Impact of beach scraping on near shore sediment transport and bar migration

This paper presents results from two sets of laboratory experiments at different scales that demonstrate a strong influence of beach scraping on sediment transport within the surf zone. For erosive conditions, the reshaping of the upper beach was found to prevent further migration of the offshore bar and reduce the offshore transport across the whole surf zone. For beach recovery tests, where the breaker bar was initially stable in the control tests, reshaping the upper beach induced rapid onshore migration of the bar. Thus, the reshaping tended to either stabilize the beach or enhance beach recovery over the surf zone. A detailed investigation of the hydrodynamics within the surf and swash zones showed that this effect was induced by the more dissipative swash zone reducing sediment concentrations within the wave backwash and within the surf zone. The experiments demonstrate the importance of the swash zone in influencing overall beach morphodynamics. Improving predictions of both beach erosion, accretion or bar behaviour therefore require inclusion of the swash zone in numerical models.