A widespread risk in high mountains is related to the accumulation of loose sediments on steep slopes, which represent potential sources of different types of geomorphic processes including debris flows. This paper combines data on 50 yr of permafrost creep at the Ritigraben rock glacier (Valais, Swiss Alps) with magnitude-frequency (M-F) relationships of debris flows recorded in the Ritigraben torrent originating at the rock-glacier front. Debris production and volumetric changes at the rock-glacier front are compared with debris-flow activity recorded on the cone and potential couplings and feedbacks between debris sources, channel processes and debris sinks. The dataset existing for the Ritigraben rock glacier and its debris-flow system is unique and allows prime insights into controls and dynamics of permafrost processes and related debris-flow activity in a constantly changing and warming high-altitude environment. Acceleration in rock-glacier movement rates is observed in the (1950s and) 1960s. followed by a decrease in flow rates by the 1970s, before movements increase again after the early 1990s. At a decadal scale, measured changes in rock-glacier movements at Ritigraben are in concert with changes in atmospheric temperatures in the Alps. Geodetic data indicates displacement rates in the frontal part of the rock glacier of up to 0.6-0.9 m yr(-1) since the beginning of systematic measurements in 1995. While the Ritigraben rock glacier has always formed a sediment reservoir for the associated debris-flow system, annual horizontal displacement rates of the rock-glacier body have remained quite small and are in the order of decimeters under current climatic conditions. Sediment delivery from the rock-glacier front alone could not therefore be sufficient to support the 16 debris flows reconstructed on the cone since 1958. On the contrary, debris accumulated at the foot of the rock glacier, landslide and rockfall activity as well as the partial collapse of oversteepened channel walls have to be seen as important sediment sources of debris flows at Ritigraben and would represent 65-90% of the material arriving on the Ritigraben cone. There does not seem to exist a direct coupling between displacement rates of and sediment delivery by the rock-glacier body and the frequency of small- and medium-magnitude debris flows. In contrast, a direct link between source and sink processes clearly exists in the case of active-layer failures. In this case, failure processes at the rock-glacier snout and debris-flow events in the channel occur simultaneously and are both triggered by the rainfall event. (C) 2010 Elsevier B.V. All rights reserved.

Debris-flow activity in a watershed is usually defined in terms of magnitude and frequency. While magnitude-frequency (M-F) relations have long formed the basis for risk assessment and engineering design in hydrology and fluvial hydraulics, only fragmentary and insufficiently specified data for debris flows exists. This paper reconstructs M-F relationships of 62 debris flows for an aggradational cone of a small ( 50 mm) in August and September, when the active layer of the rock glacier in the source area of debris flows is largest. Over the past similar to 150 years, climate has exerted control on material released from the source area and prevented triggering of class XL events before 1922. With the projected climatic change, permafrost degradation and the potential increase in storm intensity are likely to produce class XXL events in the future with volumes surpassing 5 x 10(4) m(3) at the level of the debris-flow cone. (C) 2009 Elsevier B.V. All rights reserved.

Alpine permafrost is particularly sensitive to climate change, since it's temperature is often close to the melting point of ice. In summer 1987, several hundred debris flows caused considerable damage and several victims in the Swiss Alps. Analysis showed that one out of three debris flows started at the lower boundary of mountain permafrost. A 58m deep borehole through creeping permafrost was drilled in 1987 near Piz Corvatsch (Upper Engadine, Swiss Alps). Temperatures have been measured regularly since then. Comparisons of two permafrost boreholes some 20km apart, where temperatures were measured once a year, indicated at least the regional character of the signal. Between 1987 and 1994, the uppermost 25m warmed rapidly. Surface temperature is estimated to have increased from -3.3 degreesC (1988) to -2.3 degreesC (1994), thereby probably exceeding previous peak temperatures during the 20th century. In the two-year period from 1994 to 1996, when winter snowfall was low, intensive cooling of the ground occurred, the temperatures reaching values similar to those in 1987. Since 1996, permafrost temperatures have once again been raising, followed by a cooling last winter. The variability of the observed permafrost temperatures is caused by several processes, including: (1) a reduced period of negative temperatures within the active layer due to long-lasting zero-curtains in autumn; (2) global radiation and air temperature changes influencing ground temperatures mainly in summer; and (3) variations in the duration of winter snow-cover. If the observed warming trend in alpine mountain permafrost temperatures continues into the foreseeable future, widespread permafrost degradation is likely, with potentially serious consequences with regard to mountain slope instability.