Assessment of sediment exclusion efficiency for several modified labyrinth weirs

Abstract

A trapezoidal labyrinth weir is a general transverse hydro-structure typically placed across rivers that causes longitudinal sediment discontinuity and affects riparian ecological system. A sediment exclusion labyrinth (SEL) weir is a modification designed to increase sediment exclusion from upstream to downstream over or through it. Hydraulic model experiments were carried out in straight rectangular cross-sectional open channel (15 m in length and 0.82 m in width). Four different shapes of weirs were examined. type 1 was a general labyrinth weir, whereas types 2-4 were SEL weirs with an inclined bottom slope (type 2), open slots (type 3), and reversed-wedge type holes (type 4). Standard sand (Jumunjin, Korea, d(50) = 0.56 mm, sigma(g) = 1.48, SG = 2.65) was uniformly supplied upstream using an in-house grooved drum, and sediment exclusion efficiency was calculated from the ratio of sediment amount to excluded sediment yield for each type of weir. Higher efficiencies were founded in all SEL weirs compared with the sediment exclusion efficiency of the type 1 weir. In particular, type 2 and type 4 weirs were easy to construct and their flow patterns and hydraulic performance were similar to those of type 1. Finally, their sediment exclusion efficiencies were higher under a normal water level (F-r <= 0.2), whereas the type 3 weir was the pattern favorable for a high water level (F-r > 0.2) because its discharge capacity was significantly higher. Sediment exclusion characteristics were analyzed in terms of flow rate based on the decomposition analysis of residual sediment particle size distribution. It is expected that the results from this study will be used to design SEL weirs as basic materials if more hydrodynamic parameters are studied and combined with field monitoring.