Experimental investigation on flow and scour characteristics around tandem piers in sandy channel with downward seepage

Experimental investigations have been carried out to study morpho-hydraulic characteristics such as scour geometry and turbulent flow properties around tandem piers in alluvial channels. Experiments were carried out in a plane sand bed with two circular piers of same diameter arranged in tandem manner under no seepage, 10% seepage and 20% seepage conditions. Downward seepage minimizes the scour depth around piers and restrains the development of scour depth with time. Strong reversal flow is found near the bed at upstream of piers and near free surface at downstream of piers where velocity and Reynolds shear stress are found to be negative which reduce in magnitude with downward seepage. The flow is more critical within the gap between two piers where velocity is lesser near free surface and gradually increasing towards bed. Quadrant analysis shows that contribution of each event to the total Reynolds shear stress increases with downward seepage. Sedimentation effect prevails within the scour hole whereas outside the scour hole erosive forces become more dominant. Reduced reversal flow at upstream of pier because of downward seepage results in decreasing higher order moments and turbulent kinetic energy. At downstream of piers, secondary currents are dominant due to wake vortices. Strouhal number decreases in case of seepage runs than no seepage condition. © 2017, Harbin Engineering University and Springer-Verlag GmbH Germany.