Obersee (Bielefeld)
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| Coordinates | 52 ° 3 '30 " N , 8 ° 34' 0" E | ||||||||
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| Construction time: | 1977-1982 | ||||||||
| Data on the reservoir | |||||||||
| Altitude (at congestion destination ) | 80.80 m | ||||||||
| Water surface | 20 ha | ||||||||
| Storage space | 385,000 m³ | ||||||||
| Total storage space : | 640,000 m³ | ||||||||
| Catchment area | 100 km² | ||||||||
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The Obersee (official name Johannisbachtalsperre - Obersee ) is an artificially created reservoir in the Bielefeld district of Schildesche . With a size of around 20 hectares, it is the largest dammed body of water in Bielefeld.
Planning, construction and data
Before the construction of the Obersee, the Johannisbach and the Jölle regularly led to floods in the Johannisbachtal, including in the area of today's lake. The lake, which was built from 1977 to 1982, is 950 m long and a maximum of 300 m wide, dams these two brooks and, with a permanent storage volume of 385,000 m³, is a dam under water law . The maximum storage volume is 640,000 m³, the maximum storage height 1.30 m. The lake has a catchment area of 100 km².
The name Obersee indicates that another lake was planned below the Obersee. The original concept envisaged the Obersee as a natural sedimentation basin for sediments from the tributaries in front of the actual flood protection basin. This lower, larger lake - the Untersee - should then be available for recreational use and water sports and planned and built at a later date. At the moment there are no concrete plans to carry out the planned expansion of the dam, because the project has been under discussion for several years for ecological, urban planning and, above all, financial reasons.
On the eastern shore of the lake, the Hamm – Minden railway crosses the Johannisbachtal via the Schildesche viaduct , which was destroyed by a 10-ton bomb in World War II . A few meters behind the viaduct is the dam, over which the water from the lake flows into the Johannisbach.
Silting up and remediation
By depositing the entrained from Johannisbach and Joelle sediments - about 4,000 cubic meters per year - threatened the Obersee becoming silt . Other problems were the considerable algae growth due to the large amount of nutrients flowing in and critical oxygen conditions in the appropriate weather conditions. In addition to desludging the lake, a bypass solution was therefore planned for the two streams, i.e. the Johannisbach and the Jölle were to be diverted north around the lake. By separating the lake from the rivers, their flow speed increases and the sand carried along is transported downstream. In addition, both streams will be passable again for aquatic animals.
In the summer of 2008, the dredging of the lake began. This was followed in February 2009 by the construction of a new creek bed for Johannisbach and Jölle on the northern shore of the lake with a large bulge at the inlet of the Jölle. A sheet pile wall with planks had to be erected for a watertight demarcation over a stretch of 1150 meters . The new stream bypasses the dam and uses another arch of the viaduct to flow through. This also made it necessary to build a second pedestrian bridge east of the viaduct. The work was completed at the end of 2009, and the new creek bed was flooded at the beginning of November. As a result of this measure, the Obersee has become around ten percent smaller.
Sedimentation in the mouth of the Johannisbach in September 2006
Silting up in August 2008, dredging has just started.
Viaduct with dam August 2008 (in front of the stream diversion). Today, water rarely flows over this wall.
Obersee with viaduct January 2009. Last winter before the northern bank of the lake was separated by the stream diversion.
January 2009. View of the northern shore of the lake with the half-timbered buildings of the restaurant.
Frozen suction dredger in February 2009
Mouth of the Jöllenbecker Mühlenbach ("Jölle", lower half of the picture) into the Johannisbach in September 2009
Obersee November 8, 2009: The diversion is in operation for a few days. As a result of the construction work, trees and bushes in the area of the former lake shore have been removed.
The Schildescher Viaduct in May 2014 from a north-westerly perspective.
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Recreational use
The Obersee is a popular local recreation area . In addition to the approximately 3.5-kilometer circular hiking trail around the lake, there are also paved paths in the park north of the lake and two children's playgrounds. A bird sanctuary is located in the area under the influence of the Jölle , and the lake itself is leased to a fishing club as fishing waters . An electrofishing revealed in early 2007 that the lake chub and rudd rare, perch , pike , bream , gudgeon and perch frequently and roach and bream occur very often. A half-timbered house (built in 1616) from Schildesche was rebuilt on the lakeshore, now a good 400 years old , in which there is now a restaurant and an attached beach bar as well as other leisure facilities (currently a mini golf course).
By separating the lake from the flowing water, the water quality slowly changes. The further use of the Obersee, for example as a recreational and / or bathing lake, has come up for discussion.
