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 in the Village of Monee shall be designed to convey at a minimum the stormwater runoff from the 10-year storm event. Where stormwater runoff from the undeveloped off-site areas are conveyed to the minor stormwater system, the minor stormwater system shall be designed to convey the 25-year storm event.
Minor stormwater systems shall consist of storm sewers unless otherwise approved by the Administrator.
In residential subdivisions, storm sewer systems shall be installed along rear yards. Inlets shall be placed such that overland flow lengths do not exceed 300 feet and rear yard drainage is not conveyed overland across more than the length of one home site.
C. Major Stormwater System Criteria: Major stormwater systems shall be sized to carry the base flood without causing additional flood damage.
Maximum ponding depths shall be nine (9) inches in streets, parking lots and driveways, except that parking lots and driveways intended for access only by commercial trucks may permit maximum ponding depths of twelve (12) inches. The maximum depth of ponding in non-paved drainage easements shall be twelve (12) inches.
The tail water used for the major drainage system shall be the 10-year water surface elevation in the receiving system or, in the case of stormwater detention facilities the 2-year design water surface elevation plus one (1) foot.
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 regarding 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 for drainage areas of less than twenty (20) acres. For drainage areas greater than twenty (20) acres the Administrator may require the use of hydrograph routing.
F. Design Rainfall: Any design runoff rate calculation method for conveyance shall use the data presented in Table 3: Will County Stormwater Technical Guidance Manual Rainfall Depths and Intensities, incorporated herein by reference.
G. Stormwater System Easements: All major and minor stormwater systems shall be covered by easements or right-of-way dedicated to the Village of Monee for access and for maintenance of said systems. If said systems are on private property and otherwise not intended to be dedicated to the Village of Monee, easements for access and maintenance of systems shall be granted to the Village of Monee authorizing the Village or its designated entity to access and maintain the systems if the owner fails to correct any deficiencies brought to the owner's attention by the Village of Monee.
H. Flow Depths: Maximum flow depths for new transverse stream crossings shall not exceed one foot (1') at the crown of the road during the base flood condition. The maximum flow depth on a roadway shall not exceed six inches (6") 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 (4) for the base flood condition.
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 USC section 1251 et seq., as amended. (Ord. 1893, 1-23-2019)