Comparison of Scour Patterns on Non-uniform Sediments Downstream of Arced and Triangular Labyrinth Weirs

Downstream scour of hydraulic structures such as weirs can lead to structural damage and eventual destruction. This study investigates the downstream scour of labyrinth weirs in non-uniform sediment beds. Experiments were conducted in a 12-meter-long, 80-centimeter-wide laboratory flume using one-cycle and two-cycle triangular and arced labyrinth weirs, each 1.26 meters in length, with a magnification ratio of 1.58. Flow rates of 5, 10, and 15 liters per second were applied over sediments with a median grain diameter of 1.4 mm, maintaining a constant tailwater depth of 10 cm. The experimental results showed that the maximum scour depth occurred at two key locations: (1) the junction between the weir and channel wall, and (2) the intersection of weir cycles. These can be attributed to the transverse flow curvature, nappe collision with the sidewalls, and nappe interference at cycle junctions, which increase flow disruption and ultimately the scour compared to other points. In all weir types, scour increased with a rising water head ratio and decreasing discharge coefficient. The extent of the increase in scour was determined by the decrease in the discharge coefficient. The results showed that weirs with higher performance exhibited a faster decrease in the discharge coefficient and a faster increase in scouring. Hence, the triangular one-cycle weir with the highest performance had the highest decrease in discharge coefficient (20%) and the highest increase in scour (120%). The dimensionless scour ratio (\(\:\frac{{\varvec{d}}_{\varvec{s}\varvec{e}}}{\varvec{P}}\)) was found to be 1.04 and 0.98 for the one-cycle and two-cycle triangular weirs, and 1.09 and 0.93 for the corresponding arced weirs, respectively. Although two-cycle weirs showed slightly lower hydraulic efficiency, they are more suitable from a structural point of view due to their lower scour depth.