University of Twente Student Theses


Biological influence on sediment transport and bed composition for the Western Wadden Sea

Borsje, Bas (2006) Biological influence on sediment transport and bed composition for the Western Wadden Sea.

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Abstract:Biological activity is known to have significant influence on sediment transport and bed composition on a small spatial scale. However, the large scale effects of biological activity are not known. These large scale effects could be of great importance for bringing up recommendations for the conservation and management schemes of different estuaries. Combined with a large spatial scale, also a large temporal scale is required, in order to provide a realistic assessment of the biological contribution to the fine sediment dynamics.By applying the process-based model Delft3D, the physical system combined with three biological processes is simulated. The biological influences are expressed in stabilising and destabilising of the bed by biota and the downward movement of sediment in the bed caused by the digging and feeding activities. For the Dutch Western Wadden Sea, a correlation exists between the wave direction and wave height, showing the largest wave heights for the wind directions between west and north. However, no clear seasonal variation in wind speed is observed. As a result, the seasonal variation in suspended sediment is not simply caused by the wind induced waves. To include the (de)stabilising of the bed, a parameterization of the influence of biological activity on sediment strength parameters (critical bed shear stress and erosion rate) is implemented in the model, based on measurements in the Wadden Sea. The downward movement of sediment in the bed is imitated by an increase in the porosity of the top layer of the bed. The bio-destabilisers are represented by the mud snail Hydrobia ulvae and the clam Macoma balthica. Bio-stabilisation is caused by microphytobenthos, which are known to form algae mats, which prevent the bed from erosion. Based on the temporal variation in biomass biota, it can be concluded that during spring stabilising is dominant while during autumn de-stabilising is dominant in the Western Wadden Sea. Compared to the situation without biological influences, the suspended sediment concentrations are influenced on an estuarine scale. However, every tidal basin is influence in a different way, resulting in a classification of the different tidal basins. The amount of fine sediment on the bed shows a distinct increase just outside the destabilised areas, showing the biological influences. Based on sensitivity analysis, it is determined that the suspended sediment concentrations are mainly influenced by microphytobenthos, while bio-destabilisers are responsible for the fine sediment distribution on the bed. In order to bring up recommendations for the management schema of estuaries, two systems acting on different (temporal and spatial) scales need to be mentioned. The lowest level system determines the vertical transport of fine sediment and is influenced both by the wind and the biological activity. The highest level system determines the import of fine sediment from the North Sea and is the outcome of the combined effect of the tide and the seasonal varying suspended sediment concentration at the North Sea.
Item Type:Essay (Master)
WL Delft hydraulics
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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