Beneden-Leeuwen - Side channel
Beneden-Leeuwen - Side channel
Key features of the case study
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.
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
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
Monitoring before and after implementation of the project
The following project monitoring has been carried out.
| Sample year(s) | |
|---|---|
| Species composition aquatic macroinvertebrates | 1994 1995 1996 1997 |
| Adult Odonates species composition and reproduction activity | 1994 1995 1997 |
| Species composition fish | 1994 1995 |
| Age-0 fish composition | 1997 1998 |
| Age-1+ fish composition | 1997 1998 |
| Submerged macrophytes and bank vegetation | 1993 1995 |
| Macrophyte association | 1995 |
| Wading birds during autumnal migration | 1994 1997 |
Socio-economic aspects
Contact person within the organization
Extra background information
References
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
