(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 stormwater 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 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, or 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 the spread of stormwater in the street will exceed six feet, or will disrupt traffic 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.
(B) Nature of stormwater facilities.
(1) Location. The subdivider 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 runoff from its entire upstream drainage area, whether inside or outside the subdivision. The Town Engineer shall approve the necessary size of the facility as determined by the subdivider, based on the provisions of the required construction standards and specifications assuming conditions of maximum potential watershed development permitted by the zoning ordinance.
(3) Drainage design criteria. The subdivider shall use the rational method to size all storm sewers, culverts, ditches, and other drainage structures based on runoff from a ten-year storm event using the appropriate intensity/duration curve for the area and a minimum time of concentration of five minutes. The drainage calculations shall also include the effect of the subdivision on the existing downstream drainage facilities outside the subdivided area. Where it is determined that runoff incidental to the development of the subdivision will overload an existing downstream drainage facility during a five-year storm, the Plan Commission shall also require detention facilities adequate to reduce runoff from the proposed development to the extent that the post development flow is less than or equal to the pre-development flow. A minimum detention basin volume shall be the difference in runoff volume discharged from the project area to the basin site between the pre-development and post-development 100-year storm, or such volume to sufficiently reduce post-development discharges to pre-development rates whichever is greater.
(4) 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 state’s Department of Natural Resources (DNR) in writing. No residential building site may be located within the floodway.
(5) 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 flood water; and, approval to fill the area from the state’s Department of Natural Resources has been obtained in writing. Lands below the regulatory flood elevation shall not be used for computing the area requirement for any lot.
(6) 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 flood water; 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’s Department of Natural Resources. Lands below the regulatory flood elevation shall not be used for computing the area requirement for any lot. Floodplain management regulations are contained in Chapter 154.
(7) 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 recorded.
(Ord. 2005-5, passed 6-7-2005)