Print Email Facebook Twitter Mud dynamics in the Markermeer; silt traps as a mitigation measure for turbidity Title Mud dynamics in the Markermeer; silt traps as a mitigation measure for turbidity Author Vijverberg, T. Contributor Stive, M.J.F. (mentor) Winterwerp, J.C. (mentor) Aarninkhof, S.G.J. (mentor) Kuijper, M.W.M. (mentor) Drost, H. (mentor) Faculty Civil Engineering and Geosciences Date 2008-05-23 Abstract The Markermeer is an important fresh water lake in the centre of the Netherlands. Because of its variety of functions and total scale of the lake, it is a unique area in Europe Over the years several problems have arrived in the Markermeer, which affect appropriate functioning of the lake. One of these problems is the decreasing ecological value (water quality) of the lake. This decrease in water quality is caused by a high turbidity of the water in the lake. Large amounts of mud sediment are suspended in the water column, even during normal conditions. This high turbidity has a large impact on the ecosystem. Rijkswaterstaat set up a study about several measures to decrease the high turbidity and improve the water quality of the lake. One of those measures is a deep silt trap, to catch the suspended sediment. Although some previous studies are carried out on the sediment behaviour in the Markermeer, the functioning of a silt trap is unknown. This MSc study focused on the mud dynamics in the Markermeer and the effects of a silt trap as a solution for the turbidity problem. Main question in this study is: Is a silt trap an effective measure to improve the water quality in the Markermeer? During construction of deep sand mining pits in the Markermeer, sedimentation rates in those pits were observed very high. Higher than expected from only settling of sediment from the water column. It is hypothesized that two mechanisms are responsible for the infill of the silt trap: settling of sediment from the water column and density currents. Density currents can be generated if a high sediment concentration layer exists near the lake bed and a density difference is present between two locations in the lake (for example near a silt trap). Field measurements showed that a high concentration layer (up to 800 mg/l) can exist near the bed of the Markermeer, under certain circumstances. To study the effects of a silt trap, a Delft3D model of the Markermeer is developed, which include the effect of density currents. This model showed that density currents can exists near a silt trap if the sediment concentration increases to 1 g/l near the bed. This occurs one day after a storm period. The dominant mechanism that fills the silt trap, differs for the two sediment fractions used in the model. For the fine fraction advection and settling of sediment is dominant. For the coarse fraction both mechanisms occur, although the settling process is of major importance. Also the effects of some design parameters of the trap are investigated, for example the orientation of the trap to the main flow direction (parallel and perpendicular). In both orientations a silt traps causes a reduction in concentration and an accumulation of sediment. However, a silt trap orientated perpendicular to the main flow direction has a higher efficiency. In the parallel case the trap attracts flow and causes an increase in flow velocity and turbulence intensity. This yields an increase of sediment in suspension which decreases the generation of density currents. Overall it can be concluded that a silt trap can be used as a mitigation measure for turbidity and improve the water quality. Although a silt trap will have a positive effect on the local scale, a silt trap alone will not work on the scale of the Markermeer. This is attributed to the enormous amount of fine sediment available and the extremely slow settling speed. The effect of the silt trap can be increased if it is combined with other measures to mitigate the turbidity problem. To reference this document use: http://resolver.tudelft.nl/uuid:88f7d62e-498c-442b-a6fb-2b16ce43b32e Publisher TU Delft, Civil Engineering and Geosciences, Hydraulic Engineering Part of collection Student theses Document type master thesis Rights (c) 2008 T. Vijverberg Files PDF ceg_Vijverberg_2008.pdf 7.2 MB Close viewer /islandora/object/uuid:88f7d62e-498c-442b-a6fb-2b16ce43b32e/datastream/OBJ/view