Experimental investigation downstream of USBRVI stilling basin with steeped end sill

Experimental investigation on scour downstream of USBRIV stilling basins M. Saber1, M. Ghodsian 2* 1- Ex.MSc student of Hydraulic Structures, Faculty of Civil Engineering, Tarbiat Modares University, Tehran, Iran 2- Prof. of Hydraulics, Faculty of Civil and Environmental Engineering and Water Engineering Research Institute,, Tarbiat Modares University, Tehran, Iran * ghods@modares.ac.ir Introduction Energy dissipaters are usually installed at the outlets of canals, outlets of chutes and outlets of culverts and other hydraulic structures. They are used to dissipate the excess energy of the outlet flow from the hydraulic structures. For dissipation of excess energy at the outlets, usually stilling basins are used. There are different types of stilling basins introduced and used in different parts of word. The USBR type VI stilling basin was first introduced by Bradely and Peterka (1995) and later on modified by Biechley (1978). In this paper results of experiments on the scour at the downstream of the USBR type VI stilling basin are reported. Experiments Experiments were conducted in a 0.8 m wide, 0.6 m height, and 0.58 m length rectangular channel. The bed and sides of the channel were made of still and glass respectively, and supported by metal frames. Uniform sediment with mean diameter of 1.64 mm were used as the bed material. The sediment layer with thickness of 0.4 m and length of 2.75 m were prepared at the downstream of stilling basin. Experiments were conducted for duration of six hours, at which almost equilibrium scour was reached. The bed topography and depth of scour were measured by a digital point gauge with the accuracy of ± 0.01 mm. The dimensions of different parts of stilling basing was selected following the method introduced by Biechley (1978). Total of 32 experiments were conducted for different Froude number (1.99, 3.03, 4.06, 4.07, 5.97, 8.11, 9.27 and 13.91) and four different stepped end sills (with one step, with two steps, with three steps, with four steps and with five steps). The width, height and length of all the end sills were kept equal in all the experiments. Results It was found that by increasing the Froude number, the length of scour hole increases for all the experiments. By increasing the Froude number from 2 to 4, the relative depth of scour initially increases, reach to a maximum value at Froude number equal to about 3 and then after decreases. The variations of the relative depth of scour for the range of Froude number from 4 t 6 is marginal. But the amount of the relative depth of scour for Froude number greater that about 6 significantly decreases. By increasing the Froude number (from 1.99 to 3.03), the scouring potential of the outflow from the stilling basin increase. As a result relative depth of scour increases. But for higher values of the Froude number, the weep out of the flow from the stilling basin occurs. In this case dissipating potential of the out flow jet from the stilling basin reduces, as a result the relative depth of scour reduces. This decreasing trend is more significant at Froude number greater than about 6. By increasing the Froude number, scour index (2ds/Lt) increases and enhances the performance of the stilling basin. Here ds is maximum depth of scour and Lt is distance of location of maximum scour depth from the stilling basin. The performance of the end sill with four steps was better for the Froude number in the range of 1.99 to 4.07. The amount of scour depth at the downstream of the end sill with four steps was minimum for the Froude numbers in the ranges of 9.27 to 13.91. The volume of scour hole at the downstream of the end sill with four steps was less than that due to other end sills. An asymmetry index was defined to see the symmetry/asymmetry of the bed topography after the experiments. The values of asymmetry indices for the end sills with five steps under Froude number in the range of 1 to 4, and for the end sills with four steps under Froude number in the range of 4 to 13.9 were minimum. Time variations of the scour showed that about 65 to 70 percent of the scour occurs during 60 minutes from onset of experiments. An equation was obtained for the dimensionless longitudinal profile of scour profile. Time variations of the scour showed that about 65 to 70 percent of the scour occurs during 60 minutes from onset of experiments. An equation was obtained for the dimensionless longitudinal profile of scour profile.