Experimental Study of Local Scouring at the Downstream of River Bed Protection

Abstract

Local scouring at the downstream of the river bed protection is one of the most important parts of the design criteria and sustainable management of the upstream hydraulic structures, such as dam spillway, gated or fixed weir. Especially, bed erosions in the vicinity of hydraulic structures and temporal development of maximum scour depth can threaten the safeties of hydraulic structures. From the previous researches, it has been known that dominant factors on local scouring were temporal development of the scour depth, initial flow and turbulent characteristics and properties of the river bed materials from the numerical and experimental approaches. However inappropriate the assumptions of previous researches and limitation of the measurement and monitoring caused the application of various empirical coeffcients and scour prediction. In this study, physical tests of the local scouring at the downstream of the fixed bed were conducted with movable bed materials with advanced laboratory facillities. Experimental approaches were focused on the temporal evolution of scoured hole and flow and turbulence characteristics in it at each time step in order to analyze the countermeasure of the excessive losses of the bed materials near the longitudinal transition. From the temporal change of the bed elevation at each time step, the empirical relationships between temporal change of the maximum scour depth and hydraulic conditions were concluded and can be used prediction of the value of the maximum scour depth. Especially depth averaged relative turbulence intensity at the longitudinal transition of bed and bed shear stress were revealed the dominant factors on the temporal change and equilibrium value of the maximum scour depth. To analyze the flow and turbulence characteristics in the stabilized scoured hole, similar physical process near the backward facing step were conducted with using the PIV and ADV measurement system. High resolution PIV system was helpful to analyze the flow pattern near the step and reults of the 3-D ADV system were postprocessed with despiking method and ensemble averaging which was based on consideration of the integral time scale. With the analysis of the flow and turbulence results in the stabilized scoured hole, occurrence of the reversal flow and turbulent shear layer near the scoured hole was revealed to be dominant on the additional bed losses from the bed. To find the appropriate countermeasure of the maximum scour depth, additional tests with mesh grid generated turbulent flow near the transition were conducted with same condition of the upstream water supply and water depth at the end of the laboratory flume. Results of the larger values of the depth averaged relative turbulence intensity were concluded the detention of the temporal change of scour depth and decrease of the equilibrium-maximum scour depth. Empirical equations which were suggested in this study, can be used to predict the maximum scour depth with well-known hydraulic parameters. And they can be also applied to be the quantitative index of improvement and design of the rive bed protection for the less erosion of the bed materials near the river bed protection.