748.6 High Water Surface Elevation

748.6.1 Design High Water

The design high water surface elevation is the normal water surface elevation at the centerline of the proposed roadway for the roadway design criteria discharge. The normal water surface elevation is the elevation of the water surface across the floodplain without MoDOT bridges, culverts, or roadway fills in place.

748.6.2 Backwater/Base Flood High Water

The high water elevation for the Backwater/Base Flood is the same as the design high water surface elevation except that the design discharge is based on Base (100-year) Flood discharge.

748.6.3 Freeboard High Water

The high water surface elevation for freeboard is the design water surface elevation for the freeboard design frequency discharge at the lower limit of the bridge superstructure (with structure in place). Consideration should be given to using the approach high water surface elevation, which is the water surface elevation just upstream of the bridge, as the freeboard high water elevation when the minimum freeboard is at the upstream edge/girder of the bridge. The freeboard high water surface elevation should not be lower than the normal high water surface elevation.

The location of the freeboard high water surface elevation should be noted in the hydraulics report.