Drainage and dyke building: Compensatory measures for subsidence basins. Like the other two perpetual obligations, land reclamation, or poldering, also involves a great deal of pumping. The term “polder” makes one think of the Netherlands, where plots of land that lie below sea level (the “polders”) have been reclaimed by draining them and fencing them in with dykes.
In the Ruhr region it was hard coal mining that caused many surface areas to subside. Subsidence of the land above mine workings is fairly common in this region. For example, in the area around Essen mining subsidence has been observed since the middle of the 19th century. Today the area around the former Zollverein mine lies about 20 meters lower than it did one hundred years ago.
These depressions, which are also known as polder areas, must be artificially drained in perpetuity in order to prevent surface water from accumulating there and causing flooding. As a result, more than a billion cubic meters of water must be pumped off annually throughout the entire Ruhr region. That corresponds to the volume of water that flows through the Rhine River past Duisburg over a period of five days.
The subsidence could also lead to situations in which a stream flows backwards or floods hollows because its slope has changed. In such situations, pumps located at the deepest point of the watercourse can restore the water’s original direction of flow. In addition, along large watercourses it can be necessary to increase the height of existing dykes in order to prevent flooding.
All of these methods of regulating the water level in polder areas are called polder measures. RAG Aktiengesellschaft operates a few smaller pumping stations itself, but most of the polder measures are carried out by the water management associations in the Ruhr region, of which RAG is a member. These associations were founded at the beginning of the industrial age in the early 20th century in order to cooperatively manage the water used by municipalities and industry.
All in all, about 600 polder facilities are distributed throughout the Ruhr region. In addition to the pumping stations, the associations also maintain retention basins and dykes.