Latest revision as of 11:35, 17 December 2010

Bouxweerd - Artificial pool

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Key features of the case study

Bouxweerd is an area along the Meuse river. It consists of a floodplain with an artificial pool which was dug as a side-effect of the gravel mining in the area. The pool is characterized by limited water quality and botulism is a common threat to the fish during summer. To increase inundation in the area first an adjustable weir was installed. As this did not have the initial desired effect the canal flowing out of the area was later closed off completely to stop the area from drying out. In terms of biota the area is only interesting for wading birds. All other biota have developed relatively poorly, though macrophytes have increased somewhat since restoration was started.

Site description

The Bouxweerd area is situated on the floodplain of the canalized Meuse river. Most of the area has seen a good deal of mining of both clay and gravel. The gravel mining started as early as the 1940's. A part of the area was filled up with sand and debris after the gravel was extracted. A gravel pit was also left behind however; this filled up with water and serves as an artificial pool which is characteristic to the area. In the northern part of the area a small wetland forest is situated. The forest is fed by upwelling groundwater. A ditch which was used for the hydro management of the area for agricultural use is situated near this forest.

Measures selection

The large pool was dug for non-nature restorative reasons. It was a side-product of the economically profitable gravel mining. In 2003 measures were taken to ensure that the floodplain was inundated more often. A adjustable weir was installed in the ditch in 2003 to control the water flow from the wetland forest. Later on in 2008 it became apparent that the forest was still drying out and the ditch was closed off near the forest to avoid water loss. The weir was removed as it no longer served a function.

Success criteria

Not listed

Ecological response

Macrophytes and phytobenthos

A number of macrophytes were encountered in the river. In comparison to pre-project data there is an increase in macrophytes, however the overall results are disappointing. Research has shown that this is caused by the management choices in nearby grasslands which are aimed at geese. These geese graze on the macrophytes and thereby suppress both colonisation and development of a macrophyte community. The pool is devoid of macrophytes.

Benthic Invertebrates

No known data.

Fish

Fish were not monitored in the pool, however it is expected that it is not a suitable habitat for rare fish species. Some general fish are known to occur there. During summer the pool is known for fish dying due to anoxia and botulism.

Other biota

Dry land flora

The dry land flora has developed relatively poorly since the project started. The characteristic upwelling forest has shown no signs of development. This can be explained by nearby sand excavations that accidentally dug through a soil layer which led to the severing of the upwelling in the area. The large amount of grazing by birds does not help the floral development either.

Breeding birds

The area shows remarkably high numbers of breeding birds, both regular as well as a few rare species. The area, in comparison with other areas, has a high species richness and diversity. Even though the water quality of the pool is bad it still serves as a feeding ground for the breeding birds. The shallow parts of the pool are home to a large number of insects which serve as a food source for the birds.

Hydromorphological response

Hydrological regime

The flooding frequency of the pool is determined by a step in the dam that separates the pool from the river. On average the pool is inundated more then 100 days per year. This leads to sedimentation of the pool. The upwelling was severed due to sand excavation. In 2008 the ditch near the forest was closed off. The expectation is that this will stop the area from drying out and allow the re-occurrence of the upwelling.

River continuity

Unaffected

Morphological conditions

The major morphological change in the area is the sedimentation of the pool due to inundation. The banks of the river are all fixated, leading to a lack of sediment input and bank formation. Sedimentation on banks and the dikes only takes place during high water peaks.

Monitoring before and after implementation of the project

Floristic data was collected in 2007 for the Maas in Beeld project. Historical data exists in the form of two datasets by the regional authorities, one from 1988 and the other from 1998. These two datasets can serve as 0-monitoring datasets. Fish data has not been collected systematically and only scattered individual sightings are available for the area. The breeding birds have been monitored yearly from 1994 onward. Older data is available but often consists of sightings from local bird watchers. This data goes back as far as 1976. Hydromorphology has only been examined for the Maas in Beeld project in 2007.

Socio-economic aspects

Ecosystem goods and services

Recreational area; most of the area is accessible to the public for hiking and nature experience. This has been accommodated by constructing a bird watching cabin.

Conflicts and synergies

Synergies exist with gravel and clay mining, which was at the source of the formation of the area in its current state.

Project costs

Exact costs unknown

Contact person within the organization

Rijkswaterstaat Maaswerken e-mail

Extra background information

References

• Kurstjens G., Peters B. & Calle P.; 2007. Maas in Beeld; Proefproject Meers. Uit: Maas in Beeld tussenrapport 2006. Burea Drift, Kurstjens Ecologisch Advies; Berg & Dal/Beek-Ubbergen. _{(language: dutch)}

Related Measures

• Lower river banks or floodplains to enlarge inundation and flooding
• Reduce groundwater extraction
• Remove or modify in-channel hydraulic structures
• Set back embankments, levees or dikes
• Restore wetlands

Related Pressures

• Impoundment
• Channelisation / cross section alteration
• Embankments, levees or dikes
• Sedimentation and sediment input
• Sand and gravel extraction
• Loss of vertical connectivity