Experimental investigation of effect of labyrinth and apron on local scour downstream of stepped weirs

Control of river bed erosion and consequently river banks is one of the important goals in river training projects. Instream grade-control structures [i.e. cross-vane, J-Hook vane, W-weir, log vane, block ramps, sills, etc.] are commonly used to stabilize the riverbed keeping the river slope in an equilibrium condition. When the steep river is wide, the mentioned instream structures are not the appropriate solutions. Stepped weirs are a type of spillways that creating a nappe flow over the steps, along with a high energy dissipation, lead to a reduction of downstream scour depth. Stepped weirs also can be employed as grade-control structures in the case of wide rivers. Scour depth and energy dissipation downstream of stepped weirs depend on the flow pattern over steps which depends on the weirs slope (i.e. ratio of the height to the length of the step), number of steps, constructing material and downstream hydraulic conditions (Chanson, 2002). Due to the role of scour formed at the downstream of these structures on their safety and stability, in the present study, effect of the installation of apron on variation of scour characteristics at the downstream of stepped weirs with and without labyrinths was investigated experimentally.

Experiments were performed for different flow conditions and weirs slopes of 1: 1 and 1: 2. The experimental measurements were conducted in the physical hydraulics modelling laboratory of the University of Guilan, Iran, in a flume with a rectangular cross-section that was 1.5 m width,1 m depth and 12.5 m length. A centrifugal pump supplied the required flow rate up to 90 L/s. The flow discharge was measured by ultrasonic flowmeter installed on the suction pipe with an accuracy of ± 0.1 l/s. In order to supply sediment particles for the sedimentary bed, sand with a uniform diameter of 2.68 mm was prepared and placed at the recess box with length of 2 m, width of 1.5 m and height of 0.30 m at downstream of weirs.The stepped weirs with slope of 1:1 and 1:2 consisted of five steps made of sheet metal panels. Aprons length are 0.135 (1/3P) and 0.27 (2/3P) meters. The labyrinths geometries was based on the results of research by Kazempor et al., (2019) that had the most suitable performance on reducing scour depth, was chosen. Primarily experiments indicating that the equilibrium bed condition was reached at 6 hours. For each experiment, after installing the weir, labyrinths and aprons and adjusting the sedimentary bed, the flow gradually entered into the laboratory flume and the flow depth gradually was raised. After adjusting the flow discharge, by regulating the downstream butterfly gate, the desired downstream depth was obtained. for all experiments, the temporal development of the scour depth for 6 hours at different time steps was taken by a digital camera and then digitizing using Grapher9 software. Final scour depth at the end of tests was measured using the Laser scanner Leica.

The results showed that by increasing the discharge to maximum value, installation of the apron LA1 at downstream of the stepped-labyrinth weirs SL1K1LA1 with an average 30% reduction of the temporal maximum scour depth had the best performance. The results indicate that by increasing the apron length to LA2, the SL1LA2 weir with an average of 54% reduction of the maximum temporal scour depth had the highest performance.By reduction of stepped weir to 1:2 and installing apron length of LA1 the maximum flow discharge, by the installation of apron length of LA1 stepped-labyrinth weirs SL2K4LA1 with an average reduction of 21% had the best performance on reduction of temporal scour depth. Also by increasing apron length to LA2 length, the stepped-labyrinth weirs of SL2K4LA2 had the most desirable performance with an average reduction of 34% on temporal scour depth.

Comparison of the results of the equilibrium scour depth of the yc/h range from 0.34 to 0.54 for stepped weir with slope of 1:1 showed that by installation of apron length of LA1, the equilibrium scour depth at the downstream of SL1LA2, SL1K1LA2 and SL1K2LA2 weirs reduced 38%, 34% and 45%, respectively, compared to SL1 weir. By increasing the apron length to LA2, the equilibrium scour depth at the downstream of SL1LA2, SL1K1LA2 and SL1K2LA2 decreased by 65, 65 and 63%, respectively in comparison with SL1.At a stepped weir with slope of 1:2, that by installation of apron length of LA1 the equilibrium scour depth of SL2LA1, SL2K3LA1 and SL2K4LA1 weirs was reduced 8, 1 and 10%, respectively, compared to SL2 weir. By increasing the apron length to LA2, the equilibrium scour depth downstream of SL2LA2, SL2K3LA2 and SL2K4LA2 weirs decreased 44, 50 and 48% on compared to SL2 weir, respectively.