Sediment transport under sheetflow conditions

Abstract

Important changes in the coastal profile can appear in a few hours during storm conditions. During these conditions it is difficult to conduct measurements in the field, especially in the region of most interest (very near to the bed). Because of the complex physics of the problem it is not possible to make a proper scale model. Also full scale measurements in laboratories are scarce because they demand costly facilities. To fill this lack of data oscillating water tunnels were built. An oscillating water tunnel offers the possibility to simulate the near bed velocity under waves on a full scale. During the present study experiments were conducted in the oscillating water tunnel of DELFT HYDRAULICS. The main scope of this study is to obtain data for the verification and development of sediment transport models. The series of water tunnel experiments were focused on sheetflow conditions (= flat bed) under sinusoidal waves combined with a current. Measurements were aimed on detailed time dependent research in the sheetflow and suspension layer. The following measuring devices were used to measure concentration: a conductivity concentration meter (CCM) and an optical concentration meter (OPCON) to measure respectively in the sheetflow and in the suspension layer. Further time averaged concentrations were measured with a transverse suction system, the obtained sand samples were used to obtain a D J 0 distribution in the vertical. Velocities were measured with an electro-magnetic flow meter (EMF) and a laser-doppler flow meter (LDFM) in the suspension layer. Grain velocities in the sheetflow layer were estimated with help of a high speed video technique (HSV). Time averaged sediment transport was measured with help of a mass-conservation technique. Four different test conditions were studied concerning sinusoidal waves combined with a net current all with approximately the same third-order velocity moment ((u3)). More than one hundred tests were executed during October and November 1993 in the laboratory of DELFT HYDRAULICS in De Voorst. The investigation was part of the EU program "Access to Large-scale Facilities and Installations". The analysis of the raw data was conducted by the different members of the research team. A complete overview of this processed data can be found in a data report (Katapodi et al, 1994). 'Intra-wave' concentrations and velocities were determined by ensemble averaging over a number of waves of the measurements data. The HSV-technique was not useful to. determine very accurate velocities in the sheetflow layer, but it is a useful tool to gain qualitative insight in the processes in the sheetflow layer. The obtained time dependent concentrations and velocities were combined to time dependent sediment fluxes. Fluxes in the suspension layer could be obtained quite accurate. Fluxes in the sheetflow layer were not very accurate because of the uncertainness in the Hsv-analysis. The obtained fluxes (in sheetflow and suspension layer) lead to the conclusion that the bed load transport was dominating over the suspended load transport, although the exact distribution could not be determined. Near the bed the wave-related flux is dominating over the current-related flux.