Field and laboratory experiments on the formative process of cusps on a Shingle Beach

Abstract

The present study attempted to examine the formative process of beach cusps on a shingle beach through field and laboratory experiments. Temporal changes in foreshore morphology and swash pattern were continuously monitored using a video camera suspended over an artificially flattened section at a pocket beach of Banba-ura at the tip of the Manazuru Peninsula in Sagami Bay. In the laboratory, swash-zone morphological changes were investigated on a uniformly inclined model beach in a wave tank. Cusps developed well in both field and laboratory experiments. Cusp spacings were 2.2-2.5 m in the field and 29-35 cm in the laboratory. The Froude similarity law indicates that the laboratory cusp spacing corresponds to the spacing of 7.7 9.5 m in the field. Although this value is much larger than the cusp spacing observed in the field experiment, they were still on the same order of magnitude; and therefore, the results of field and laboratory experiments are considered comparable. Cusp formation was triggered by the presence of a small depression or a mound associated with an obstacle such as boulders in the field and the side wall of the tank in the laboratory. A series of cusps gradually and successively developed alongshore from the location of obstacles, simultaneously changing an incipient two-dimensional swash pattern into three-dimensional features. Along-shore migration of the interaction between topography and flow structure in the swash zone appears to promote the development of beach cusps.