Hydraulic Design of Highway Culverts - DOT On-Line Publications

4. Add the culvert flow and the roadway overtopping flow at the corresponding headwater
elevations to obtain the overall culvert performance curve.
Using the combined culvert performance curve, it is an easy matter to determine the headwater
elevation for any flow rate, or to visualize the performance of the culvert installation over a range
of flow rates. When roadway overtopping begins, the rate of headwater increase will flatten
severely. The headwater will rise very slowly from that point on. Figure III-16 depicts an overall
culvert performance curve with roadway overtopping. Example problem III-4 illustrates the
development of an overall culvert performance curve.
Figure III-16--Culvert Performance Curve with Roadway Overtopping
C. Culvert Design Method
The culvert design method provides a convenient and organized procedure for designing
culverts, considering inlet and outlet control. While it is possible to follow the design method
without an understanding of culvert hydraulics, this is not recommended. The result could be an
inadequate and possibly unsafe structure.
1. Culvert Design Form. The Culvert Design Form, shown in Figure III-17, has been
formulated to guide the user through the design process. Summary blocks are provided at the
top of the form for the project description, and the designer's identification. Summaries of
hydrologic data of the form are also included. At the top right is a small sketch of the culvert with
blanks for inserting important dimensions and elevations.
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