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Beneden-Leeuwen - Side channel

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Key features of the case study

The Beneden-Leeuwen project illustrates how flood defence through the strengthening of dikes with clay from the site can be combined with nature development. A large variety of habitats was created in this project by connecting old side arms, clay and sand pits. The effects of the morphology are apparent, though there are signs that the dynamics are relatively low in the side channel (no sedimentation, no specific benthic invertebrate species). The project has a clear positive effect on fish, mostly by providing nurturing habitats. Benthic invertebrates seem to be less affected, and macrophytes are scarce. The later is likely caused by the relatively steep banks of the side channel which is less suited for macrophytes.

Site description

Beneden-Leeuwen is a site situated along the river Waal, a tributary of the Rhine in the Netherlands. It is characterized by a number of old sand and clay excavation pits and a cut of meander which was part of the Waal river prior to canalization. The floodplain was largely cut off from the main channel due to dikes situated on the banks of the river. The banks are fortified and groynes are in place in the channel. This has led to a decrease in hydromorphological processes and a subsequent decrease in ecological value of the area.

Measures selection

In an attempt to restore the ecological potential of the area as well as fortifying the dikes a deep sand and a clay excavation pit were connected. During high water this clay pit was connected to a former river bend. In years to follow a number of clay pits were dug and three secondary branches were connected, leading to a side channel. This side channel was connected to the main river, creating a side channel which flows with the river during 95% of the time. During low water levels the flow in the channel is reduced by a sill between the sand pit and the channel. The clay that was dug up from the clay pits was used to fortify dikes in the area. These dikes were not yet up to the set national standard. This measure made it possible to both fortify the dikes and at the same time facilitate ecological development of the area.

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

Macrophyte-zonation due to slow rising slopes is absent in this side channel due the steep banks of the side channel. This creates a relatively uniform macrophyte community with relatively low diversity and abundance of species. No rare species were found.

Benthic Invertebrates

Benthic invertebrates showed a comparable total number and abundance of taxa throughout the monitoring period. Some rheophilic taxa were found, but permanent settlement of rare species proved problematic. The flowing parts of the channel showed an increase in rheophilic taxa, but held no unique species. The occurrence of the rheophilic species seems to be largely related to the available substrate, which is more abundant at higher water levels. Also, a lot of exotic species were found in the area. Due to the large number of habitat (oxbow lake, sand/clay pits, flowing channel) a good number of benthic invertebrates were able to settle in the area. The absence of certain species of benthic invertebrates indicates that there is a shortage of dynamics in the channel. Species composition is largely similar to that of the groynefields of the main channel, indicating a limited effect of the side channel on bentic invertebrates.

Fish

Age-1+ eurytopic fish species were dominant in abundance throughout the side channel. Rheophilic fish were rare and occurred only in the flowing parts of the side channel, and only in relatively low abundance. The total fish abundance was highest in the clay pits. The site shows a strong occurrence of eurytopic fish species, which seems to increase throughout the years. The Age-0 fish showed increases in abundance of both eurytopic and rheophilic species. This indicates that the channel is a suitable nursery habitat.

Other biota

Dry land Flora

The dry land flora flourishes due to the large variety of habitats. Due to management (grazing) and the dynamics of flooding there is a diversity in succession in the area which gives a great diversity of plant species from different succession stages.

Wading birds

The number of wading birds is relatively low for both the clay pits as the flowing channel. Though some species visit, their presence is not very proclaimed. This is likely caused by the steep banks which are not an ideal habitat for most wading birds.

Hydromorphological response

Hydromorphological monitoring showed an improvement towards a natural situation with greater floodplain inundation and a flowing side channel. The lateral connectivity was improved by the measure. Also, the natural processes of erosion and sedimentation can take place in the channel, though no sedimentation has been observed. The slopes of the banks are rather steep which hampers the development of certain characteristics of the side channel. Large parts of the site are stagnant during shorter or longer parts of the year.

Monitoring before and after implementation of the project

The following project monitoring has been carried out.

Data was collected by RIZA, Klink ecological advisory and FREE. It should be noted that nearly all monitoring was carried out as the first stages of the project were already underway.

Socio-economic aspects

Ecosystem goods and services

Clay and sand mining.

Conflicts and synergies

The clay which was mined from the clay pits was used for strengthening the dikes of the site which were not up to safety standards. The main channel has not been negatively affected by the side 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 Beneden-Leeuwen and the measure type are presented in this report. _{(language: dutch)}

References

• Peer Reviewed Paper: SIMONS J.H.E.J, BAKKER C., SCHROPP M.H.I., JANS L.H, KOK F.R. & GRIFT R.E.; 2001. Man-made secondary channels along the River Rhine (The Netherlands); results of post-project monitoring. Regul. Rivers: Res. Mgmt. 17 pp. 473–491 _{(language: english; file: abstract)}
• Klink A. Mulder J., Wilhelm M., Jansen M.; 1994. De Leeuwense Waard in 1994. Opname van de aquatische makro-evertebraten voor de aanleg van een permanent meestromende nevengeul. Hydrobiologisch Adviesburo Klink bv Wageningen Rapporten en Mededelingen 55 (20 november 1994)In opdracht van het RIZA en Rijkswaterstaat Direktie Gelderland, 12 pages _{(language: dutch)}
• Klink A.; 1994. De Leeuwense Waard in 2005. Monitoring aquatische macrofauna in de Kaliwaal en Leeuwense Waard (2005). Hydrobiologisch Adviesburo Klink Rapporten en Mededelingen nr. 87. Project 161 December 2005 In opdracht van de Delgromij, Elst (Gld), 22 pages _{(language: dutch)}
• Linnartz-Nieuwdorp E.; 2009. Leeuwense waard, FLORA EN FAUNA, Inventarisatie 2008. Bureau Apis In opdracht van FREE, 22 pages _{(language: dutch)}

Related Measures

• Lower river banks or floodplains to enlarge inundation and flooding
• Increase flood frequency and duration in riparian zones or floodplains
• Reconnect backwaters and wetlands
• Construct semi-natural/articificial wetlands or aquatic habitats

Related Pressures

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