Vreugderijkerwaard - Side channel
Vreugderijkerwaard - Side channel
Key features of the case study
Site description
The Vreugderijkerwaard is a floodplain situated on the river IJssel, a tributary of the Rhine in the Netherlands. The area is characterized by a number of river dunes, which are a rare occurrence for the Netherlands. Since 1982 the area has been subjected to extensive grazing to preserve and develop the rivePart of the area, namely the river dune part, has been the property of a nature conservation organisation since 1962. This has led to a variety of rare plant species occurring in the area. The IJssel river was artificially connected to the Rhine in roman times, leading to the Rhine being the main factor of the IJssel's flow. The river was largely fixated through the use of groynes. Part of the natural meanders were removed through channelization measures.
Measures selection
Through the Vreugderijkerwaard a side channel was dug. This was done with for the Room for the River project in which flood protection,by increase in water retention area, and ecological recovery are linked. The project was co-funded by the EU-IRMA project. The measure of a flowing side channel was chosen to increase dynamic river processes in the otherwise fixated IJssel system. The side channel is artificially limited in its flow by divers. This measure was taken to avoid sedimentation of the main channel. The side channel has a flow-through of 1.5% of the average flow-through of the main channel. The flow speed of the side channel is lower then that of the main channel. This leads to sedimentation of sand and clay. To avoid the formation of islands in the channel and clogging of the channel a sand trap was installed.
Success criteria
In the project plan four main goals were formulated for the project:
- Maintaining the unique river dunes in the area
- Increasing diversity
- Increasing river dynamics and promoting communities of organisms that benefit from higher dynamics
- Attention for relations and connections to tributaries of the IJssel.
Another (secondary) goal was to keep recreational use of the area intact.
Ecological response
Macrophytes and phytobenthos
Macrophytes have shown a strong improvement over the last few years in both abundance and species diversity. In 2006 only a few scarce plants were present while in 2008 the channel was filled with them at some places, especially near the diver at the inlet of the channel. The fast colonization of the channel is likely caused by the presence of source populations. Also, the side channel has a continues low flow regime which is a suitable habitat for the macrophytes. Also, the river has a relatively low water fluctuation. All in all macrophytes have been positively affected by the measure.
Fish
No rare fish species were encountered prior to the project implementation. After the implementation a limited sampling of fish was undertaken. A number of fish of a rare species type were encountered. It is speculated, though unproven, that a larger number of rheophilic species use the side channel as habitat.
Other biota
Dry land flora
The dry land flora of the area are very diverse and conform the expected ecotype.
Hydromorphological response
Hydrological regime
The regime is affected in such a way that water is abstracted from the main channel into the side channel. To avoid sedimentation in the main channel the maximum flow abstraction of the side channel has been limited to 1.5% of the average discharge of the main channel.
River continuity
Unaffected.
Morphological conditions
The measure has led to an increase in river dynamics. Due to the sand trap at the entrance of the side channel natural sedimentation processes are hampered though. This was done to avoid unwanted sediment accumulation in the side channel. The effect is that sand banks and islands do not form in the side channel. The banks of the side channel are unprotected and can freely develop. The banks of the IJssel river on the other hand are almost exclusively covered by bank protection. This makes it impossible for sandy river banks to develop and due to the lack of bank erosion there is not enough sediment deposition to sustain the river dunes in the long run.
Monitoring before and after implementation of the project
Monitoring of the Vreugderijkerwaard has been carried out in a relatively unstructured way. A number of datasets concerning flora and fauna development exist, but no structured monitoring program was set up for the project. In 2007 a report by Bureau Drift tried to collect data on the natural development in the area. An effort was also made to find and analyse historical data. A number of datasets were found reaching back as far as the 1930s. Most of the research carried out focuses on the river dune area and its rare species. Data on the floristic development of the side channel was collected by FLORON and KNNV Zwolle between 2003 and 2006.
Socio-economic aspects
Ecosystem goods and services
The area serves as a recreation area with hiking paths and a bird watching cabin. The area is not freely accessible except for the designated paths, this to minimize human disturbance of the area.
Conflicts and synergies
This project has a synergy between flood defense and ecological development. By giving the river more room the chances of dangerously high water levels is reduced.
Project costs
Exact costs unknown. Estimated to be around 2.5 million euros.
Contact person within the organization
Extra background information
References
Related Measures
- Improve water retention
- Trap sediments
- Increase flood frequency and duration in riparian zones or floodplains
- Link flood reduction with ecological restoration
- Allow/increase lateral channel migration or river mobility
- Remove sediments
- Remove or modify in-channel hydraulic structures
- Lower river banks or floodplains to enlarge inundation and flooding
- Reconnect backwaters and wetlands
- Remove hard engineering structures that impede lateral connectivity
- Restore wetlands
