(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.
(a) A subdivision plat shall not be considered for preliminary approval until the subdivider shall submit to the Planning Commission a written report by a 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 additional run-off for the storms noted below which would be generated by the planned future development of the land within the area according to long range plan. There shall be no increase in the rate of run-off as a result of new construction.
(b) 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:
1. Estimates of the quantity of storm water entering the subdivision naturally from areas outside the subdivision on each inlet;
2. Quantities of flow at each pick-up point (inlet);
3. Location, sizes and grades of required culverts, storm drainage sewers and other required appurtenances. This shall be recorded on the preliminary plan as well as the design calculation of the following factors for each culvert or channel: drainage area discharge-Q; amount of D.A. imperviousness-C; intensity of rainfall-I; time concentration-TC; amount of head-H; length of runoff-L; and D.A. in acres-A; and the design-Q for each structure, and the resulting velocity; and
4. The basic standard for design of drainage systems for subdivisions will be to keep run-off characteristics after development at the same level as existed prior to development. No excess stormwater runoff is permitted to occur. To achieve these objectives, storm water retention and/or detention systems will be required in most cases. The analysis used for the design of the required retention/detention systems shall consider impact on downstream properties for a ten-year/24-hour storm, a 25-year/24-hour storm, and a 100-year/24-hour storm. Likewise, where conditions and engineering calculations indicate benefit would not occur, storm drainage detention facilities may be deleted from the development requirements in favor of channel improvements, off-site improvements to improve flow, or other alternative wherein the alternate is less burdensome to the developer than providing detention facilities. In computing storm water run-off and designing the drainage system, the developer’s engineer shall use standard engineering practice which shall be approved by the Planning Commission Engineer.
(2) Drainage requirements (grading).
(a) No final grading or sidewalk or pavement construction or installation of utilities shall be permitted in any proposed street until the preliminary plan has been approved by the Planning Commission.
(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 plat shall be prepared for the streets along with street improvement details. The grading of the roadway shall extend the full width of the right-of-way. Planting strips shall be graded at a gradient of not less than 2% upward from the curb to the sidewalk or property line.
2. Lot grading. Lots shall be graded so that water drains away from each building at a minimum grade of 2%. Surface drainage swales shall meet the standards and specifications of the county’s Conservation District and shall be designed so that the surface water will drain into a driveway, street gutter, storm sewer, drain inlet, or natural drainage way. Contractors shall return lots to approved grade if disturbed during building construction.
3. Top soil. If upgrading results in the stripping of top soil, the top soil shall not be removed from the site or used as spoil and shall be uniformly spread over the lots as grading is finished.
4. 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.
(a) The design criteria for storm drainage systems shall be based on information from the Manual of Instruction for Drainage Design prepared by the state’s Department of Highways.
(b) Culverts shall be designed for 50-year storm frequency unless the culvert is a part of the stormwater retention/detention system. Storm sewers in all streets shall be designed for a 25-year storm frequency.
(c) For curbs and gutters, curb and gutter inlets and open channels an intensity of four inches per hour shall be used in all computations.
(d) A minimum of “TC” of eight minutes shall be used as well as the “C” valve shall be based on the planned future development of the watershed area according to the Comprehensive Plan.
(4) Road drainage system.
(a) All roadways shall be provided with an adequate storm drainage system.
(b) 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 water shall be permitted to be run into the sanitary sewer system within the proposed subdivision. Since the road drainage is a major contributing factor to excess surface runoff, it must be routed through the storm water retention facilities.
(c) Curb drainage box inlets shall be type A or B (unless otherwise ap proved by the Commission) and provided at intervals along roadways. Inlet spacing shall be adequate to limit the spread of water to two feet into the roadway. Storm drain inlets will be placed so that crosswalks will not be flooded during the design storm intensity of four inches per hour.
(5) Off-road drainage systems. The design of the off-road drainage system shall include the watershed affecting the subdivision and shall be extended to a watercourse or ditch 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 an easement to provide for the future maintenance of said system.
(b) If a watercourse or ditch is left open it may be required to be protected by a fence as determined by the Planning Commission. The watercourse or ditch easement shall be wide enough to contain said ditch slope with ample clearance for the operation of maintenance equipment. The side slopes of the ditch shall not be greater than three to one.
(6) Drainage easement. Easements for drainage purposes shall be a minimum of 20 feet in width. Where the watercourse is large, easement widths shall be increased as determined by the Planning Commission. Where watercourses cross platted lots diagonally, the subdivider shall straighten such courses where practicable and shall substantially follow sub-lot lines. Easements shall be shown on the record plat and shall cover all existing or reconstructed watercourses.
(7) Protection of drainage system.
(a) The subdivider shall adequately protect all ditches to the satisfaction of the Planning Commission. Ditches and open channels shall be seeded, sodded, or paved depending on grades (slopes) and types of soils.
(b) As a general rule, ditches and channels with grades up to 1% shall be seeded, with grades from 1% to 4% shall be sodded and with grades over 4% shall be paved. Paving operations shall be in compliance with the state’s Department of Highways, Division of Design, Guidance Manual issued by the state’s Department of Highways. Seeding and sodding operations shall be in accordance with the recommendations of the county’s Conservation District.
(8) Material specifications. Material and construction specifications for all drainage projects (i.e., pipe, tile, seed, sod) shall be in compliance with the “Standard Specifications for Road and Bridge Construction” issued by the state’s Department of Highways.
(Prior Code, § 74.850) (Ord. passed 3- -1995; Ord. passed 10- -2005; Ord. 2018-03, passed 2-14-2018; Ord. passed 1- -2020)