A predictive model for channel scour on alluvial fans using morphometric analysis

CA Lau

Dans les comptes rendus d’articles de la conférence: GeoVancouver 2016: 69th Canadian Geotechnical Conference

Session: GEOHAZARDS - V Climate ChangeFloods & Landslides

ABSTRACT: Proposedinfrastructure watercourse crossingsrequireanassessment of the expected depth of vertical channel scour fordesign specification ofburial depth and/or additionalmitigationmeasures.At present neitherbest-practice guidelines orscientificliteratureprovides guidancefor estimating scour depth onalluvial fan creek channels.In this contribution wepresentpreliminary results ofa statistical modelusing morphometric parametersfor predicting channel scour for steepcreek fans.Channel scour along the creek profile is measured from LIDAR digital elevation models as the differencebetween the average fan surface and the channel bed.The average and maximum channel scour from these channelprofilesareused as the predictor variables in abootstrappedlinear multivariate regression analysis of the fan dataset.Theresults of this study suggest average fan gradient, watershed area, and total watershed channel length can be used topredict average channel scour measurements on fans. However, further work using alternate statistical methods will becarried out to reduce under-fitting within the predictive models.R•SUM•Les endroits o† des infrastructures propos‡es traversentdescours d…eau n‡cessitent une ‡valuation de la profondeurpr‡vue d…affouillementdecanal pour la sp‡cification delaprofondeur d…enfouissement et/oula conceptiond…autresmesures d…att‡nuation.— l…heure actuelle, il n…existe aucune directivedemeilleurespratiques ou dans lalitt‡raturescientifiquequifournissedes indications pour estimer l…affouillementdes canauxsur lesc–nesalluviaux.Dans cettecontribution, nous pr‡sentons les r‡sultats pr‡liminaires d…un modƒle statistique ⁄ l…aide de paramƒtres morphom‡triquespour pr‡dire l…affouillement de canal pour lesc–nesderuisseau escarp‡.Affouillement de canal le long du profil ruisseauest mesur‡e ⁄ partir des modƒles altim‡triques num‡riques LIDAR comme ‡tant la diff‡rence entre la surfacemoyenneduc–neet le lit du canal.Les valeursmoyenneset maximalesd…affouillement de canalsont utilis‡s comme variablespr‡dictives dans une analyse de r‡gression multivari‡e lin‡airede la banque de donn‡e de c–nes alluviaux.Les r‡sultatsde cette ‡tude suggƒrentque la pente moyenne du c–ne,lasuperficie du bassin versant,etla longueur totale des canaux⁄l…int‡rieurd…unbassin versantpeuvent ‹treutilis‡s pour pr‡dire les mesuresmoyennes d…affouillementd…uncanal surunc–ne alluvial. Cependant, des travaux suppl‡mentaires ⁄ l…aide de m‡thodesalternatives destatistiques seront effectu‡spourr‡duirelesous raccord ausein de modƒles pr‡dictifs.1INTRODUCTIONAlluvial fans form where a streamexitsaconfined drainagesystemand deposits sediment onto a lower gradientsurface. In British Columbia, alluvial fans commonly occurat the base ofupland areas, such as mountains orplateaus,where streams encounter valley bottoms. The fanforms a cross section of a geometric cone in plan view, withthe deposits radiating outward from the fan apex (Bull1977).Stream channels flowing over the surfacedepositsediments onto the fan.While fans are classified asdepositional landforms, existing fan sediments arereworked through bed scour and bank erosion, creatingchannel networks on the surface. Thiscontrastof fanreaches undergoing cycles of aggradation and degradationis often not fullyappreciated.Designs of buried infrastructure watercourse crossingsrequireanassessment of the expected depth of verticalchanneldegradation (scour)fordesign specification ofburial depth and/or additionalmitigationmeasures.Thereareminimum requirements for pipeline depth of cover(DOC)providedby theCanadian Standards Association(CSA)(CSA-Z662-15)(CSA 2015)and theCanadianEnergyPipelineAssociation (CEPA)provides best practiceguidelinesforpipeline watercourse crossing assessments(CEPA2015). However, neither documentdistinguishespotential increasedscour hazards forsteep (>3›) creekchannels,common on alluvial fans.Typical estimates offluvialscourin creeks and riversrely onthe measurementof theanticipateddesigndischarge of the creek(e.g. 1:200yearflood).However, ifdebris floodsor debris flowsaffectthe fan, thepeakdischarge can beup to50timeslargerthan −clearwater‰ floods(Hungr 2005).Buried infrastructureexposed by scouron fanscould be damaged or ruptured.Little work has been completed to determine ifmorphometrics could be successfully applied to predictscour depth on alluvial fans. Morphometrics aremeasurements that quantify landscape features, such as

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