Experimental and Theoretical Study of Local Scour around Three-Pier Group

The prediction of bridge-pier scour has been mostly aimed for a single pier of various sizes, shapes, and alignments with flow. For the case of multiple piers, widely used manuals still recommend using single-pier scour formulas with an equivalent diameter, regardless of pier separation and angle of attack. In this paper, we present the results of a new set of laboratory experiments conducted to address the local scour around a group of three piers with different sizes, spacing, and attacking angles. Based on the phenomenological theory of turbulence, we also formulate a semitheoretical framework for calculating the maximum scour depth in the multipier condition, including the single-pier counterpart as a special case. The few coefficients in the final equation were determined by experimental data from this and other studies. Our results provide an understanding of the eddies responsible for the scour process, and show that pier diameter, pier spacing, actual pier width, flow depth, Froude number, and sediment size are all important variables that need to be considered in order to obtain an accurate prediction of the maximum scour depth. Therefore, these results shed new light on current engineering procedures based on using single-pier expressions in the case of multiple piers.