Experimental and numerical study of local scour around bridge piers with different cross sections caused by flood hydrograph succeeding steady flow

Design of the bridges by considering structural and hydraulic problems is necessary for their safety. Local scour around bridge piers and foundations, as a result of flood flows, is one of the major causes of their failure. In this thesis, local scour around bridge piers in both steady and unsteady flow cases were studied under clear water conditions, by using different shaped piers. The main goal of the experiments is to examine the final scour depths, the time dependent scour depths and the geometry of scour holes around five different types of piers under unsteady flow conditions which succeed a steady flow of 5 minutes. The other objective is the comparison of the experimental findings with those available in the relevant literature and also with the results obtained from the numerical simulations in order to make some practical suggestions. The final and the temporal evaluations of the scour depths were measured for circular, square, rectangular, rectangular with circular nose and rectangular with trapezoidal nose piers. The scour hole lengths, widths and areas were recorded by means of two cameras. The equilibrium scour depths were determined after translating the time varied scour depth curves to analytical expressions. Regression analyses were performed in order to derive empirical relations about temporal variation of the scour depths for both steady and unsteady cases. The shape factor values were investigated based on the experimental findings and the obtained values were compared with those obtained previously by different researchers. The numerical solution was realized by using the software ANSYS FLUENT with RNG k-ε turbulence model for circular and square piers. The numerical and experimental values are compared and interpreted. The high values of determination coefficients revealed that the suggested empirical relations are significant statically. An acceptable accordance was observed between numerical and experimental results.