EXTENSIVE HIGH-ACCURACY DREDGING OF CONTAMINATED SEDIMENTS IN HOLLAND

摘要

In the Netherlands, located in the delta of the rivers Rhine, Meuse, Scheldt and Ems, contaminated sediments form a huge problem. Polluted sediments have a bad impact on ecology, and poisonous chemicals deposited in lakes and harbours cause depth restriction for ships. This article describes a large high-accuracy dredging project actually in execution in the Netherlands. The focus is aimed on the dredging operations in Lake Ketelmeer. Lake Ketelmeer is located at the end of the river IJssel, a branch of the River Rhine. At the mouth of the IJssel there has always been natural silt deposition and delta forming. People have also contributed to land forming by reclaiming large parts of the Zuiderzee. Thus Lake Ketelmeer was created between the polders Noordoostpolder and Oostelijk Flevoland. Because industrial plants along the Rhine and IJssel discharged into the river in the period between 1950 and 1980 large quantities of pollutants ended up in the river water. This happened in Holland as well as in Germany, Switzerland, and France. The pollution settled in Lake Ketelmeer, where the current of the river loses its velocity, causing a reduction of the transport capacity of the river sediments. Together with the adhered pollutants, the silt was deposited in thick layers. The lake covers an area of 3800 ha, of which 2800 ha now is polluted. On average the polluted layer is 50 cm thick, varying from a few centimetres to over a metre. Overall, a quantity of 15 million cubic metres of heavily polluted sediments have been deposited. However, since the beginning of the 1990s, the incoming sediments have been less polluted. The accumulated pollution causes deviations in animals that live in Lake Ketelmeer. The pollution also slowly seeps through to the groundwater and is spread by wind, current, and ship propeller turbulence. For all these reasons, in 1994 it was decided to clean the Ketelmeer. As treatment of the contaminated material is not economically feasible, it was decided to built a large confined disposal facility (CDF) to be called "Ijsseloog", in the middle of the lake. The storage volume will be 23 million m~3, making it capable of storing contaminated sediments from other locations as well.