Optical Estimation of Swash Zone Sediment Transport

The swash zone is a region of high societal impact and highly-variable morphology, yet sediment transport in the swash is not well understood. Sediment transport measurements, needed for improved models of swash morphodynamics, are rare due the hostile nature of the environment, the large excursions of bed elevation over which sampling is required, and the complication of extracting net transport signals as the small residual between large onshore and offshore components. Moreover, most in-situ measurements sample only one component of transport, usually the suspended load, not the desired total load transport. Our objective is to develop a methodology to measure the net swash zone sediment transport, resolving time scales of variation of several incident waves periods. To resolve total transport, sediment transport will be estimated by cross-shore integration of the time rate of change of the beach profile (assuming conservation of mass). Observations of beach profile change with adequate accuracy and resolution will be made non- intrusively using optical stereo methods, analyzing synchronous imagery from two spatially displaced video cameras. The method was tested during laboratory experiments at the Delta Flume at Delft Hydraulics, The Netherlands, during Winter 2005/2006. Synchronous image pairs from two cameras, mounted above the wave flume, were analyzed using stereographic techniques to determine temporal changes in the cross-shore profiles of the subaerial beach with a typical vertical resolution of 0.5 cm. Experimental conditions were representative of winter storms on the Dutch coast and included episodic dune erosion. Cross-shore transport signals from these transients should be easily resolvable and should help constrain swash transport models.