Scours creation in riverbed at the Hričov weir is a permanent problem since its construction. It is caused by the shortened stilling basin of the weir. In almost all cases of flow control at the weir the energy is not dissipated sufficiently. A 3D physical model was built in the hydraulic laboratory to investigate the measures for reduction of the scour creation. To simulate uneven loads on the downstream riverbed, a flood discharge controlled by the weir in symmetric and asymmetric operations was used for simulations. The scours were evaluated using short-range photogrammetry for contactless measurements. Based on this method digital models of the riverbed for each simulation were created and the scours were assessed to determine the effect of the investigated measures on scour reduction.

One of the main problems at the Hričov weir is the scour development in the riverbed just downstream. It is caused of construction the size of the stilling basin was significantly shortened. Flow energy is dissipating just partially. Each flood makes scour close to the foundations of the structure, which potentially endangers its stability. A permanent solution was experimentally investigated in the hydraulic laboratory at the 2D model in a scale of 1:40. Different variants of the secondary stilling basins were designed to minimize creating scours. The investigation and its results are described in this paper.

The weir of the Hričov water structure has an untypical shortened stilling basin, which affects significant scour development in the downstream riverbed. A rockfill embankment structure adjacent to the stilling basin was constructed to prevent the formation of scours in the riverbed. However, increased flows at the weir erode this fortification and it is necessary to replenish the material back to the embankment after the flow situations. A 2D hydraulic model constructed in a flume of the Hydraulic laboratory was used for the research of new downstream fortification, which would be better resist the scouring effects. The model was created according to Froude’s modelling similarity as a 2D model of the weir structure with control gates and part the downstream riverbed consisting of gravel. Different designs of the embankment were not allowed to interfere with the existing construction of the water structure. The construction of the embankment was designed as a concrete “secondary” stilling basin adjacent to the existing one. Tested variants of the embankment were of different length and depth of the stilling basin. The length varied from 11 to 24 m and the depths varied from 0 to 1,8 m. Overall 11 different design of the embankment were investigated. Each design was tested for three different flow rates on the model simulating common and extreme flow situations at the weir. The flows were selected to simulate also different hydraulic phenomena at the weir – overflow, outflow, and free overflow. After each flow simulation the scour in the riverbed was measured. The measured scour parameters were depth of the scour, the distance of the deepest point from the existing water structure and the amount of the material carried away. The resulting parameters were assessed with respect to each other for the corresponding flow rates. The best results in scour reduction were achieved for horizontal embankment with the depth of 0 m. The designs with increased depth were causing secondary hydraulic jump in the riverbed which increased the scours. The tested designs of the embankments were able to reduce scour size by up to 45% for the outflow, by less than 20% for free overflow and by more than 80% at the overflow.

Stilling basins are commonly used to efficiently dissipate energy of flow at weirs. Different types of stilling basins are used at weirs due to different conditions – hydraulic, operational, constructional. At the Hričov water structure a short stilling basin has been built. Its operation over the years showed that it does not dissipate the energy of the flowing water sufficiently, which causes intense scouring in the riverbed downstream. To partially deal with this problem and to protect the riverbed from scours, a rockfill embankment supported with a steel construction was constructed adjacent to the stilling basin’s toe. Despite this riverbed fortification, scours are being created in the riverbed and even in the fortification itself for different cases of operation conditions of the weir. A hydraulic research on a scaled model of the weir was used to investigate the problem and to propose a permanent solution significantly improving the scouring downstream the weir. The proposed fortification of the riverbed downstream the weir was tested at different operational conditions, which simulated extreme situations at the weir. To assess the effects of the investigated fortification, the simulations were performed for the weir without and with the fortification. After each simulation, the deformations in the riverbed (scours) were measured and evaluated. For measuring the riverbed deformations on the model, the method of short-range photogrammetry was used as a very effective and contactless method. This method allowed to examine the investigated area with a very high accuracy and speed. Digital models of the riverbed deformations created after each simulation on the hydraulic model were used to determine the locations and sizes of the deepest scours. Final assessment of the results showed the improvement in the reduction of scouring by the proposed fortification by almost 50% in the size of the scours. The investigations and the results are described in this paper.