Abutment Scour Computation: Step 1. Water Surface Profile

Prepare a water surface profile using HEC-RAS or WSPRO to model flow conditions upstream of, through and downstream of the bridge. The discharge selected for analysis should represent the worst case scour condition for the bridge. This may require more than one run to develop the anticipated worst case conditions at the bridge. Typically, scour studies should evaluate the 100-year, the 500-year and the overtopping floods.

Carefully select Manning's "n" values for the flood plain areas. There appears to be a tendency of some engineers to underestimate the flood plain roughness, thereby assigning too much flow to the flood plain areas. This results in higher estimates of overbank contraction scour and abutment scour.

The approach section must be selected so that it is upstream of the influence of the bridge contraction.