Cohesive sediment transport modelling - application to the Lister Dyb tidal area in the Danish Wadden Sea

A numerical 2D one-layer cohesive sediment transport model has been set up for the Lister Dyb tidal area in the Danish Wadden Sea. The applied modelling system MIKE 21 consists of several separate modules. The modules used in this study are a hydrodynamic module (MIKE 21 HD) calculating the water movements in the area, a near shore waves module (MIKE 21 NSW) for calculating a number of wave fields, and a mud transport module (MIKE 21 MT) for calculating the cohesive sediment dynamics in the area. The simulated suspended sediment concentrations are compared to measured time series during a six-week period. It shows that the suspended sediment concentration (SSC) phase is in accordance with the observed patterns showing relatively low concentrations at high water and high concentrations at low water. The magnitude of the concentrations also to a high degree correlate with the observed values, although the lowest concentration levels around high water may be too high. Further, the bed level change has been validated on the time scale of a few tidal periods. This shows that the model is capable of creating the correct pattern of bed level change. However, on longer time scales (months), the net deposition pattern and hence the calculated sediment budget may not be realistic. As the cohesive sediment transport model is well calibrated, it has been possible to calculate the distances of the settling and scour lags individually. The combined lag effects amount to distances of around 1500 m, with the scour lag being of greatest importance. Through this study, it has been shown that it is possible to use numerical modelling to simulate suspended sediment concentrations that fit well with observed values. In addition, it is possible to recreate measured patterns of relative bed level change. This leads to the assumption that cohesive sediment transport models in the future can be used to elaborate on cohesive sediment transport problems with a high degree of reliability.