SEDIMENT STORAGE IN ALPINE VALLEYS: VARIABILITY AND QUANTIFICATION

Spatial patterns of sediment storage types and associated volumes using a novel approach for quantifying valley fill deposits are presented for alpine catchments in the Bavarian Alps/Germany and in the Dolomites/Italy. The different sediment storage types were analysed with respect to geomorphic coupling and sediment flux activity. Spatial information of storage types was based on geomorphological mapping, airphoto-interpretation and GIS-analysis. The most dominant landforms in the valley of the Bavarian Alps in terms of surface area were found to be talus slopes (sheets and cones) followed by rockfall deposits and alluvial fans and plains. More than two thirds of the talus slopes are relict landforms, completely decoupled from the geomorphic system. Notable sediment transport is limited to avalanche tracks, debris flows and along alluvial plains. Temporal information was derived from concepts on paraglacial slope adjustments, surface activity and dating results obtained from several drilling cores. Sediment volumes were calculated by means of polynomial functions of cross-sections and GIS modelling. In addition, 84 seismic refraction profiles were carried out throughout the valleys for a more precise determination of sediment thicknesses, and to check the bedrock data generated from geomorphometric analysis. The seismic refraction data showed that large alluvial plains and sedimentation areas, which have been developed through damming effects from large rockfalls, are in general characterised by shallow sediment thicknesses (< 20 m). By contrast, the thickness of several talus slopes is more than twice as much. For some locations (e.g. narrow sections of valley) the polynomial generated cross-sections resulted in overestimations of up to one order of magnitude, whereas in sections with a moderate valley shape the modelled cross-sections are in good accordance with the obtained seismic data. In the Reintal basin (Bavarian Alps) the overall sediment volume is calculated to be 0.07 km3 resulting in a mean sediment thickness of 23 m. For the quantification of valley fill deposits a combined application of bedrock data derived from polynomials and geophysical prospecting is highly recommended.