Design flood frequency

The following table provides the minimum design standard recurrence interval of the event for which each type of stream crossing must be designed. Nothing contained in this chapter is intended to restrict an entity from providing greater capacity. Type of Crossing State Highway System County Urban System Rural System Rural System Regional Urban Roads Principal Arterial Minor Arterial Major Collector Major Collector Off4 System Interstate Other Bridges & Reinforced Concrete Boxes 25 year2 25 year2 50 year2 50 year2 50 year2 25 year2 25 year2, 3 15 year2, 3 Roadway Culverts 25 year2 25 year2 50 year2 25 year2 25 year2 25 year2 25 year2, 3 15 year2, 3, 5 Storm Drains 10 year1 5 year1 10 year2 10 year2 10 year2 10 year2 Underpass Storm Drains 25 year1 25 year1 50 year2 25 year2 25 year2 25 year2 1Discharges must be computed using the rational method or other recognized hydrologic methods. 2Discharges must be computed using United States geological survey report 2015-5096 or other recognized hydrologic methods. 3If an overflow section is provided, the pipes and the overflow section, in combination, must pass the appropriate design event within the headwater limitations provided in this chapter. 4Off system roads include all nonmajor collector public roads open for public travel. 5For public roads open for public travel under the jurisdiction of a board of township supervisors of a civil township, the recurrence interval is 10 years.