Introduction:
Labyrinth weirs are used in river sections that have limited width, to increase the flow capacity. Due to the turbulence of the flow and the increase in velocity, scour is one of the important issues downstream of the weirs. Moreover, the type of materials used downstream of the structure usually has a great impact on the amount of scour downstream of the weirs. For this reason, it is essential to evaluate the effect of material type on scour. Given that weirs are exposed to numerous floods, the most important challenge is to ensure the stability of the structures against erosion and scour, and the amount of scour must be estimated. In this research, the erosion downstream of labyrinth weirs is investigated, focusing on the effect of sediment bed compaction and non-uniformity of gradation on scour.
Materials and Methods:
Experiments were conducted in a glass flume with a length of 12 meters and a width of 80 cm. Also, three discharges of 5, 10 and 15 liter/s were considered. A triangular labyrinth weir with L/W equal to two, height of 35 cm and thickness of 4 mm was constructed. Downstream of the weir, the examined sediments were placed in the channel with a height of 20 cm. Two types of sediment were used: uniform sediment (A) and non-uniform sediment (B), both with a mean diameter of 2 mm. Sediment A had uniform gradation with the particles diameter varied between 0.7 and 3 mm, and the uniformity coefficient of 2.4. Sediment B had non-uniform gradation, with particles diameter of 0.07 to 6 mm, and a uniformity coefficient of 5.96. The uniformity coefficient, is defined as ratio d₆₀/d₁₀, indicates uniformity when less than 5 and non-uniformity when greater than 5. The important point was that d₅₀ of both sediments was the same and equal to 2, meaning that both sediments had the same mean diameter, but different in uniformity. Therefore, the effect of uniformity and non-uniformity of sediments could be investigated. The tailwater depth in the experiments was 11 cm, and the equilibrium time was 6 hours. After each test, the scour depth was measured on a 2x2 cm grid and the scour profile was plotted in Sigma Plot software.
Result and Discussion:
Investigations revealed that the maximum scouring depth occurred near the junction of the weir and the channel wall. With increase in discharge, the scour rate increased by up to 68 percent in sediment A with 80 percent compaction. Also, the scouring profile of sediment B for a discharge of 15 Lit/s and 80 percent compaction showed a 40 percent increase compared to a discharge of 5 Lit/s. Therefore, higher discharges significantly increased the scour rate. Increasing the compaction from 80 percent to 90 percent, slightly reduced the scour depth. For sediment A, at a discharge of 5 Lit/s, increasing compaction resulted in over than 50 percent reduction in scour depth, but at a discharge of 15 Lit/s, only 5 percent change occurred. For sediment B, at a discharge of 5 Lit/s, a 35 percent reduction was obtained in scour depth with an increase in compaction from 80 percent to 90 percent; while at a discharge of 15 Lit/s, 2 percent change occurred in scour with an increase in compaction. This indicated that in high discharge, compaction had not significant effect. In comparison to sediment A, the sediment B had a 20 percent reduction in scour on the sides at a discharge of 15 liters per second and 90 percent compaction. Regarding the results, the non-uniformity of sediment B caused less erosion than sediment A. The sediment A was almost uniform and the sediment particles had less overlap and interlocking.

Studies confirmed that the maximum scour depth occurs near the junction of the labyrinth weir with the channel wall and this location remains consistent with increasing discharge. The second maximum scour occurs near the labyrinth weir apex. Therefore, to prevent erosion downstream of the labyrinth weir, necessary considerations should be taken regarding the maximum erosion locations. Increasing the compaction from 80 percent to 90 percent, the scour depth decreases to some extent, but at high discharge, compaction is less effective. However, the non-uniformity of the sediment affects the scour rate, because it enhances sediment particles contact and resistance to erosion.