Sediment loads due to fluvial transport and debris flows during the 2005 flood events in Switzerland

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Severe floods in August 2005 in Switzerland were associated with enormous morphologic changes along the stream channels and with large sediment transport, including many debris flows in the headwater catchments and fluvial bedload transport along the mountain rivers. We compiled data on transported sediment volumes and estimated surface runoff volumes in the streams determined from radar-based rainfall data and stream gauge measurements lower in the catchments. A quantitative analysis of the data shows that the transported bedload volumes are related to the flood runoff volume and the relevant channel gradient. For channel gradients flatter than about 5%, our data agree with a simplified version of the Meyer-Peter and Müller equation but two other equations tested tend to under predict bedload volumes. For channel gradients steeper than about 5%, the observed sediment volumes transported by fluvial processes are over predicted by all bedload transport equations tested, though two of them were specifically developed for this slope range, which likely results from non-accounted energy losses and supply limitation. For events with debris flows as the dominant sediment transfer mechanism, sediment volumes show a large variability for given flood runoff volume and channel gradient. The sediment volumes of the torrent catchments show a similar functional dependence on catchment parameters as for two empirical power law equations for the estimation of a ‘design’ event magnitude of debris flows based on data from Northern Italy but our events are associated with a much smaller coefficient. Finally, the majority of our debris-flow data is in line with the trend defined by our fluvial transport events, indicating a continuous transition to debris-flood and debris-flow conditions.

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