Analysis of concentration and grain size distributions based on a diffusion concept

Gradients in sediment transport due to waves and currents cause coastal changes and sediment transport is an important subject in coastal engineering research. It is therefore necessary to predict the sediment transport rates in the coastal region. Various formulae are available to predict the sediment transport rates, for example the sediment transport formulae of Bijker (1971) and Van Rijn (1993). These formulae use a diffusion concept for the calculation of the concentration and the velocity distribution over the water depth. In order to calculate the concentration and velocity distribution, the diffusion coefficient distribution has to be known. This is not always the case. Therefore, the main objective of this study is to increase our knowledge of the diffusion coefficient distribution. In many coastal areas, non-uniform grain size distributions of bottom material and suspended sediment occur. It is thought that the diffusion coefficient distribution does not or hardly depends on the grading of the sediment and will be mainly dominated by the hydraulic conditions. From measurements (Jacobs & Dekker (2000) and Sistermans (2000)) higher concentrations were found for experiments with well-graded bottom material when compared to experiments with uniform bottom material. From these measurements the diffusion coefficient distribution can be calculated. Most calculations in this study have been executed with the program CALCEPS. The program CALCEPS calculates several possible diffusion coefficient distributions from measured concentration distributions. When a calculation is done with the so called 'fraction method', more or less the same diffusion coefficient distributions are found for experiments with the same hydraulic conditions. The vertical distribution of the diffusion coefficient can be calculated from measured concentration distributions. It is assumed that the diffusion coefficient distribution as has been determined is correct only when both the concentration and grain size distribution over the water depth are being calculated correctly. The grain size distribution was extensively measured in the above mentioned experiments. In the first analysis the diffusion coefficient distribution described both concentration and grain size distribution very well. From these calculations it has been concluded that a diffusion concept can be used to calculate the concentration and grain size distribution of each experiment. In practical applications, measurements of the grain size distributions over the water depth are not available. Sometimes, the grain size distribution of the bottom material is known. Therefore, it is tried to impose the bottom material at a certain height and to describe the concentration and grain size distribution over the water depth as good as possible. It was found that the height above the bed where the bottom material can be imposed, should be in the order of magnitude of the measured ripple height. It can be concluded that the diffusion concept can be used for the modelling of concentration distributions of (well-) graded sediment.Civil Engineering and Geoscience