Engineering–geological investigation of the river bottom near a dam’s stabilization threshold based on two different evaluation methodologies

The article concerns engineering and geological studies of the river bed near the threshold stabilizing the water flow of the Włocławek hydroelectric power plant in Poland. We paid attention to the differences in river erosion behind the dam, as one section of the study area was dominated by erosion of potholes and the other by sedimentation of river material. A protective threshold (partly concrete, partly gabion) was built to stabilize the water flow and optimize the hydrological regime near the dam. As a result of the erosion process (behind part of the hydroelectric power plant) and sedimentation (behind part of the weir), this threshold is constantly being destroyed. This threatens uncontrolled changes in water level, instability of water masses and, consequently, constitutes a threat to the stability of the hydrotechnical structure. The principal aim of the study is to propose a new methodology to perform a risk assessment and to indicate how to protect the threshold structure. Initially, geodetic and bathymetric measurements were performed of the river bed, potholes and the threshold. Then the results were analysed, the risks were estimated and two independent risk assessment procedures were proposed. One suggestion concerning the area of potholes was to backfill them to protect the threshold made of concrete slabs. In the second section, there is a sedimentation of the river material around the gabion threshold, which is damaged, especially during flooding. To protect this gabion threshold, a different approach has been taken of modelling the water flow based on digital bottom models (DBMs). An engineering–geological investigation of one river bottom benefited from two types of protection features (a concrete threshold and gabion threshold) using two different methodologies to evaluate hazard and design subsequent protection of the thresholds. The results of our study support the structure's managers in maintaining its safety.