Hydraulic Design of Highway Culverts - DOT On-Line Publications

This approximate method works best when the barrel flows full over at least part of its length
(Figure III-9-B). When the barrel is partly full over its entire length (Figure III-9-C), the method
becomes increasingly inaccurate as the headwater falls further below the top of the barrel at the
inlet. Adequate results are obtained down to a headwater of 0.75D. For lower headwaters,
backwater calculations are required to obtain accurate headwater elevations.
The outlet control nomographs in Appendix D provide solutions for Equation (5) for entrance,
friction, and exit losses in full barrel flow. Using the approximate backwater method, the losses
(H) obtained from the nomographs can be applied for the partly full flow conditions shown in
Figures III-7 and III-9. The losses are added to the elevation of the extended full flow hydraulic
grade line at the barrel outlet in order to obtain the headwater elevation. The extended hydraulic
grade line is set at the higher of (dc+ D)/2 or the tailwater elevation at the culvert outlet. Again,
the approximation works best when the barrel flows full over at least part of its length.
Figure III-10--Roadway Overtopping
3. Roadway Overtopping. Overtopping will begin when the headwater rises to the elevation of
the roadway (Figure III-10). The overtopping will usually occur at the low point of a sag vertical
curve on the roadway. The flow will be similar to flow over a broad crested weir. Flow
coefficients for flow overtopping roadway embankments are found in HDS No. 1, Hydraulics of
Bridge Waterways (21), as well as in the documentation of HY-7, the Bridge Waterways
Analysis Model (22). Curves from reference (22) are shown in Figure III-11. Figure III-11-A is for
deep overtopping, Figure III-11-B is for shallow overtopping, and Figure III-11-C is a correction
factor for downstream submergence. Equation (8) defines the flow across the roadway.
Qo= CdL HWr 1.5 (8)
Qo is the overtopping flow rate in m³/s (ft³/s)
*Cd is the overtopping discharge coefficient (*for use in SI units, see note)
L is the length of the roadway crest, m (ft)
HWr is the upstream depth, measured from the roadway crest to the water surface
upstream of the weir drawdown, m (ft)
*Note: Cd determined from Figure III-11 and other English unit research must be corrected by
a factor of 0.552 [Cd (SI) = 0.552 (Cd English)]
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