Experimental study on the flow characteristics of unstructured block ramps

Unstructured block ramps (UBR) are fish-friendly structures to stabilize a river bed. They are increasingly used to replace drops and represent large macro-roughness elements randomly placed on the river bed. Laser Doppler Anemometry measurements were conducted to determine the mean flow characteristics in a laboratory channel covered by block ramps. The resulting local time-averaged flow quantities allow for considerations of the flow field heterogeneity and for the identification of migration corridors for a certain target fish. Double-averaging (in time and in space) was applied to characterize the flow conditions taking into account the strongly heterogeneous bed of an unstructured block ramp. By applying this method, the characteristic vertical profiles describing the general flow conditions are presented. Based on geometrical and physical considerations, a suggestion is made to sub-divide the interfacial sublayer of macro-rough beds into upper and lower parts. The aim of this research is to present not only the flow field, but also to highlight the ecological value of UBR.