748.6 High Water Surface Elevation

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748.6.1 Design Flood Elevation

The design flood elevation is the high water surface elevation at the location of the backwater with MoDOT bridges, culverts and roadway fills in place. The frequency (for example, 25-year, 50-year) and discharge of the design flood is based on the roadway design criteria.

748.6.2 Base Flood Elevation

The base flood elevation is defined the same as the design flood elevation except that the discharge is based on Base (100-year) Flood frequency.

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 water surface elevation.

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

748.6.4 Accuracy

See EPG 751.5.2.1, Hydraulic Data.

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