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Opijnen - Side Channel

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Key features of the case study

The artificial side channel of Opijnen was created by opening up groynes along a longitudinal dam. This has led to a flowing side channel which has been extensively monitored for ecological response. This manner of side channel can be used to improve navigation while at the same time restoring characteristic habitat for rheophilic fish and benthic invertebrates. Macrophytes have not shown a clear positive effect in this case.

Site description

The river Waal is a major tributary of the Rhine. Near the village of Opijnen a number of groyne fields were closed off in 1984 by the construction of a longitudinal dike. This construction improves navigation conditions for ship traffic on the Waal by directing the flow more towards the center of the channel. This prevents unwanted sedimentation of the deep middle parts of the channel which are used by ships. The site is situated in the outer bend of a meander. By closing off these groyne fields the hydromorphological conditions were altered. Water flow was stopped, leading to static water bodies in the groynes. Also the sedimentation and erosion of the banks was hampered.

Measures selection

National natural and ecology policy led to measures being taken to restore part of the natural flow and dynamics in the channel that the dike had severed. This was undertaken in 1994. To do this without influencing the main channel, openings were made at two points in the dike (entrance & outlet) and between the groynes. This allowed water from the main channel to flow through. In essence a side channel was created. The side channel has a flow during 99% of the year due to the bed level and the bottom sill at the entrance of the channel. The measure was taken in hopes of artificially creating biotopes that no longer exist along the heavilly modified river Rhine.

Success criteria

The project served as a pilot to monitor and analyse the effects. Scientific research was done on the channel to identify if the channel indeed served to (re)create habitat and biotopes that had disappeared due to channelization and artificial alterations of the Rhine and its tributaries in the Netherlands. The primary success criteria in this research were an increase in characteristic wading birds, fish and macro invertebrates.

Ecological response

Macrophytes and phytobenthos

Macrophytes showed a change in assemblage structure after the measure was implemented. The overall abundance remained relatively low, specifically for the characteristic species who are only found at low densities. The general consensus for the WFD is that the BQE macrophytes is not positively or negatively affected by the measure.

Benthic Invertebrates

Benthic invertebrates showed an increase in species diversity a few years after the opening of the channel. The species diversity was roughly equivalent to the diversity found prior to opening of the channel. This can be explained by the loss of stagnant water species when the channel was opened up. A significant increase in rheophilic taxa was visible; increasing from 4% of the total amount of found taxa prior to the opening of the channel to 14-21% after the opening. Subsequent years of monitoring have led to the conclusion that the benthic invertebrates have been positively affected by the measure. Rare rheophilic species have not been found at the site.

Fish

Eurytopic fish species were dominant in abundance throughout the side channel. This was the case for both the age-0 and the age-1+ fish. The site showed a significant increase in rheophilic fish species, including a number of rare fish that are uncommon in the Dutch rivers. Limnophilic species decreased and become (nearly) absent in the years after the measure was implemented. This was in line with the expectations however, as the measure led to the destruction of stagnant water habitats. It can be stated that fish were positively influenced by the measure.

Other biota

Wading birds

The habitat needed for seasonally migrating wading birds is available in the side channel; the very shallow waters and gradually increasing banks serve as ideal habitat. Despite this the number of wading bird species observed showed high fluctuations throughout the years. This may in part be related to the breeding success of wading bird colonies throughout the years. Different species made use of the area in different months of the year, conform their migratory behaviour.

Hydromorphological response

Hydromorphology has not been monitored specifically. What can be noted is that the hydrology in terms of flow conditions (stagnant water - flowing side channel) has been altered by the measure.

Monitoring before and after implementation of the project

The following project monitoring has been carried out.

Data was collected by RIZA & Klink ecological advisory.

Socio-economic aspects

Ecosystem goods and services

Not affected.

Conflicts and synergies

The lateral dike which is part of the side channel system helps to guide water flow in the main channel in such a way that navigation is positively affected. Though this dike was build prior to the side channel project, it does serve as an example of a potential restoration technique which benefits both natural dynamics and ecology as well as navigation along the main channel.

Project costs

Exact costs unknown.

Contact person within the organization

Extra background information

• Report on the success of river restoration projects. The project of Opijnen and the measure type are presented in this report. _{(language: dutch)}

References

• Simons, J. H., Bakker, C., Schropp, M. H., Jans, L. H., Kok, F. R. and Grift, R. E. (2001), Man-made secondary channels along the River Rhine (The Netherlands); results of post-project monitoring. Regulated Rivers: Research & Management, 17: 473–491. doi: 10.1002/rrr.661] _{(language: english; file: abstract)}
• Klink A.; 2000. Inventarisatie van de macrofauna in de nevengeul bij Opijnen 2000. Hydrobiologisch Adviesburo Klink Rapporten en mededelingen nr. 67 november 2000. In opdracht van Rijkswaterstaat Directie Oost-Nederland, 24 pages _{(language: dutch)}

Related Measures

• Lower river banks or floodplains to enlarge inundation and flooding
• Remove or modify in-channel hydraulic structures
• Develop riparian forest
• Reconnect backwaters and wetlands

Related Pressures

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