The 4 : 1 ratio is a value that was suggested by the US Army Corps of Engineers when doing bridge studies for determining ineffective flow areas. This is an approximate value, and you need to use engineering judgement in determining where the ineffective flow areas are.
I have used between 1 : 1 to 1 : 4, depending on the size of the breach, the flow velocity, and other considerations.
Sincerely,
Chris E. Maeder, M.S., P.E.
Senior Technical Engineer
chris.maeder@bossintl.com