Résumé
Effective erosion control requires both a sound understanding of the physical context and sufficient motivation on the part of farmers to act, and in Europe this almost inevitably depends on financial subsidies. In southwest France, intense spring and summer storms lead to large scale sediment deposition on roadways, and cleanup costs are a major concern for local authorities. Current erosion control subsidy rates are not sufficient to encourage farmers to limit erosion and reduce cleanup costs. The objective of this study was to define the topographic characteristics which control sediment deposition on roadways in order to compare the cost of planting grass strips along these roads to current cleanup costs. Topographic conditions were characterised by midslope inclination (with a range of from 6 to 8%) and distance of this slope to the road (range of 50-200 m), and these were defined by a combination of field visits, air photos, and GIS techniques. The effect of grass buffer strips on sediment deposition was estimated using the revised universal soil loss equation (RUSLE). The financial benefits of planting grass strips were based on the estimated efficiency of the buffer strips and the length of roads affected according to the topographic conditions. More than 70% of roads requiring regular cleanup occur within 200 m of a midlsope inclination of 6%. Errors in predicting sediment deposition sites according to topography occur mainly when small scale changes in topography are beneath the resolution of the Digital Elevation Model (DEM) or when sediment deposition occurs due to the intersection of talwegs with roads. Modelling the effects of grass strips on soil erosion rates suggests that buffer strips of 12 or 24 m wide reduce sediments leaving the field by 32 and 54%, respectively. Planting grass buffer strips along predicted problem road sections can reduce cleanup costs associated with sediment deposition. Savings in the first year of planting are in the order of about 2% of current estimated cleanup costs (100,000) for 12 m wide strips, and this amount increases to almost 35% in subsequent years for 24 m strips. Savings in cleanup costs can be used to increase erosion control subsidies, thereby ensuring a higher level of farmer participation. In addition to reducing sediment deposition on roadways, the grass buffer strips serve as demonstration plots for the farming community. The strategy does not directly eliminate soil degradation upslope, but it is a first step towards better erosion control.