Pilot study concerning the behaviour of coastal supply on the Dutch Wadden Island of Terschelling

Abstract

In 1993 an artificial sand nourishment was carried out at the coast of Terschelling. The total amount of supply was deposited in a trough between two breaker bars, up to a level of about NAP -5m. The present report gives a prediction of the development of the coast in the next 10 years. For this, a mathematical model has been used, in which the coast is characterized by 3 lines, roughly representing the beach, the inshore and the area between NAP-6m and NAP- 9m. The model simulates the diffusion of the supply in longshore and cross-shore direction. Two extremes cases are considered: 1. cross shore diffusion without longshore diffusion 2. cross shore diffusion with longshore diffusion The diffusitivity is determined by the wave climate and by a sand transport formula (the Svasek variation of the CERC formula, (Bijker & Svasek, 1969)). As the effect of the curvature of the inshore on the curvature of the beach is not included in this case, the diffusitivity of sand along the beach will be rather overestimated. Starting from the assumptions, given in section 2.2, the computations give the result that the present erosion of the beach, estimated as 2.75 m/year, will be compensated for 13 years in case 1 and for 7 years in case 2. The values of the longshore diffusitivity constants have apparently a rather high influence on the results of the computations. The effect on the beach of the supply between NAP-9m and NAP-6m is found to be approximately one third of the result with the supply at the inshore (between NAP-6m and NAP-3m). The recommendation is therefore to carry out the supply as high as economically attractive.