Evaluation of Wave-Generated Longshore Current Velocities and Sand Transport Rates on Beaches

Abstract Currents associated with the nearshore cell circulation, including rip currents, redistribute beach sands into a variety of rhythmic topographies, but do not produce longshore sand transport continuously along the shoreline. Waves breaking at an angle to the beach generate longshore currents flowing parallel to the shoreline. These currents in turn interact with the wave surf to produce a longshore transport of sand. A simple equation has been found with which this longshore current velocity can be evaluated for the mid-surf position where data are available. Theoretical relationships have been formulated for the complete longshore current distribution across the beach width, but data are lacking. However, the distribution can be made to agree with the available data at mid-surf by the proper selection of the drag coefficient. Equations have also been obtained for the evaluation of the sand transport rate, caused either by waves breaking at an angle to the shoreline or by longshore currents generated in other ways. This gives the total sand transport rate. Theoretical relationships have been determined for the distribution of longshore sand transport across the beach width, but again data are lacking to test the equations. The distribution can be calibrated such that, when summed across the beach, it gives the correct total sand transport rate.