TRIGGERING OF DEBRIS FLOWS BY RAINFALL AT THE ILLGRABEN CATCHMENT, SWITZERLAND: A SUMMARY OF OBSERVATIONS FROM 2001 TO 2013

Since 2001 we have been observing the triggering of debris flows by rainfall at the Illgraben Catchment, southwest Switzerland, using a combination of three rainfall gauges and a debris-flow detection system, as well as other instrumentation such force plates which can be used to estimate the water content of debris flows. The Illgraben experiences several debris flows and many debris floods every year, between May and October. Using the first six years of data (before a change in how the rainfall data were collected) we find that debris flow triggering is most closely correlated with 10-minute rainfall intensity. We established a rainfall intensity-duration threshold for debris flow activity, which we compare with the subsequent 7 years of rainfall data. Half of the rainstorms which exceed the threshold result in a debris flow that reaches the distal end of the fan. We found no correlation between debris flow triggering and antecedent rainfall, however the correlation between rainfall intensity and duration is weaker in the spring snow-melt period (May–June), suggesting that antecedent moisture may be important. Runoff coefficients are somewhat larger during spring snowmelt, however the water content of the debris flows is more difficult to predict. Although other studies have shown that the Illgraben tends to accumulate sediment in the winter and spring seasons, there is no significant change in the position of the rainfall intensity–duration threshold in any given year, suggesting that debris flow activity is more limited by a lack of intense rainfall than by a lack of sediment.