Bakenhof - Dyke relocation
Bakenhof - Dyke relocation
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
Contact person within the organization
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
