(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 or 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 five-year design storm, and adequate surface drainage (swales, ditches and the like) facilities shall be provided to handle the additional flow.
(2) Consistency with plans or regulations. Drainage facilities shall be designed in conformity with any adopted local, regional or levee, or sanitary district plans or regulations, including the county stormwater drainage and detention and sediment control ordinance.
(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 County 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) Retention discharge. The volume and rate of stormwater runoff leaving the subdivision shall not substantially exceed what would have occurred under natural, undeveloped conditions if it appears that existing or potential downstream developments would be damaged by a greater or faster flow. Thus, if necessary in the opinion of the Building and Zoning Committee and the County Engineer, the subdivider shall install stormwater retention facilities in his or her subdivision.
(C) Technical requirements. Subdivision drainage facilities shall be designed or 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, USGS Small Watershed Method, 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 Figure 6.
(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 county, unless the County Engineer approves an alternative design.
(b) Adequate measures (seeding, sodding, riprap, paving or other techniques) 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 County Engineer’s written permission.
(5) High water table. Whenever special drainage problems resulting from a high water table are encountered, the subdivider shall take reasonable remedial or compensatory measures in accordance with current engineering practices, as determined by the County Engineer. Such measures might include anchoring of water or sewer lines, subsurface drain tiles, added subbase or pavement thickness, or other measures.
(1993 Code, § 92.32) (Ord. passed 10-17-1956; Ord. passed 10-18-1978; Ord. passed 9-18-2002; Ord. 2002-14, passed - -2002; Ord. 2024-01, passed 3-5-2024) Penalty, see § 92.999