The Influence of Large Roughness Elements on Natural Morphological Changes in a Mountain River Bed

Abstract

The presence of large roughness elements in a river bed influences the existing fluvial processes. Large roughness elements change the flow regime, continuity of river and morphological conditions, such as bed material size, water depth, and bed slope. Large roughness elements (for example boulders) are also often used to improve fish habitat and to change the hydraulic conditions of water flow (velocity magnitude, water flow direction). As long as the riverbed stays under their influence, one can observe that on the border between liquid and solid phases there is an extremely high turbulence. This causes the high erosion potentials in the zone of large roughness area. Determination of influence of large roughness elements on flow conditions is of crucial importance with regard to a proper engineering approach when designing flow channels. In the chapter, the results of investigations carried out in Carpathian mountain rivers: Raba, Porębianka and Czarny Dunajec are presented. Performed measurements included cross-section and longitudinal leveling, bed load sampling and hydrometric measurements. Differentiation of flow velocity in the region of large roughness element and its influence of the bed load size in the stream channel was shown. The aim of the chapter was to find the changes of fluvial processes due to the presence of large roughness elements and to connect it with the fish habitat conditions for different species. This improves the ecological state of water flow which is the requirement of the Water Framework Directive of the European Union.