Effects of the internal structure of a rock–avalanche dam on the drainage mechanism of its impoundment, Flims sturzstrom and Ilanz paleo-lake, Swiss Alps

The Holocene sturzstrom of Flims, Grisons, Switzerland mobilized 12 km3 of Jurassic limestone making it one of the largest events of this kind on the earth. The material dammed a 10-km-long reach of the upper Rhine River Valley with thicknesses up to 400 m. A 40-km-long natural lake was formed upstream. The body of the avalanche deposits displays a well-preserved internal fabric. Five major sedimentary facies were identified related to the confinement of the deposits in a narrow valley that restricts the spreading of transported material. In the deepest parts of the avalanche deposit, a stratified facies attests that the movement was mainly non-turbulent, whereas a granular facies appears in the lateral and upper parts. The relative position of these facies within the natural dam has played an important role in the way the water flow eroded the dam crest and emptied the lake as suggested by the present valley slope morphology. Ledges corresponding to the levels of the different facies on north and south slopes of the valley correspond to steps in the narrowing of the incision. The outburst of the dam, if it occurred, only affected the upper part of the deposits that corresponds to the poorly cohesive granular facies. One hundred kilometres downstream, in Lake Konstanz (Bodensee) lacustrine sediments, the presence of two layers, contrasting with the normal grey varved deposits and separated by six laminites, are contemporaneous with the mass movement. These layers, which could correspond to the outburst of the dam, point to not one, but two major overflooding events.