Roadways: Storm Drain Systems

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Storm Drains

Primary Guidance

  • Design storm drain systems to maintain nominal traffic speeds and prevent stormwater runoff damage to surroundings
  • Convey the design storm flows and prevent adverse impacts to adjacent properties and outfalls
  • Locate inlets to minimize spread, ponding, and reduce vehicular hydroplaning potential
  • Provide:
    • Adequate system capacity to accommodate runoff entering the system
    • Adequate accessibility for inspection and maintenance
    • A system that requires minimal maintenance to remain functional and durable
  • Consider the age, condition, and costs versus benefits of improving system function when existing infrastructure is encountered and found to be functionally deficient
  • Consider the roadway safety and integrity when existing infrastructure is encountered and include appropriate repair or replacement when safety or structural integrity is or may be compromised


A storm drain system may be a closed-conduit, open-conduit, or some combination of the two. Ensure storm drain systems have adequate capacity to accommodate the runoff that enters into it. Consider future development or system expansion/extension when appropriate.

The minimum storm drain system design frequency is based on a 10 year storm full-flow condition with the 25 year HGL staying within the system. Use a higher design frequency (or return interval) for storm drain systems located in major sags/vertical curves to decrease the depth of ponding on roadways and bridges, and potential inundation of adjacent property, typically by ensuring that the 100 year HGL remains below the top of curb for closed-section roadways. Where feasible, design storm drain systems to avoid existing utilities and provide adequate clearance. Ensure storm drain system outfalls are designed to minimize the potential for erosion. In instances where a storm drain system also serves as a culvert roadway crossing, use the Culverts criteria to evaluate overtopping.

Limit placing manholes in travel lanes to prevent vehicles from swerving to avoid them and facilitate maintenance access without the need for lane closures.

Locate grate inlets and depressed curb-opening inlets outside the through traffic lanes to minimize vehicles from swerving to avoid them. Ensure all grated inlets use “bicycle-friendly ” grates when used on roadways that allow bicycle travel. Curb inlets are preferred to grate inlets at major sag locations because of their debris handling capabilities. When grate inlets are used at sag locations, assume that they are half plugged with debris and size accordingly to reduce ponding potential. In locations where significant ponding may occur (e.g., underpasses, sag vertical curves in depressed sections), place flanking inlets on each side of the inlet at the low point in the sag to reduce spread as much as practicable. Locate inlets to minimize spread and provide safer driving conditions using 6 inches per hour rainfall intensity.

System maintenance and durability is dependent on the materials used and system access. Select pipe materials to meet the same service life and selection criteria as Culverts. Provide access at pipe junctions to facilitate maintenance and cleanability. Avoid long pipe runs without adequate access. Provide self-cleaning pipe runs.

There may be instances where it is not practical to upgrade existing infrastructure to current design standards. At a minimum, existing drainage conditions may not be degraded at any location. Worsened flow conditions in both peak discharge and velocity require some overall improvement. Seek design exceptions for systems that do not meet the standard design criteria, and provide adequate documentation supporting why system upgrades to current standards may not be warranted.

See Also