(A) Generally. Every residential subdivision shall be provided with facilities which can satisfactorily accommodate the runoff incident to the ten-year design storm. The drainage facilities in any commercial or industrial subdivision shall be designed to handle the runoff from the 25-year design storm.
(B) General design considerations.
(1) Underground/surface system. The drainage system may provide for the design storm indicated above by either an underground or surface drainage system or a combination of both. If an underground system is utilized, it shall at least accommodate the ten-year design storm; adequate surface drainage (swales, ditches, and the like) facilities shall be provided to handle the additional flow.
(2) Consistency with plans/regulations. Drainage facilities shall be designed in conformity with any adopted local, regional, or levee/sanitary district plans and/or regulations.
(3) Accommodation of upstream drainage areas. Drainage facilities large enough to accommodate potential runoff from the entire drainage area upstream from the proposed subdivision shall be provided in accordance with the City Engineer’s specifications. Potential runoff shall be determined on the basis of the maximum development of the upstream area that is permitted under the current zoning district regulations.
(4) Effect on downstream areas. The volume and rate of storm water runoff leaving the subdivision shall not damage existing or potential downstream developments or overload existing downstream drainage facilities. Thus, if necessary in the opinion of the City Council and the City Engineer, the subdivider shall install additional storm water drainage improvements in or downstream of his or her subdivision that would alleviate any damages or overload problems.
(C) Technical requirements. Subdivision drainage requirements shall be designed/installed in accordance with the following requirements in such a way that natural drainage is impeded as little as possible during construction.
(1) Pipe.
(a) All pipes shall be sized according to the “rational method,” “United States Geological Survey (USGS) small watershed method,” “United States Department of Agriculture (USDA) soil conservation services method,” or other recognized engineering method.
(b) Crossroad pipe shall meet the requirements of the “Standard Specifications for Road and Bridge Construction” and the “Administrative Policy Manual” of the Bureau of Local Roads and Streets, Illinois Department of Transportation.
(2) Inlets. Inlets shall be installed wherever necessary so that surface water is not carried across any street or intersection, nor for a distance of more than 600 feet in the gutter. Inlets shall be built to the specifications indicated in Appendix H, Figure 8. Other inlet configurations in conformance with IDOT standards are also acceptable.
(3) Storm sewers.
(a) Any storm sewer system shall be separated from and independent of the sanitary sewer system.
(b) Storm sewers shall be built in accordance with current engineering practices; they shall be at least 12 inches in diameter, and an inlet or manhole shall be installed at each change of alignment or grade.
(4) Ditches.
(a) Ditches shall be designed and built to the specifications established by the city unless the City Engineer approves an alternative design.
(b) Adequate measures (seeding, sodding, riprap, paving, and the like) shall be taken to prevent erosion of ditch banks.
(c) The natural drainage system shall be used as far as feasible for the storage and flow of runoff, but no existing ditch, stream, drain, slough, retention basin, or drainage channel shall be deepened, widened, filled, or rerouted without the City Engineer’s written permission.
(5) High water table. Whenever special drainage problems resulting from a highwater table are encountered, the subdivider shall take reasonable remedial/compensatory measures in accordance with current engineering practices as determined by the City Engineer. Such measures might include anchoring of water/sewer lines, subsurface drain tiles, added sub-base and/or pavement thickness, and the like.
(Ord. 1402, passed 2-24-1998)