Frequency dependent cross-shore suspended sediment transport. 1. A non-barred shoreface

Abstract

The local, time-varying and time-averaged suspended sediment transports across a marine, non-barred shoreface were obtained from field measurements of the near-bed velocity and sediment concentration vectors using electromagnetic current meters and optical backscatterance suspended solids sensors. Co-spectral analyses of velocity and sediment concentration revealed the contributions of waves of different frequencies to the total transport; the transport attributable to quasi-steady currents was determined from the product of the time-averaged cross-shore velocity and sediment concentration. Estimates of the local, time-integrated sediment volume flux (total and net) and the associated erosion or accretion were determined using depth-of-activity rods. Suspended sediment transport was associated with: (1) local wind-forced waves; (2) swell; (3) low frequency waves (group-bound long waves) and (4) offshore-directed mean currents (undertow). These transport components varied spatially and temporally in response to changes in the wave height to water depth ratio. The local net oscillatory transport rate at wind-wave and swell-wave frequencies was directed onshore predominantly, and increased as water depth decreased in association with wave shoaling. The local mean sediment transport rate was predominantly offshore and dominated the net transport rate wherever wave-induced components were weak relative to the mean currents. Under breaking waves the suspended sediment transport contribution associated with wind-waves and swell-waves was reduced as a result of dissipation. The net suspended sediment transport rate exhibited a distinct vertical structure, reflecting the balance at each elevation between the opposing mean and oscillatory components of transport. The local time-averaged total and net sediment volume flux and resulting erosion and accretion patterns support the hypothesis of a near balance in sediment flux for the complete storm event.