Doctor of Philosophy (Ph.D.)
Virginia Institute of Marine Science
Robert J. Byrne
John M. Zeigler
Paul V. Hyer
John S. Fisher
Beach cusps are crescentic shoreline accumulations of unconsolidated sediment that are characterized by a quasi-equal longshore spacing. The processes controlling the development of the cuspate form and the spacing of cusps were investigated. Field measurements showed that the cuspate form was derived through erosion of beach ridges, The ridges oriented alongshore in the swash zone are characterized by quasi-equally spaced channels cut normal to the shoreline whose widths are much less than the channel spacing. The upwash flows up the front face of the ridge and ponds shoreward of the ridge crest. The ponded fluid returns seaward through the channels. This circulation system tends to migrate the ridge shoreward on flood tide since sediment is transported from the front face of the ridge and deposited shoreward of the crest. After high water slack the swash zone retreats seaward to a position where the upwash can no longer effectively mount the ridge crest and supply the channel currents, The upwash now flows shoreward through the channels since the floor of the channels slope more gently than the front face of the ridge and the seaward channel currents have been depleted, The channelling of the upwash tends to erode the side walls of the channels and widens the mouths until adjacent mouths meet effecting a cuspate shape. The process controlling the spacing of cusps is then the same as the process controlling the spacing of the channels. During the field observations of cusp formation it was additionally observed that cusps can form when incident wave crests are parallel to the shoreline. Of the numerous hypotheses on the process controlling the spacing of cusps presented in the literature only three (with modifications) are consistent with these results. Each hypothesis involves processes which could create rhythmical longshore variations of energy in the surf zone. The sheetflood hypothesis contends that the swash segments unto equally spaced salients as it flows up the foreshore (Gorycki, 1973). Plateau's rule hypothesis suggests that the breaking wave approximates the form of a liquid cylinder and, therefore in light of Plateau's early work the wave should segment at equally spaced intervals (Cloud, 1966). The interaction of an edge wave with the incident wave will set up a rhythmical longshore variation in wave height (Bowen and Inman, 1969, and others). In each ·of these hypotheses the swash will tend to diverge at the areas of high swash and converge in areas of low swash. Conceivably, the channels could be eroded by return flows over a ridge in the longshore positions of swash convergence. Each hypothesis can satisfy the criteria that waves whose crests are parall~l to shore can form cusps. Each of the hypotheses were analyzed in respect to the characteristics of cusp spacing (trends in the magnitude of cusps both within and between shoreline environments) and the edge wave hypothesis (the only one amenable to quantitative analysis) was analyzed in respect to results of experiments conducted during cusp formation in the field. Each hypothesis could reasonably satisfy the characteristics of cusp spacing. The edge wave hypothesis additionally showed correlations between the calculated spacings due to subharmonic, zero order edge waves and the measured spacing of cusps found in relatively low energy environments. The edge wave hypothesis was considered the strongest of the three hypotheses on the basis of these results and the fact that edge waves have been shown to exist in the field by Huntley and Bowen (1973). The other two processes have not as of yet been shown to be of importance in fluid movements in the nearshore. This dissertation is from the Joint Program Degree from the College of William & Mary and University of Virginia and awarded by the University of Virginia.
© The Author
Sallenger, Asbury H., "Mechanics of beach cusp formation" (1974). Dissertations, Theses, and Masters Projects. William & Mary. Paper 1627407611.