(A) A drainage system shall be designed and constructed by the subdivider to provide for the proper drainage of the surface water of the subdivision and the drainage area of which it is a part.
(B) The following requirements and methods shall be followed.
(1) Storm drainage; report required. The subdivider shall submit to the Planning Commission’s duly authorized representative, a report by a registered professional engineer, as to the ability of existing watercourse channels, storm sewers, culverts and other improvements pertaining to drainage or flood control within the subdivision, to handle the runoff which would be generated by the development of the land within the subdivision, and the additional runoff to be anticipated from the upstream drainage area, according to the land use identified in the adopted comprehensive plan for the county. Additional information shall be submitted to adequately indicate that provision has been made for disposal of surface water without any damage to the developed or undeveloped land downstream or below the proposed subdivision. This report shall also include:
(a) Estimate of the quantity of storm water entering the subdivision naturally from areas outside the subdivision;
(b) Quantities of flow at each pickup point (inlet); and
(c) Location, sizes and grades of required culverts, storm drainage sewers and other required appurtenances.
(2) Drainage requirements (final grading).
(a) No final grading or sidewalk or pavement construction or installation of utilities shall be permitted in any proposed street until the improvement drawing and specifications have been submitted and approved by the Planning Commission’s duly authorized representative.
(b) The subdivider shall grade each subdivision in order to establish street, block and lot grades in proper relation to each other and to topography, as follows.
1. Street grading plan. A grading plan shall be prepared for the streets along with street improvement details, as per § 157.104 of this chapter.
2. Block grading. Block grading shall be accomplished in one or more of the following methods:
a. A ridge constructed along the rear lot lines providing for drainage into the streets;
b. Parts of all lots draining to a sidewalk or ditch along the rear lot lines; or
c. Draining across rear or side lot lines; provided that, drainage onto adjoining properties shall be controlled.
3. Lot grading. Lot grading shall be accomplished as follows: lots shall be graded so that water drains away from each building at a minimum grade of 2%. Surface drainage swales shall have a minimum grade of 2% and shall be designed so that the surface water will drain into a driveway, street gutter, storm sewer, drain inlet or natural drainage way. The minimum grades of driveways shall be 0.4% and the maximum grade shall be as specified in § 157.105 of this chapter.
4. Top soil. If grading results in the stripping of top soil, top soil shall be uniformly spread over the lots as grading is finished.
5. Trees. As many trees as can be reasonably utilized in the final development plan shall be retained and the grading adjusted to the existing grade of the trees where practicable.
(3) Drainage system requirements. The design criteria for storm drainage systems shall be based on information from the Manual of Instructions for Drainage Design prepared by the state’s Department of Transportation or any other acceptable drainage manual.
(a) Culverts and storm sewers in all streets shall be designed for at least a ten-year storm frequency and in no case shall a storm sewer be less than 12 inches in size. In no case shall the time of concentration be less than eight minutes.
(b) For open channels, roadway ditches, curb and gutters, and all inlets, a storm frequency of ten years and a time of concentration of 20 minutes which yields an average intensity of four inches per hour shall be used in all computations.
(4) Road drainage system. All roadways shall be provided with an adequate storm drainage system.
(a) The road storm sewer system shall serve as the primary drainage system and shall be designed to carry roadway, adjacent land and building storm water drainage. No storm sewer shall be permitted to be run into the sanitary sewer system within the proposed subdivision.
(b) Curb drainage inlets and catch basins shall be provided at intervals along roadways. Inlet spacing shall be adequate to ensure that the flow of water should not encroach on the outside lane by more than six feet for a storm of 20 minutes duration and ten-year storm frequency.
(5) Off-road drainage systems. The design of any off-road drainage shall include the watershed affecting the subdivision and shall be extended to a watercourse or drainage way adequate to receive the storm drainage.
(a) When the drainage system is outside of the road right-of-way, the subdivider shall make provisions for dedicating of an easement to the applicable legislative body or fiscal court to provide for the future maintenance of the system.
(b) The size and location of all off- street watercourses or ditches running through the subdivision shall be enclosed or left open, subject to the approval of the Planning Commission’s duly authorized representative.
(c) Roof and area drain pipes shall not be connected into or through the curb and gutter section of the public street, but rather directly connected to a storm sewer system or discharged onto the same parcel of land from which the water is generated.
(6) Drainage easement. Easements for drainage purposes shall be a minimum of ten feet in width. Where the watercourse is large, easement widths shall be increased as determined by the Planning Commission’s duly authorized representative. Where watercourses cross platted lots diagonally, the subdivider should straighten such courses where practicable and should substantially follow sublot lines. Easements shall be shown on the record plat and shall cover all existing or reconstructed watercourses.
(7) Protection of drainage systems. The banks of newly created ditches and channels shall be stabilized with adequate material to prevent erosion. Newly created ditches and open channels shall be seeded, sodded, rip-rapped, paved or other acceptable erosion control methods used depending on grades, slopes and type of soils. Wherever practical, existing ditches and channels shall be retained in their natural state.
(8) Material specifications. Material and construction specifications for all drainage projects shall be designed in accordance with the requirements of App. C of these regulations.
(1984 Code, § 157.71) (Ord. O-86-78, passed 8-17-1978)