Control of bed scour downstream of ski-jump spillway by combination of six-legged concrete elements and riprap

Abstract

Local scouring is one of the key issues at the downstream of ski-jump spillways which may result in failure of the hydraulic structures. Therefore, adopting some measures in order to control and reduce this phenomenon is very crucial. In the present study, the effect of a special shaped element referred to six-legged concrete (SLC) elements along with riprap materials at the downstream of a ski-jump spillway as the scour countermeasure was experimentally evaluated. Experiments were conducted in unprotected (baseline tests) and protected bed (armed with SLC elements of the fixed size and two sizes of riprap with mean diameters of 9 and 20 mm) at five different flow discharges, with Froude numbers ranged from 3.42 to 15.51, and three tail-water depths. The results of baseline tests showed that increasing the flow discharge at different tail-water depths increased the maximum scour depth. Also, at each state, with the increase in the stability number, the maximum scour depth increases. From the results of the protected bed tests, it was concluded that with the application of SLC elements the jet penetrated from the pores between the protected cover and scour was developed, eventually resulting in the collapse of the protected layer. In addition, when the bed was protected by the combination of SLC elements and proper riprap size, the maximum scour depth beneath the protected material was reduced by 56.6%−100%. Using the experimental data, several empirical relations were developed to predict the maximum scour depth for different protection alternatives under different conditions. By defining the bed scour as a criterion for the stability of SLC, a design graph was also proposed to determine the required riprap size.