Outburst and rainfall-induced peak runoff events in highly glacierized Alpine basins

Abstract

The effect of rainfall on runoff from the highly glacierized basins containing Findelengletscher and Gornergletscher, Pennine Alps, Switzerland, is assessed for the periods May–September 1987 and 1993. Several major rainstorms produced high peak discharges in both seasons. Runoff response to prolonged heavy rainfall varied during the ablation season in both basins, but also differed between the two. Annual outbursts from the ice-dammed Gornersee also produced peak discharges from Gornergletscher. The transient snow-line partitions a glacierized basin into snow-covered and snow-free portions which, respectively, retain and return rapidly to runoff both meltwater and rain. Elevation of the 0 °C isotherm in the atmosphere interacts with basin hypsometry to determine the proportion of the catchment area that receives rain in a given event. Progressive rise of the transient snow-line in summer increases the size of the snow-free area, and hence the area of basin that rapidly responds to rainfall. Heavy rainfall appears to have produced an outburst from Findelengletscher in September 1993. The largest rainfall-induced floods are most likely to occur in summer between mid-July and late-August, terminating periods of elevated air temperature, when thermally induced runoff is at a maximum and subglacial drainage pathways are well developed, or when rainfall induces outbursts in spring and late summer at times of declined internal drainage capacity. Copyright © 1998 John Wiley & Sons, Ltd.