Longshore currents in the vicinity of a breakwater

Several theoretical models of the wave-induced current circulation in the vicinity of a breakwater extending from shore are presented. The models,which are patterned after a local field site, include several numerical models which take into account variable sea-floor topography and which compare the effects of linear and non-linear bottom friction as well as an analytical model characterized by semi-infinite beaches and uniform sea-floor topography. In general (for a given angle of wave incidence) the circulation patterns show two counter-rotating cells driven by wave-induced longshore currents which flow along both the breakwater and natural beaches toward their common intersection corner, with an offshore return flow in the form of a rip current. The qualitative features of the models are consistent with observations of sediment transport taken at the study site. Differences in the linear and non-linear bottom friction models do not become apparent until an off-shore trench parallel to the breakwater is introduced to the sea-floor topography. The non-linear model shows a deflection of the off-shore return flow into the trench in agreement with preliminary analysis based on a one-dimensional model. The linear results, however, differ considerably from those of the non-linear model and are difficult to interpret, showing an inordinate increase in transport over the trench. During the development of the analytical model difficulties were encountered due to the complexities of the analysis which necessitated that part of the solution be solved numerically. The results, while showing the same general features as the numerical models, exhibit a much more strongly divergent off-shore return flow. This difference, while unresolved, appears to be one of scale rather than of form.