Effects of a downstream submerged weir on local scour at bridge piers

This paper describes 64 experiments conducted in a sand-recirculating flume using coarse sand (d50 = 0.85 mm), investigating the interference of local scour between a circular pier (diameter D = 48 mm) and a downstream submerged weir (weir height z = 30 mm) with different separation distances L. The experimental results show that, for clear-water scour, the scour depth at the pier can be reduced by up to 54% by the downstream submerged weir when L = 0. This reduction decreases with increasing L until it approximately vanishes at L = 6D. For live-bed scour, besides the upstream bed aggradation, the downstream submerged weir can affect the local scour at a pier through two mechanisms: (i) superposition of local scour areas; and (ii) altering flow patterns. The mechanism (i) can increase the scour depth at the pier significantly when the pier is located in the upstream scour area of the submerged weir. The mechanism (i) vanishes at a distance L1, where the pier is located out of the upstream scour area of the submerged weir. The mechanism (ii) is similar to that of the clear-water scour tests, and can reduce the scour depth at the pier. The reduction effectiveness of mechanism (ii) decreases with increasing L until it becomes zero at a distance L2. Based on the experimental data, L1 and L2 are found to be 2D and 12D, respectively. For safe design, L1 < L < L2 is suggested to avoid the mechanism (i) and take the advantage of the mechanism (ii) for reducing the scour depth at the pier.