Short Abutments Setback Case (Case A)
When setback distance of abutments from channel no greater than 5 times the flow depth in the channel at Section 2, it is considered
short setback case. When the setback is more than 75% the upstream flow width of the overbank, it is considered long setback case.
The medium setback case is located between the short and long setback cases.
The assumption used for developing the velocity and flow distribution under bridge for the short setback is that the channel and
overbank flow is entirely mixed together. The average velocity of flow (Vave ) under the bridge can be calculated as:
Va = Q / A
where:
Q = total flow under the bridge, and
A = total channel and flood plain flow area under the bridge.
The discharge per foot (q) is computed as
q = Va * yo
where:
yo = average depth of flow on the flood plain or in the channel
Note that the value of yo can be expected to be different for the left overbank, channel and right overbank areas.