Sediment Balance Alteration wird Significant Water Management Issue SWMI an der Donau

Am 10.12. wurde im Ordinary Meeting der Donauschutzkommission IKSD von Prof. Habersack das Ergebnis des EU Projektes DanubeSediment vorgestellt und beschlossen, dass künftig Sedimente erstmals ein SWMI im Donau Flussgebietsbewirtschaftungsplan 2021 sind. Die Donau weist meist entweder Sedimentüberschuss oder -defizit auf, wobei der Eintrag ins Schwarze Meer um ca. 60 % reduziert wurde, was zu Küstenerosionen von bis zu 24 m pro Jahr führt.


Danube Sediment Management - Restoration of the Sediment Balance in the Danube River

Socio-economic development has gradually altered the Danube River and its tributaries and consequently changed the sediment regime. The DanubeSediment project identified the following key drivers of these changes as flood protection, hydropower, navigation, water supply, land use and dredging. Further boundary conditions may arise, e.g. from ecology and climate change.

These drivers cause key pressures that strongly impact the sediment regime, such as transversal structures, river training and maintenance works. Transversal structures for hydropower use and water supply, like dams and weirs, interrupt the sediment continuity to a large extent. Bank protection measures and cut-off side channels as well as flood protection dykes hinder the lateral exchange of sediments.

Especially in the Upper and Middle Danube, large-scale engineering transformed the formerly complex river morphology to a uniform channel over large stretches. The river length was reduced by 134 river kilometres (rkm), which means the Upper Danube was shortened by 11% and the Middle Danube by 4%. The average width of the river was reduced by 39% in the Upper and by 12% in the Middle Danube. The results of the project DanubeSediment show, that the lateral restrictions due to river training are less severe in the case of the Lower Danube River. Here, the length was marginally reduced by around 1% and the average width by 4%.

Consequently, the sediment regime in the Danube River Basin has severely changed: free-flowing sections are prone to erosion due to higher transport capacities and a lack of sediment continuity, while the reduced energy slope in the impoundments leads to sedimentation. In total, about 733 rkm (29%) of the Danube River are dominated by erosion and 857 rkm (34%) by sedimentation. In the Lower Danube, 670 rkm (27%) show an erosional trend, but a lack of data hinders a detailed analysis, for example local relative sedimentation exists in stretches showing general riverbed erosion. Thus, about 56% of the river length, including reaches without sufficient data for a detailed analysis, are facing erosional tendencies. Along 241 rkm (10%) of the Danube River, a dynamic balance prevails, or no significant changes occur.

The interruption of river continuity prevents bedload transport, which leads to a lack of those sediments that shape the riverbed. Additionally, in some river stretches, the dredging amounts exceeded the sediment supply from upstream. The results of the DanubeSediment project clearly show the effects of sediment alterations from the Upper Danube through to the Danube Delta. The total suspended sediment input to the Danube Delta and the Black Sea decreased by more than 60%, from former amounts of about 60 Mt/yr and 40 Mt/yr to approximately 20 Mt/yr and 15 Mt/yr nowadays.

The data collected within the project DanubeSediment highlight, that at the moment the data base is too incomplete to be able to set up a sediment balance for the whole Danube River. Thus, the project recommends establishing a harmonized transnational sediment quantity monitoring network and the setting-up new monitoring stations. The most important sediment monitoring elements are: suspended sediments, bedload, bathymetry data, bed material, dredging and feeding and floodplain deposition.

Nevertheless, the numbers above show that the sediment balance is disturbed and they underline the need for action. Therefore, sediment management in the Danube River Basin should aim to achieve a balanced sediment regime with a dynamic equilibrium between sedimentation and erosion, providing type-specific natural bed forms and bed material.

The DanubeSediment project concluded that sediments are a Significant Water Management Issue (SWMI). According to the resolution of the ICPDR Heads of Delegations, the sediment balance alteration has been identified as a new sub-item under the existing SWMI “Hydromorphological alterations” in the 3rd Danube River Basin Management Plan 2021. Additionally, sediments, respectively sediment management, should be an integral part of the National River Basin and the Flood Risk Management Plans.

The Danube Sediment Management Guidance (DSMG) defines the baseline for the future sediment management in the Danube Basin and major tributaries. It aims to improve the sediment continuity and to reduce the gap between surplus and deficit of sediments leading to a sustainable use and protection of the Danube River. The DSMG as a strategic document seeks to improve awareness on challenges related to sediment quantity. It suggests measures that can be implemented to reduce sediment-related problems in the Danube River Basin. Improving the sediment balance can benefit both humans and nature alike, for example by improving the ecological status and decreasing flood risk. This document provides a strategy for better sediment management and thereby directly contributes to transnational water management and flood risk prevention.

The Sediment Manual for Stakeholders (SMS) offers assistance for sediment related actions in the Danube River Basin and future programmes of measures. In order to mitigate the impacts a collection of good practice examples of sediment measures is available to support targeting measures to improve the sediment balance and continuity. The sediment management measures in the SMS are described according to different spatial scales and in a harmonized way in form of fact sheets for the different stakeholders, hydropower, navigation, flood risk management as well as river basin management and ecology.

DanubeSediment was co-funded by the European Union funds ERDF and IPA in the frame of the Danube Transnational Programme (Project reference number: DTP-1-1-195-2.1). The overall budget is: 3 558 581.62 Euros, whereby the ERDF contributes 2 827 421.16 Euros and the IPA contributes 197 373.19 Euros.