BAFU-Project: Sediment Transport in Alpine Catchments

Deposition of bed load in the Brenno streambed, Kt. Ticino (Picture: M. Böckli, WSL)

Background and objectives

After the severe flood and debris-flow events in August 1987 the task group for operational hydrology (GHO) initiated the compilation of a Swiss national data base for bedload in alpine rivers and torrents. The data is derived mainly from surveys of deposits in torrential retention basins and of excavations in alpine rivers. Using these data, a field based method to estimate the potential bedload in torrents (catchment size < 20 km²) during major events was developed at the geographical Institute of the University of Berne. However, this method is not applicable in larger mountain rivers with a catchment size > 20 km².

Here we present a procedure to estimate the potential bedload in alpine rivers during events with a recurrence interval of 100 years The procedure is developed and tested on the basis of data from three Swiss catchments (the Brenno in the canton Ticino, the Lonza in the canton of Valais and the Grosse Schliere in the canton Obwalden). The project is financially supported by the Federal Office for the Environment FOEN.

Approach and methods

A mountain river catchment is divided in various sub-catchments. The bedload potential of the lateral torrents is estimated with field based methods or with simple empirical equations. For different scenarios, we will define how much sediment per sub–catchment is delivered to the alpine river.

To calculate bedload transport in the mountain river systems we then carry out numerical simulations. For this purpose we use the one-dimensional bed load transport model sedFlow. The model was developed in the research unit mountain hydrology and mass movements at WSL and is optimized for long simulation perimeters in steep mountain streams. The simulation procedure can be divided into two steps: (i) First, past bedload events are recalculated. The calculations can be verified either with data of cross section alterations or with local bedload measurement data. The aim of this recalculation is a general validation of the transport model and an analysis of the sensitivity of the system to different parameters such as channel geometry, discharge and grain size distribution. Particularly the impact of the sediment supply by lateral torrents has to be investigated. (ii) In a second step, potential bedload events with a recurrence interval of 100 years are determined in a scenario based analysis. For different combinations of the factors sediment supply by lateral torrents, discharge and grain size distribution, bedload transport is then calculated. Therefore, realistic scenarios have to be identified.

Participants

• Martin Böckli
• Dieter Rickenmann
• Alexandre Badoux
• Jens Turowski

Contact

• Dieter Rickenmann