Shoreline Processes and Sediment Responses Related to the Origin of Beach Cusps on Whidbey and Fidalgo Islands, Washington

Beach cusps are rhythmically spaced, crescentic beach features which consist of cusp shaped seaward pointing depositional ridges or mounds, often referred to as horns, separated by concave seaward troughs or bays. They have longshore spacings that vary from a few centimeters up to 25 meters. The purpose of the project was to conduct a field investigation of the shoreline processes and sediment responses associated with beach cusp formation on Whidbey and Fidalgo Islands, Washington. Despite the many theories published in the literature for the origin of beach cusps, many of which are contradictory, there were only two theories proposed that could explain the origin of the cusps being studied: the shore drift in the form of sand waves theory proposed by Schwartz (1972), and the edge wave theory proposed by Huntley and Bowen (1973). The goals of the investigation were to determine which theory explained the origin of the beach cusps and also to document the processes and responses associated with their formation. Five beaches were selected as field sites due to their different sediment mix and exposure to incoming waves. These beaches often had beach cusps on them. Associated with the cusps were ridges that were symmetric in profile, linear and normal to the water line in plan, and directly down beach from the cusp horns. They were referred to as symmetric beach normal ridges and one question was whether or not these ridges were actually sand waves formed by longshore currents perturbing the foreshore bed. The investigation indicated that edge waves, not shore drift in the form of sand waves, explained the origin of the beach cusps. The steep beaches (4° to 9°), relatively low wave energy (wave height of 13 cm ± 3.5 cm) and small angle of incidence (12° ± 3.5°) were not conducive to the formation of sand waves. The spacings of beach cusps computed by the edge wave theory compared very favorably with the actual measured beach cusp spacings. The interaction of edge waves and incoming waves developed a cell circulation of swash-backwash and a corresponding transportation of beach sediment in an arcuate trajectory. The beach cusp horns were probably directly up-beach of intensified swash from the edge wave antinode - incoming wave interactions. This intensified swash deposited and sculptured the gravel mound comprising the cusp horns. A reinforcing feedback to the edge wave induced cell circulation was observed to be developed by an initial topography or even longshore variations in sediment sorting. This feedback aided in maintaining the circulation cells and continual redistribution of beach material. This redistribution occurring through an ebbing tide formed the symmetric beach normal ridges associated with the beach cusps