Pilot Investment Retentionsraum Zell
(River Mümling) in Germany
Wasserverband Mümling, Internet: www.wv-muemling-gersprenz.de
Project leader: Matthias Sottong; E-mail: m.sottong@wv-muemling-gersprenz.de
The River Mümling catchment area (360 km²) is located in the Odenwald, a low mountain range in the South of the Federal State of Hesse in Germany. It is a tributary to the River Main which flows into the River Rhine. The maximum elevation within the catchment is 560 m asl. The average discharge is about 3 m³/s. However, the maximum discharge ever recorded was 65 m³/s. In statistical terms this flood event had a reccurrence interval of 25 years only.
The River Mümling catchment represents well the flood risk situation in most German low mountain ranges. The area is densely populated and the damage potential located next to streams is high. Space in the valleys is a scare resource. In 1993 and 1995 two flood events occurred. The latter event caused flood damage which was estimated to be around € 6.5 Mill..
In 1999, as a consequence of this and because of the European Water Framework Directive the Waterboard Mümling decided to implement a new strategy, which aims at increasing water storage and nature development by means of a decentralised network of small to medium scale measures located all over the catchment. It replaces the former policy of the Waterboard which has focused on large-scale flood protection infrastructure like the Marbach Reservoir which is located in the upper catchment of the Mümling. This reservoir provides a total storage volume of 3.1 Mio. m³.
The Retention Area Zell is a first pilot investment that implements the new strategy. This retarding basin provides a total storage volume of 200.000 m³. It will reduce the downstream maximum flow from 34.2 m³/s to 29.1 m³/s in case of a 50-years flood event. The ecological impact could be minimized because of its adapted structure. The retarding basin will be operated without permanent backwater. Under normal flow conditions and minor flood events the hydrological regime remains unchanged. The design of the throttle was optimized in a way that the impact on the hydromorphological condition is minimal. There are no steep ramps or other obstacles which might hinder fish or sediments to pass through the throttle.
Special attention was paid to the GIS-based retention cadastre in which locations of natural and potential retention areas in the Federal State of Hesse (Retentionskataster Hessen — RKH) and along the River Mümling are stored. This innovative system covers about 350 different streams with a total length of 4000 km. In addition to existing retention areas the RKH particularly emphasises those areas with potential retention capacity, which need to be activated by means of small to medium scale measures. One of these locations is the “Retention area Zell”. In future this measure is part of a catchment-wide de-centralised network of possibilities for water retention and for avoiding further growth of damage potential.
