the beach sands freeze and an ice foot forms, the beach typically remai~s static for about 10 weeks. There are some ice-push features that form, but for the most part, normal shore processes slow or stop (Davis et al. 1976). Owens and McCann (1970) concluded that offshore, nearshore and beach ice limit wave action on the beach and bank, but do form ice-push and ice-melt features along the shore. However, when the beach and nearshore zone are frozen, the sediment there is stable and unaffected by either ice push or storm waves. .... The configuration of the shoreline also influences the amount of bank erosion that could result from actions of a mobile ice cover. Along a reach of reservoir with a straight shoreline, ice erosion of the shoreline or bank could be more evenly distributed, with no one location being eroded more than any other. Along a reach with an irregular shoreline, however, ice would be more likely to erode the banks on the promontories than along the bayse Along reservoir shorelines with gentle offshore slopes it is more likely that ice could become attached to or gouge offshore bottom sediment and never reach or erode the shorelines or banks. As the water level drops in these areas, ice becomes grounded sooner than in areas with steep offshore slopes and less ice pressure is directed to shorelines or banks. Along shorelines with steep offshore slopes, ice can advance to the shore- line or banks and erode them directly. 12 [ [ c [ c p u [ fl L c n [ E c c 1J \.;:i c c _[ [ EROSION FEATURES CAUSED BY ICE COVERS There are several reviews of how ice erodes and reshapes nearshore zones and shorelines (Bryan and Marcus 1972, Code 1973, Mackay and Mackay 1977). The ice creates a variety of erosion features: linear and cellular scars, basins, ice-pushed and ice-deposited ridges or mounds (ice ram- parts), ice-contact cusps, and ice-rafted sediments including boulders, pebbles and ice-cemented blocks (Dionne and Laverdiere 1972, Lawson 1972, Short and Wiseman 1973, Dionne 1974b, 1981, Brochwicz-Lewinski and Rudowski 1976, Kovacs and Sodhi 1979). However, I did not find any references that describe features left by ice erosion on reservoir banks. It may be that the water level of most reservoirs is low enough when there is ice that the ice seldom erodes the bank directly. This is probably true for most flood-control reservoirs. Ice could be driven up a reservoir bank, but this would require a large fetch and sustained winds. Also, if the water level was low, it is unlikely that winds would be strong enough to drive the ice cover over the shore up to the banks. Some of the factors that limit the amount of shoreline rideup are water current drag, friction, jamming, ice pileup on the beach slope, steep beach slopes, ice flexural strength and thickness, and high freeboard (Croasdale et al. 1978). Tsang (1975) reported that a shore lead and gently sloping shores allow more ice rideup. Field observations along Whitefish Bay in Lake Superior show that ice can be driven by winds up to and over banks that are as high as 25-30 ft (Fig. 11). Ice driven by currents can also ride up and directly erode a riverbank (F~g. 12). The erosion of the banks and the damage to bank vegetation can be extensive (Fig. 13). Outhet (1974) also described how ice can be pushed onto and over low·banks and small willow trees along the Mackenzie River delta, Canada. Reservoir bank erosion can occur when winter floods rapidly raise· the water level of flood-control reservoirs. The ice cover on the reservoir is lifted, broken up, and made mobile. It can then erode the banks directly and damage bank vegetation (Fig. 14). This type of vegetation damage could eventually lead to increased bank instability if the damage is severe enough to kill the vegetation. The root systems which tend to bind the upper sediment would decay, and the added stability would be lost. As previously stated the disruption of the beach and nearshore zone by ice can also lead to increased bank instability. The following sections 13 ~ aa.Iceoverthebank(March1979).bytheice.Figure11.Michigan).intoMay1979.NotethesedimentdislodgedWind—drivenice(WhitefishBay,LakeSuperior,ii;--rb.Iceremained14 Figure13.Bankdamageduetoiceride—up(WhitefishBay,Michigan,Nay1979.-4$-kFigure12.Icepile—uponBelleIsle,DetroitRiver,Michigan,duringanicerun(March1978)a.Treedamage._,C--.b.Upperbankdamage.15 addresssomeoftheerosionprocessesandfeaturesleftbyiceerosionalongthebeachandnearshore.Themostfrequentlyreportedfeaturesalonglakesareice—pushfeatures.Ice—pushfeaturesDionne(1974a)summarizedtheliteratureonice—pushfeatures,whichincludeiceramparts,ice—pushridges(Fig.15)andice—thrustridges.Fieldobservationsofice—pushfeaturescitedintheliteraturearenumerous;thereisadebateonwhethertheyresultfromiceexpansionorfromwind—drivenice(Jones1970,Montagne1963).SinceIammoreconcernedwiththeextentratherthanthecausesoftheshoredisruption,Iwillonlysummarizethedebate.Someinvestigators(LaskarandStrenzke1941,Jennings1958)feelthaticepushduetoiceexpansiondominatesonlakeswithdiameterslessthan3—4km(Worsley1975),smalllakesabout5km2inarea(Dionne1979),andnarrow,shallowbaysonlargelakes(Pessl1969).Wind—generatedicepushoccursmoreoftenonwidelakes(Dionne1979,Varjo1960)orlargelakeswithfetchesgreaterthan4km(Worsley1975).Whenwindsarethecauseoftheicemovement,moderateprevailingwindscanpushicesufficientlytoFigure14.Treesbrokenbyicebeingletdownasthereservoirwaterleveldroppedafterawinterflood(December1973,FranklinFallsReservoir,Franklin,NewHampshire).16 formashorelineridge(BruunandJohanneson1971,BruunandStraumsnes1970);strongwindsarenotnecessary.Taylor(1978)observedice—pushridgesandpittopography185minlandacrossthebeachofSomersetIsland,NorthwestTerritories,Canada.Ice—pushridgeswereobservedalongtheSouthArmofYellowstoneParkinWyoming(Montagne1963),WamplersLakeinsoutheasternMichigan(ZumbergeandWilson1952,1953),ElevenMileCanonReservoirincentralColorado(Monfore1952),andGeneratorLakeonBaffinIsland(Ward1959).AlongalakenearResoluteBayonCornwallisIsland,NorthwestTerritories,Canada,theridgeswereseveralfeethighandusuallyhadanasymmetricalcrosssection(Nichols1953).BackshorebrushandtreeshavebeendamagedorbrokenalongKnobLakeinLabrador,Canada(Jones1970)andalongmanylakesincentralQuebecandLabrador(Pyokari1981).Wagner(1970)observedthatsmallrampartsformedalongshoresofembaymentswithfinesedimentandgentleslopesandthatlargerrampartsformedalongstraightshoreswithintermediate—sizesedimentandmoderateslopesalongShelburneBay,LakeChamplain,Vermont.Heconcludedthatice—rampartformationwaslimitedbyacontinuoussnowcover,thedevelopmentofpressureridgesinice,shortperiodsofpartiallyopenwater,and•1;r•4-Figure15.Ice—pushridge(about90cmhigh)formedattheendofNovember1979duringastormwith45—mphnorthwesterlywinds;theicewas10—12cmthick(30January1980,OrwellReservoir,Minnesota).17 4;.Figure16.Ice(A)andshoresediment(B)uplifted(about60cm)whereanicepressureridgeintersectstheshoreline(30January1980,OrwellReservoir,Minnesota).iceweakenedbybreakup.Thecontinuoussnowcoverinsulatestheicefromairtemperaturefluctuations;consequentlytheicetemperaturefluctuationsandresultingexpansionandcontractionareless.Fieldobservationsshowthatwhenanicecoveriscomplete,thecompressiveforcesthatformpressureridgesinanicecovercanalsocauseshoresedimenttobethrustedwherethepressureridgesintersecttheshoreline(Fig.16).Worsley(1975)describedthefollowingice—pushfeaturesaroundGrasvatn,Scandinavia,whereicethicknessesarenormally1.25m:boulderconcentrationsalongthebeach,boulderpavements,bouldersandtrails,ramparts,planedsubaqueousmoraineridges,andgroovedorscrapedsurfacesduetofloegrounding.TheaveragenetshorewardmovementoftheicecoveronGardnerLake,Connecticut,wasapproximately1min30daysandproducedanice—pushbeachridge1mhigh(Pessi1969).Ice—pushridges0.6mhighcausedbyiceexpansionwerereportedalongmanyGermanlakes(Goebeler1972).Adams(1977)observedice—pushfeaturesandiceactionalongAstrayLake,Labrador,Canada.Hereportedthatthefeaturesareduetoiceexpansionandwind;hefoundice6mabovetheJunewaterleveland1$minland.Itisclearthaticedisruptsanderodeslakeshorelines.Whenwindsarestrongenoughorwaterlevelsarehighenough,icecanerodelakeorA‘IAI-18 [ [ c [ n L [ c r u c r c c ·L [ c h u c [ [ [ reservoir banks and damage vegetation. How frequently this occurs and how much bank erosion results are unanswered questions. Ice-scour features Ice-scour features are documented along marine and estuary coasts and rivers, but I found no references for lakes or reservoirs. Ice along the Beaufort Sea coast typically striates, planes and gouges the bottom and beaches, leaving mounds and ridges (Hume and Schalk 1964, Barnes et al. 1977, FENOO 1975). Dionne (1969, 1974b) determined that most of the erosion along the tidal flats in the St. Lawrence River estuary occurs in the spring by ice gouging and ploughing. These processes form linear and arcuate scars and basins; they scratch and polish soft rock, dislodge previously broken rock fragments, and level weathered rock platforms. The bed of the Tana River in Norway has also been gouged and flattened by ice (Collinson 1971 ). Many of these features could form along lake and reservoir shores in ~ the spring, especially where bottom slopes are gentle and the water is shallow. As the water level rises, the ice breaks and floats. It can then be pushed over shallow areas and modify the bottom configuration. If this occurs near the toe of the bank, it may add to bank instability; if it occurs far from the bank toe, the bank would probably be unaffected. SUMMARY AND CONCLUSIONS There is considerable information in the literature on the ice pro- cesses that erode and deposit sediment along lake shores as well as on the features created by these processes. The general forces applied to a lake or reservoir shoreline by an expanding or wind-driven ice cover have been described and occasionally measured. The shoreline features fcrmed and the changes resulting from ice actions along the waterline are documented. However, the direct effects of an ice cover on bank erosion, the importance of ice rafting, and the effects of an ice cover on the erodibility of bank sediment by other processes have not been documented. The importance of ice-related erosion along a reservoir bank is determined by water level. If the reservoir water level is high enough for ice to act directly on the bank face, the amount of ice erosion can be substantial. If the water level is below the bank, ice may have no direct effect on the bank but could indirectly cause increased bank instability by disrupting the nearshore and beach zanes. 19 [ [ ., c c 0 [ ~ [ n u c r c E c c f1 u c c c [ The relative importance of the factors and processes that con~~ibute to bank erosion has not been determined. Simons et al. (1979) developed a qualitative assessment using available data, a review of current theory, personal experience, and sound professional judgment. Field studies currently underway will provide measurements that can be used to evaluate the relative importance of ice and other processes contributing to reservoir bank erosion. LITERATURE CITED Adams, P. (1977) How spring ice breakup alters our shorelines. 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Proceedings of the Fourth Coastal Engineering Conference, Chapter 13, p. 201-205. 26 tfU ~ r.nVI=RII.IMI=NT PRIII.ITIII.Ir. nCC:If'C. 100., _ A .,.,nA 1'>4.,