General. Each primary road shall be designed, constructed or reconstructed, and maintained to have adequate drainage, using structures such as but not limited to bridges, ditches, cross drains, and ditch relief drains. The drainage control system shall be designed to safely pass peak runoff from a 2-year, 24-hour precipitation event. Drainage pipes and culverts shall be installed as designed and maintained in a free and operating condition and to prevent or control erosion at inlets and outlets. Drainage ditches shall be constructed and maintained to prevent uncontrolled drainage over the road surface and embankment. Culverts shall be installed and maintained to sustain the vertical soil pressure, the passive resistance of the foundation, and the weight of vehicles using the road. Ditches. A ditch shall be provided on both sides of a through-cut and on the inside shoulder of a cut-and-fill section, with ditch relief cross drains spaced according to a grade. Water shall be intercepted before reaching a switchback or large fill and drained safely away. Water from a fill or switchback shall be released below the fill, through conduits or in riprapped channels, and may not be discharged onto the fill. Drainage ditches shall be placed at the toe of all cut slopes formed by the construction of roads. Culverts. Culverts shall be designed to safely pass a 2-year, 24-hour precipitation event. The applicant shall be responsible for obtaining all other local, State, and federal permits pertaining to waterway crossings or obstructions. Drainage pipes and culverts shall be constructed to avoid plugging, collapse, or erosion at inlets and outlets. Culvert openings installed on the road may not be less than 12 inches in diameter. All culverts shall be covered by compacted fill to a minimum of 1 foot, or half the culvert diameter, whichever is greater. Culverts for road-surface drainage only shall be constructed in accordance with the following: Spacing may not exceed 1,000 feet on grades of 0 to 3 percent; Spacing may not exceed 800 feet on grades of 3 to 6 percent; Spacing may not exceed 300 feet on grades of 6 to 10 percent; Spacing may not exceed 200 feet on grades of 10 percent or greater; Culverts at closer intervals than the maximum listed above shall be installed if required by the Bureau as appropriate for the erosive properties of the soil or to accommodate flow from small intersecting drainages; Culverts may be constructed at greater intervals than the maximum if authorized by the Bureau upon a finding that greater spacing will not increase erosion; The inlet end shall be protected by a rock headwall or other material approved by the Bureau as adequate protection against erosion of the headwall. The water shall be discharged below the toe of the fill through conduits or in riprapped channels and may not be discharged onto the fill. If it is necessary to control sediment, culverts shall have a vertical riser on the inlet end. The top of the riser shall be lower than the centerline of the road to ensure adequate freeboard (a minimum of 1 foot is recommended). These structures shall be maintained and cleaned out when the sediment reaches 60 percent of their capacity. Natural Drainage. Natural channel drainageways may not be altered or relocated for road construction or reconstruction without the prior approval of the Bureau. The Bureau may approve alterations and relocations only if: The natural channel drainage is not blocked; No significant damage occurs to the hydrologic balance; and There is no adverse impact on adjoining landowners. Stream Crossings. Drainage structures are required for stream channel crossings. Drainage structures may not affect the normal flow of gradient of the stream, or adversely affect fish migration and aquatic habitat or related environmental values. The applicant shall be responsible for obtaining all other local, State, and federal permits pertaining to waterway crossings or obstructions.