IPCC_Managing Risks of Extreme Events.pdf - Climate Access

Chapter 3Changes in Climate Extremes and their Impacts on the Natural Physical Environment(Gruber and Haeberli, 2007; Huggel, 2009) and other mountain regions(Niu et al., 2005; Geertsema et al., 2006; Allen et al., 2011). This evidenceincludes several recent rock falls, rock slides, and rock avalanches inareas where permafrost thaw in steep bedrock is occurring. Landslideswith volumes ranging up to a few million cubic meters have occurred inthe Mont Blanc region (Barla et al., 2000), in Italy (Sosio et al., 2008;Huggel, 2009; Fischer et al., 2011), in Switzerland, and in BritishColumbia (Evans and Clague, 1998; Geertsema et al., 2006). Very largerock and ice avalanches with volumes of 30 to over 100 million m 3include the 2002 Kolka avalanche in the Caucasus (Haeberli et al., 2004;Kotlyakov et al., 2004; Huggel et al., 2005), the 2005 Mt. Steller rockavalanche in the Alaska Range (Huggel et al., 2008), the 2007 Mt. Steeleice and rock avalanche in the St. Elias Mountains, Yukon (Lipovsky et al.,2008), and the 2010 Mt. Meager rock avalanche and debris flow in theCoast Mountains of British Columbia.Quantification of possible trends in the frequency of landslides and iceavalanches in mountains is difficult due to incomplete documentationof past events, especially those that happened before regular satelliteobservations became available. Nevertheless, there has been an apparentincrease in large rock slides during the past two decades, and especiallyduring the first years of the 21st century in the European Alps (Ravaneland Deline, 2011), in the Southern Alps of New Zealand (Allen et al., 2011),and in northern British Columbia (Geertsema et al., 2006) in combinationwith temperature increases, glacier shrinkage, and permafrost degradation.Research, however, has not yet provided any clear indication of achange in the frequency of debris flows due to recent deglaciation.Debris flow activity at a local site in the Swiss Alps was higher duringthe 19th century than today (Stoffel et al., 2005). In the French Alps nosignificant change in debris flow frequency has been observed since the1950s in terrain above elevations of 2,200 m (Jomelli et al., 2004).Processes not, or not directly, driven by climate, such as sediment yield,can also be important for changes in the magnitude or frequency ofalpine debris flows (Lugon and Stoffel, 2010).Debris flows from both glaciated and unglaciated volcanoes, termedlahars, can be particularly large and hazardous. Lahars produced byvolcanic eruptions on the glacier-clad Nevado del Huila volcano inColombia in 2007 and 2008 were the largest rapid mass flows on Earthin recent years. Similarly, large mass flows occur on ice-covered activevolcanoes in Iceland (Björnsson, 2003), including Eyjafjallajökull in 2010.Large rock and ice avalanches, with volumes up to 30 million m 3 , havehappened frequently (on average about one every four years) on theglaciated Alaskan volcano, Iliamna, and are thought to be related toelevated volcanic heat flow and possibly meteorological conditions(Huggel et al., 2007). Glacier decay on active volcanoes can lead to areduction of lahar hazards due to less potential meltwater available forlahar generation, but it is difficult to make a general conclusion as localconditions also play important roles. In 1998, intense rainfall mobilizedpyroclastic material on the flanks of Vesuvius and Campi Flegreivolcanoes, feeding approximately 150 debris flows that damaged nearbycommunities and resulted in 160 fatalities (Bondi and Salvatori, 2003).In the same year, intense precipitation associated with Hurricane Mitchtriggered a small flank collapse at Casita volcano in Nicaragua. This slopefailure transformed into debris flows that destroyed two towns andclaimed 2,500 lives (Scott et al., 2005). Following the 1991 Pinatuboeruption in the Philippines, heavy rains associated with tropical stormsmoved large volumes of volcanic sediment. The sediment dammed rivers,causing massive flooding across the region that continued for severalyears after the eruption ended (Newhall and Punongbayan, 1996).A variety of climate and weather events can have geomorphologicaland geological impacts. Warming and degradation of mountainpermafrost affect slope stability through a reduction in the shearstrength of ice-filled rock discontinuities. For example, the 2003European summer heat wave (Section 3.3.1) caused rapid thaw andthickening of the active layer, triggering a large number of mainly smallrock falls (Gruber et al., 2004; Gruber and Haeberli, 2007). Permafrost thawin sediment such as in talus slopes may increase both the frequency andmagnitude of debris flows (Zimmermann et al., 1997; Rist and Phillips,2005). The frost table at the base of the active layer is a barrier togroundwater infiltration and can cause the overlying non-frozen sedimentto become saturated. Snow cover can also affect debris flow activity bysupplying additional water to the soil, increasing pore water pressureand initiating slope failure (Kim et al., 2004). Many of the largest debrisflows in the Alps in the past 20 years were triggered by intense rainfallin summer or fall when the snowline was elevated (Rickenmann andZimmermann, 1993; Chiarle et al., 2007). Warming may increase theflow speed of frozen bodies of sediment (Kääb et al., 2007; Delaloye etal., 2008; Roer et al., 2008). Rock slopes can fail after they have beensteepened by glacial erosion or unloaded (debuttressed) following glacierretreat (Augustinus, 1995). Although it may take centuries or evenlonger for a slope to fail following glacier retreat, recent landslidesdemonstrate that some slopes can respond to glacier down-wastingwithin a few decades or less (Oppikofer et al., 2008). Twentieth-centurywarming may have penetrated some decameters into thawing steep rockslopes in high mountains (Haeberli et al., 1997). Case studies indicate thatboth small and large slope failures can be triggered by exceptionallywarm periods of weeks to months prior to the events (Gruber et al.,2004; Huggel, 2009; Fischer et al., 2011).The spatial and temporal patterns of precipitation, the intensity andduration of rainfall, and antecedent rainfall are important factors intriggering shallow landslides (Iverson, 2000; Wieczorek et al., 2005;Sidle and Ochiai, 2006). In some regions antecedent rainfall is probablya more important factor than rainfall intensity (Kim et al., 1991; Glade,1998), whereas in other regions rainfall duration and intensity are thecritical factors (Jakob and Weatherly, 2003). Landslides in temperateand tropical mountains that have no seasonal snow cover are nottemperature-sensitive and may be more strongly influenced by humanactivities such as poor land use practices, deforestation, and overgrazing(Sidle and Ochiai, 2006).Rock and ice avalanches on glaciated volcanoes can be triggered byheat generated by volcanic activity. Their incidence may increase with187