The Effect of Sill to Reduce Scour Around Bridge Piers in Flood Flow Conditions

Scouring around bridge piers due to strong storm currents is a main threat to the stability of bridges in rivers. Hence, in this conditions the protection against excessive scouring is required. Attempts have been made by several investigators to reduce scour depth using several devices around bridge piers. One of the successful methods for reduction of scouring was the application of bed sills. In this study, the effect of applying bed sill at the downstream of the pier in reducing the scour around bridge piers under strong storm currents is studied. For this purpose, experiments were conducted by placement of bed sills at the downstream of the pier throughout the flume width with its top position at the bed level and by varying the distance between bridge pier and bed sill. The experimental tests were done under uniform flow with clear water condition and storm currents. Storm currents were simulated by two group of hydrographs with different base time and three different peak flow. A circular pier was installed on the bed with uniform sediments and final scour depth was measured in different flow conditions. The results under uniform flow showed that the efficiency of the bed sill is reduced by increasing the distance between the bed sill and the base of pier. The placement of a bed sill sticking to the pier resulted in the most significant scour reduction of about 36 and 77 % in depth and volume, respectively. This result confirms the findings of previous studies. Under storm flow condition, the placement of bed sill at distances smaller than three times of the pier diameter resulted in the reduction of scour depth and volume. The results show that the efficiency of the bed sill is reduced significantly by increasing the distance from the pier and it results in the increase of scour depth at the distance of three pier diameter. The maximum reduction in scour depth and volume was observed about 25 and 48 %, respectively, at the condition with the bed sill sticking to the pier. Based on these findings, it can be concluded that the efficiency of the bed sill is reduced under storm flow conditions when compared with its efficiency under uniform flow with clear water condition.