Hydro-ecologically based operation of run-of-river reservoirs for effective sediment management and energy production

Freshwater is indispensable and one of the most important resources for humans. With the growing human population and increasing water demand for urban, economic, and industrial development, as well as due to climate change, water scarcity has become a serious global threat worldwide. Climate change is reflected mainly in a gradual increase of temperatures, which result in longer dry periods, frequently followed by heavy rainfalls, causing increased intensity and occurrence of floods and erosion processes. The changed hydrological conditions require proper adjustments of water management practices. Construction of water reservoirs, used for hydropower generation, offers the possibility to adapt to changed hydrological conditions, especially in terms of multipurpose water use. However, hydropower plant (HPP) reservoirs disrupt the dynamics of sediment transport and may have a negative impact on the riverine environment and water organisms. At the lower Sava, Slovenia, 5 dams were built in a cascading system between 1993 and 2017. Sediment deposition in water reservoirs and the need for removal of potentially contaminated sediment from the reservoir areas is a Slovenian, as well as worldwide problem. Sediment management under changing hydrological conditions is a challenge of global proportions. Existing sediment management practices in water reservoirs worldwide are mostly unsustainable and lead to the loss of the multifunctional role of such facilities, such as loss of water availability for different uses and reduction of the riparian space, which worsen habitat conditions and self-cleaning capacity of the water body. The additional problem represents contaminated sediment. Advanced, holistic sediment management strategy, which includes all elements of the natural sedimentation cycle and environmental concerns related to potential sediment pollution from point and nonpoint sources, offers sustainable management solutions. In the proposed project, a novel, active river sediment management strategy in hydropower reservoirs of the HPPs on the lower Sava, under changing hydrological conditions, will be developed. The strategy will assure to the highest possible extent of the restoration of natural dynamics (continuity) of sediment transport, considering also the environmental status of sediments. To establish the proposed management strategy, a holistic, interdisciplinary approach, which includes a detailed analysis of hydraulic conditions in the reservoirs and associated sedimentation processes, as well as analysis and origin of pollutants trapped in the deposited sediment layers, will be applied. Sediment transport/deposition simulations using detailed bathymetry and sediment grain-size data, applying different simulation tools (2D and 3D), including also corresponding data on sediment pollution will be employed. On the basis of the gathered data on the presence of hazardous substances, and identification and quantification of the sources of pollution based on advanced mass spectrometry methods, it will be further possible to define potential measures related to the removal of sediments and the alternatives of their disposal or re-use. The developed sediment management plan for the chain of HPP on the lower Sava River will contribute to the restoration of sediment connectivity along the river course and the improvement of the river channel's ecological/habitat role. Despite the global problem of sediment management in water reservoirs, to the best of our knowledge, such holistic, innovative approach, considering all aspects of sediment cycle, characterization and identification of sources of pollution as well as modelling of hydrological and hydraulic processes supported by environmental data, has not been yet documented and implemented. The developed active sediment management strategy could be applied also to other HPP reservoirs on rivers with torrential character in Slovenia and worldwide.