Vén Duna - side arm reopening
Vén Duna - side arm reopening
Key features of the case study
The Vén-Duna sidearm is situated in the Gemenc Floodplain of the Danube river, a nature reserve area. The hydrological flow of the sidearm was severed by the construction of a rock dam. At the same time siltation of the sidearm started due to deepening of the Danube river. This lead to an up-filling which further hampered natural flow conditions. --ADD RESULTS/CONCLUSIONS--
Site description
The Vén-Duna sidearm is a water body located in the Gemenc floodplain, a nature reserve on the Danube river. The sidearm was largely blocked off by an artificial rock dam. This dam isolated the Vén-Duna sidearm during low and average flow conditions. It was build to improve navigation on the Danube. During high water levels in the Danube main channel the rock dam allowed water to flow into the sidearm through a small opening. This led to exceptionally high flow speeds near the dam. The flow speed decreased further downstream, but a well defined through-flow was still apparent. An up-filling of the side arm took place at the same time. The sidearm was characterized by periods of extremely bad water quality, caused by the hydrological isolation. Also, the flora and fauna were changing from rheophilic species towards stagnant water species. The forest surrounding the arm has undergone significant changes in the last decades due to an increase in logging and forestry industrial pressure. The hardwood oak (Quercus forests have largely been replaced by artificial plantations of Populus trees, a fast growing softwood species.
Measures selection
The selected measure taken was to remove the rock dam which was causing the (partial) hydrological isolation of the sidearm water body. This measure was selected based on analyses made by Hungarian and Dutch experts in the field. By reopening the side arm the rheophilic species are expected to recover and the biodiversity of the system as a whole is expected to increase. Also the hope was to stop the seasonal deterioration in water quality due to the lack of flow in the arm. The measure was also carried out as a means of testing the effects of it on a system so that it may serve as an example for similar future restoration projects.
Success criteria
The goal of the project was to rehabilitate the side arm for both hydromorphology as well as ecology; recovery of the rheophilic species and restoration of flow through the sidearm channel being main success criteria. However, perhaps more important was the goal to study the effects of the taken measure so that future restoration projects may benefit from this knowledge.
Ecological response
Macrophytes and phytobenthos
Benthic Invertebrates
Fish
Hydromorphological response
Hydrological regime
- Flow conditions: The through-flow in the channel was improved by the removal of the dam. Prior to its removal the sidearm water body was stagnant during all but high flow conditions in the main channel of the Danube. After the implementation of the measure the sidearm showed clear signs of flow during much lower discharges of the main channel then before.
River continuity
Unaffected
Morphological conditions
- Sediment composition: A change in the bed sediment composition near the site of the former dam took place as a result of this measure. Upstream of the dam a coarsening of sediment was observed, while downstream the sediment became finer then it was prior to the removal of the dam.
- Sedimentation processes: Upstream of the dam the erosion processes started to play a more significant role due to the restoration of flow conditions. This resulted in a deepening of the riverbed and erosion of the banks. The majority of the suspended bed load was transported out of the sidearm and into the main channel, leading to a net erosion in the Vén-Duna. During high flood events large amounts of sandy sediment were deposited just beyond the relatively narrow opening made by the removal of the rock dam.
Monitoring before and after implementation of the project
Socio-economic aspects
Ecosystem goods and services
Not affected.
Conflicts and synergies
At the start of the project it was unclear if the project would influence navigation on the adjacent river Danube. Measurements conducted up to three years after the implementation do not show any affect on flow direction of the Danube. Neither the inflow into the sidearm nor the backflow into the Danube seem to have significant affects on the flow direction or flow speed.
Project costs
Exact costs unknown.
Contact person within the organization
Béla Csányi, VITUKI, e-mail
Extra background information
References
Related Measures
- Remove or modify in-channel hydraulic structures
- Reconnect backwaters and wetlands
- Improve backwaters
