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• overflow in subdivisions having enclosed systems. This system shall be an aboveground system consisting of swales or other drainage mechanisms with the capacity to carry excess water, to an approved drainage facility, not carried by the underground system. This system shall have the capacity for a one in 100-year storm design frequency. (2) In determining a drainage plan for a development, the project engineer shall assume a fully developed watershed in calculating the stormwater runoff. The engineer shall refer to city zoning maps to determine the classification of development planned for the undeveloped area in determining a "C" factor. The minimum runoff coefficient ("C" factor) for single-family detached residential areas is 0.50. (Refer to the following runoff coefficient tables for the "C" factor.) RUNOFF COEFFICIENTS FOR RATIONAL METHOD Runoff Coefficients Frequency Land Use Types 10 25 100 Residential: Single-family(detached) .50(.30-.60) .65 .70 Single-family(attached) .60(.40-.70) .65 .75 Multifamily .70(.60-.80) .75 .80 1/2 AC lots or larger .40(.25-.50) .45 .65 • Commercial: All commercial zones .85(.70-.95)* .90 .95 Industrial: Light areas .80(.50-.85) .82 .85 Heavy areas .85(.60-.90) .87 .90 Parks and cemeteries .30(.10-.40) .40 .60 laygrounds .35(.20-.40) .50 .70 Schools and churches .60(.50-.75) .65 .75 Off-site flow analysis(when land use is not defined) .55(.45-.65) .67 .70 *Note: The range of runoff coefficients is based on soil type: The low value is for sandy soils, while the high value is for clay soils. The given runoff coefficient outside the parentheses is to be used for design unless the engineer of record receives approval from city staff for another value located within the given coefficient range. (3) All open drainage ditches shall conform to the requirements of the master drainage facilities plan, and section 3.11 for open ditches in industrial subdivisions when the ditch is within the street right- of-way. Open drainage ditches along lot lines of residential property shall not be permitted unless approved by city staff,and only then in extraordinary circumstances. (4) The minimum allowable pipe size shall be 18 inches in diameter unless otherwise approved by city staff. Reinforced concrete pipe (RCP) shall be used for crossings under all collector and arterial . streets as defined by the master street plan. Coated metal pipe(CMP) and high-density polyethylene (HDPE) shall be allowed to be used for crossings under local streets, private streets and for all other underground drainage systems. -175 - Batesville Subdivision Regulations