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Sediment concentrations and diffusivity profiles under skewed waves from ripple to sheet flow regime

By C.E. Huisman

Abstract

Morphological evolution of coasts is mainly determined by hydrodynamics and sediment transport processes. One of the present-day bottlenecks in predicting morphological change, concerns the description of the vertical distribution of cross-shore sediment transport and the description of associated vertical sediment concentration profiles. The prediction of these sediment concentration profiles requires specification of the sediment diffusivity, which is proportional to the vertical derivative of the sediment concentration. In existing models the distribution of the sediment diffusivity over depth varies between a constant, linear, parabolic or even a constant-linear-constant distribution. This thesis aims to determine diffusivity profiles based on time-averaged concentration profiles under skewed waves, from ripple to sheet flow regime. Full-scale experiments were done in the large scale wave flume, GWK, Gro{\ss}er Wellen Kanal, Hannover, Germany. The experiments have produced a data set of bed states, hydrodynamic conditions and time- and height-varying concentration measurements for the ripple and sheet flow regime for two different sediments Analysis of time-averaged data from a triple-frequency Acoustic Backscatter Sensor (ABS), attached to the `Twente-Utrecht measurement frame', provided time-averaged vertical sediment concentration profiles, from which the sediment diffusivity profiles were computed. For all conditions, the measured diffusivity profile has a parabolic shape. Uncertainty of the magnitude of the background signal of the ABS causes large uncertainty in the dimensions of sediment concentrations. However, the dimensions of sediment diffusivities are comparable to sediment diffusivities determined from existing empirical formulations. A comparifson to existing formulations resulted in a best fit of shape and dimensions to the diffusivity predictor of Bijker (1971)

Topics: Geowetenschappen, Acoustic Backscatter Sensor, ABS, sediment concentrations, sediment diffusivity, flume experiments, wave experiments
Year: 2009
OAI identifier: oai:dspace.library.uu.nl:1874/36751
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