An assessment of the impact of experimental brushwood fences on foredune sand accumulation based on digital elelvation models

A 400 m2 experimental plot, covering the transitional zone from the upper beach to the dune in Leffrinckoucke, on the southern North Sea coast of France, was monitored over 24 months via 9 high-resolution terrain surveys in order to determine the role of cost-effective and ecological brushwood fences (1.2 m high, 4 m long and spaced 3.5-6.7 m apart) in sand accumulation on a fragile dune front subject to severe past erosion. The dunes on this coast protect densely populated former tidal wetlands that form the Flemish coastal plain, much of which lies below mean sea level. The wind regime consists of a fairly balanced mix of moderate (80% of winds are below 8 m s−1) onshore, offshore and shore-parallel winds. The experimental plot showed moderate beach-dune mobility and accretion. The pattern of dune front accretion throughout the surveys showed a close relationship with wind directions. The fences induced rapid accretion on the dune front in just 10 weeks that accounted for 20% of the overall plot sand gain and 41% of the total gain over the 2-year study by this dune front unit. This rapid accretion also created hummocks of sand between which deflation corridors, oblique to the dune front, developed, in response to oblique onshore and offshore winds. However, the subsequent prevalence of variable wind approaches embracing longshore, offshore and onshore directions finally favouring the sealing of these corridors, thus illustrating the efficiency of this fence design in inducing foredune accretion. This efficiency has been favoured by mild sand supply from the nearshore zone combined with the absence of significant erosive storm events. Longer-term foredune management will need to take into account the future potential effects of storm damage due to climate change.