Effects of Span on Local Scour Depth around Four Columns of Tandem Piers in Clear Water

This research explores the effect of the span on the local scour depth around four columns of tandem piers by experimenting under clear water conditions in six cases. The local scour depth is the largest around the first pier in each column in all cases and is not affected by the span, and can be determined by using the 65-2 formula in China. When the span is shorter than 30.0D, local scour depths around other piers in each column gradually decrease along the flow direction and finally become stable. Except for the piers in the first row, local scour depths around the piers close to the channel edge are larger. If the span is longer than 30.0D, local scour depths around piers in each column are close in different cases, and the local scour depths around piers close to the middle of the channel are larger. Those depths around the subsequent tandem piers can be determined according to the reduction factor of local scour depth (R-LSD). If the span is shorter than 30.0D, the maximum R-LSD is about 0.78. If it is greater than 30.0D, the R-LSD of piers near the channel edge is similar for all cases, which is about 0.85. In each case, the R-LSD of piers near the middle line of the channel is close and increases along with the increase in the span, tending eventually to 1.0.