The correct answer is A. 1.2 D.
The launching apron is a structure that is used to launch and recover floating equipment, such as barges and cranes. It is typically located at the end of a pier or jetty, and it consists of a concrete slab that is sloped towards the water. The width of the launching apron is important because it determines the maximum size of equipment that can be launched or recovered.
The depth of scour below the original bed is the amount of material that is removed from the bottom of the water body by the action of waves and currents. This can be a significant problem for launching aprons, as it can reduce the width of the apron and make it difficult to launch or recover equipment.
To account for the depth of scour, the width of the launching apron is typically taken as 1.2 times the depth of scour. This ensures that there is enough space for the equipment to be launched or recovered, even if the bottom of the water body is scoured.
The other options are incorrect
because they do not take into account the depth of scour. Option B, 1.5 D, is too wide and would be unnecessary if the bottom of the water body is not scoured. Option C, 2.0 D, is too narrow and would make it difficult to launch or recover equipment if the bottom of the water body is scoured. Option D, 2.5 D, is even narrower and would be impractical for most applications.