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:
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:
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.