MEASUREMENT AND ANALYSIS OF SUSPENDED SAND CONCENTRATION BY WAVES IN A RIPPLED BED REGIME

The importance of the wave related suspended sediment concentration in the near shore zones as a major part of the total sediment load in different flow regimes implies a strong need to measureand analyse precisely the suspended concentration transport. Since sandy sea beds are rarely flatand often covered with bed structures such as ripples, the study of the sediment entrainment above the sea bottom with a high temporal and spatial accuracy is of a great importance. Thecharacteristics of these bed structures depend on the strength and the nature of the flow, including steady or tidal currents, waves or a combination of them. Above a sea bed covered with long crested vortex ripples, the coherent flow separationmechanism at the steep ripple crests generates lee side vortices, which are laden with sediment particles. The sediment-laden lee vortices are detached from the ripple crest, when the flow startsto reverse, lifted into the water and are finally shed under the flow reversal. Many previous experiments involving temporal and spatial measurements of sedimentconcentration under regular asymmetric waves [2, 4, 6, 8] showed that above steep ripple crests,within a layer of almost two ripple heights, the net total sediment flux are offshore-directed. Above this layer, the net current-related sediment flux is onshore-directed, whereas the net waverelatedflux is still offshore-directed. Due to the dominance of the wave-related sediment fluxabove the rippled bed, the total net sediment transport under regular asymmetric waves is offshore-directed. In contrast, above plane beds, the net suspended sediment transport is found tobe in the onshore direction. Experiments with movable sand bed were carried out recently 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) [1]. Primarily, the paper focuses on the comparative analysis of the measured vertical distribution of the suspended sand concentration using an acoustic measuring device (ABS), an opticalmeasuring device (Optical Turbidity meter) and a mechanical measuring device (Transversesuction system, TSS). The aim of the investigation is to demonstrate that the acoustic measuring method (ABS) represents the most appropriate technique for the measurement of the suspensionprocesses especially over the steep ripples. 2Second, the paper also addresses the sediment entrainment processes above a steep vortex ripple by showing details of the temporal sediment entrainment due to the intra-wave processes around the steep ripple under non breaking weakly asymmetric regular waves.

Keywords:

Non-breaking Waves , Suspended Sediment Concentration , sandy sea bed , Acoustic Backscatter Sensor , Intra-Wave Processes , Steep Vortex Ripple