allen 2009 exploring steep bedrock permafrost and its relationship with recent slope failures in the...

Exploring Steep Bedrock Permafrost and its Relationship with Recent Slope Failures in the Southern Alps of New Zealand S. K. Al len , 1 * S. Gruber 2 and I. F. Owens 1 1 Department of Geography, University of Canterbury, Christchurch, New Zealand 2 Glaciology, Geomorphodynamics & Geochronology, Department of Geography, University of Zurich, Switzerland ABSTRACT The central region of New Zealand’s Southern Alps is characterised by steep, glaciated slopes prone to rock mass failure, but permafrost conditions and any relevance to past or future slope instabilities have rec ei ved litt le pre vio us attent ion. A ne twork of 15 dat alo gge rs was us ed to rec ord ne ar- sur fac e temperatures on steep rock walls located about the Main Divide of the Alps and further leeward where the cli ma te is muc h drie r. Mea n annua l roc k temper ature (MART) ran ged from 1.9 to 5.4 C, corresponding to local 0 C ele vat ions (E0) of 2465 –3514 m, with no signi fican t diffe renc e obse rved between the humid and drier mountain ranges. On extremely shaded slopes, the permafrost limit may exte nd down towards 2000 m, but further measure ments are needed to confi rm this. E0 leve ls were model led as a funct ion of pote ntial solar radia tion, allowi ng stee p perma frost distribu tion to be map ped across the re gio n. Fro m an inven tor y of 19 be drock fai lur es occ urring about the Mai n Divide since the mid-20 th century , 13 have initiated from source areas where MART in the range of þ / 1.8 C is considered to indicate marginal permafrost conditions. None of these events was triggered by se ismic ac tiv ity , and mos tly exh ibi t scarp areas tha t inc lud e or ori gin ate in clo se- to- ridge topog raph y , wher e the most rapid permafros t degr adat ion migh t be expe cted . Copy right # 2009 John Wiley & Sons, Ltd. KEY WORDS: rock temperat ures; mountain perma frost ; bedrock stabi lity; Southe rn Alps; New Zealan d INTRODUCTION In high mountain environments, geotechnical hazards are increasi ngly be ing co ns idered in rela tion to possible interactions wi th changing gl acial and pe rma fros t con diti ons (e. g. Har ris , 200 5; Hug gel , 2008). Thes e st udies ha ve st emmed from init ia l theoretical discussions linking atmospheric warming, pe rma fros t deg rad atio n and the sta bil ity of ste ep bedr ock (Haeb erli et al., 1997; Har ris and V ond er Mu ¨ hll, 2001). Laboratory studies have since demon- strated that the shear strength of an ice-bonded rock dis con tinuit y sig nifi can tly red uces with war ming, possibly having a factor of safety lower than that of an ice -fre e joi nt whe n war med abo ve 1.5 C (Davies et al ., 2001). Oth er fac tor s inc lud ing ice segregation and volume ex pansion ar e li kely to predispose a rock discontinuity to subsequent failure upo n war min g (Gr uber and Hae ber li, 200 7), and mel twa ter or gro und wat er flow int o a pre vio usl y frozen fracture may cause elevated water pressure and reduced frictional strength (Harris, 2005). Reduced or absent snow and debris cover on steep slopes means tha t sub sur fac e temper atu res and related bedrock stability will respond rapidly to atmospheric warming, as was wi tnesse d du ri ng the ex tr emel y warm European summer of 2003, when numerous rockfalls were associated with extreme heat fluxes in the active PERMAFROST AND PERIGLACIAL PROCESSES Permafrost and Periglac. Process. 20: 345–356 (2009) Published online 13 August 2009 in Wiley InterScience (www.interscience.wile y.c om) DOI: 10.1002/ppp .658 * Correspondence to: S. K. Allen, Department of Geography, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand. E-mail: [email protected] Copyright # 2009 John Wiley & Sons, Ltd. Received 11 March 2009 Revised 11 May 2009 Accepted 19 May 2009

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