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.
