Restoration measures of LIFE projects in the Dabube river branch system
The territory of the Danube Floodplains between the villages of Dobrohošť and Sap is part of the so-called Danube inland delta – a unique and rare system of river branches. Once upon a time they meandered freely in the alluvium of Danube gravel and sand brought here by the river current. An important reservoir of drinking water has been created in the huge alluvial cone, which can be 80 to 200 meters thick. At the same time, this space was one of the most important areas for fish spawning and the reproduction of other animals bound to the water habitats of the Danube.
Over the course of the last two centuries, people gradually changed the Danube branch system by building protective flood dams, regulating the Danube bed for navigation conditions, and later also using the river for energy purposes in connection with the construction of an artificial navigation channel (Gabčíkovo waterworks). The originally extensive and, above all, regularly and with varying intensity, flooded area of the Danube branches found itself cut off from the main river bed as a result of the modifications. That’s when a long-term process began, the consequences of which were not so obvious in the first years, but after decades of negative impacts, the situation is really critical. Due to the lack of water and low flow speed, the branches have been clogged with fine sediments for decades, they are overgrown with vegetation, their capacity is decreasing and the ecosystem of the branches is in an advanced stage of degradation. Some arms are disappearing due to the disturbed water regime. Where there is a lack of water flow, deposits of mud form and decomposition of organic residues occurs. Stagnant water that overheats in the summer causes an increase in putrefactive processes and a subsequent lack of oxygen in the water. The mud that replaced the beautiful gravel bottom is unsuitable for spawning of rheophilic fish species and equally unsuitable for many foraging species.
After diverting the main stream of the Danube into the supply channel of the hydroelectric power plant, only a low regulated flow of water, averaging 400 m3/s, reaches the original old bed. For comparison, before the construction of the Gabčíkovo waterworks, it was around 2,000 m3/s.
Subsidy of water to the cut-off part of the branch system on the Slovak side of the Danube is carried out only through the inflow facility near the village of Dobrohošť. In the long term, a constant amount of water reaches the branch system – around 20 m3/s in the winter (non-vegetation) season and 30 m3/s in the summer (vegetation) season. Such a stable flow, and thus steady levels, are not suitable for dynamic river processes, wetlands or floodplain forest vegetation, originally tied to periodic floods.
So that the entire cut off river branch system does not dry out or did not flow into the old bed of the Danube, which due to the lack of water is 3-4 m lower than the branch system itself, a system of weir lines (A – J) was built in the entire area. One line usually crosses several branches, so a total of up to 30 barriers are located in the branch system. On each of them there is one or more objects for transferring water – frame culverts, pipes or overflow fords.
The purpose of the dams is to retain water in the locality and increase the levels in the cascade of the flooded sections. With the help of the dam system, water retention in the territory and level heights comparable to the situation before the damming of the Danube were ensured, but without the natural fluctuation of levels and flow dynamics, which are necessary for the river ecosystem and therefore also for the branch system. Stagnant or only very slowly flowing water does not create the conditions for natural river processes – there is no fluctuation of flows and levels and cyclic flooding and drying of shallow sites within river banks and extensive surrounding wetlands. The flow conditions in the branch system are not good and require correction. All the dams are also an obstacle for migrating fish, so far the only exception is dam C2, which was modified in 2021 to allow upstream fish migration. The continuous flow of the entire system is necessary for regular flushing of the branches during floods and slowing down their clogging with sediments. In addition, the system of branches is cut off from the main channel due to the subsidence of the bottom of the Danube and the gradual siltation of the flood, when the water level in the old channel can be as much as 3.5 meters lower. This disrupts the interaction between the Danube and its branch system.
During the last years, within the framework of LIFE project activities, the conservation association BROZ, in cooperation with strategic partners, strives to restore the water dynamics of the river branches of the inland Danube delta and to overcome migration barriers for fish. A pilot measure within the LIFE BeeSandFish project was the removal of the non-functioning dam C2 on line C, its reconstruction and the simultaneous construction of a fish channel, which fulfills its function at low flows, but also during high flows in the period of simulated floods. The dam crosses the Šulianske arm, which was extremely rich in the past. The water flow in this part of the arm was minimal before the restoration and fish migration was almost impossible. In the branch, the riverbed was continuously clogged with fine sediments, the inlet area was significantly narrowed compared to its original width. As part of the modifications, the dam was reconstructed and parts of the side arm above and below it were restored. The inlet area was expanded to its original size with the preservation of the central island, making it easier for water to flow into the arm. In order to achieve a significantly higher flow rate in the arm, the original narrow pipe culverts were replaced by four frame culverts with a sufficient diameter. In the extension of the culverts, a fish channel was built at a suitable slope, the parameters of which were set so as to enable the migration of aquatic animals during almost the entire year under the expected flow rates.
Subsequently, as part of the LIFE Danube Floodplains project, comprehensive restoration measures were proposed for the entire branch system of the Danube. Based on field measurements and the results of hydrodynamic and physical modeling, experts from the Water Research Institute prepared an expert study aimed at restoring the flow and longitudinal continuity of the entire Danube branch system in the mentioned section. BROZ and Water Management Construction, state enterprise implement the proposed measures.
On the basis of the developed technical documentation for the restoration of the water regime of the lower Vojčianske side arm and after obtaining all the necessary approvals and building permits, construction works began in August 2023. The goal is to improve the original unsatisfactory condition such as lack of water, weak flow or completely dry sections and to replace insufficient pipe culverts with more capacious frame culverts with higher luminosity and better patency. The reconstruction of two dams, the deepening of two branches of the arm and the widening of narrow sections will improve the water regime on a length of 3.7 km. The plan was also closely communicated with the representatives of the Slovak Fishermen’s Association, who for a long time registered the location as problematic due to the lack of water and had to carry out rescue fishing here several times.
In October 2023, restoration works began also on the E3 weir of the E line. The problem here was the lack of water dynamics together with an unsuitable concrete slip originally built for recreational vessels, which did not meet the requirements for either recreational navigation or fish migration. It was therefore necessary to rebuild the dam and add a functional fish pass. During the simulated floods, there used to be more water at this location, but by flowing over the dam and the steep slide, the flow reached unsuitable speeds for fish migration. In addition, the adjustments will also serve the recreational sports. In the preparatory phase, a request arose from the boaters association to create safe places for embarking and disembarking vessels at the dam level.
In the fall of 2023 under the management of Water Management Construction, restoration works were also started on the D2a dam of line D, the aim of which is to ensure the restoration of the flow of the smaller arm connecting the Šulianske side arm with the Bodícke side arm. There will be a significant water drop under the constructed culvert, so the mitigation of the impact on fish migration and the stability of the bottom of the banks at this place will be ensured by the construction of special technical objects. In this case too, the demands of boaters were taken into account, larger culverts were installed and places for handling vessels were built.
In addition, the Water Management Construction will also improve the migratory conditions for fish at the old dam Bst., where the crown of the dam will be lowered by 1.3 m on almost 30 meters of its length.
Another location that BROZ is planning to bring back to life is Kráľovská lúka. It is an originally flowing, but currently completely dry arm, which, together with its two tributary branches, will be restored with a total length of 4,470 meters. The main branch of Kráľovská lúka has the possibility to be supplied with water from the Bakanská arm system. Currently, however, the problem is that just a few meters below the area of the inlet, the branch is filled with soil, and a little below it is crossed by a forest road with a clogging insufficient pipe culvert. These obstacles prevent the free flow of water. Pipe culverts in the area of the inlet as well as the outlet will be replaced by more capacious frame culverts. According to an agreement with the State Forests of the Slovak Republic managing this location, the frame culverts will be equipped with manipulable sluice gates to be able to regulate the water flow if necessary. In addition, the channel of the arm will be deepened in several places, especially in places where accumulated soil or alluvial material creates obstacles in the flow. Larger stones will be placed locally in selected places in the modified bed to reduce flow velocities according to the requirements of fish migration.
In addition to bridging the barriers within the branch system, BROZ, together with experts from the field of water management and nature protection, has been working for a long time within the LIFE Danube Birds Conservation, LIFE NATURA 2000 BA and LIFE Microtus II projects to water the relics of smaller branches and depressions, build supply channels to wetlands or the creation of migration biocorridors (Dunajské Kriviny, Bačianske ramená, Ostrov orliaka morského, Istragov, Dedinský ostrov,…), which, together with the currently ongoing measures, amounts to 18,518 meters of positively affected water courses and 76.55 hectares of restored wetlands in the area of the left branch system of Danube between the villages of Dobrohošť and Sap.
The whole concept for improving the water regime and above all for improving fish migration and the return of dynamics to the area is only at the beginning. The challenge is great, the degradation of the entire area has been going on for decades and the decline in the number of fish in the Danube and its branches is dramatic (decrease in numbers by 70% in the last 50 years). So there is slot to do. The restoration of other locations of the branch system will be continued by BROZ together with partners, for example, within the LIFE Living Rivers project, which is aimed at the implementation of the Water Plan of Slovakia in selected basins.
Moreover, in addition to the technical modifications on the Danube river branches, artificial floods are necessary for the inland delta area, which significantly help to improve the area’s water regime. The aim of the artificial flood is to subsidize the area, which has been significantly altered by man, the originally regularly flooded Danube landscape, with an amount of water approaching the natural flow at least for a few days a year (during the spring and summer floods). Over a period of more than two decades, increased flows were not properly implemented. In the first step, it was therefore necessary to simulate floods or to literally revive and start the increased flows, which was achieved thanks to the communication of conservationists and water managers. With the increase of the flow in the period of artificial floods to 90 m3/s, small connecting arms are also flooded, thanks to which terrain depressions and the remains of the branches are flooded. In August 2023, after a long effort, it was possible to test an increased flow of 120 m3/s, which flowed into the branch system for less than four days.
We owe the Danube landscape regular simulated floods. It is an injection after a long period during which this territory suffered from drought. After the temporary flooding of floodplain forests and the filling of shoulders and wetlands with water, suitable conditions are created for the reproduction of amphibians and the spawning of fish. Increased water flow during simulated floods should be a societal concern for the protection of biodiversity, climate and water resources.
The water regime of the branch system of the Danube is gradually being restored thanks to the cooperation of conservationists, water managers, representatives of urban districts and managers of the surrounding lands. The restoration measures are part of the LIFE projects, financed from the resources of the European Commission and the Ministry of the Environment of the Slovak Republic and taking place at locations within the NATURA 2000 network of protected areas.