ABSTRACT
The paper presents a study case of Boštanj reservoir on the Sava River in Slovenia. The reservoir for a run-of-the-river dam has been in operation since 2006. The total capacity of the reservoir is 8,000,000 m3, with 1,170,000 m3 of live storage, the maximum operational denivelation in the reservoir is 1 m. The combined rated discharge of the powerhouse is 500 m3/s, while the mean annual discharge of the Sava River is 229 m3/s. The Sava River is a sub-Alpine River with a predominantly torrential character, the discharge conditions in the river can vary substantially, e.g. 3100 m3/s during a 100-year flood. Approximately 3.3 km upstream of the Boštanj dam, in the convex on the right riverbank, a large amount of sediment has been already deposited. During a regular reservoir denivelation, in a length of approximately 400 m, the sediment deposition area is now peaking above the reservoir level. Based on available documentation the material in this part of the reservoir has been already removed in the past, while the aim of this study was to find a permanent solution which would contribute to improved sediment management.
Using the HEC-RAS hydrodynamical model, a full 2D hydraulic model of the reservoir has been constructed. In this study the entire Boštanj reservoir area was considered since local conditions at the convex are affected by global behavior of the sediment in the reservoir. As input data for the hydraulic model a 1×1 m digital terrain model was created using combined lidar and bathymetry measurements. Furthermore, the model was calibrated based on hydrometric measurements in a reference profile. The model consisted of 50,950 calculation cells and the results of the model were used to redesign the area of the reservoir to provide for the sediment continuity and consider the multipurpose use of the reservoir.
Results of the hydraulic model confirm that flow velocities within the area of the reservoir with an increased sediment deposition are low under all flow conditions, meaning the area is generally prone to sediment deposition. Even if the sediment is now completely removed, the natural processes in the river will again start to create a sediment dune in the same location. Therefore, the final solution is designed as a nature-based solution, a plateau of approximately 5900 m2 and 400 m of length, re-designed as a riparian habitat for various aquatic and riparian plants and animals. Instead of fighting the natural processes a redesign of this section of the reservoir is a more appropriate solution; the final solution has been confirmed with a hydraulic model. By raising the plateau level at the mean reservoir level and by providing proper vegetation cover we can create good conditions for the riparian habitat and provide sufficient protection against the erosive power of water.
