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(A) Stormwater facilities shall be required and designed so that runoff exits the site at the point whereit exited prior to development (unless a change is required and approved in writing by the village), and in a manner so as not to increase flood damage downstream. Concentrated discharges from new developments must enter conveyance systems capable of carrying the design flow rate without increasing flood damage, erosion or maintenance costs downstream.
(B) Minor stormwater systems shall be sized to convey runoff from the tributary watershed under fully developed conditions consistent with the design requirement of the permitting authority.
(C) Major stormwater systems shall be sized to carry the base flood without causing additional flood damage.
(D) Stormwater systems shall properly incorporate and be compatible with existing subsurface and surface discharge systems, including agricultural systems. Designs shall not cause new or increased damage to existing drainage systems, or to existing adjacent or tributary agricultural land uses.
(1) Offsite 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 new or increased damage to downstream structures and land uses. If the outfall drain tile and surface drainage systems prove to be inadequate to handle the design release rate, one of the following solutions shall be used:
(a) The site stormwater system and offsite outfall shall be designed so that it does not exceed the existing capacity of the downstream drainage system; or
(b) Modify the existing systems or construct new systems that will not conflict with the existing systems and will not impact existing land uses; or
(c) Alternate solutions may be submitted to the Administrator for approval.
(2) Onsite. Agricultural drainage systems shall be located and evaluated. All existing onsite agricultural drain tiles not serving a beneficial use shall be abandoned by trench removal prior to other development, and recorded on engineering details for the project. If any existing drain tiles continue to drain upland watersheds, the developer must maintain drainage service during construction until new storm sewers can be installed for a permanent connection.
(3) Offsite 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 stormwater system that includes observation structures located at the limits of the site and shall provide a free flow discharge. Agricultural tributary surface conveyance shall be accepted by the new development, with consideration given to the water quality and the ability to filter out sediment.
(4) Preservation of existing systems. New roadway construction shall preserve existing subsurface systems within the right-of-way. Inspection wells shall be placed in the right-of-way. Tiles found not to be flowing between inspection wells at the end of construction shall be replaced.
(E) Design runoff rates shall be calculated using event hydrograph methods. Design runoff rates for minor stormwater systems may be calculated using the rational method if the design watershed is less than 20 acres.
(F) Any design runoff rate calculation method shall use Bulletin 70, “Northeast Sectional Rainfall Statistics”, and shall calculate flow from all tributary areas upstream of the point of design. Peak discharges for conveyance design purposes shall be based on the critical duration considering the appropriate rainfall distribution.
(G) Major and minor stormwater systems shall be located within easements or rights-of-way, explicitly providing for government access or maintenance of such facilities by the grantee.
(H) Maximum flow depths for new transverse stream crossing shall not exceed one foot at the crown of the road during the base flood condition. The maximum longitudinal flow depth on a roadway shall not exceed six inches at the crown for flow depth (in feet) and velocity (in feet per second) shall not exceed four feet per second for the base flood condition.
(I) Transfers of waters between watersheds (diversions) shall be prohibited, except when such transfers will not violate this subchapter and are otherwise lawful.
(J) Developments shall incorporate best management practices in accordance with the Clean Water Act (33 U.S.C. §§ 1251 et seq., as amended), and as required by the IEPA’s National Pollutant Discharge Elimination System (NPDES) permit program.
(Ord. 2019-03, passed 4-15-2019)