Erosion irregularities resulting from series of grade control structures: The Mszanka River, Western Carpathians

Grade control structures (GCSs) are one of the most often used hydraulic structures in a river regulation in mountain catchments. The purpose of their use is to reduce the river bed gradient and prevent downcutting. The aim of this study is to evaluate the influence of GCS on changes in river bed morphology. The study was conducted in a downstream stretch (5.66 km long) of the Carpathian river - the Mszanka, regulated with 25 GCSs. Research has shown that erosion is here a predominant process despite the use of GCSs. The river bed has been cut into bedrock along approximately 40% of a studied stretch. A total of 56 scour holes were identified, which vary substantially in terms of their depth, length, and mechanism of formation. The erosion depth depends mainly on the length of reaches between GCSs and on the water surface gradient. A surprising research discovery was scour holes found upstream of the crests of the GCSs - 16 such scour holes were found. The formation of these scour holes is explained through the drawdown profile characterized by the increasing of the velocity that favored the bed erosion. The studied river is characterized by deficit of sediment. The insufficient sediment supply and imbalance between the river's load and sediment transport are two of the greatest problems affecting the most Carpathian rivers. The novelty of this research is the analysis of such a long and complex reach of mountain river engineered with GCSs in terms of variances in erosion processes. Also a new knowledge presented here is an information about scour holes located upstream of GCSs and explanation of the mechanism of their formation. Results from this research could serve geomorphologists, engineers and ecologists as well as help river managers in decision-making processes when river regulation is planned.