Hydraulic Design of Highway Culverts - DOT On-Line Publications

Figure III-11--English Discharge Coefficients for Roadway Overtopping
The length and elevation of the roadway crest are difficult to determine when the crest is defined
by a roadway sag vertical curve. The sag vertical curve can be broken into a series of horizontal
segments as shown in Figure III-12-A. Using Equation (8), the flow over each segment is
calculated for a given headwater. Then, the incremental flows for each segment are added
together, resulting in the total flow across the roadway.
Representing the sag vertical curve by a single horizontal line (one segment) is often adequate
for culvert design (Figure III-12-B). The length of the weir can be taken as the horizontal length
of this segment or it can be based on the roadway profile and an acceptable variation above
and below the horizontal line. In effect, this method utilizes an average depth of the upstream
pool above the roadway crest for the flow calculation.
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