Bridge (Contracted) Section

A basic limitation of the current programs used to model flow through bridges such as HEC-RAS and WSPRO is that they provide for the distribution of flow under the bridge based on conveyance calculations. This approach does not appear to adequately reflect the three dimensional flow patterns observed in the field at bridge contractions. For scour calculations, it is important to account for the high local flow velocities and turbulence near the abutments caused by the contracting flow in the overbank areas upstream of the bridge.

Findings from recent laboratory studies of compound channels indicate that the velocity of flow under a bridge tends to be highest at the abutments (due to rapid acceleration and turbulence of the overbank flow entering the bridge contraction) and in the channel. Converging flows under bridges with abutments near the channel banks tend to distribute uniformly, with higher local velocities observed at piers and abutments. This phenomenon has been observed in field surveys conducted by the U. S. Geological Survey and is consistent with the theory of potential flow at a contraction. On the other hand, if an abutment is set well back from the channel bank, the overbank flow and the main channel flow tend to remain separated from each other and do not mix as the flow passes under the bridge. This concept is applied in the ABSCOUR model in the following manner.

Click here to see the sketch for sections

Click here to the the profile through the bridge