(1) General requirements. The Commission shall not recommend for approval any subdivision plat which does not make adequate provision for stormwater or floodwater runoff channels or basins. The stormwater drainage system shall be separate and independent of any sanitary sewer system. Storm sewers, where required, shall be designed as approved by the Commission and the County Drainage Board, and a copy of the design computations shall be submitted along with the plans. Inlets shall be provided so that surface water is not carried across or around any intersection, nor for a distance of more than 600 feet in the gutter or when the encroachment of stormwater into the street disrupts traffic. When calculations indicate that curb capacities are exceeded at a point, no further allowance shall be made for flow beyond that point, and catch basins or inlets shall be used to intercept flow at that point. Surface water drainage patterns shall be shown for each and every lot and block.
(2) Nature of stormwater facilities.
(a) Location. The applicant may be required by the Commission to carry away by pipe or open ditch any spring or surface water that may exist, either previously to, or as result of, the subdivision. Such drainage facilities shall be located in the street right-of-way, where feasible, or in perpetual, unobstructed casements of appropriate width and shall be constructed in accordance with the county’s construction standards and specifications.
(b) Accessibility to public storm sewers.
(i) Where a public storm sewer is accessible, the applicant shall install storm sewer facilities, or if no outlets are within a reasonable distance adequate provision shall be made for the disposal of stormwater, subject to the specifications of the County Engineer. However, in such subdivisions containing lots of less than 15,000 square feet in areas and in business and industrial districts, underground storm sewers systems shall be constructed throughout the subdivision and be conducted to an approved outfall. Inspection of facilities shall be conducted by the County Engineer.
(ii) If a connection to a public storm sewer will be provided eventually, as determined by the County Engineer (see comment above) and the Commission, the developer shall make arrangements for future stormwater disposal by the public utility system at the time the plat received final approval. Cost provision(s) for such connection(s) shall be incorporated by inclusion in the amount of the performance bond or equivalent required for the subdivision plat.
(c) Accommodation of upstream drainage areas. A culvert or other drainage facility shall in each case be large enough to accommodate potential runoff from its entire upstream drainage area, whether inside or outside the subdivision. The County Engineer shall determine the necessary size of the facility, based on the provisions of the required construction standards and specifications assuming conditions of maximum potential watershed development permitted by the Zoning Ordinance.
(d) Effect on downstream drainage areas. The County Engineer (see comment under subsection (b)(i) above) shall determine the effect of each proposed subdivision on existing drainage facilities outside the area of the subdivision. County drainage studies together with such other studies as may be available and appropriate, shall serve as a guide to needed improvements. Where it is anticipated that the additional runoff incident to the development of the subdivision will overload an existing downstream drainage facility, the Commission may withhold secondary approval of the subdivision until provision (such as a storage facility) has been made for the improvement of said potential condition in such sum as the Commission shall determine. No subdivision shall be approved unless adequate drainage from it will be provided to an adequate drainage water course or facility.
(e) Areas of poor drainage. Areas which are not in the floodplain but contain soils which are subject to flooding may be approved for subdivision by the Commission, provided that the subdivider fills the affected areas of said subdivision to an elevation sufficient to place building sites and streets two feet above ponding levels.
(f) Areas of high seasonal water tables. In areas characterized by soils having a high seasonal water table as determined by the County Soil and Water Conservation District, lots shall be limited to slab-type construction unless the Commission determines that appropriate engineering techniques will be applied to alleviate the subsurface problem.
(g) Floodway areas. If a subdivision of land is proposed within the floodplain, floodways shall be preserved and not diminished in the county by filling or obstruction, except as approved by the Natural Resources Commission in writing. No residential building site may be located within the floodway.
(h) Floodway fringe areas. Where a subdivision is proposed within an area of the floodplain designated as a floodway fringe, the Commission may approve such subdivision provided, that: all streets are elevated sufficiently to be above the regulatory flood elevation; all lots for residential usage have a flood protection grade two feet above the regulatory flood elevation; where provided, water and sanitary sewer facilities are constructed to eliminate contamination of or by, floodwater; and approval to fill the area from the Natural Resources Commission has been obtained in writing. Lands below the regulatory flood elevation shall not be used for computing the area requirement for any lot.
(i) Floodplain areas. Where a subdivision is proposed within an area of the floodplain for which floodway and floodway fringe designations have not been made, the Commission shall not approve such a subdivision unless all streets are raised sufficiently to be above the regulatory flood elevation; all lots for residential usage have a flood protection grade of two feet above the regulatory flood elevation; where provided, public water and sanitary sewer facilities are constructed to eliminate contamination of or by floodwater; and filling to achieve the above will not raise the level of the regulatory flood elevation more than one-tenth of one foot for that reach of the stream. All filling in the floodplain must be approved in writing by the State Natural Resources Commission. Lands below the regulatory flood elevation shall not be used for computing the area requirement for any lot.
(j) Recording of plats in the floodplain and floodway fringe. All final plats having within their boundaries areas whose elevation is below that of the regulatory flood elevation shall show and label the regulatory flood boundary and elevation, as of the date the final plat is drawn, on the final plat for recording.
(3) Dedication of drainage easements.
(a) General requirements. Where a subdivision is traversed by a drainage course, drainage way, channel or stream, a stormwater easement or drainage right-of-way shall be provided, granted or dedicated to the county conforming substantially to the lines of such watercourse, and of such width and construction or both as will be adequate for the purpose of both drainage and maintenance of the right-of-way. Wherever possible it is desirable that the drainage be maintained by an open channel with landscaped banks and adequate width for maximum potential volume of flow.
(b) Drainage easements.
(i) Where topography or other conditions are such as to make impractical the inclusion of drainage facilities within street rights-of-way, perpetual unobstructed easements at 15 feet in width for such drainage facilities shall be provided across property outside the right-of-way lines and with satisfactory access to the street. Easements shall be indicated on the plat. Drainage easements shall be carried from the street to a natural watercourse or to other drainage facilities.
(ii) The applicant shall dedicate, either in fee or by drainage or conservation, easement land on both sides of existing watercourses of a width to be determined by the Commission and in the case of legal drains, the County Drainage Board.
(iii) When a proposed drainage system will carry water across private land outside the subdivision, appropriate drainage rights must be secured and indicated on the plat.
(iv) Low-lying lands along watercourses subject to flooding or overflowing during storm periods, whether or not included in areas for dedication, shall be preserved and retained in their natural state as drainage ways. Such land or lands subject to periodic flooding shall not be included in the computations for determining the number of lots allowable under average density procedures nor for computing the area requirement for any individual lot.