Case study:Opijnen- Side channel

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Location: 51° 49' 34", 5° 18' 40"
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Project overview

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Status Complete
Project web site
Themes Habitat and biodiversity, Monitoring, Water quality
Country Netherlands
Main contact forename Rijkswaterstaat
Main contact surname Rijkswaterstaat
Main contact user ID
Contact organisation Rijkswaterstaat
Contact organisation web site http://wiki.reformrivers.eu/images/d/d9/RWS WD 2008.pdf
Partner organisations
Parent multi-site project
This is a parent project
encompassing the following
projects
No
Arial photo courtesy of Rijkswaterstaat

Project summary

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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.

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.

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.

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.

Monitoring surveys and results

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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 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.

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.

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.

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.

Lessons learnt

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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.


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Catchment and subcatchment

Catchment

River basin district Rhine
River basin Millingerwaard

Subcatchment

River name Waal
Area category more than 10000 km²
Area (km2)
Maximum altitude category 100 - 200 m
Maximum altitude (m)
Dominant geology Calcareous
Ecoregion Central Plains
Dominant land cover Extensive agriculture, Urban
Waterbody ID



Other case studies in this subcatchment: Beneden-Leeuwen-Side channel, Buiten Ooij, Millingerwaard-Floodplain rehabilitation


Site

Name Opijnen
WFD water body codes
WFD (national) typology R7; intertidal
WFD water body name
Pre-project morphology Single channel, High width:depth
Reference morphology
Desired post project morphology
Heavily modified water body Yes
National/international site designation
Local/regional site designations
Protected species present No
Invasive species present No
Species of interest
Dominant hydrology Tidal, Estuary
Dominant substrate Gravel
River corridor land use Urban
Average bankfull channel width category 10 - 50 m
Average bankfull channel width (m)
Average bankfull channel depth category more than 10 m
Average bankfull channel depth (m)
Mean discharge category
Mean annual discharge (m3/s)
Average channel gradient category
Average channel gradient
Average unit stream power (W/m2)


Project background

Reach length directly affected (m) 1000 m
1 km
100,000 cm
Project started 1994/01/01
Works started 1994/01/01
Works completed 1994/01/01
Project completed
Total cost category
Total cost (k€)
Benefit to cost ratio
Funding sources

Cost for project phases

Phase cost category cost exact (k€) Lead organisation Contact forename Contact surname
Investigation and design
Stakeholder engagement and communication
Works and works supervision
Post-project management and maintenance
Monitoring



Reasons for river restoration

Mitigation of a pressure
Hydromorphology Quantity & dynamics of flow, Flow velocities, Continuity of sediment transport
Biology Fish: Species composition, Fish: Abundance, Fish: Disturbance-sensitive species, Fish: Age structure, Invertebrates, Macrophytes
Physico-chemical Nutrient concentrations
Other reasons for the project Landscape enhancement


Measures

Structural measures
Bank/bed modifications Improvement of natural flows
Floodplain / River corridor
Planform / Channel pattern Creation of side channel
Other
Non-structural measures
Management interventions
Social measures (incl. engagement)
Other


Monitoring

Hydromorphological quality elements

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative
Flow velocities Yes Yes Improvement
Quantity & dynamics of flow Yes Yes Improvement

Biological quality elements

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative
Invertebrates Yes Yes Yes No change
Fish: Species composition Yes Yes Yes No change
Fish: Disturbance-sensitive species Yes Yes Yes No change
Fish: Abundance Yes Yes Yes No change
Fish: Age structure Yes Yes Yes No change
Macrophytes Yes Yes Yes No change

Physico-chemical quality elements

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative

Any other monitoring, e.g. social, economic

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative


Monitoring documents



Additional documents and videos


Additional links and references

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Supplementary Information

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