Vén Duna - side arm reopening

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Vén Duna - side arm reopening

Factsheet: Vén Duna - side arm reopening

Country HU
River Name Danube
Site Name Vén Duna - side arm reopening
River Characterisation
    River typology
    Location (Lat Lon) 46.2124769066052, 18.9001750946045
    Altitude lowland: < 200 m
    Catchment area very large: > 10000 km2
    Geology Calcareous
    National code/
    River type name
    Hydromorphological quality elements

    Biological quality elements
    Ecosystem Services
    EU Directives
    Project size 3 km2
    Approximate costs 100 000 - 1 000 000 Euros
    Synergy none
    Status Realised
    Period of realization 1998
    Evaluation Hydromorphological and ecological changes

    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. The removal of the dam led to an increase in erosion of both the channel and the banks. The flow through the sidearm was restored. The phytoplankton showed a decrease in diversity, but the species composition was typical for the created habitat. The benthic invertebrates flourished and rheophilic species recolonized Vén-Duna. Fish species composition became more like that of the Danube which can be seen as evidence of a good connection between the sidearm and the main river channel.

    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


    Prior to restoration the phytoplankton showed clear divergence from the main channel of the Danube. After restoration however this difference in species composition disappeared. This is logical as the sidearm became directly connected to and started to show similar conditions as the main channel in terms of flow speed and chemical water composition. The phytoplankton found after restoration is typical for the channel type.

    Benthic Invertebrates

    The restoration led to a recolonization of rheophilic benthic invertebrate species. Also, the overal biodiversity increased after the measure was implemented. Prior to the measure the site was characterized by stagnant water species.


    The fish data collected on the sidearm is hard to interpreter due to changes in sampling method throughout the years of monitoring. What has been shown is that the fish species and species composition of the side arm has become more similar to that of the Danube. An increase in rheophilic species is also observed in fish. Whether the measure has led to an increase or decrease in fish is difficult to say as the baseline value collected for the fish species diversity in the Danube main channel is remarkably low. This is in contrast to species diversity indicated by local fishermen. If the number of fish in the Danube is used as an indicator for the number of fish in the sidearm then it should be concluded that the relative fish diversity has decreased since the measure was implemented. However, at the same time the absolute fish diversity has increased in both the Danube and the sidearm since the measure was implemented. The only sure conclusion is that the fish were influenced by the measure in terms of species composition (more rheophilic) and that their diversity shows a higher degree of similarity to the Danube river then it did prior to the measure.

    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


    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

    The measure effects were monitored intensively as part of the project. A baseline pre-project monitoring was done just prior to the removal of the rock dam. After the removal annual or biannual monitoring was done for hydromorphological variables (flow rate, sediment deposition, bed material composition). This was carried out by Eötvös József College (EJC), Baja. Also, fish and benthic invertebrates were monitored on an annual basis. Chemical and phytoplankton analyses were also carried out. These measurements were collected and summarized by VITUKI.

    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

    hydrological, morphological, water quality and ecological data of the Vén-Duna, River Danube and Nyéki-Holt-Duna in 1997-1998, prior to restoration. Project number: RI-2314 (language: english)

    hydrological, morphological, water quality and ecological data of the Vén-Duna and River Danube in 1998, after reopening the dam. Project number: RI-2314 (language: english)

    Hydrological, morphological water quality and ecological data of the Vén-Duna and River Danube in 1999, one year after reopening the dam. Project number: RI-2314 (language: english)

    Hydrological, morphological water quality and ecological data of the Vén-Duna and River Danube in 2000, two years after reopening the dam. Project number: RI-2314 (language: english)


    Related Measures

    Related Pressures