Morphodynamic equilibrium is a widely adopted yet elusive concept in the in the setting) so that the concept of morphodynamic equilibrium should be 13 * Corresponding

The influence of sand and mud transport on the morphological behaviour of a short tidal basin is investigated in this paper. For this purpose, a morphological model is applied in which sand and mud transport are included and the temporal and spatial bed composition variations are taken into account. Initially, the morphological development shows a sand wave near the entrance of the basin and a mud deposition wave more landward. A quasi equilibrium bed level profile is found after a long period (order century) with a sandy bed surface over almost the entire basin and only a small muddy area near the landward end. The dimensionless ratio between the deposition and erosion flux turns out to be a crucial parameter for the understanding of the observed behaviour. Comparison with previous studies on short tidal basins for sand indicates only that the presence of mud in a combined sand mud model does not change the equilibrium bed level profile considerably for the applied parameter settings herein, but drastically decreases the morphological time scale. Comparison between model results and field data of the Wadden Sea suggests that the obtained bed level and bed composition profile are realistic, indicating that the process-based sand mud model is a first step towards a better understanding of sand mud distributions in tidal basins.

Large-strain consolidation theory is widely used for the management of dredged disposal sites. The theory is universally accepted to deal with this problem, though the determination of the material properties is not yet standardised. Decisions made on this level can lead to the prediction of a totally different consolidation history. This paper describes the results of a prediction exercise, performed using a batch of sediment from the river Schelde (Antwerpen, Belgium). Numerical modellers were given the data of four calibration experiments and were then asked to predict another experiment. Settling column experiments (0·2–0·6 m in height) with density and pore pressure measurements provided the basis for the calibration data. The prediction demonstrated the significance of the soil compressibility at low effective stresses, when time-dependent behaviour is observed.

This paper discusses the application of a simple algorithm for the buffering of fines in a sandy seabed. A second layer is introduced in which fines may be stored during calm weather and from which fines may be resuspended during storms. The algorithm is applied first in a one-dimensional vertical (1DV) point model at a location in the North Sea, Noordwijk 10, 10 km offshore. It is able to reproduce the observed temporal variability of suspended particulate matter satisfactorily. Apart from the second layer, also the applied first order erosion rate is an important element of the algorithm. This allows for an equilibrium sediment mass per unit area for any combination of bed shear stress climate and sediment supply. The classical Partheniades-Krone formulation with zero th - INTERCOH'07 -Brest, France -25-28 September, 2007 2 order erosion (i.e. an erosion rate that is independent from the sediment mass per unit area) does not have such equilibrium.As a next step, the algorithm is incorporated into a 3D model for suspended particulate matter (SPM) transport in the Dutch coastal zone. It is demonstrated that the model is able to reproduce the observed spatial and temporal variability reasonably well. An essential feature of the 3D mud model is that it is sufficiently fast to compute equilibrium bed composition. This implies that the results are completely independent from the applied (uniform) initial conditions.Finally, the mud model is applied to assess the impact of a large-scale release of fines in the Dutch coastal zone. The computed impact turns out to be very sensitive to the assumed buffer capacity of the seabed. However, information on transient system response (such as the dissipation of a sediment pulse in the system) from which the buffer capacity may be estimated is most often lacking. For the time being, estimates on the residence time of fines in the seabed and its mixing depth are derived from the literature. Additional field and laboratory test on the exchange mechanisms of fines between the water column and a sandy seabed are recommended.