Revision as of 12:59, 16 June 2010

Bakenhof - Dyke relocation

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Key features of the case study

Site description

The Nederrijn (or Lower Rhine) is a branch of the Dutch part of the Rhine river. It starts at the city of Arnhem, near the site of this project, and continues up to the city of Wijk bij Duursteden where the river continues as the river Lek. Along the Nederrijn on the outskirts of Arnhem a number of floodplains are situated on the southern bank. On the northern bank there is an entrance to a harbour. The southern floodplains were largely cut off from river dynamics due to a laterally placed dyke. Also, river banks were fixated and groins were placed in the channel to keep it open for navigational purposes.

Measures selection

To increase the available natural area and to promote natural processes the dyke on the southern side of the river was relocated 200 meters inland over a reach of 1,4 kilometres. This meant that a number of previously agricultural lands were claimed for natural development. Also, a side channel was dug to increase the lateral connection to the floodplains. This side channel was created by creating two openings in the old dyke and deepening the floodplain between the new and the old dyke. This led to an artificial channel without bank fixation where natural processes like erosion and sedimentation can take place.

Success criteria

No clear success criteria for the project were found.

Ecological response

Macrophytes and phytobenthos

Macrophytes have been monitored in the side channel for the project 'Rijn in Beeld'. The conclusions of this study where that the side channel is exceptionally poor in macrophytes compared to other study areas (e.g. Gameren & Vreugderijkerwaard). Only one macrophyte species was encountered, and only sporadically. Possible explanations for this lack of macrophytes were:

• Nearly drying up of the side channel during summer months
• Low flow in the side channel
• Heavy silt deposition in the side channel, possibly related to the low flow conditions
• Lack of nearby source populations for macrophyte colonization

Benthic invertebrates

Not monitored

Fish

Not monitored

Other biota

Flora

Vegetation monitoring shows a great deal of changes throughout the time. Clear is that the creation of the side channel and the relocation of the dyke have led to a greater diversity in habitats, and with that a greater diversity in species. At the same time the species vary greatly throughout the time. In monitoring in 2003 and 2009 a total of 342 species were found, however only 213 were found in both monitoring periods. Also, in 2009 the total amount of species was lower then in 2003. Closer examination shows that pioneer species have disappeared, indicating succession taking place in the area. In due time this may lead to a further decrease in vegetational diversity. To set back succession large grazers have been deployed in part of the project area.

Hydromorphological response

Hydromorphology was not monitored specifically, though some information is available as it was used to explain the absence of water plants (see above).

Monitoring before and after implementation of the project

The area has only been monitored for vegetation, not for fish or macro-invertebrates. A number of datasets exist, most of which are from the KNNV, the Royal Dutch Association for Natural-history. They have had a multitude of monitoring campaigns in the Bakenhof, even prior to the dyke relocation. Data on vegetation is available for 1998, 2003-2005 and 2009. In 2009 the project Rijn in Beeld also monitored the Bakenhof side channel for macrophytes for Rijkswaterstaat.

Socio-economic aspects

Ecosystem goods and services

The Bakenhof project has a major secondary function as a recreation area for the nearby city of Arnhem. To facilitate this while maintaining its function as ecological corridor a number of measures were taken. This included making parts of the area inaccessible to dogs to avoid unpleasant confrontations with grazers. Also, a barbecue spot was made to give the people a chance to barbecue in nature. For the children a playground which fits into the style of the natural surroundings was constructed. The area is surrounded by fencing to keep the grazers in the area, but can be freely accessed by eight gates divided fairly evenly over the fenced area. The area is freely accessible, making it ideal for hikers.

Conflicts and synergies

The major synergy of the Bakenhof project is the coupling between nature and recreation (see above).

Project costs

Exact costs unknown

Contact person within the organization

Vereniging Vrienden Speeluiterwaard Bakenhof e-mail

Extra background information

References

Related Measures

• Set back embankments, levees or dikes
• Adjust land use to reduce nutrient, sediment input or shore erosion
• Reconnect backwaters and wetlands
• Remove bank fixation
• Construct semi-natural/articificial wetlands or aquatic habitats

Related Pressures

• Impoundment
• Channelisation / cross section alteration
• Alteration of riparian vegetation
• Alteration of instream habitat
• Embankments, levees or dikes
• Loss of vertical connectivity