Wide Green Dike (WGD) concept for grass revetment under coastal conditions

Abstract

The Dike along the Dollard estuary in the northeast of the Netherlands has a grass cover. This revetment is not sufficiently strong for the local hydraulic conditions. Traditional dike strengthening leads to the replacement of grass with a hard revetment. An alternative under investigation is the “Wide Green Dike” (WGD) concept: a grass-covered dike with a gentle slope of around 1:7, naturally merging into the salt marsh located in front. The WGD concept is easily adaptable to future challenges, such as sea level rise. Furthermore, sediment of the estuary is currently being ripened to clay near the dike, potentially saving numerous truck movements needed for strengthening. The WGD is therefore the example of a circular, innovative and sustainable concept. However, the WGD needs to comply with the national flood safety standards to make it a feasible alternative, while the current assessment methods and instruments are not yet suited for this particular type of dike. During normative conditions, the grass cover is allowed to fail and the underlaying clay experiences erosion. A clay erosion model is used to design the thickness of the clay layer and proof that the WGD has sufficient residual strength to meet the norms of flood risk.