Hydraulic Design of Highway Culverts - DOT On-Line Publications

• Flood plain ordinances or other legislative mandates limiting backwater or
encroachment on the flood plain.
• Channel stability considerations which would limit culvert velocity or the amount
of constriction.
Data collection and analysis to perform a LTEC design requires more effort than traditional
culvert design. Data collection efforts include land use information, flood plain geometry,
hydrologic and hydraulic data, geologic and soils investigation, construction cost, traffic data,
and cost of embankment and pavement repair. Hydrologic and hydraulic analyses comprise
flood frequency determination, water surface profile generation, stage-discharge relationship,
preparation, overtopping analysis, and hydrograph generation.
Based on the hydrologic and hydraulic analyses, a full accounting of economic losses is
required. These costs include but are not limited to:
• Embankment damage
• Traffic restoration time
• Increased running cost (detour)
• Time losses
• Accident costs
• Backwater damage losses
• Damage to culvert
• Erosion and sedimentation damage
The final step of the LTEC analysis requires the computation of TEC's over the range of flood
frequencies and design alternatives. An optimum design referred to as the LTEC is the end
result. A sensitivity analysis may be performed on the LTEC and the overtopping frequency can
be determined. Figure VI-26 depicts the design process. Figure VI-27 represents a typical
solution surface with the optimum culvert size and embankment height highlighted.
F. Safety
The primary safety considerations in the design and construction of a culvert are its structural
and hydraulic adequacy. Assuming that these major considerations are appropriately
addressed, attention should be directed toward supplementary safety considerations. These
considerations include traffic safety and child safety. The safety of errant vehicles should be
provided for by the appropriate location and design of culvert inlets and outlets. Safety barriers
and grates may substitute or add to this protection. Safety grates also provide a degree of
protection against inquisitive youngsters by inhibiting access to a culvert.
1. Inlet and Outlet Location and Design. The exposed end of a culvert or culvert headwall
represents an unyielding barrier to vehicles leaving the roadway. Safety provisions must be
made to protect occupants of such vehicles against injury or death. One technique employed is
to locate the culvert ends outside of the safe recovery area. Traffic safety standards provide
distance from pavement limitations based on speed limits. Culverts should also extend through
medians unless safe distances can be maintained.
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