During heavy rainfalls, overflowing sewage water flows from the Combined Sewer Overflow (CSO) chambers and pollutes the Trnávka River in Trnava, Slovakia. This paper aims to propose water retention measures for the Trnávka River as a remediation technique for CSO-affected watercourses, which can contribute to the ‘flushing’ of the riverbed. During heavy rainfalls, the Trnávka River is polluted by solid, non-soluble materials, which produce unpleasant odors and are the subject of numerous complaints by citizens, particularly during low water levels. Three inflatable rubber weirs were designed, and their design was verified using a 1D numerical model of the Trnávka River. The simulations of the proposed measures performed in the HEC-RAS 5.0 software excluded the adverse effect of the backwater on the functioning of the CSO chambers in the city of Trnava during normal flow rates and confirmed that, even after installation of the weirs, the transition of the flood wave will pass in the riverbed, not causing the flooding of the adjacent area. The chemical–physical study of the Trnávka River confirmed our assumption that higher flow rates, which can be secured by the regulation of the proposed weirs, can contribute to the purity of the watercourse in the city of Trnava.

The Hydrologic Engineering Center River Analysis System (HEC-RAS) software package developed by the U.S. The Army Corps of Engineers is a widely used software package designed mainly for modeling steady and unsteady water flow in rivers. Although the software includes tools for implementing basic control elements, HEC-RAS users often require additional unique functions including integration with other software. This is made possible by means of the HEC-RAS Controller, an application programming interface (API) that allows manual interaction with HEC-RAS to enable automated control through user-defined code directly from other programming environments. The main goal of this study was to design a universal software architecture for linking HEC-RAS with Visual Basic using the API, which enables users to enter the unsteady flow boundary condition directly as the demand power output if the river system under analysis includes a hydropower plant. The effectiveness of the proposed approach and its applicability to real-world operational management environments were demonstrated through a case study of the Gabčíkovo hydropower plant on the Danube River in Slovakia. The integration of the HEC-RAS with Visual Basic enabled the simulation of the Danube River's water-level regime within a reasonable time and with adequate accuracy, allowing for the analysis of the impact of daily flow regulation on navigation conditions downstream of the hydropower plant.

The hydropower plant together with its reservoir makes it possible to modify the natural flow regime. These changes can affect sediment transport dynamics and cause morpho-logical changes in the river. If the river is also used as a waterway, the operational scenar-io of the hydropower plant can have a significant impact on sediment deposition, thereby reducing its navigable depths and increasing the risk of vessel–riverbed collisions. In this study, a 2D hydrodynamic model of the Danube River downstream of the Gabčíkovo Hy-dropower Plant (GHP) in Slovakia was developed to evaluate the influence of operational scenarios on maintaining the required navigable depths and to determine the most suita-ble scenario in terms of fairway maintenance costs. The operational scenario of the GHP influences the amount of sediment deposited downstream of the plant. The volume of deposition in the critical ford was approximately 50% smaller under hydropeaking than under run-of-river operation. The increase in riverbed elevation during hydropeaking was 33% to 64% lower than under run-of-river operation. The study results indicate that this reach of Danube can remain navigable for a longer period without intervention (dredging), thanks to sufficient navigable depth maintained by erosion caused by hydropeaking, compared to run-of-river operation.

The hydropower plant, together with its reservoir, makes it possible to modify the natural flow regime. These changes can affect sediment transport dynamics and cause morphological changes in the river. If the river is also used as a waterway, the operational scenario of the hydropower plant can have a significant impact on sediment deposition, thereby reducing its navigable depths and increasing the risk of vessel–riverbed collisions. In this study, a 2D hydrodynamic model of the Danube River downstream of the Gabčíkovo Hydropower Plant (GHP) in Slovakia was developed to evaluate the influence of operational scenarios on maintaining the required navigable depths and to determine the most suitable scenario in terms of fairway maintenance costs. The operational scenario of the GHP influences the amount of sediment deposited downstream of the plant. The volume of deposition in the critical ford was approximately 50% smaller under hydropeaking than under run-of-river operation. The increase in riverbed elevation during hydropeaking was 33% to 64% lower than under run-of-river operation. The study results indicate that this reach of the Danube can remain navigable for a longer period without intervention (dredging), thanks to sufficient navigable depth maintained by erosion caused by hydropeaking, compared to run-of-river operation.

Reservoir storage capacity is a fundamental parameter for design and operation. Over time, sedimentation—driven by factors such as geology, hydrology, and flow velocities—reduces usable volume, often significantly. In the context of climate change, opportunities for natural sediment flushing have become increasingly rare, while extensive dredging is often economically unsustainable. Moreover, sedimentation is closely linked to erosion processes that may endanger bank stability or compromise bridge piers. This study presents a case analysis of the Sĺňava Reservoir on the Váh River in Slovakia, a typical shallow, lowland reservoir within the Váh Cascade hydropower system. In recent years, sediment-related issues have intensified due to declining flow rates and limited options for reusing excavated material. To address these challenges, several mitigation measures were proposed, including riverbed dredging, groins, and sedimentation pits. These measures were evaluated using a two-dimensional numerical model developed in HEC-RAS (Hydrologic Engineering Center's River Analysis System) to simulate sediment transport and morphological changes. The results indicate that the most effective approach combines targeted dredging with the implementation of sedimentation pits. Additionally, the findings highlight the importance of monitoring erosion processes near bridge piers to ensure long-term structural stability.