Characteristics of sediment deposits formed by intense rainfall events in small catchments in the Belgian Loam Belt

Erosion and deposition patterns within two agricultural catchments in the Belgian Loam Belt were mapped and their volumes measured after an intense rainfall event. From these data, the total erosion and deposition budget was calculated. The surveys clearly indicated that deposits could be differentiated according to the type of process that caused deposition. For most deposits topography was the controlling factor. However, important deposits were also found at field borders where a vegetation barrier caused deposition. Vegetation-controlled deposition occurs at significantly higher slope gradients than slope-controlled sediment deposition. This implies that vegetation-controlled deposition has an important effect on the spatial distribution of deposited sediment and on the sediment delivery ratio at the catchment outlet. The undispersed aggregate-size distribution of sediment deposits in front of vegetation barriers is finer than the sediment deposited under topographically-controlled conditions. However, the dispersed particle-size distributions of both types of sediment are very similar and only slightly coarser than the dispersed particle-size distribution of the source material. During these extreme rainfall events, sediment is eroded, transported and deposited in aggregated form. The aggregates themselves have a particle size distribution, closely resembling the source material. Consequently, considerable quantities of fine material and associated pollutants, which are expected to be exported to the river system, are trapped within the catchment.