Temporal variation of scour depth around bridge abutment in presence and absence of permeable spur dike under unsteady flow conditions

The position and depth of a scour hole around a bridge abutment are the important design parameters. The existing models have been mostly developed for steady flows, whereas real-life abutments often fail due to intrinsically unsteady flood streams. This study used nine hydrographs with different durations and skewness values and evaluated the ultimate scour depth and temporal scour depth variations for a rectangular abutment using the precise underwater image recording devices and then image processing method. Moreover, an ecofriendly permeable spur dike was employed to protect the abutment for the first time, and the testes were re- performed in the presence of the permeable spur dike. Although the hydrograph skewness had no significant effect on the ultimate scour depth, scour variations are dependent on the skewness of the hydrograph. The scour depth increased by up to 17% after the time of the peak flood, and then its variations became highly smaller, with the rising limb having a larger effect than the falling limb on the scour depth. Finally, equations were developed to calculate the scour depth during hydrographs. These equations enable bridge designers to have deeper insights into scouring around abutments during a flood and provide more reliable and economical estimates in the design of the abutment foundation size and depth.