Water surface elevation upstream of the structure

The water surface elevation just upstream of the structure as determined from the water surface profile (HEC-RAS) model. The ABSCOUR program compares this elevation with the upstream bridge low chord or culvert crown elevation to determine whether pressure flow occurs. If it does, the thickness of the pressure flow separation zone, t, is computed to determine the increased the contraction scour at the bridge due to the pressure flow. Please refer to FHWA Manual HEC-18, Fifth Edition, Section 6.10 for a discussion of the procedure for estimating pressure flow.