Prevention of outburst floods from periglacial lakes at Grubengletscher, Valais, Swiss Alps

Flood and debris-flow hazards at Grubengletscher near Saas Balen in the Saa s Valley, Valais, Swiss Alps, result from the formation and growth of sever al lakes at the glacier margin and within the surrounding permafrost. In or der to prevent damage related to such hazards, systematic investigations we re carried out and practical measures taken. The evolution of the polytherm al glacier, the creeping permafrost within the large adjacent rock glacier and the development of the various periglacial lakes were monitored and doc umented for the last 25 years by photogrammetric analysis of annually flown high-resolution aerial photographs. Seismic refraction, d.c. resistivity a nd gravimetry soundings were performed together with hydrological tracer ex periments to determine the structure and stability of a moraine dam at a pr oglacial lake. The results indicate a maximum moraine thickness of > 100 m; extremely high porosity and even ground caverns near the surface may have resulted from degradation of sub- and periglacial permafrost following 19th /20th-century retreat of the partially cold glacier tongue. The safety and retention capacity of the proglacial lake were enhanced by deepening and re inforcing the outlet structure on top of the moraine complex. The water lev el of an ice-dammed lake was lowered and a thermokarst lake artficially dra ined.