Experimental study of launching apron around abutment

Abstract

An investigation was undertaken to study the behaviour of launching apron in a straight rectangular laboratory channel. Since launching apron is a counter measure adopted to limit scour in sand bed around some structure, initially the investigation was incorporated on local scour study. The scouring information obtained during the scour study was used for designing launching apron. Then the behaviour of launching apron was studied. The study was made under clear-water condition and channel bed formed ripples during test runs. The hydraulic parameters were set so that the flow is close to the critical condition of sediment transport that is clear water condition. For scour study, three vertical-wall and three sloping-wall abutments were used. Vertical-wall abutments were made of half-inch thick particleboards and sloping-wall abutments were made of metal. Around these structures, the scour depth, scour contour and scour pattern were observed. Each test was run for 7 hours and 30 minutes. This test running time was selected based on the limitation of laboratory pump and in an attempt to reach equilibrium stage. Results were compared with some existing prediction methods and data. It was found that, in the case of vertical-wall structures results were comparable with existing prediction methods and data of other investigators. However, in case of sloping-wall structures, the existing prediction methods consider the sloping face of structures to be extended beyond the maximum scour level. Where as, structures that were used during present study, had their sloping face extended up to initial bed level only. From the initial bed level, the structures had vertical faces. Thus, the existing formulae might not be able to predict the scour depth around those structures. Therefore, for this type of structures probably new shape factors or slope factors need to be found out. After the scour study, one vertical-wall abutment was selected for apron study. Apron was laid around that abutment in six different settings. During the apron study also each test was run for 7 hours and 30 minutes to maintain uniformity among the tests. It was found that, when apron material was laid around a structure, the apron behaved like a submerged extension of the original structure. The effect of the submerged extension was found to be quite significant in scour formation and in vertical velocity distribution within the scour hole formed.