Buiten Ooij - Sluice operation
Buiten Ooij - Sluice operation
Key features of the case study
Site description
The area of Buiten Ooij is a floodplain area along the river Waal, a major tributary of the Rhine in the Netherlands. The area is part of a larger nature area, the Geldersche Poort. The area is situated between the summer and the winter dyke. In the area there are a number of pools that originated from historical gravel and sand mining pits. A remainder of an old river arm, the Oude Waal, is located in the south-west corner of the area. This old arm is permanently filled with water, but not connected to the current river. The summer dyke around the area was constructed in multiple phases, leading to differences in height in comparison to the water level. It is also outfitted with a sluice which was historically used to let in water during peak discharges. The dyke is higher on north-eastern part of the area. Flooding of the area is rare; an active water flow through the area is even rarer due to the height differences in the summer dyke. This leads to an unnatural situation in the floodplain where flooding occurs only sporadically and incompletely.
Measures selection
To improve flooding of the area and stimulate the formation of floodplain marshland an experiment was started with letting water into the area through the sluice during winter and closing the sluice during summer to retain the water in the area. This was both a cheap and easy way to improve the inundation of the area. This measure by no means fully removes the pressures acting upon the system, but it is a first step. Plans are being made to further improve the area by means of floodplain lowering and active side channel constructions.
Success criteria
The project was carried out as an experiment, no clear success criteria were formulated though the goal was to create wet natural habitat.
Ecological response
Macrophytes and phytobenthos
In 1999 the coverage of macrophytes was quite high. The current situation shows a very different image with only sporadic occurrences of macrophytes in the area. This would suggest that the macrophytes have been negatively affected by the measure. Data from monitoring in 2005 shows that this conclusion is a tad too easy. The macrophytes in the area show a collapse in 2005 and a (nearly) complete absence in 2006. As this collapse occurred prior to the sluice operation experiment it seems unlikely that the project has anything to do with the difference in macrophytes observed between the present day situation and the historical data (1999). If there has been a positive effect or not is difficult to say definitively, however macrophytes have increased in the area since 2006. The area has an 'acceptable' Ecological Quality in WFD terms. This is mostly caused by the low species diversity and relatively low abundance of macrophytes.
Benthic invertebrates
The benthic invertebrates show a significantly higher abundance and species richness than they do in the main channel. This leads to the conclusion that the wet floodplain is a suitable habitat for benthic invertebrates. These invertebrates in turn serve as food for both fish and water birds, leading to the expectation that in due time those groups will also be positively affected.
Fish
The (near-)stagnant water bodies of the area form excellent breeding and mating grounds for fish. Especially the areas where macrophytes are present show a good deal of limnophilic species. The presence of spawning and rearing habitat is vital to the structural presence of fish species. The first indicators are there that the measure is positively affecting the fish population in the river Waal.
Hydromorphological response
There has not been any monitoring of hydromorphology. It should be noted that it is known that the area is affected by sedimentation of the floodplain and water bodies. This sedimentation takes place at a rate which is fairly standard for Waal floodplains (1-2 cm/year). This sedimentation may become a problem in time. To avoid this plans are being developed to lower the floodplain by digging off the top soil.
Monitoring before and after implementation of the project
Socio-economic aspects
Contact person within the organization
Extra background information
References
Related Measures
- Lower river banks or floodplains to enlarge inundation and flooding
- Improve water retention
- Increase flood frequency and duration in riparian zones or floodplains
- Adjust land use to develop riparian vegetation
- Retain floodwater
- Improve backwaters
