Effectiveness of erosion mitigation measures to prevent muddy floods: A case study in the Belgian loam belt

Abstract : During the previous decade, 68 per cent of the municipalities in the Belgian loam belt have been confronted with muddy floods from agricultural catchments after intense rainfall. Runoff concentrates in dry valleys and causes damage to infrastructure and housing property downstream. A typical problem area is the village of Velm where a permanent river is constrained by a culvert designed to accommodate its peak discharge. However, the design of the culvert does not take the local flooding from seven dry valleys just upstream into account. This study focuses peak discharge from one of these agricultural catchments (c. 300 ha). The Meshed Hydrological Model (MHM) is used to evaluate the effectiveness of mitigation measures to reduce flooding under seasonal variation of soil cover in cropland and difference in land use patterns i.e. before and after land consolidation. The land cover spatial pattern was mapped at regular intervals during 2003. The largest potential of runoff generation occurs in December, and therefore represents a worst-case scenario. Mitigation measures implemented after the extreme event of August, 2002 (a 12 ha grassed waterway and a retention dam in the thalweg) alleviate the flooding risk in Velm. The model simulates a peak discharge and a runoff volume reduction of more than 40%. The retention pond would buffer all the generated runoff volume for the selected worst-case scenario. Land consolidation carried out in the 1970s has led to a 33 per cent rise of peak discharge and to a 19 per cent increase of runoff volume. The major role played by a new consolidation road built in the thalweg on runoff concentration is highlighted. Implementation of additional soil conservation measures is therefore needed to limit runoff generation within the catchment.