Experimental study of the effect of the apron installation on reducing scour depth at the downstream of stepped weirs with labyrinth sill

Document Type : Research Article


1 Department of water engineering, University of Guilan, Rasht, Iran

2 Department of water Eng., university of Guilan

3 Assistant Professor of Hydraulic structures university of Guilan, Iran

4 IA.ING, Lecce, Italy


Introduction: The movement of water flow in rivers and streams of the erodible bed causes the cycle of erosion and sedimentation. Although this is a natural process, its occurrence occurs along sections of a river course that conflict with different uses, ranging from agricultural damage to structures built along rivers or riverbeds (Esmaeili Varaki et al., 2021).
Stepped weirs are one of the effective structures in flow energy dissipation. Due to their shape and geometry, these structures implement to reduce energy and erosive power of water flow and also to reduce the cost of energy consuming structures that should be built downstream of dam weirs (Chanson, 1995; Khatsuria, 2005). In the present study, the simultaneous effect of creating an apron at the downstream of a stepped weir and installing sills with different geometries on its stairs on the downstream scour depth under different flow conditions and apron length was examined in a laboratory.
Methodology: The experiments of this research were performed in the hydraulic laboratory and physical-hydraulic models of the Department of Water Engineering of the University of Guilan in a flume of 12.5 meters long, 1.5 meters wide and 1 meter high with glass walls and metal floors. In order to provide the flow rate, a centrifugal pump was used that can provide a flow rate of up to 90 L/s. In this research, two different weir slopes (1:2 and 1:3), aprons with lengths of P/3 (0.135 m) and 2P/3 (0.27 m) and sill with different geometries, were examined. In order to supply sediment particles, mineral sand with a uniform diameter of 2.68 mm was prepared and placed in the sediment bed with a length of 2 m, a width of 1.5 m and a height of 0.3 m at the downstream of the weir. Long term experiment was conducted to find the corresponding time of equilibrium scour depth. Comparison of results showed that after 6 hours, the scour depth reached equilibrium condition and no noticeable change occurred, so in all experiments, measurements were performed during the mentioned period. For each experiment, after installing the weir, the sills and aprons the downstream sedimentary bed was leveled. Then, according to the desired flow, the necessary adjustments were made for the relevant engine speed. After the flow entered the flume, the flow depth gradually increased and by adjusting the downstream tail gate, desired tail water depth was adjusted. In each experiment, instantaneous scour profiles were recorded at 5, 10, 15, 30, 45, 60, 90, 120, 180, 270 and 360 minutes from the start of the experiment using digital camera and then digitized using Grapher9 software. The final scour profile was also measured at the end of each experiment using a laser meter with an accuracy of ± 1 mm.
Results and discussion: Experimental observation showed that by installation of sills, nappe thickness increased from yc/2 to yc, in which yc is critical flow depth. Furthermore, installation of sills reduced angle of imping jet to sedimentary bed from 58 to 34 degree. Consideration of the length of falling jet form the last step of weir to the downstream sedimentary bed indicated that by reduction of the weir slope from 1:2 to 1:3, length of falling jet increased. Comparison of the temporal development of scour depth showed that at the low discharge, installation of sill increase temporal scour depth. However, by increasing flow discharge and corresponding flow velocity over steps, installation of sills reduced temporal scour depth. From different geometry of sills and length of apron, weir of S2Si2LA2 have the best performance and decrease dse/p form 0.23 and 0.45 in range of low and high flow discharge to 0.1 and 0.24. by reduction of the weir to 1:3, installation of sill have not positive effect to reduction of the temporal scour depth.
Conclusion: Comparison of the results of the installation of apron with different lengths on the maximum scour depth in the range of minimum and maximum flow discharge, i. e., relative critical depth (yc/ h) from 0.06 to 0.34, showed that stepped weir with and without sill at a slope of 1:2 showed by installation apron of lengths LA1 (P/3) and LA2 (2P/3), the relative maximum scour depth (dse/p) reduced from 0.23 to 0.19 and 0.11, respectively. By installation of different sills, the relative maximum scour depth decreased to 0.22 and 0.17, respectively. By reduction of weir slope to 1:3, installation of apron with length of P/3 and 2P/3, reduced the relative scour depth to 0.18 and 0.14. by installation of different sills geometries, dse/p reduced to 0.24 and 0.1, corresponding to the length of aprons P/3 and 2P/3, respectively.


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