The EU Interreg Danube Transnational Programme has published its data analysis for the future sediment management plan for the river. The programme investigates how to tackle adverse changes in the sediment load caused by riverbed straightening and hydropower dams and dykes, which contribute to flood risks and reduce navigation and hydropower production. Those stressors also contribute to the loss of biodiversity within the Danube basin.
“In the Upper and Middle Danube, large-scale engineering transformed the formerly complex river morphology to a uniform channel over large stretches. By using historic maps, the changes in the length of the Danube were calculated. The river length was reduced by 134 km, which means that the Upper Danube is now 11 % and the Middle Danube is 4 % shorter. In addition, the average river width was reduced by about 39 % in the Upper and by about 12% in the Middle Danube. In the Lower Danube River, the length was marginally reduced by around 1% and the average width by 4%,” the report states. These comparisons are based on charts and maps dating back to the beginning of the 19th century. However, they depend on availability of data from the different countries, a project representative told DPC.
“The total suspended sediment input to the Danube Delta and the Black Sea has decreased by more than 60%. Former amounts ranged from about 40-60 tonnes per year, whilst nowadays between 15-20 tonnes per year enter the delta and the sea,” it is further stated in the report.
See the below interactive map, which was produced as part of the project, for current and historical flow of the river as well as sediment monitoring stations.
For the analysis, the project team collected and analysed sediment transport data along the river from the Black Forest in Germany to the Black Sea.
The collected data included information on morphological changes, the longitudinal profile, dredging, sediment variations, and water levels. This data was referenced against historical data to validate the changes.
Dredging data was considered important. “Overdredging for commercial purposes has often caused riverbed degradation leading to a fall in the surface and ground water levels in certain stretches of the Danube River. Sediment feeding has been performed downstream of the hydropower plants in order to reduce the impact of riverbed degradation, only in several stretches of the Danube in Germany and Austria,” it is stated in the report.
Sediment dredging along the Danube River has been performed mainly for water management, such as river training works, navigation and flood protection, construction of hydropower plants, and for commercial purposes, such as the sale of gravel and sand for construction.
“The most complete data were collected for the period between 1991–2016. Some of the partner countries provided fairly detailed data on dredging, such as Germany, Austria, Slovakia, [and] Hungary, including the annual volume, locality, and purpose of dredging with smaller or no data gaps. By contrast, some countries provided only the total volume of sediments dredged in longer river stretches in selected years,” the report authors stated.
One of the main conclusions was therefore that a unified approach to collecting data was needed to simplify future monitoring.
A manual of the morphological monitoring based on data collection in this report, more detailed analysis, and available scientific knowledge are available in the report titled Long-term morphological development of the Danube in relation to the sediment balance.