Experimental study on flow behaviour around launching apron

Abstract

Launching apron is widely used at the toe of all riverbank protection works. Use of launching apron around revetment is very popular for protection of structures against scour failure. A systematic experimental investigation was conducted to study the flow behaviour around launching apron in a laboratory channel situated in the Hydraulics and River Engineering Laboratory of the Department of Water Resources Engineering, BUET. Since launching apron is a measure to limit scour in sand bed for riverine structures, initially the investigation incorporated flow behaviour and then the scour study were observed. The extent of aerial coverage of launched apron was determined during the test runs in connection with the study. Before the test runs, few trial runs were conducted initially to gather information on flow behaviour and scour using fixed as well as mobile bed with a view to visualizing some idea about the size of apron materials. Afterwards necessary test runs were conducted. Each test was run for 8 hours to maintain uniformity among the tests. Sixty test runs were conducted in total of which 24 test runs were conducted using fixed bed and 36 test runs were conducted using mobile bed. For fixed bed bank slopes of 1:1.25,1: 1.5 and 1:2 were used. For mobile bed also the same three bank slopes were utilized. Three discharges of 40 lis, 50 lis and 60 lis were used for test runs. The apron materials fabricated using sand-cement were of sizes 3mm, 4mm, 5mm and 6 mm. Geobags were also used as apron materials which was prepared by filling with sands inside properly sewn cloths and weighing approximately 1 gm. These materials were placed on the toe of sloping revetment. Test runs were conducted for specified conditions and measurement were taken. The study was made under clear-water condition and the channel bed formed ripples during test runs, while mobile bed test runs were conducted. Test runs were conducted for different apron setting configuration and velocities were measured using a programmable 2-D electromagnetic liquid velocity meter. The flow behavior was analyzed critically by examining the measured data on velocity. The flow field analys~s were conducted by examining the variation of stream wise velocity and changes along vertical direction. The study showed that presence of apron at the toe of revetment caused a change in the velocity distribution near bed. In most of the cases, shifting away of maximum velocity was observed that caused shifting of maximum discharge flux away from the revetment. Analysis of velocity contour also revealed this trend of shifting maximum velocity away from the revetment. It was found that, when apron material was laid around a structure, the apron behaved like a submerged extension of the revetment. It was also observed that, launching apron was protecting the revetment structure in two ways - one was by forming a protective layer on the sloping face of the developed scour hole and the other was by forcing the scour hole to be formed away from the structure for mobile bed tests. Scour depth was found to vary with velocity variation of flow and an increasing tendency of scour depth has been observed with increasing flow intensity. It was found that placing of apron in different arrangement not only reduced the maximum scour depth but also had changed the deposition pattern around revetment. Attempts were made to establish non-dimensional relationship between relative scour depths with projected area of apron provided. Using data obtained for different test arrangements regression analyses were performed to derive expressions. These expressions are expected to be useful for practical applications .