Tom Rommens

Soil erosion and sediment deposition in the Belgian loess belt during the Holocene: establishing a sediment budget for a small agricultural catchment

A method to establish a Holocene sediment budget for a 103 ha agricultural catchment representative for the Belgian loess belt is presented. Soil erosion and sediment deposition were determined based on 185 coring locations and a large excavation in the valley bottom. Results were integrated in a GIS and interpolation techniques applied to derive spatial patterns of erosion and sedimentation. Total soil erosion, sediment deposition and sediment export were calculated and the results show that volumes are highly dependent on the interpolation technique used. Sediment delivery ratios between 20% and 42% are derived and are consistent with data reported in previous studies. This clearly shows that the majority of the sediments produced during the Holocene have been stored near their source area and have not been delivered to the downstream rivers. The spatial distribution of soil erosion and sediment deposition within the catchment is strongly dependent on slope gradient and position within the catchment, which suggests that, since human impact began, topography has been the main factor determining long-term soil erosion and sedimentation.

Reconstruction of late-Holocene slope and dry valley sediment dynamics in a Belgian loess environment

To unravel the evolution of a dry valley in the Belgian loess area soils and sediments along a slope catena were studied in detail. A 67 m long trench was opened from the upper slope to the centre of the valley bottom. The exposed soils and sediments showed evidence of severe soil erosion and other human disturbances that significantly changed the valley topography. The early-Holocene slope gradient was up to 25%, whereas now it is less than 10%. In the thalweg, a remnant of the early-Holocene soil was found underneath colluvial deposits, which were more than 3 m thick. A chronology for the valley evolution was established based on AMS 14C dating of charcoal fragments and optical dating of colluvial sediments. The first sediment deposition occurred in the early Iron Age, with an average sedimentation rate of approximately 3.4 ± 1.3 t/ha per yr. This increased to c. 5.4 ± 2.2 t/ha per yr during the Roman Period and further to 18.0 ± 2.2 t/ha per yr in the Middle Ages. Although sediment accumulation in the valley was substantial, soil-erosion processes were mainly low-magnitude and signs of gullying are absent in the thalweg until the last few centuries.