Heavy metal pollution and sediment transport in the rhinemeuse estuary, using a 2D model Delft3D: Water quality and calamities. Case study Biesbosch

Different measures will be executed in the framework PKB “Room for the River” in order to lower the water levels in the river area during normative high discharges. The plan Ontpoldering Noordwaard is one of them and has as aim to inundate parts of the Noordwaard during high discharges. Water coming from the Nieuwe Merwede as result of high water levels will flow into the Noordwaard and leave the area through the south part. As consequence the creeks in the Brabantse Biesbosch will process more water and flow velocities will increase. This may result in higher sediment transport en possible erosion of the gullies. Dankers et al. (2008) studied the flow velocities in the Brabantse Biesbosch under different discharge conditions and the possible effects of the velocities on erosion and transport of contaminated bed material. This study was a combination of computer model simulations and expert judgment. The model simulations were obtained with the combination of boundary conditions for discharges with a return period of once in 100 year for the Rhine and Meuse and a water level of the sea of 1 m above normal. In this case flow velocities higher than 1 m/s and shear stresses higher than 1 N/m2 were found at different locations of the Brabantse Biesbosch. This means that a transition may occur from a situation without spreading of contaminated bed material towards a situation where spreading can occur. By comparing the quality of the top layer of the areas with risk of erosion according to De Straat (2004) with the intervention values for river beds (“Circulaire Sanering Waterbodems 2008”) and the MACsediment (Maximum Allowed Concentration in the Sediment), Dankers et al. (2008) found that different areas in the Brabantse Biesbosch may exceed the intervention values and/or the MAC for sediments. The areas Gat van de Visschen, Gat van Den Kleine Hil and Gat van de Noorderklip present concentrations of metals in the top layer that are higher than the intervention values and the MAC. In the areas Gat van Van Kampen, Gat van de Binnennieuwensteek and Spijkerboor a small violation of the MACsediment was observed. This study concluded that there are unacceptable risks of spreading of contaminated mud to the surface water