(A) General requirements. The Commission shall not recommend for approval any subdivision plat which does not make adequate provision for storm or flood water runoff channels or basins. The storm water drainage system shall be separate and independent of any sanitary sewer system. Storm sewers, where required, shall be designed by the rational method, or other methods as approved by the Commission and a copy of all computations shall be submitted along with the plans. All computations shall be based upon a ten year frequency storm. Surface water drainage patterns shall be shown for each and every lot block.
(B) Nature of storm water facilities.
(1) 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 easements of appropriate width.
(2) Accommodation of upstream drainage areas. A culvert or other drainage facility shall in each case be large enough to accommodate potential run-off from its entire upstream drainage area, whether inside or outside the subdivision. The applicant with subsequent approval of the Plan Commission shall determine the necessary size of the facility based on the provisions of the required constructions standards and specifications assuming conditions of maximum potential watershed development permitted by the zoning ordinance.
(3) Effect on downstream drainage areas. The Plan Commission shall require the applicant to adequately determine the effect of each proposed subdivision on existing drainage facilities outside the area of the subdivision. 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.
(4) Areas of high seasonal water tables. In areas characterized by soils having a high seasonal water table as determined by the Johnson 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.
(5) Floodway areas. If a subdivision of land is proposed within the flood plain, floodway shall be preserved and not diminished in capacity by filling or obstruction, except as approved by the Natural Resources Commission in writing. No residential building site may be located within the floodway.
(6) Floodway fringe areas. Where a subdivision is proposed in an area of the flood plain designated as a floodway fringe, the Commission may approve such subdivision provided that: all lots for residential usage have a flood protection grade three feet above the regulatory flood elevations; where provided, water and sanitary sewer contamination of or by, flood water; 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.
(7) Flood plain areas. Where a subdivision is proposed within an area of the flood plain for which floodway and floodway fringe designations have not been made, and 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 three feet above the regulatory flood elevation; where provided, public water and sanitary sewer facilities are constructed to eliminate contamination of or by flood water; and, filling to achieve the above will not raise the level of the regulatory flood elevation more than 1/10 of one foot for that reach of the stream. All filling in the flood plain 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.
(8) Recording of plats in the flood plain 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.
(C) Dedication of drainage easements.
(1) General requirements. Where a subdivision is traversed by a drainage course, drainage way, channel, or stream, a storm water easement or drainage right-of-way conforming substantially to the lines of such watercourse, and of such width and construction or both as will be adequate for the purpose shall be provided. 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.
(2) Drainage easements.
(a) Where topography or other conditions are such as to make impractical the inclusion of drainage facilities within street right-of-ways, perpetual unobstructed easements at least 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.
(b) 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 the County Drainage Board in the case of legal drains.
(c) 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.
(d) 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.
(Ord. 174, § 413, passed 6-11-90) Penalty, see § 153.999