Bridge Opening:
The hydraulic flow depth (yo) at the downstream side of the bridge equals A/T. The solid black line represents the bottom of the ABSCOUR cross section and the dashed brown line represents the existing bed elevation.
Bottomless Culvert Opening:
The hydraulic flow depth (yo) at the downstream side of the bottomless culvert equals A/W. Notice that on the left overbank, the cross hatched area is equal to the light blue shaded area on the ABSCOUR cross section.