UNDERSTANDING THE BEHAVIOUR OF LAUNCHING APRONS AROUND ABUTMENT: AN EXPERIMENTAL STUDY

Abstract

This study aims to investigate the behaviour of a launching apron in a straight sand bed laboratory channel. The apron wasmade under clear-water condition and channel bed formed ripples during test runs. The hydraulic parameters have beenset so that the flow is close to the critical condition of sediment transport. Three vertical-wall and three sloping-wall abutments have beenconsidered for this test. Around these structures, scour depth, scour contour and velocity distribution have been observed. Results have been compared with some existing prediction methods and data. It has found that in the case of vertical-wall structures, results are comparable with existing prediction methods. However, in case of sloping-wall structures, the existing prediction methods do not produce so close agreement. Results show that when apron material has been laid around a structure, the apron behaves like a submerged extension of the original structure. The effect of the submerged extension has been found to be quite significant in scour formation and in vertical velocity distribution. The study shows that the launching apron protects the structure by forming a protective layer on the sloping face and forcing the scour hole to be formed away from the structure.