For an abutment with an medium or long setback, use the characteristic average flow velocity on the flood plain between the channel and the bridge abutment.
This flow velocity is computed as follows:
V = Q/A = Q overbank/ (overbank hydraulic depth * overbank top width between the abutment and the channel.)
For an abutment with short setback, but other abutment with medium or long setback, the flow velocity is calculated as follows:
V=(Q overbank + Q Channel)/(Flow Area of Overbank + Flow area of the channel)
Please note that this information can be obtained from the ABSCOUR calculations