Estimation of the local scour around the cylindrical pier over the gravel bed for a low coarseness ratio

This study investigates the local scour around the cylindrical pier mounted on a gravel bed for a low coarseness ratio (d/d(50)), which is defined as the ratio of pier diameter d to the median particle size (d(50)) of the bed material. An experimental study was conducted in a flume with varying pier diameters and flow conditions. The effect of flow depth on the scour was observed for a limiting condition, i.e. the ratio of flow depth (h) to the pier diameter (d) is less than 4.33. The critical limit of h/d for affecting the scour depth for the gravel bed was found to be three times more than that of the sand bed. From the experimental results, an empirical relationship is proposed based on the dimensional analysis for the estimation of scour depth for a low coarseness range (d/d(50) < 25). The proposed relationship estimates the scour depth with an error of less than +/- 15%, within the acceptable limit. Furthermore, it is also performing well for the coarseness ratio up to 100 with an error within the range of +/- 30%. The results from this study can be useful for scour prediction and for the design of a bridge pier in a gravel bed with a low range of coarseness value.