Shore-parallel sediment transport by waves and tides in inner shelf waters

Abstract

In shallow waters the combined action of waves and currents significantly affects the motion of sediment on the bottom, and hence the nature and phase of their interactions with each other contribute to the shaping of the beach. Using pressure and velocity data collected just outside the surfzone (in two meters depth) on the south shore of Martha's Vineyard, MA, we compared the variations in net alongshore transport during different wave-current conditions, studying the effect of the wave-current phase to show that it can have a significant impact on transport rates in shallow water conditions. Three distinct intervals were looked at in particular, representing respectively neap tide, calm conditions, spring tide, calm conditions, and spring tide, storm conditions. For each of these time intervals wave and current characteristics were computed and presented. Water depth, wave height and period and were obtained from the pressure data using linear wave theory and wave orbital velocity was computed using two different approaches: one based on gross characterisation of the wave conditions and treating them as a bulk, the other from spectral analysis of the surface spectra. These results were shown to have very good agreement with the same wave characteristics obtained from the velocity data. Current characterization was also computed from the velocity data. These were shown to be dominantly tidal and hence we focused our interest on identifying a similar signature in the slow-variation of the waves to correlate the two. Based on the results from the hydrodynamic analysis, an estimate of the net transport rate was made, assuming a 90° angle between the waves and the currents, for each of the three scenarios, highlighting the effect of varying wave shear velocity and net sediment transport rates. To further understand the impact of the wave-current phase, a fourth analysis was performed on synthesized data (based on real data) in which the phase could easily be altered. These calculations showed how accounting for this phase-shift greatly altered the sediment transport, and how it is affected by increased wave velocity due to meteorological conditions.