Author(s): M. H. P. Jansen; A. L. De Jongste; M. Klinge; S. Schep; P. Stook; G. Egbring
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Keywords: Fines; Shallow lakes; Water quality; Ecological functioning; Silt screens
Abstract: Many shallow lakes suffer from accumulation of fine sediment. The sediment may have several origins, such as erosion of banks or shallows, degradation of peat layers, internal organic production, external inflow etc. Its composition may range from organic to inorganic. The accumulation may cause several problems, ranging from problems for human use of the water (boating, swimming, drinking water) to ecological problems (increased turbidity, loss of vegetation cover and associated biodiversity) . Removing the sediment by dredging is usually expensive because of the large quantities, the large surface area that needs to be treated and the costs associated with transport, storage and/or disposal. Moreover, dredging usually does not restore the water ecologically. An interesting solution to reduce or solve the problems with fine sediment in a cost effective manner is to steer the transport of fine sediment with relative small measures. Fine sediment could be caught by the creation of lee areas, where silt can settle. Depending on water depth and transparency submerged and/or emerged vegetation may develop, thus increasing further settlement conditions and local biodiversity recovery. Depending on the specific functioning of the system and other measures to be taken, steering the fines may contribute significantly to the ecological recovery of the entire lake. Also the human use of lakes (e. g. ship navigation) can increase significantly. The principle of steering fines in shallow lakes is applied in the Wormer-and Jisperveld, The Netherlands. The Wormerand Jisperveld consists of three shallow lakes connected to each other by small channels or‘reeven’. Banks of both the lakes and the channels are eroding, especially the banks with orientation in south western direction. Due to the erosion, large parts of the lakes and channels are filled with organic silt. This limits the flows and gives problems to boats, because parts are too shallow for navigation. In order to get insight in the wave attack and present transport patterns of fines, a numerical hydrodynamic model was set up. Special attention was paid to the orientation of the grid and the interaction between currents and waves. After that, the model was used to examine the best way to create lee areas and to optimise the effectiveness of measures. The most promising measures were implemented in Wormer-and Jisperveld last year. Since then the silt transports decreased significantly. In some lee areas emerged vegetation developed, demonstrating the potential of this approach to solve problems with fines and to contribute to ecological recovery of the lakes.
Year: 2015