No subdivision shall be approved unless adequate drainage will be provided to an adequate drainage watercourse or facility.
A. The City Council shall not approve any plat or subdivision that does not make adequate provision for drainage of storm or floodwater runoff. The storm water drainage system shall be separate and independent of any sanitary sewer system. Storm sewers, where required, shall be designed according to City of Prairie City Standard Specifications for Public Works dated July, 1994, and a copy of design computations shall be submitted along with plans. Inlets should be provided so that surface water is not carried across any intersection, nor for a distance of more than 600 feet in the gutter. When calculations indicate that curb capacities are exceeded at a point, no further allowance shall be made for flow beyond that point, and basins shall be used to intercept flow at that point. Surface water drainage patterns shall be shown for each and every lot and block.
B. The applicant may be required by the City Council to carry away by pipe or open ditch any spring or surface water that may exist either previously to, or as a result of the subdivision. Such drainage facilities shall be constructed in accordance with the construction standards and specifications approved by the City Council.
C. Underground storm sewer system shall be constructed throughout the subdivision and connected to an existing storm sewer if feasible. If no suitable outlets are available within a reasonable distance but a public storm sewer will be provided eventually, as determined by the City Engineer and the City Council, the developer shall make arrangements for future storm water disposal by a public utility system at the time the plat receives final approval. Provision for such connection shall be included in the performance bond or escrow agreement required for the subdivision plat. The subdivider shall also provide a suitable interim outfall or drainage system as approved by the City Council.
2. Accommodation of Upstream Drainage Areas. A culvert or other drainage facility should in each case be large enough to accommodate potential runoff from its entire upstream drainage area, whether inside or outside the subdivision. The City Engineer shall determine the necessary size of the facility, based on the provision of the construction standards and specifications assuming conditions of maximum potential watershed development as established by land uses projected in the Comprehensive Plan, and as permitted by the zoning regulations applicable to such land uses.
3. Effect on Downstream Drainage Areas. The City Engineer shall also study the effect of each subdivision on existing downstream drainage facilities outside the area of the subdivision. Local government drainage studies, together with such other studies as shall be 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 City Council may withhold approval of the subdivision until provision has been made for the improvement of said potential condition in such sum as the City Council may determine.
4. Dedication of Drainage Easements.
A. Where a subdivision is traversed by a watercourse, drainage way, channel, or stream there shall be provided a storm water easement or drainage right-of-way conforming substantially to the lines of such watercourse, and of such width and construction as will be adequate for the purpose. 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. Where topography or other conditions are such as to make impractical the inclusion of drainage facilities within the road rights-of-way, perpetual unobstructed easements at least 15 feet in width for such drainage facilities shall be provided across property outside the road lines and with satisfactory access to the road. Easements shall be indicated on the plat. Drainage easements shall be carried from the road to a natural watercourse or to other drainage facilities.
C. When a proposed drainage system will carry water across private land outside the subdivision, appropriate drainage rights shall be secured and indicated on the plat.
D. The City Council may require dedication to the City, either in fee or by drainage or conservation easement, of land on both sides of existing watercourses, to a distance to be determined by the Planning and Zoning Commission or the City Engineer, if such dedication is deemed necessary to assure preservation of said watercourses, drainage ways, channels, or streams.
E. 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 computed in determining the number of lots to be utilized for planned unit development nor for computing the area requirement of any lot.
