Abstracts Earth and Environmental Sciences

Further analysis of sand concentration distributions based on a diffusion concept:

by E. Van Herwijnen

Morphological changes of coastal areas are caused by sediment transport. Sediment transport is one of the most important subjects in coastal engineering research. Various models are available to predict the sediment transport rate as a function of wave height, current velocity and grain size. The reliability of these models is unknown, because data under field conditions are scarce. Also the sediment transport processes are not fully understood. Considering that in the coastal areas the waves are superimposed on a current (longshore current), the problem becomes more complicated. In this study a sediment transport model based on a diffusion concept is used. The sediment transport is based on the product of a velocity profile u(z) and a sediment concentration profile c(z) over the depth, resulting in a sediment transport profile over the water depth. The concentration profile can be described by a bed concentration and a shape of a concentration distribution over the water depth. To calculate the concentration distribution over the depth, a diffusion (or mixing) coefficient distribution can be used. To predict the sediment transport it is important to understand the influence of wave height, current velocity, water depth and grain size on the diffusion coefficient distribution (&8distribution). It is tried in this study to determine the reliability of the diffusion model for predicting the sediment concentration over the height. Nine existing diffusion coefficient methods (methods to describe the shape of the diffusion coefficient distribution) are used, on several sets of experiments, to determine the performance of these diffusion coefficient methods. Each ofthe methods can be determined with 2, 3 or 4 parameters (degrees of freedom). For the five best performing diffusion coefficient methods the influence of the boundary conditions on the involved parameters of the diffusion coefficient methods is determined. It shows that the diffusion coefficient method with 4 degrees of freedom performs best. However, the difference with the diffusion coefficient methods with 3 degrees of freedom is not very large. The influence of the boundary conditions on the involved parameters (degrees of freedom) of the diffusion coefficient methods is not always what one expects. One parameter does not show very much consistency with the wave height or current velocity. There can not be made a distinction between the five best performing methods, since these methods perform approximately the same for calculating the sediment concentration over the height. With the calculated diffusion coefficient distributions, the velocity profiles of certain experiments (experiments with current wave angle of 90°) are determined. The measured velocity profiles are compared with the calculated velocity profiles. It shows that for most experiments the calculated velocity profiles do not fit to the measurements very well. The resulting bottom shear stresses for the experiments are discussed. It appears that the bottom shear stress decreases for increasing…