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Vreugderijkerwaard - Side channel

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Key features of the case study

In the Vreugderijkerwaard, a floodplain located near the city of Zwolle along the IJssel river a side channel was constructed. The side channel has reintroduced river dynamics, though the flow through the channel is artificially limited. Erosion and sedimentation of the banks of the side channel is possible and occurs. Ecological response appears quite good, though it is hard to make a definitive judgement as no monitoring plan was set up at the start of the project. Despite this, the studies carried out in the area all sketch a positive image for species diversity in the area.

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:

1. Maintaining the unique river dunes in the area
2. Increasing diversity
3. Increasing river dynamics and promoting communities of organisms that benefit from higher dynamics
4. 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.

Birds

The project has led to an increase in rare or endangered bird species. The increase is mostly caused by the presence of wading birds who have an ideal habitat in the low gradient slopes of the side channel.

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. Additional data on the flora and fauna in the area is currently being collected and reported on for the 'Rijn in Beeld' project.

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

W. Tiggeloven, Dienst Landelijk Gebied, e-mail

Extra background information

References

• Herwijnen M. van, Asselt H.D. van, Oosterhuis F.H., Vermaat J.E. en Goosen H.; 2003. Succes- en faalfactoren van natuur-ontwikkeling in en langs het water. IVM rapport R-03/12, ISBN 90 5383 9054 _{(language: dutch)}
• Huiskes, H.P.J.; Navis, P.; Sykora, K.V.; Nijs, L.J. de; Melman, T.C.P.; 2007. Natuurontwikkeling onder de loep : een evaluatiemethode voor de inrichting van nieuwe natuurgebieden. Alterra, Wageningen. Alterra-rapport 1492 _{(language: dutch)}
• Maas G.J., Makaske B., Hommel P.W.F.M., Nijhof B.S.J., Wolfert, H.P.; 2007. Verstoring en successie: rivierdynamiek en stroomdalvegetaties in de uiterwaarden van de Rijntakken. Alterra, Wageningen. Alterra-rapport 759. _{(language: dutch)}
• Peters, B. & G. Kurstjens; 2007. Rivierenland in ontwikkeling; resultaten van natuurontwikkeling in het rivierengebied. Bureau Drift, Berg en Dal. Kurstjens Ecologisch Adviesbureau, Beek-Ubbergen. Studie in opdracht van het Ministerie van Landbouw, Natuur en Voedselkwaliteit. Februari 2007 _{(language: dutch)}
• Peters, B. & G. Kurstjens; 2009. Waterplanten in nevengeulen. Inventarisatie 2009. Bureau Drift, Berg en Dal. Kurstjens Ecologisch Adviesbureau, Beek-Ubbergen. Studie in het kader van project Rijn in Beeld, Oktober 2009 _{(language: dutch)}

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

Related Pressures

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