| Debris flows consist of a mixture of water and sediments of various sizes.
Apart from few exceptions, the water is usually contributed directly from
precipitation. In a high mountain environment like the Alps, it appears
necessary to consider infiltration of water into the ground during rainfall
events, the runoff characteristics and the potential supply of sediment as a
function of a multitude of climatic and hydrogeological factors. This paper
outlines several new processes - either linked to ice formation in the
ground before an event, or to the presence of snow avalanche deposits -
that change the probability of observing an event.
These processes were identified during field observations connected with
extreme weather events that occurred recently in the Valais Alps
(south-western Switzerland): they can be seen as factors either amplifying
or reducing the potential of slope instability caused by the precipitation
event. An intense freezing of the ground during the week preceding the
exceptional rainfall event in mid-October 2000 amplified the probability of
triggering debris flows between roughly 1800 and 2300m asl. Both
growth of ice needles and superficial ground freezing destroyed soil
aggregates (increasing the availability of sediments) and/or, a deeper
ground freezing resulted in decreased infiltration rate (increased runoff)
during the first hours of heavy rainfall. The presence of snow avalanche
deposits in a gully could be simultaneously an amplifying factor (the snow
deposits increase the base flow and create a sliding plane for the
sediments, mainly at the time of summer storms) or a reducing factor
(reduction in the impact energy of the raindrops, mainly at the time of
winter storms) of the risk of triggering debris flows.
If it is not currently possible to establish rainfall threshold values for
debris flow triggering, the knowledge and the implementation of these
processes in the analysis of the potential triggering (for example by
comparing the catchment hypsometric curve with the meteo-climatic situation)
would nevertheless make the analysis of debris flows and forecasting more
efficient. |