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(A) Stormwater facility discharges. Stormwater facilities shall be required and designed so that runoff exits the site at a point where it exited prior to the subject development and in a manner so that flows will not increase flood damage to adjacent property except when otherwise approved by the Administrator. Concentrated discharges from new developments must enter conveyance systems capable of carrying the design flow rate without increasing flood damages or maintenance costs downstream.
(B) Minor stormwater system criteria. Minor stormwater systems shall be sized to convey runoff from the tributary watershed under fully developed conditions consistent with the design requirements of the local jurisdiction.
(C) Major stormwater system criteria. Major stormwater systems shall be sized to carry the base flood without causing additional flood damage.
(D) Existing subsurface and surface drainage systems.
(1) Stormwater systems shall properly incorporate and be compatible with existing subsurface and surface drainage systems including agricultural systems. Designs shall not cause damage to the existing drainage system(s) or the existing adjacent or tributary land including those with agricultural uses.
(2) The following principles and requirements shall be observed in the design.
(a) Off-site outfall. Agricultural subsurface and surface drainage systems shall be evaluated with regard to their capacity and capability to properly convey low flow ground water and site runoff storage facility release without damage to downstream structure and land use on the adjacent property. If the outfall drain tile and surface drainage systems prove to be inadequate, it will be necessary to modify the existing systems or construct new systems which will not conflict with the existing systems and will not impact the existing agricultural land use. Existing subsurface systems shall only be used with extended detention design.
(b) On-site. Agricultural drainage systems shall be located and evaluated on-site. All existing on-site agricultural drain tile not serving a beneficial use shall be abandoned by trench removal prior to other development and recorded on record plans. If any existing drain tiles continue to upland watersheds, the developer must maintain drainage service during construction until new sewers can be installed for a permanent connection.
(c) Off-site tributary. Existing drainage systems shall be evaluated with regard to existing capabilities and reasonable future expansion capacities. All existing tributary drain tiles shall be incorporated into the new conduits including observation structures located at the property limits, shall provide a free flow discharge and shall not allow surface runoff to enter the system.
(d) New roadway construction. New roadway construction shall preserve existing subsurface systems within the right-of-way. Inspection wells shall be placed at the right-of-way (ROW) and tiles found to not be flowing between inspection wells at the end of the construction shall be replaced.
(E) Design runoff rate. Design runoff rates for conveyance may be calculated using the Rational Method.
(F) Design rainfall. Any design runoff rate calculation method for conveyance shall use data from Illinois State Water Survey Updated Bulletin 70 (March 2019) Northeast Sectional Code.
(G) Stormwater system easements. For projects involving subdivision major and minor stormwater systems shall be located within easements or rights-of-way explicitly providing for public access for maintenance of the facilities. For all other projects requiring a permit, easements are required for public access for maintenance of stormwater facilities only for new construction or modifications involving components of a drainage system that conveys runoff from off-site properties. For I-2, I-3 or equivalent zoning property, the stormwater system does not have to be located within an easement or public rights-of-way. Instead, the owner’s representative shall provide a signed agreement to the appropriate government entity authorizing it to enter the facility to maintain the stormwater system if the owner fails to correct any deficiencies brought to the owner’s attention by the governmental entity.
(H) Flow depths. Maximum flow depths for new transverse stream crossings shall not exceed one foot at the crown of the road during the base flood condition. The maximum flow depth on a roadway shall not exceed six inches at the crown for flow parallel to the roadway. For flow over a new roadway or parallel to a new roadway, the product of the flow depth (in feet) and velocity (in feet per second) shall not exceed four for the base flood condition.
(I) Diversion of flow to another watershed. Transfers of waters between watersheds (diversions) shall be prohibited except when the transfers will not violate the provisions of § 55.021(A) and are otherwise lawful. Watersheds for purpose of regulation under this section shall be the major watershed divides as defined in the County Plan.
(J) Best management practices requirement. Developments shall incorporate all best management practices as may be required pursuant to the United States Clean Water Act, 33 U.S.C. §§ 1251 et seq., as amended.
(Res. 02-441, passed 10-17-2002; Res. 02-495, passed 11-21-2002; Res. 04-87, passed 3-17-2004; Ord. 19-239, passed 9-19-2019)