Hydraulic Design of Highway Culverts - DOT On-Line Publications

When a tailwater exists, the control device may be submerged so that more than one flowversus-elevation
relationship exists. Then, the performance curve is dependent on the variation
of both tailwater and headwater. In the case of a weir or orifice, the device is called a
submerged weir or a submerged orifice, respectively. For some cases, submergence effects
have been analyzed and correction factors have been developed. (21,22,24)
Culvert performance curves are similar to weir and/or orifice performance curves. In fact,
culverts often behave as weirs or orifices. However, due to the fact that a culvert has several
possible control sections (inlet, outlet, throat), a given installation will have a performance curve
for each control section and one for roadway overtopping. The overall culvert performance
curve is made up of the controlling portions of the individual performance curves for each
control section.
1. Inlet Control. The inlet control performance curves are developed using either the inlet
control equations of Appendix A or the inlet control nomographs of Appendix D. If the equations
of Appendix A are used, both unsubmerged (weir) and submerged (orifice) flow headwaters
must be calculated for a series of flow rates bracketing the design flow. The resultant curves are
then connected with a line tangent to both curves (the transition zone). If the inlet control
nomographs are used, the headwaters corresponding to the series of flow rates are determined
and then plotted. The transition zone is inherent in the nomographs.
2. Outlet Control. The outlet control performance curves are developed using Equations (1)
through (7) of this chapter, the outlet control nomographs of Appendix D, or backwater
calculations. Flows bracketing the design flow are selected. For these flows, the total losses
through the barrel are calculated or read from the outlet control nomographs. The losses are
added to the elevation of the hydraulic grade line at the culvert outlet to obtain the headwater.
If backwater calculations are performed beginning at the downstream end of the culvert, friction
losses are accounted for in the calculations. Adding the inlet loss to the energy grade line in the
barrel at the inlet results in the headwater elevation for each flow rate.
3. Roadway Overtopping. A performance curve showing the culvert flow as well as the flow
across the roadway is a useful analysis tool. Rather than using a trial and error procedure to
determine the flow division between the overtopping flow and the culvert flow, an overall
performance curve can be developed. The performance curve depicts the sum of the flow
through the culvert and the flow across the roadway.
The overall performance curve can be determined by performing the following steps.
1. Select a range of flow rates and determine the corresponding headwater elevations for the
culvert flow alone. These flow rates should fall above and below the design discharge and cover
the entire flow range of interest. Both inlet and outlet control headwaters should be calculated.
2. Combine the inlet and outlet control performance curves to define a single performance
curve for the culvert.
3. When the culvert headwater elevations exceed the roadway crest elevation, overtopping will
begin. Calculate the equivalent upstream water surface depth above the roadway (crest of weir)
for each selected flow rate. Use these water surface depths and Equation (8) to calculate flow
rates across the roadway.
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