The (dis)balance of sediments in the Danube River Basin
A s a lifeline for both people and nature, the Danube River has to meet a wide range of needs: it provides drinking water, is a trans-European navigational corridor, serves tourism and local recreation, provides energy and ecosystem services and is essential for the natural environment. In order to make better use of this essential lifeline, the Danube and its tributaries have been gradually altered over the years to enhance flood control, navigation and, more recently, hydropower. Point and diffuse pollution, increased agricultural use and forestry development are intensifying the aforementioned impacts. According to the ICPDR, when compared to the 19th century, less than 19% of the Danube River Basin’s former flood plains remain. The rich and unique biodiversity and river habitats along the Danube are at risk.
As a result of numerous interventions, the sediment regime of the Danube has changed drastically over the last century. The consequences of these interventions on the river and the sediment regime are currently being investigated by the EU-funded project DanubeSediment. The project will receive the first sediment budget for the entire Danube River and will improve our understanding of the process and close knowledge gaps (for more information about the project see the article “Transnational Cooperation for Sediment Management in the Danube River” in this issue of Danube Watch). Only with the help of this Danube-wide sediment analysis, can we propose measures and make recommendations to improve sediment management.
Sándor Baranya, Associate Professor at the BME Budapest, coordinates data collection and monitoring in the DanubeSediment project and says: “Our project will deliver the first comparison of historical and current sediment transport data along the whole Danube River.” A graph has been prepared, based on the monitoring data provided by the project partners, showing the development of suspended sediments along the Danube River (Figure 1). The results clearly show the effects of interventions from the Upper Danube through to the Danube Delta. Today, the Danube transports less than half of the historic amount of about 60 million tons of suspended sediments into the Black Sea. These sediments are primarily trapped in the reservoirs of the hydropower plants where sedimentation occurs due to reduced flow velocities, shear stress and transport capacities. During major flood events, these fine sediments can be remobilised, for example in the reservoirs of the Upper Danube, leading to siltation and thus an increase in damage to the flooded areas in the event of inundation.
In contrast, the remaining five free flowing sections of the Danube are experiencing a sediment deficit due to the longitudinal and lateral interruption of the sediment continuum. Furthermore, the width of the Danube is being reduced (Figure 2) and the gradient increased because of a reduction in the length of the river, which – in combination with the canalisation and disconnection from floodplains – has led to an increase in the sediment transport capacity of the river. In addition, the Danube River lacks lateral self-forming processes due to river training measures such as bank protection, resulting in correspondingly reduced morphodynamics in the non-impounded sections. As a consequence, there are various forms of riverbed degradation and a lack of instream structures in the remaining free-flowing sections. The restricted lateral erosion and lateral branching also limits the sediment input from the side and riverbanks, thereby reducing the lateral exchange of sediments (deposition and erosion).
Suspended sediments along the Danube River: past and present (source: DanubeSediment deliverable“Analysis of Sediment Data collected along the Danube”)
These combined factors are leading to an erosion of the riverbed of up to several centimetres per year in the free-flowing sections. As a result, man-made structures such as bank protection measures are undermined, putting their stability at risk. Additional consequences are that groundwater levels are lowered, side arms become disconnected, instream structures are lost and habitat quality deteriorates, affecting the ecological status of the river and valuable floodplains. This affects coastal morphology, leading to increased coastal erosion of up to 20 to 25 m per year. In the Danube Delta, the lack of sediments combined with the cutting-off of meanders, regulation and dredging are also leading to erosion in the main branches of the Danube. Overall, the data shows a strong disbalance of the sediment regime, resulting in a severely disturbed system throughout the whole Danube River Basin.
Katarína Holubová from the Slovakian Water Research Institute, who coordinates the assessment of the sediment balance, explains: “DanubeSediment provides an agreed sediment balance for the whole Danube River that is based on sediment transport data, the comparison of the historical and the present state of morphology and dredging data.” Setting up a sediment balance helps to understand the functional behaviour of the river system at various levels, which in turn allows for the assessment of the consequences of human intervention. This is important because interruption of the sediment balance results in an increasing difference between surplus and deficit of sediment, which leads to increased reservoir sedimentation, risk of flooding, problems for navigation at bottlenecks and deterioration of the ecological status of the river. In order to develop sustainable sediment management for the Danube River, we need an improved understanding of sediment processes. “DanubeSediment not only analyses the processes and status, but also puts together measures that are capable of improving the sediment balance and related river management” says Florin Vartolomei from the National Administration "Romanian Waters", who is responsible for coordinating the collection of measures. Sustainable sediment management and the implementation of good practice measures in the Danube Basin can improve navigation conditions, reduce flood risks, enhance ecological status and sustain hydropower production. “DanubeSediment clearly shows that sediments are of serious importance for the future Danube River Basin and Flood Risk Management Plans.” says Ivan Zavadsky, Executive Secretary of the ICPDR.
Width of the Danube River: past and present (source: based on DanubeSediment deliverable “Long term-morphological development of the Danube in relation to the sediment balance” in prep.)