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 B elgian Loam Belt were mapped and their volumes measured after an intense ra infall event. From these data, the total erosion and deposition budget was calculated. The surveys clearly indicated that deposits could be differenti ated according to the type of process that caused deposition. For most depo sits topography was the controlling factor. However, important deposits wer e also found at field borders where a vegetation barrier caused deposition. Vegetation-controlled deposition occurs at significantly higher slope grad ients than slope-controlled sediment deposition. This implies that vegetati on-controlled deposition has an important effect on the spatial distributio n of deposited sediment and on the sediment delivery ratio at the catchment outlet. The undispersed aggregate-size distribution of sediment deposits i n front of vegetation barriers is finer than the sediment deposited under t opographically-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 materia l. During these extreme rainfall events, sediment is eroded, transported an d deposited in aggregated form. The aggregates themselves have a particle s ize distribution, closely resembling the source material. Consequently, con siderable quantities of fine material and associated pollutants, which are expected to be exported to the river system, are trapped within the catchme nt. (C) 2000 Elsevier Science B.V. All rights reserved.