If D is the scour depth measured below the bed, then the width of launching apron will generally be equal to: 

Explanation

Launching Apron:

• To protect the face of the guide bank at the river bed level a thick stone cover is laid on the bed. It is called an apron. When the scour undermines the river bed the apron comes down or launches to cover the face of the scour. Hence it is called Launching apron also.
• Heavy scour of the river bed at curved heads and shanks of guide banks can cause undermining of stone pitching thereby resulting in failure of guide banks. Such a failure can be prevented by providing a launching apron beyond the toe of guide banks.
• The width of the launching apron is generally taken as 1.5D.