The importance of marshes providing soil stabilization to resist fast-flow erosion in case of a dike breach

Salt marshes provide valuable ecosystem services including coastal protection by reducing wave loading on dikes and seawalls. If the topsoil is erosion-resistant to fast flowing water, it may also reduce breach depth if a dike fails. In this experiment we quantified the topsoil erosion resistance from marshes and bare tidal flats with different soil types to understand the extent to which they can help reduce breach depth. Intact soil samples were collected from eleven locations in the Netherlands at different tidal elevations and then exposed for three hours to 2.3 m s-1 currents. To the samples that remained stable after flow exposure, an artificial crack was made to test their stability following soil disturbance. All samples from the tidal flats were completely eroded, regardless of sediment type. In contrast, all samples from well-established marsh plateaus were stable as long as no disturbances were made, including those with sandy subsoils. After creating artificial cracks, samples with a thin cohesive top-layer on top of sandy subsoil collapsed, while marshes with silty subsoils remained stable. Pioneer marshes on sandy substrate without a cohesive top-layer were the only vegetated soils that completely eroded. The lower erosion of marshes with either sandy or silty soils compared to bare tidal flats, was best explained by the presence of a top-layer with belowground biomass, high organic content, high water content and low bulk density. When analysing the erodibility of marshes only, fine root density was the best predictor of erosion resistance. This study demonstrates the importance of preserving, restoring or creating salt marshes, to obtain a topsoil that is erosion-resistant under fast flowing water, which helps reducing breach dimensions if a dike fails. The probability of topsoil erosion in established marshes with sandy subsoil is higher than in silty marshes. A silty layer of cohesive sediment on top of the sand provides extra erosion resistance as long as it does not break. Pioneer marshes that have not developed a cohesive top-layer are erosion sensitive, especially in sandy soils. For future marsh creations, using fine grained sediments or a mixture of sand with silt or clay is recommended.