Roadways: Design Storm

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Design Storm

Primary Guidance

  • Size bridges and culverts to safely pass the design year storm without overtopping as based upon the roadway functional classification and risk assessment
  • A scour evaluation is required for bridges and bottomless culverts over a waterway
  • See BridgeA structure including supports erected over a depression or an obstruction, such as water, highway, or railway, and having a track or passageway for carrying traffic or other moving loads, and having an opening measured along the center of the roadway of more than 20 feet between undercopings of abutments or spring lines of arches, or extreme ends of openings for multiple boxes; it may also include multiple pipes, where the clear distance between openings is less than half of the smaller contiguous opening. For lengths, all dimensions shall be parallel to the center line of the roadway. The dimensions of handrails will not be taken into account in measuring bridge lengths. Any bridge or highway grade separation structure includes the connecting highways, substructure, superstructure, roadway approaches, entrance plazas, interchanges, overpasses, underpasses, and other structures which the Administration may deem necessary together with all property rights, easements, franchises, and interests acquired by the Administration for the construction and operation of the bridge. – Stream Crossing for bridges and large culverts
  • See Drainage/Hydraulics – Culverts for small culverts


Design Storm for Bridges and Culverts over Waterways

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.

AASHTOAmerican Association of State Highway and Transportation Officials Classification Design Storm Frequency
Interstate,other Freeways, and Expressways and Principal Arterials 100 Year
Intermediate and Minor Arterials 50 Year
Collectors 25 Year
Local Roads 10 Year

At a minimum, size structures 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, when there is no history of overtopping, or when based on value-engineering. Site conditions, such as existing development, may also constrain solutions that do not meet the design storm, resulting in in-kind replacements.

In addition, hydrology and hydraulic analyses must comply with COMARCode of Maryland Regulations 26.17.04 regarding impacts to floodplains. Water surface increases must be limited to 0.1 ft. on improved properties; up to 1.0 ft. on unimproved properties with property owner notification and/or concurrence and mitigation. If the structure is within a FEMA floodplain, the water surface increase must be kept to 0.00 ft.


A scour evaluation or assessment must be performed and approved for any bridge or bottomless culvert over a waterway that is to be rehabilitated or replaced with federal or State funds. StructuresBridges, culverts, catch basins, drop inlets, retaining walls, cribbing, manholes, end walls, buildings, sewers, service pipes, underdrains, foundation drains, steps, fences, and other features that may be encountered in the work and not otherwise classified. with paved bottoms (pipes, pipe arches, box culverts, etc.) do not require a scour evaluation. A scour evaluation is a detailed scour study to estimate scour depths at substructure foundations, and a scour assessment consists of a field and office review of plans and records to determine the degree of risk of scour damage. If the scour damage risk is low, no further study is needed. If the risk is high, detailed scour evaluations or additional studies are needed. Action is needed to address and minimize the potential for scour damage and resulting risk to the public. Scour evaluations should be performed for the 100 year storm regardless of the roadway functional classification. This is to ensure that the bridge will not wash out in a major storm event, which can lead to a loss of life. The resulting cost increase to the substructure foundations are significantly lower than the cost of replacing an entire bridge.

See Also