Sediment transport in Swiss torrents

Abstract

Sediment loads have been measured in six Swiss mountain torrents over several decades. Most of these torrent catchments are situated in the prealpine belt. They have catchment areas of between 0·5 and 1·7 km 2 . Bedslopes at the measuring sites vary between 5 and 17 per cent, and peak discharges up to 12 m 3 s −1 have been recorded. Geophone sensors installed in the Erlenbach stream allow bedload transport activity to be monitored and sediment volumes associated with each flood event to be determined. A detailed analysis of the measurements in this stream results in an empirical equation in which the sediment load per flood event is expressed as a function of the effective runoff volume (discharges above the threshold for bedload motion) and of the normalized peak discharge. For the total of 143 investigated flood events in the Erlenbach stream, the deviation of the predicted from the measured value is within a factor of two for more than two-thirds of all events. A distinction can be made between summer and winter events in analysing the bedload transport events. The summer events, mainly caused by thunderstorms, transport comparatively larger sediment loads than the winter events. For the other investigated streams, the periods of the deposited sediment volume surveys cover in general several flood events. An analysis is performed analogous to that for the Erlenbach stream. The sediment loads show a similar dependency on the two factors effective runoff volume and normalized peak discharge. However, the exponents of these factors in the power law expressions differ from stream to stream. A comparison of the investigated stream shows that some of the variation can be explained by considering the bedslope above the measuring site. The inclusion of a bedslope factor is in agreement with laboratory investigations on bedload transport.

Bedload transport measurements with impact plate geophones in two Austrian mountain streams (Fischbach and Ruetz): system calibration, grain size estimation, and environmental signal pick-up

“…The first indirect bedload transport measurements using impact plates were made in the Erlenbach from 1986 to 1999 using a piezoelectric crystal as sensor, with the aim of continuously monitoring the intensity of bedload transport and its relation to stream discharge (Bänziger and Burch, 1990;Rickenmann, 1994Rickenmann, , 1997Hegg et al, 2006;Rickenmann and McArdell, 2007). A geophone sensor was used at the Erlenbach and at all other field sites that were set up in the year 2000 and later (Rickenmann and Fritschi, 2010).…”

Rainfall intensity–duration thresholds for bedload transport initiation in small Alpine watersheds

“…9). Even though the scatter in the data is considerable, this probably reflects the fact that event bedload volumes in the Erlenbach depend both on effective runoff volume and on peak discharge (Rickenmann, 1997;). …”

Magnitude‐frequency analysis of bed load data in an Alpine boulder bed stream

“…[3] Furthermore, research aiming at validating these bed load formulae with field data has been rather scarce [Johnejack and Megahan, 1991;Blizard, 1994;Rickenmann, 1994Rickenmann, , 1997Rickenmann, , 2001Asti, 1999;D'Agostino and Lenzi, 1999] because of the obvious problems of measuring very coarse sediment transport occurring only at very high flow rates. Frequency of bed load movement can be idealized to be a function of the ratio hydraulic driving force over channel resisting forces, and poorly sorted mountain rivers may require extremely high, infrequently occurring discharges for the mobilization of the coarsest clast size, so that annually, only a portion of finer (gravel and cobble) particles is likely to be entrained [Knighton, 1998;Wohl, 2000].…”