The Effects of the distance between ground-sill and double cylinder-piers against the scour patterns

Abstract

Flood disaster causes local scouring around the bridge piers. Some efforts to protect piers locally have been widely practiced, but the protective effect is only temporary. This is due to the riverbed being degraded within a few years, and eventually the pier protector is also broken. Ground-sill has a dual function of setting the ground level of the channel and stopping local scouring on the pier to continue. The present study is about physically modelling in the laboratory to observe scour depths due to the distance between the ground-sill and double piers. The position of the double piers remain, i.e. a line of flow direction, so there are upstream pier (called pier 1) and downstream pier (called pier 2). Only the ground-sill positions are varried. The ground-sill is located on the downstream of the pier 2. Some experiments are conducted to get the two best positions of the ground-sill by trial and error, that is W1 (position 1) and W2 (position 2) using the biggest discharge flow that can be. The position W1 is better than W2. This is based on two criteria: the scour depth has to be stable (no more scour) at the specified time range (0 - 120 minutes), and the maximum scour depth formed has to be the shallowest among those in the other experiments. The resulting two best positions of the ground-sill are then tested with three variations of discharge to observe the scour depth, and to observe the differences of the scour depths due to both piers in point 1 (behind the pier 1), point 2 (right side of the pier 1), point 3 (left side of the pier 1), point 4 (in front of the pier 1), point 5 (behind the pier 2), point 6 (right side of the pier 2), point 7 (left side of the pier 2), and point 8 (in front of the pier 2). The experimental results show that the maximum scour depth with the position W1 occurs in points 1 and 3 with a 2.9 cm deep and a maximum flow of 3.161 l/s. Meanwhile the maximum scour depth with the position W2 occurs in point 1 with a 3.8 cm deep and a maximum flow of 3.254 l/s. The scour characteristics with the point W1 is that a local scouring occurs significantly since the beginning of the flow, but channel bed has been stable since several minutes after the flow starts overtopping the ground-sill. The flow with the position W2 shows that the backwater affects significantly the flow condition so that scour and sedimentation process continuously occur. This experimentation also confirms that the control slope (the slope between riverbed where the piers are and the top of the ground-sill) has a optimum distance. In this study the optimum control slope is −0.014.