Abstract
Sediment entrainment processes due to the oscillatory flow above rippled and plane sea beds are fundamentally different. Whereas above plane beds the sheet flow or bed load regime dominates and the momentum transfer is primarily caused by turbulent diffusion, above a sea bed covered with long crested vortex ripples the well organised coherent vortex mechanisms induce sediment-laden lee vortices at the ripple crest, which will be detached from the bed ejected into the water column and finally shed when the flow reverses. To investigate the sediment entrainment processes above vortex ripples, a study was carried out in the Large Wave Flume (GWK) aiming firstly to find out the most appropriate measuring technique to determine the suspended sediment concentration both temporarily and spatially. and secondly to analyse the intra-wave sediment entrainment processes around a steep ripple.
Highlights
Non-cohesive sediment, which is permanently transported along the world’s coast lines by the action of tides and waves, modify over long time periods both the sea water-land boundary and the coastal bathymetry
The goal of the presented paper was (i) to comparatively analyse the appropriateness of the most commonly used concentration measurement devices and (ii) to interpret the first results related to the intra wave entrainment sediment processes over two selected steep vortex ripple under weakly asymmetric non breaking regular waves by means of a comparative analysis of concentration events measured by Acoustic Backscatter Sensors (ABS)
The results of the comparative measurement of the suspended sand concentration showed clearly that in most cases there were no significant differences between the time-averaged concentrations measured by means of the ABS and the optical turbidity meter with pump sampled mean concentrations conducted with Transverse Suction System (TSS)
Summary
Non-cohesive sediment, which is permanently transported along the world’s coast lines by the action of tides and waves, modify over long time periods both the sea water-land boundary and the coastal bathymetry. The recently innovative acoustical and optical measuring techniques with a high temporal and spatial accuracy aiming at the experimental investigation of the sediment transport fundamentals are providing unique insights allowing a reliable description of the sediment transport mechanisms and an improved prediction of the evolution of the coastlines and coastal bathymetry. Experiments with movable sand bed were recently carried out in the Large Wave Flume (GWK) of the Coastal Research Centre (FZK) to measure the suspended sediment concentrations (SSC) under regular and irregular waves using comparatively Multi Frequency Acoustic Backscatter Sensors (ABS), Optical Turbidity meter and Transverse Suction System (TSS). In the first part of the paper a comparative analysis is discussed to show why acoustical transducers represent the most appropriate technique to measure the temporal and spatial suspended sediment concentration movement above different see bed forms, especially over rippled beds, including the simultaneous measurement of the sea bed profile evolution. In the second part of the paper the process of the intra-wave sediment entrainment in oscillatory flow around a steep ripple is probed using high multi frequency acoustic backscattering technique
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