Roadways: Culverts

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Primary Guidance

  • Size culverts to safely pass the design year storm without overtopping based upon the functional classification and risk assessment
  • Choose the culvert slope, length, and skew to approximate existing topography to the degree practicable
  • Align the culvert invert with the channel bottom, unless aquatic passage needs require a depressed bottom
  • Consider the backwater/headwater impacts on floodplains and adjacent properties
  • Select culvert material as based upon the roadway functional classification and consider replacement cost, construction difficulty, access, and maintenance of traffic impacts associated with installation, repair, and replacement activities
  • Consider culvert end treatment, long-term stability, and accessibility
  • Assess the need to resize existing culverts
  • Provide outfall protection using the same design storm as the culvert.
  • See Bridge – Stream Crossing for large culverts


Roadway Overtopping Allowable roadway overtopping is based upon the roadway classification , the level of risk associated with culvert and roadway failure, potential backwater increases, possible redirection of floodwaters, FEMA floodplains location, and an economic assessment or analysis justifying flood frequencies different than the minimum flood frequencies listed below.

At a minimum, size culverts to preclude overtopping by the design storm for the roadway classification . Design exceptions may be granted on a case-by-case basis in instances where overtopping is considered low risk, there is no history of overtopping, or based on value-engineering. Site conditions, such as existing development, may dictate solutions that do not meet the design storm, resulting in in-kind replacements.

Provide 1 ft. freeboard from the water surface elevation to the lowest shoulder point. Ensure that flow does not divert away from the culvert.

Do not create a condition where the roadway embankment may be considered a dam; HW/D > 2.0.

Material Selection Material selection affects culvert performance and longevity. Select culvert materials that will withstand loads and environmental factors and provide for a Service Life commiserate with the roadway functional classification . Some of the factors to consider are:

  • Durability as it relates to service life
  • Longevity as it relates to long-term outfall and channel stability
  • Structural strength
  • Hydraulic roughness
  • Bedding conditions
  • Abrasion and corrosion resistance
  • Water tightness requirements
  • Difficulty and costs associated with replacement including height of fill, access and maintenance of traffic impacts

End Treatment Provide end treatments at all culvert ends. Ensure culvert ends within the clear zone are traversable or protected. Protect all culvert outlets for the culvert design flood. Where computations indicate that no protection is required, provide a minimum 10 ft. long riprap apron . Whenever practicable, consider and provide access to both culvert ends for future maintenance cleaning, lining, replacement, and entrance and outfall repair/stabilization.

Existing Structures Where existing infrastructure does not have a history of overtopping, is in good condition, and improvements could cause downstream impacts, consider leaving the existing culvert in place with design exception.

Outfall Protection Design outfall protection according to HEC-14(link). Consider impacts to stream stability and aquatic passage.

See Also