(A) General requirements. The Commission shall not recommend for approval any subdivision plat that does not make adequate provision for storm or flood water run-off channels or basins. The storm water drainage system shall be separate and independent of any sanitary sewer system. 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 storm water 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 lot and 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) Accessibility to public storm sewers.
(a) Where a public storm sewer is accessible, the applicant shall install storm sewer facilities. If there are no outlets within a reasonable distance, adequate provision shall be made for the disposal of storm water subject to the specifications of the Commission. However, in subdivisions containing lots of less than 14,000 square feet in area and in business and industrial districts, underground storm sewer systems shall be constructed throughout the subdivision and be connected to an approved outfall. Inspection of facilities shall be conducted by the town’s appropriate officials.
(b) If a connection to a public storm sewer will be provided eventually, as determined by the Commission, the developer shall arrange for future storm water disposal by the public utility system at the time the plat receives 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.
(3) 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 Commission shall determine the necessary size of the facility based on the provisions of the required Construction Standards assuming conditions of maximum potential watershed development permitted by Ch. 156 of this code of ordinances.
(4) Effect on downstream drainage areas. The Commission shall require the applicant to determine adequately the effect of each proposed subdivision on existing drainage facilities outside the area of the subdivision. Where it is anticipated that the additional run-off incident to the development of the subdivision will overload an existing downstream drainage facility, the Commission may withhold secondary approval of the subdivision until adequate provisions (such as a storage facility) have been made. No subdivision shall be approved unless adequate drainage from it will be provided to an adequate drainage watercourse or facility.
(5) Areas of poor drainage. Areas that are not in the floodplain, but contain soils which are subject to flooding may be approved for subdivision by the Commission. Such approval shall be given only if the applicant fills the affected areas of said subdivision to an elevation sufficient to place building sites and streets two feet above ponding levels.
(6) Areas of high seasonal water tables. In areas characterized by soils having a high seasonal water table as determined by the county’s 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.
(7) Floodway areas. If a subdivision of land is proposed within the floodplain, floodways 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.
(8) Floodway fringe areas.
(a) Where a subdivision is proposed in an area of the floodplain designated as a floodway fringe, the Commission may approve such subdivision if the following conditions are met:
1. All lots for residential usage have a flood protection grade two feet above the regulatory flood elevation;
2. Where provided, water and sanitary sewer facilities are constructed to eliminate contamination of or by flood water; and
3. Approval to fill the area from the Natural Resources Commission has been obtained in writing.
(b) Lands below the regulatory flood elevation shall not be used for computing the area requirement for any lot.
(9) Floodplain areas.
(a) 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 the following conditions are met:
1. All streets are raised sufficiently to be above the regulatory flood elevation;
2. All lots for residential usage have a flood protection grade of two feet above the regulatory flood elevation;
3. Where provided, public water and sanitary sewer facilities are constructed to eliminate contamination of or by flood water; and
4. 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.
(b) All filling in the floodplain must be approved in writing by the state’s Natural Resources Commission. Lands below the regulatory flood elevation shall not be used for computing the area requirement for any lot.
(10) Recording of plats in the floodplain and floodway fringe. All final plats, having within their boundary areas, any elevation of which 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. The final plat shall also include all areas of the floodway within a public utility and drainage easement.
(C) Minimum design standards. In designing a drainage system, the applicant shall be guided by the following minimum standards and requirements.
(1) Storm street inlets placed in a low point shall be sized to accept a ten-year storm volume with 50% of the inlet clogged and no more than one foot of water pooling above the inlet.
(2) Storm swale inlets shall be sized to accept a ten-year storm volume with 50% of the inlet clogged and no more than one foot of water pooling above the inlet.
(3) The storm detention design shall outlet storm water at a two-year pre-developed rainfall event rate for a ten-year post developed storm. Also, the 100-year post developed storm shall be limited to the ten-year pre-developed outlet rate.
(4) Storm pipes shall be reinforced concrete, Class III, with type B wall thickness. The minimum pipe size shall be 12 inches in diameter. The minimum pipe flow velocity shall be two and one-half feet per second.
(5) Drainage swales with longitudinal slopes flatter than 1% shall have a six-inch thick reinforced concrete swale, a width (minimum three feet) and shape as approved by the town’s consultant. Alternative type swale treatments shall be subject to approval of the town’s consultant.
(6) Exposed ends of storm pipes shall have six-inch thick reinforced concrete slope walls. Pre-cast reinforced concrete end sections are a permitted alternative.
(7) All streets shall be provided with an adequate storm drainage system consisting of curbs, gutters and storm sewers, or ditches and culverts, as determined by the Commission. A six-inch perforated tile shall be placed on each side of all streets and:
(a) Be two feet below the soil subgrade and parallel with the longitudinal pavement grade;
(b) Flow to the low point and into the storm drainage system;
(c) Be placed below pavement at the inside face of the curbing; and
(d) Be backfilled with #8 washed gravel.
(8) Inlets in streets shall be placed a maximum of 600 feet apart, or 600 feet from the high point in the street.
(9) Downspouts and sump pump outlets shall discharge onto grass surface no closer to the street than the building setback line.
(10) The on-site drainage system shall be designed and sized to handle a minimum of a ten-year rainfall event. The developer is responsible for analyzing the ponding and results of a 100-year rainfall event and establishing flood protection grade for all structures and verifying and adequate outlet for the 100-year storm.
(11) Detention design shall adhere to current soil conservation service methods for drainage. The rational method is acceptable for pipe design only.
(12) Wet detention ponds shall have a minimum six-foot wide safety ledge placed below water level at a maximum water depth of 30 inches. Also, wet detention ponds shall have at least 25% of the pond surface with a minimum water depth of eight feet.
(13) Pre-developed run-off rates shall be based on pasture, meadow, brush or woods ground cover type. Any existing farm ground will be based on the pasture condition.
(14) Any dry detention facilities must be designed with sub-surface drainage.
(15) Whenever evidence available to the Commission indicates natural surface drainage to be inadequate, the applicant shall provide an adequate storm water sewer system. When the surface drainage is adequate, easements for such surface drainage shall be provided.
(D) Erosion control. When vegetation has been removed from a slope and the possibility of soil erosion occurs, the applicant and subsequent building contractors shall be required to seed or otherwise prevent damage to adjacent property or accumulation on street surfaces. These erosion control measures shall be according to standards and specifications on file with county’s Soil and Water Conservation District.
(E) Alteration of swales and ditches. Drainage swales or ditches along dedicated roadways and within rights-of-way, or on easements dedicated to the town are not to be altered in any way without written permission from the town. Driveways may be constructed over these swales or ditches only when appropriate sized culverts or other approved structures have been permitted by the Commission. Swales on private property shall be regulated by covenants.
(F) Dedication of drainage easements.
(1) General requirements. If a subdivision is traversed by a drainage course, drainage way, channel or stream, then a storm water easement or drainage right-of-way shall be provided. Such easement or right-of-way shall conform substantially to the lines of such watercourse, and be of such width and construction or both 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.
(2) Drainage easements.
(a) 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 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, the land on both sides of existing watercourses of a width to be determined by the Commission and the county’s 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 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.
(2004 Code, § 4-417)