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Postgraduateresearchgrantreport:Rockslopeinstabilityinalpinegeomorphicsystems,Switzerland–Installationofequipmentforalong-termmonitoringofrocktemperature

KarolineMessenzehl,GeomorphologicalandEnvironmentalResearchGroup(Prof.Dikau),UniversityofBonn,Germany

k.messenzehl@uni-bonn.deIntroductionRockfallsofdifferentmagnitudesandfrequenciesaretypicalphenomenainmountainenvironments(Whalley1984,Sass2005).Duringrecentdecades,thenumberofperiglacialrockslopeinstabilitiesisstated to have increased substantially, likely related to rapid changes of the alpine cryosphere(Gruberetal.2004,Huggel2009,BallantyneandStone2013). Inparticular, theoutmeltingof ice-filledfracturescausedbypermafrostdecayandglacierretreatimpactsthethermal,hydrologicalandmechanicalpropertiesofbedrockandhighlycontributestothedestabilisationofrockwalls(Murtonetal.2006,GruberandHaeberli2007,Krautblatteretal.2013).Whenusingperiglacialtalusslopesas indicator forrockwall instabilities, the inhomogeneousspatialpatternof those landformswithinmountainvalleysaswellastheirvaryingsedimentthicknessessuggestacomplicatedsetofdifferentkey controls governing the spatio-temporal dynamic of rockwall failures (Hales and Roering 2005,Mooreetal.2009,McColl2012).However,identifyingthemajordestabilisingfactorsprovestobeadifficult taskowing to thedynamicandnon-linear interplaybetween themechanicalpropertiesofthe rockmass and its discontinuities, the environmental settings and, in particular, the synergyofdifferentweathering processes including freeze-thaw cycles, thermal stress and shock (Viles 2013,Krautblatter and Moore 2014, Hall and Thornes 2014). Therefore, apart from a natural hazardperspective, but also due to the overwhelming geomorphic importance of rockfall processes forsedimentbudgetsandreliefevolution,furtherknowledgeisneededconcerningthekeycontrolsofrockslopeinstabilitiesinmountainsettings.

AimsandstudyareaThe field campaign in summer 2014,supported by the BSG PostgraduateResearch Grant, was the secondfieldwork of my PhD project “Rockslope instability inalpinegeomorphicsystems, Switzerland”. The project’soverall aim is to decipher the keycontrols on the spatio-temporalvariability of rockwall instability andassociated talus evolution inmountain systemswith focus on thegeomorphic consequences forsediment fluxundbudget.Thestudyarea is the 100km2 large catchmentofthehigh-alpineTurtmannvalley in

the Swiss Alps, being a southern tributary of the Rhone valley (Fig. 1). The valley represents anexcellent study site, as it has 14 hanging valleys (2300-3000m a.s.l.), strongly E-W-oriented, withmore than 200 talus slopes all indicating rockfall events of different magnitude and frequency(Messenzehletal.2014).Therefore,theobjectivesofthefieldcampaignin2014werea)identifyingcriticalrockmechanicalparametersdrivingbedrockinstabilitiesb)quantifyingthepostglacialactivityof rockfallprocessesandc) installingequipment fora long-termrock temperaturemonitoring.Forthelattertask,theinstrumentswerefinancedbymeansoftheBSGPostgraduateResearchGrant.Fieldworkinsummer2014GeotechnicalmappingoftherockmassanditsdiscontinuitiesTo evaluate the role of rock mechanical characteristics for slope destabilisation, detailedgeotechnical investigations of the rock mass and its discontinuities were performed at selectedrockwallswithtalusslopes.Inacomprehensivesurveyinoneofthehangingvalleys(Hungerlitälli),a

Fig. 1: Study area. Fieldwork was performed in three hanging valleys of the Turtmann valley (1:Pipjitälli, 2:Brändjitälli,3:Hungerlitälli).

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total of 15 rockwalls were studied (Fig. 2). Major rockmechanicalpropertieshavebeenanalysedincludingtherockmass strength, aperture and spacing of discontinuities bymeansof scanlinesurveysbasedon ISRM(1997)standards.Additionally, the rockmassstrength (RMS)ofeachrockwallwasquantifiedusingtheapproachbySelby(1987).Basedonthe first evaluation of the data, the lowest RMS values,indicating unstable rockwalls, are mainly related to N-exposed rockwalls, where the dominance of freeze-thawweatheringasdestabilisingmechanismisverylikely.

GeophysicalsurveysoftalusslopesandrockwallsToquantify the long-termactivityof rockfalls since lastglacialretreat and to evaluate the efficiency of destabilisingparameters, the sediment thickness of talus slopes wasmeasured based on geophysical surveys. For this, electricalresistivitytomography(ERT)andseismicrefractiontomography(SRT)wereperformedalongtwonorth-exposed,varyingactivetalus slopes (200m long transects) in the Hungerlitälli.Additionally, the two corresponding rockwalls weregeophysicallyinvestigatedalong50mlongcrosstransects(Fig.3). The geophysical data of the bedrock will provide valuableinformationonthebedrockstructureandthespatialpatternofmoistureand/orpermafrost.Installationofequipmentforlong-termtemperaturemonitoring:Since bedrock temperature and, particularly, their temporal variations (e.g.freeze-thawcycles)areexpected tobemajorparameters controlling the rockbreakdown inalpineenvironments,equipment fora long-termmonitoring (atleast for 2 years) was installed in three neighbouring hanging valleys of theTurtmann valley (Fig. 1, 1-3). Thanks to the Postgraduate Research Grantprovided by the BSG, a total of 17 miniature temperature loggers (iButton®DS1922L, Fig. 4) were installed inside the near surface of selected rockwallswith talus slopes, spanning a diversity of lithological settings, rockmass properties (i.e. rockmassstrength, Fig. 5a), slope gradients, aspects, altitudes, proximity to the hanging glacier (paraglacialsignal)andbioticfactors.Allenvironmental,geotechnicalandgeomorphometricalcharacteristicsoftherockwallswererecorded.Usinga28mmhammerdrill(Hilti),7-10cmdeepboreholesweredrilledintothe intactbedrock,ca.1.5mabovetheapexof thetalusslopes (Fig.5b).Aftercleaningof theboreholes,theiButtonswereinserted(Fig.5c).UsingtheAndroidAppiAssist(Kelleretal.,2010),alliButtonswereprogrammedwithasamplingrateofthreehoursandaspecificstartingtime(duringAugust).Accordingtothemanufacturer,theiButtonsrecordtemperaturesfrom−40°Cto85°C,with

Fig. 3: Electric resistivity tomographymeasurementsonbedrock.

Fig. 5: Installation of the long-term rock temperature monitoring at 17 selected rockwalls in the Turtmann valley. A) Testing of the rock strength using the Schmidt Hammer, b) drilling a 10cm deep borehole with a diameter of 28mm, c) iButtton inside the borehole, waterproofed by a plastic capsule, d) closing and sealing the borehole with a rubber plug and silicone.

Fig.2:Geotechnicalsurveyofrockmass.

Fig.4:iButton®DS1922L.

(a) (b) (c) (d)

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a resolutionof 0.0625°C. To avoid adestructionof the iButtonsdue towater contact, the loggerswerewaterproofedbyspeciallymanufacturedplasticcapsules.Finally,theopeningoftheboreholeswere closedby a rubberplug and sealedwith silicone (Fig. 5d).Due to thehigh strengthof somebedrock(amphibolite),inconjunctionwiththebadwetterconditionthissummer,thedrillingsoftheboreholesturnedouttobeverytime-consuming.Therefore,amaximumonly17ofthe30iButtonsfinancedbytheBSGgrantwere installedthissummer,but theremainingwillbeusedfor thenextfieldcampaign,e.g.assubstitutematerialinthecaseofdataloggerfailures.In summer2015, thedata loggerswill be readout. Then, theywillprovideahigh-resolution,one-year time-series of the near surface rock temperature of 17 different rockwalls in the Turtmannvalley. In conjunctionwith the geotechnical data of the rockwalls aswell as theirmechanical andhydrological properties assessed by means of the geophysical surveys, the next step will be toevaluatethethermalprocessesinsidethebedrock(near)surfaceandtodrawconclusionontheroleand efficiency of the destabilising processes. Once all data of the fieldwork are analysed, theoutcomeswillbesubmittedforpublication(ESP&L)andpresentedinconferences(EGU2015).AcknowledgmentsIwouldliketoexpressmysincerethankstotheBritishSocietyforGeomorphologyfortheirfinancialsupport, by which the cost-intensive equipment for the long-term rock temperature monitoringcouldbebought.Thankstotheverygenerousgrant, Igottheuniquechanceto installavery largenumberofiButtons,whichisapartfromafewexceptionsunusualforbedrockstudiesinhigh-alpinesystems.Moreover, thegrantmadepossible tostartwith the temperaturemonitoring in theearlystageofmyPhD,sothatessentialempiricaldatacanbecollectednextyear,whenalliButtonswillbereadoutforthefirsttime.TheBSGmadethereforeanessentialcontributiontothefurtheradvanceofmy PhD-Project, as it prevented to extendmy PhD for one additional year andmade the fieldcampaignthisyearacompletesuccess.Finally,IalsothanktheHumboldt-Ritter-PenckfoundationoftheBerlinGeographicalSocietyforthetravel grant, by which the travelling and accommodation expenses of the field campaign werepartiallyfinanced.ReferencesBallantyne,C.K.,Stone,J.O.(2013):Timingandperiodicityofparaglacialrock-slopefailuresintheScottishHighlands.In:Geomorphology

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