THE DEVELOPMENT OF COAST PROFILES ON A RECEDING COAST PROTECTED BY GROYNES

Abstract

When a continuously receding sandy coast is protected by groins the immediate result is a considerable reduction of the rate of recession of the coastline chiefly due to a reduction in the beach drift. This fact is generally accepted and has been confirmed empirically under prototype conditions. However some disagreement still seems to exist regarding the long term effect of groynes under the conditions mentioned above. Clearly, the groynes do not directly affect the sand transport at some distance seaward of the groynes and therefore erosion seaward of the groynes will continne to occur after the construction of the groynes. Since the coastline and the depth contours must eventually attain the same rate of movement some coastal engineers have drawn the conclusion that the effect of groynes on a receding coast will be of a temporary character only. This reasoning is obviously rather superficial. A better understanding of the manner in which the coast profile develops after the construction of groynes may be obtained by considering the fundamental physical factors involved in the sand transport outside the breaker zone (the offshore bar).