Hydraulic Design of Highway Culverts - DOT On-Line Publications

The central portion of the design form contains lines for inserting the trial culvert description and
calculating the inlet control and outlet control headwater elevations. Space is provided at the
lower center for comments and at the lower right for a description of the culvert barrel selected.
The first step in the design process is to summarize all known data for the culvert at the top of
the Culvert Design Form. This information will have been collected or calculated prior to
performing the actual culvert design. The next step is to select a preliminary culvert material,
shape, size, and entrance type. The user then enters the design flow rate and proceeds with the
inlet control calculations.
Figure III-17--Culvert Design Form
2. Inlet Control. The inlet control calculations determine the headwater elevation required to
pass the design flow through the selected culvert configuration in inlet control. The approach
velocity head may be included as part of the headwater, if desired. The inlet control
nomographs of Appendix D are used in the design process. For the following discussion, refer
to the schematic inlet control nomograph shown in Figure III-18.
a. Locate the selected culvert size (point 1) and flow rate (point 2) on the appropriate scales of
the inlet control nomograph. (Note that for box culverts, the flow rate per foot of barrel width is
used.)
b. Using a straightedge, carefully extend a straight line from the culvert size (point 1) through
the flow rate (point 2) and mark a point on the first headwater/culvert height (HW/D) scale (point
3). The first HW/D scale is also a turning line.
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