Sand-mud distribution in the Amelander inlet: Sand and mud transport computations in a tidal inlet

Sand-mud distribution in the Amelander inlet: Sand and mud transport computations in a tidal inlet

Nieuwenhuis, O.

Stive, M.J.F. (mentor)
Van Ledden, M. (mentor)
Wang, Z.B. (mentor)
Winterwerp, H. (mentor)
Roelvink, J.A. (mentor)

Civil Engineering and Geosciences

Hydraulic Engineering

2001-04-01

In the tidal inlets in the Dutch Wadden Sea, the most important sediment types are sand, silt and clay. A distinction between the sediment types is made, because clay particles have cohesive properties. Consequently, the clay particles do not behave as individual particles but tend to stick together. Floes are formed whose size and settling velocity are larger than those of the individual particles. The bed is mostly composed of a mixture of different sediment types. The bed has cohesive properties (and is called a mud bed) if the clay percentage is higher than 5-10%. At present, computer-models are not able to simulate the sand-mud distribution in tidal inlets. The bed composition variation cannot be taken into account in the computations. This is caused by a lack of knowledge about the processes and mechanisms that take place during mud transport and sand-mud interaction. Therefore, the goal of this study is to gain more knowledge about sand-mud distribution in tidal inlets. In this study the tidal basin of the Amelander Inlet has been chosen as area of interest. The first part of the study consists of a data analysis. The data is taken from the Sediment Atlas (Rijkswaterstaat, 1998) and consists of the grain size distribution at sample points in the tidal basin of the Amelander Inlet. To obtain these grain size distributions, samples were taken every kilometre or every five hundred meters. The samples were taken in the period April-July 1995. The grain size distribution was determined with the Malvern method. It is know that the Malvern method underestimates the finer fractions in a sample. The second part of the study consists of using numerical models to try to simulate the observed sand-mud distribution in the Amelander Inlet. In the data analysis, research questions were raised which can be answered with the models in this part. These questions concern the wave penetration, the tidal penetration, the water depth at high water and the level of the sand and mud flats. The computations are made within the Delft3D modelling system. First, hydrodynamic computations are made in Delft3D-FLOW. After that, sediment transport computations are made separately for the sand bed and the mud bed. The main conclusions from this study are: 1) The areas where deposition takes place are mainly determined by the occurring tidal action. The influence of waves is large at small water depths, and determines how much and how fast the mud particles are suspended. Whether the mud particles are transported after being stirred-up, is determined by the actual tidal action. 2) Waves also are of influence for the deposition of mud layers. Waves in the tidal basin spread the mud particles from areas with high wave attack (high concentration) towards areas with low wave attack (low concentration). Due to this redistribution of mud, the mud particles, on a large scale, are transported from west to east in the tidal basin. 3) Qualitatively, the computed mud deposition agrees with the measured mud contents on a large scale. With the help of the second deposition computation and the erosion computation, part of the differences can be explained and the measured mud contents are understood better. Further experimenting with the wind speed, computing different phases in a certain sequence and the computation times for the different phases can improve the results and help understand and predict the measured mud contents on a more accurate level.

tidal inlet
sediment distribution
simulation

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Student theses

master thesis