Hydraulic Design of Highway Culverts - DOT On-Line Publications

APPENDIX C
CULVERT DESIGN OPTIMIZATION USING PERFOREMANCE CURVES
NOTE: Figures in this Appendix are only provided in English Units since they are only
illustrating a concept and are not example problems.
A. Introduction
Performance curves are an integral part of the culvert design process and can be used to
optimize the selected culvert design, particularly when using tapered inlets and/or upstream
depressions. This optimization may involve further reduction in the barrel size required to pass
the design flow at the design headwater, provision of a factor of safety against damages, or a
more balanced design. The visualization of culvert performance provided by performance
curves may lead to a further reduction in the size of the culvert barrel. At many culvert sites,
designers provide a safety factor in the design. The safety factor may compensate for: (1)
uncertainty in the design discharge estimate, (2) potentially disastrous results in property
damage or damage to the highway from headwater elevations which exceed the design
headwater, (3) the potential for development upstream or downstream of the culvert, or (4) the
chance that the design frequency flood will be exceeded during the life of the installation. The
procedures described here enable the designer to maximize the performance of the selected
culvert or to optimize the design in accordance with his evaluation of site constraints, design
parameters, and costs for construction and maintenance.
B. Outlet Control Performance Curves
The outlet control performance curves for various barrel sizes and inlet configurations are used
first to evaluate the operation of the selected barrel. The full flow outlet control performance
curve for a given culvert (size, inlet edge configuration, barrel shape, material) defines its
maximum performance. Inlet improvements beyond the beveled edge or changes in inlet invert
elevations will not reduce the required outlet control headwater elevation. Therefore, the outlet
control performance curve is an ideal minimum limit for culvert design.
When the barrel size is increased, the outlet control curve is shifted to the right, indicating a
higher capacity for a given head. Also, it is generally true that increasing the barrel size will
flatten the slope of the outlet control curve (Figure C-1).
The outlet control curve passing closest to and below the design point (design Q and design
headwater elevation) on the performance curve graph defines the smallest possible barrel which
meets the hydraulic design criteria. The curve for the smallest possible barrel may be very
steep (rapidly increasing headwater requirements for discharges higher than the design
discharge) and use of such a small barrel may not be practical due to high outlet velocities or
flooding from flows exceeding the design flow.
To define the outlet control performance curves, perform the following steps:
1. Calculate the headwater elevations at the design discharge for a selected series of culvert
sizes, inlet configurations, shapes, and materials.
209