Case study:Flood detention basin (HRB) Mengede

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Location: 51° 34' 58.57" N, 7° 21' 27.28" E

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Project overview

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Status	Complete
Project web site
Themes	Habitat and biodiversity, Hydromorphology
Country	Germany
Main contact forename	Mechthild
Main contact surname	Semrau
Main contact user ID	User:EGLV
Contact organisation	Emschergenossenschaft
Contact organisation web site	http://eglv.de
Partner organisations
Parent multi-site project
This is a parent project encompassing the following projects	No

Aerial view of the HRB Mengede during construction

Project summary

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Part of the integrated river basin management for the Emscher Region (865 km², 2.700 inhabitants/ km²) is the revitalization of the river Emscher and its tributaries. Due to industrialization the waterbodies were systematically developed as open wastewater sewers in the beginning of the 20th century. Economic and technical alternatives such as closed sewer systems could not be implemented due to constant subsidence caused by coal mining. Momentarily, the river system is being restructured in order to enable the Emscher to once again be a fully functioning, continuous water-based ecosystem with typical topology and vegetation. An ecological concept based on the assumption of a non-interrupted river as a connection from the source to the mouth has been developed.

Studies on the feasibility of restructuring the Emscher show, that due to coal mining, industrial impacts and population changes the “original” conditions can never be reached again. It will not be possible to return the Emscher to its original, meandering state. Therefore, the reference conditions are no direct goals for the river restoration, but they give orientation in the planning procedure. One of the first hot spots that has been put into practice is the HRB Mengede. The HRB comprises an area of 30 ha, where a typical lowland river with natural substrats, features and vegetation can be formed. A varied mosaic of biotopes, which are typical for floodplains (alder floodplain forest, sedge, typha and phragmites reed, floodgrass and potamogeton) is expected to develop. Two million m³ of native soil had to be excavated. Half of which have been used to build dikes.

The river Emscher is integrated into the basin, hence even smaller floods will reach the floodplain that has been created. Parts of the basin will be flooded about 80 days a year. Since the bottom of the HRB is uneven, parts of it will constantly be a body of standing water, whereas some elevated parts will be islands even during high floods. This diversity of topologic forms will lead to a great diversity in habitats - and therefore species.

As of now, the basin is still divided into three smaller parts by the Emscher which is encased by high dykes, to protect the basins from the wastewater which the river is still carrying. Only during high floods diluted wastewater will cross the dykes to flood the basins. Once the wastewater is fully removed from the river, there will be a slight remodelling of the basins (removing of the dykes), thus creating the possibility for the river to mold its own body and have access to the floodplain.

Monitoring surveys and results

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Image gallery

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View on one of the basins

View into basin nr. 2, with deadwood and Salix spec. visible

Catchment and subcatchment Select a catchment/subcatchment Edit the catchment and subcatchment details (affects all case studies in this subcatchment) Catchment River basin district Rhine River basin Rhine Subcatchment River name Emscher Area category Area (km2) 865865 km² <br />86,500 ha <br /> Maximum altitude category Maximum altitude (m) Dominant geology quarternary, holocene Ecoregion Central Plains Dominant land cover Urban, Intensive agriculture (arable), Grassland, Broadleaf/mixed woodland (semi natural) Waterbody ID DE_NRW_2772 Other case studies in this subcatchment: Upper Emscher Site Edit site Name HRB Mengede WFD water body codes DE_NRW_2772_O WFD (national) typology Type 15: small sand-dominated lowland river WFD water body name Emscher Pre-project morphology Artificial bed, Over deepened, Straightened, Single channel Reference morphology Actively meandering Desired post project morphology Actively meandering, Set back defence Heavily modified water body Yes National/international site designation Local/regional site designations Protected species present Yes Invasive species present Yes Species of interest Dominant hydrology Groundwater, Quick run-off Dominant substrate Sand, Silt River corridor land use Urban, Broadleaf/mixed woodland (semi natural), Improved/semi-improved grassland/pasture, Intensive agriculture (arable) Average bankfull channel width category 5 - 10 m Average bankfull channel width (m) Average bankfull channel depth category 0.5 - 2 m Average bankfull channel depth (m) Mean discharge category Mean annual discharge (m3/s) Average channel gradient category Less than 0.001 Average channel gradient Average unit stream power (W/m2) Project background Edit project background Reach length directly affected (m) Project started 2005/01/01 Works started 2010/01/01 Works completed 2013/12/31 Project completed Total cost category more than 10000 k€ Total cost (k€) 6200062,000 k€ <br />62,000,000 € <br /> Benefit to cost ratio Funding sources Cost for project phases Phase cost category cost exact (k€) Lead organisation Contact forename Contact surname Investigation and design Emschergenossenschaft Stakeholder engagement and communication Emschergenossenschaft Works and works supervision more than 10000 k€ 4370043,700 k€ <br />43,700,000 € <br /> Emschergenossenschaft Post-project management and maintenance Emschergenossenschaft Monitoring Emschergenossenschaft Reasons for river restoration Edit reasons for restoration Mitigation of a pressure Hydromorphology development of typical structures for the river and the riparian zone Biology Create a new natural habitat with continuity for benthos, fishes and other organisms Physico-chemical Other reasons for the project Ensuring flood protection, development of an ecological hot spot, landscape enhancement Measures Edit Measures Structural measures Bank/bed modifications removal of bottom slab formwork, Adding sinuosity Floodplain / River corridor Increase water retention area, connection to wider floodplain, create new wetland habitats Planform / Channel pattern Channel naturalisation, creation of meandering channel, improvement of channel morphology, enabling dynamics Other Non-structural measures Management interventions Social measures (incl. engagement) Public participation process during the planning phase, 2006 a public workshop was organised by the EG on the design aspects of the construction (e.g. bike trails) Other Monitoring Hydromorphological quality elements Edit Hydromorphological quality elements Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Structure & condition of riparian/lake shore zones No Yes Yes No No Awaiting results Channel pattern/planform No Yes Yes No No Awaiting results Width & depth variation No Yes Yes No No Awaiting results Biological quality elements Edit biological quality elements Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Invertebrates No Yes Yes Yes No Awaiting results Fish No Yes Yes Yes No Awaiting results Macrophytes No Yes Yes No No Awaiting results Physico-chemical quality elements Edit Physico-chemical quality elements Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Nutrient concentrations Yes Yes Yes Yes No Awaiting results Oxygen balance Yes Yes Yes Yes No Awaiting results PH Yes Yes Yes Yes No Awaiting results Salinity Yes Yes Yes Yes No Awaiting results Temperature Yes Yes Yes Yes No Awaiting results [[]] No No No No No Any other monitoring, e.g. social, economic Edit Other responses Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Monitoring documents Upload monitoring documents Additional documents and videos Upload additional documents Additional links and references Edit links and references Link Description Supplementary Information Edit Supplementary Information