(1) Location.
(a) The subdivider may be required by the Plan Commission 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 located in the street right-of-way or in perpetual, unobstructed easements of appropriate width, and shall be constructed in accordance with the town construction standards and specifications.
(b) 1. Whenever a change of land use occurs, concentrated storm water discharge to adjacent areas shall not be permitted unless the discharge is conveyed through a suitable drainage feature to:
a. An approved outlet within the right-of-way;
b. An existing drainage easement;
c. A defined drainageway as approved by the Town Engineer; or
d. An open ditch which appears as a dashed or solid blue line on the 7.5 Minute Series Topographic Quadrangle Map as published by the United States Geological Survey.
2. The drainage feature used to convey the site discharge to the approved outlet shall be located in the right-of-way or easement of appropriate width.
(2) Public storm sewers. With the exception of storm water conveyance along nonresidential local streets within the I-2, I-3 and I-4 Districts, all storm water conveyance within or parallel to and adjacent to the public right-of-way shall be fully contained in storm sewers with an adequate and approved outlet. Residential subdivisions containing lots of less than 15,000 square feet and nonresidential subdivisions shall be provided with storm sewers to convey all on-site drainage. However, on-site drainage swales with required conveyance capacity of less than five cubic feet per second under the ten year post-development design storm may be allowed.
(3) Accommodation of upstream drainage areas. A culvert, storm sewer, open ditch or other drainage facility shall, in each case, be large enough to convey potential runoff from its entire upstream drainage area, whether inside or outside the subdivision. The minimum conveyance provided for upstream, off-site drainage areas shall be the runoff resulting from the 100-year storm over the present state of upstream, off-site drainage areas. The conveyance provided for upstream waters may be decreased to the extent that hydrograph routing and detention on land controlled by the subdivider and dedicated to the town as a drainage easement allows for decreased capacity; provided, that the runoff resulting from the 100-year storm is staged less than one foot above the stage on the natural drainage way at the interface of the development with the upstream drainage area. Culvert capacities for conveyance under interior local, collector and arterial streets without roadway overtopping shall be the runoff resulting from the 25-year, 50-year and 100-year storms respectively over off-site areas in present state and on-site areas in post development state. Driveway culvert capacities shall be the capacities required for the street classification to which the driveway connects. Greater culvert capacity shall be required to protect the finished floor elevation of buildings from the post-development 100-year storm when, in the opinion of the design engineer or the Town Engineer, the finished floor elevation is threatened. It is recommended that drainage of loading docks or other building features which are proposed at an elevation lower than adjacent roadways be considered with respect to culvert drainage impacts.
(4) Effect on downstream drainage areas. The release rate of storm water from development, redevelopment and new construction shall not exceed the storm water runoff rate from the land in its present state of development.
(5) Areas of poor drainage. Areas which are not in the floodplain, but which contain soils subject to flooding, may be approved for subdivision by the Plan 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 the 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 Soil and Water Conservation District, lots shall be limited to slab type construction unless the Plan Commission determines that appropriate engineering techniques will be applied to alleviate the subsurface problem.
(7) Special flood hazard areas. If any portion of a proposed subdivision lies within the floodplain, the subdivision shall comply with Ord. 25-2009, “Management of the Floodplain and Other Areas of Special Flood Hazard of the Town of Plainfield”.
(Ord. 22-97, passed - -1997; Ord. 4-2000, passed - -2000)