Medium Abutment Setback Case (Case B)

When the abutment setback is between 5 times the channel flow depth at the bridge section and 75% of the upstream overbank flow width, it is considered to be the medium setback case.

The average velocity at the overbank area is adjusted as following equation


Vmix=Qt/At

(Va)c=Vmix

Vo=Qo/Ao

(Va)o=Vmix-(Vmix-Vo)/(0.75*Wo-5*yc)*(Setback-5*yc)
Where:
Vmix is the velocity of the totally mixed flow condition., i.e., average total flow under bridge.
(Va)c is the average channel flow velocity and is equals to Vmix in this case
Vo is the overbank flow assume separate flow condition (long setback condition)
(Va)o is the average overbank velocity for this medium setback case

This case provides for a smooth transition between the short and long setback cases.

There is a special case where the above interpolation method for a medium setback may over-predict the flood plain velocity. The ABSCOUR program prints a warning for this case when the overbank flow velocity is greater than the channel flow velocity.

The special case typically applies to crossings of wide swamps/flood plains where most of the flow is carried on the flood plain and where the channel flows in a meandering pattern with shallow depths, low velocities and small discharges.

The recommended procedure in this case is to evaluate the flow distribution under the bridge to see if it is reasonable. If this is the case, use the program results and calculated scour depths.

If the flow distribution is not reasonable, there are two approaches that can be used to modify the ABSCOUR input values:

  1. On the downstream bridge data card, use the over-ride discharge under bridge (the line immediately below the HEC-RAS discharge under bridge) to revise the flow distribution between the channel and right and left overbank sections.

  2. Check the bridge section unit discharge over-ride on the project info card, then go to the downstream bridge data card, and modify the unit discharges for the channel and left and right overbank sections. Calculate the unit discharge under the bridge as indicated below for an example where the medium setback is on the left overbank:
q (left overbank under bridge) = Q (left overbank at approach section)/ medium setback distance between channel bank and the left abutment.

In both cases, the user should check the total discharge under the bridge to assure that it is the same as the HEC-RAS discharge under bridge.

The user needs to evaluate the answer obtained by the override procedure to see if it is reasonable. For example, compare the override unit discharges with the unit discharges obtained from the HEC-RAS program.