Sediment residence time in alluvial storage of black marl badlands

- Field survey of alluvial storage and monitoring of bedload sediment yield
- A scaling law to predict the volume of alluvial deposits in the stream network
- Assessment of sediment residence time in alluvial storage
- Rate of increase of alluvial storage with drainage area similar to the sediment yield

The aim of this study is to estimate the sediment residence time in the stream network of two small headwater catchments (Laval and Moulin) characterized by a badlands landscape entrenched into Jurassic black marls of the Southern French Prealps. The method is based on an intensive field survey of the alluvial storage along the main stream reaches from which a scaling law between the average thickness of alluvial deposits and their width was established in order to predict the volume of alluvial deposits in the entire stream network. To complete this approach, bedload sediment yield monitored over the last 30 years with topographic surveying of sediment retention basins are used. The assessment of sediment residence time is performed according to a steady-state assumption, validated by the long-term dynamic equilibrium of bedload sediment yields. The results highlighted very close values of residence time between the catchments, around 3 years, despite a one order of magnitude difference in drainage area. It is shown that the rate of increase of alluvial storage with drainage area is the same as for sediment yield. This is likely attributed to the high degree of confinement of the stream network, which prevent the formation of a floodplain or large internal alluvial fans. Implications of these results for the prediction of the effects of bioengineering works in controlling erosion are discussed.