• Fascines and shrub hedges are vegetative barriers used in north-west France.
• Their hydraulic and sedimentation efficiency has been assessed.
• Their main benefits are flow slowdown and suspended sediment trapping.
• Their most relevant situations at the catchment scale are given.
• Combining fascines, hedges and grass gives higher efficiency for a longer period.

Résumé

Despite a smooth landscape and gentle rain, the deep loamy soils of the open-field intensive arable crop-farming area known as the Pays de Caux in north-west France are subject to runoff and erosion every year. These muddy floods are induced by a low infiltration capacity of the soil due to crust sealing, and occur in late autumn and winter, as well as in spring. They turn into safety and environmental issues in many catchments. Thus, in these areas, buffer zones are of particular interest to mitigate runoff, erosion and related pollution on the catchment scale. During the last decade, two kinds of buffer zone have become popular: fascines (also known as brushwood fences) and shrub hedges, which are types of vegetative barriers. Being less space-consuming, they are more easily accepted by farmers than are grassed waterways, but their efficiency needs to be evaluated in that context. To provide such an evaluation, seven fascines and three shrub hedges of different shapes and ages have been tested in situ for hydraulic roughness, sedimentation capacity and infiltration evaluation, using a field runoff simulator. This latter allowed the record of in- and out-flow from a channel placed across the vegetative barrier, while determining topography, waterlines and wetted sections along the channel. The flow rates were generally between 0.5 and 12 l/s/m. Sedimentation measurements and observations could be lead as well, using several sediment size distributions. Well-made dense fascines show Manning coefficients n between 0.5 and 1.0 s/m^1/3, sometimes more. Shrub hedges have n values ranging from 0.4 to 0.8 s/m^1/3 when the stem density is greater than 50 per linear metre. Sedimentation measures show high efficiency for catching coarser aggregates. Mean infiltration is about 400 mm/h for hedges and 35 mm/h for dead fascines. These results allow a description of the optimal design of these vegetative barriers, the magnitude of the efficiency that can be expected and the time needed to reach that efficiency. Thus, on the catchment scale, these vegetative barriers are more efficient when placed immediately downstream of erosion sources, across channels of concentrated runoff, or immediately upstream of local assets at risk. Combination of several kinds of buffer zone can be used to increase efficiency.