(A) General requirements.
(1) A stormwater management (SWM) permit is required for all regulated development under this chapter.
(2) No regulated activities shall commence until the village issues written approval of a SWM permit.
(3) The SWM permit approved by the Plat Officer is required to be on site throughout the duration of the regulated activity.
(4) All regulated activities (pre- and post-construction, erosion and sediment control measures and stormwater management BMPs) shall be designed, implemented, operated and maintained to meet the purposes and requirements of this chapter. Various BMPs and their design standards are listed in the Illinois Urban Manual.
(5) All regulated activities shall include such measures as necessary to:
(a) Protect health, safety and property; and
(b) Meet the water quality goals of this chapter by implementing measures to:
1. Protect and/or improve the function of floodplains, wetlands and wooded areas.
2. Protect and/or improve native plant communities, including those within the riparian corridor;
3. Protect and/or improve natural drainage ways from erosion;
4. Minimize thermal impacts to waters of the state; and
5. Minimize runoff to impervious surfaces by directing runoff to pervious areas.
(6) (a) The measurement of an impervious area shall include all of the impervious areas in the total proposed development, even if development is to take place in stages or phases.
(b) For development taking place in stages or phases, the entire development plan must be used in determining conformance with this chapter.
(c) Any areas designed to initially be gravel or crushed stone shall be assumed to be impervious.
(d) For new permeable pavement, designers should use one-half of the measured infiltration rate during design to approximate long-term infiltration rates or similar design standards based on the specifications of pavement used in the design.
(7) A minimum ten-foot wide access easement shall be provided for all proposed stormwater facilities with tributary areas equal to or greater than 1,000 square feet and not located within a public right-of-way. Easements shall provide for ingress and egress to a public right-of-way.
(8) Drainage easements shall be provided where the conveyance, treatment or storage of stormwater, either existing or proposed, is identified on the SWM permit. Drainage easements shall be provided to contain and convey the 100-year frequency flood.
(9) A plan for the ongoing maintenance of all stormwater management system components, including wetlands and buffer areas, is required prior to plan approval. The plan shall include:
(a) Maintenance tasks;
(b) The party responsible for performing the maintenance tasks;
(c) A description of all permanent public or private access maintenance easements and overland flow paths and compensatory storage areas; and
(d) A description of dedicated sources of funding for the required maintenance.
(B) Site runoff controls. Site runoff control for large storms, up to the 100-year event, is essential to protect against immediate downstream erosion and flooding.
(1) Post-development discharge rates shall not exceed the exiting conditions discharge rates for the two-, ten-, 50- and 100-year critical duration storm events. If it is shown that the peak rates of discharge indicated by the post-development analysis are less than or equal to the peak rates of discharge indicated by the existing conditions analysis for two-, ten-, 50- and 100-year critical duration storms, then the requirements of this section have been met. Otherwise, the applicant shall provide additional controls as necessary to satisfy the peak rate of discharge requirement. Peak runoff rates shall be based on the critical duration storm.
(2) Any concentrated stormwater discharges leaving a site must be conveyed into an existing channel, storm sewer or overland flow path with adequate downstream stormwater capacity and will not result in increased erosion, flood damage or other drainage hazard. Demonstration of this shall include a comparison of existing and post-development velocity, discharge and hydrograph at the location where stormwater leaves the site. Post-development velocity, peak discharge and runoff volume shall be less than 110% of the existing conditions of velocity, peak discharge and volume for the 100-year critical duration storm.
(3) The design of stormwater management systems shall not result in any transfer of water between watersheds unless no reasonable alternative exists as determined by the Plat Officer.
(4) Concentrated stormwater discharge shall not be connected to an existing field tile or any other drainage tile system unless the applicant submits a maintenance agreement, recorded easement and a report that indicates the existing system, from the connections to the discharge point in an open channel, has adequate hydraulic capacity and structural integrity. The recorded easement and maintenance agreement must extend from the connection to the discharge point in an open channel. The recorded easement and maintenance agreement must be approved by the Plat Officer prior to issuance of a stormwater management permit.
(5) Calculation methodology:
(a) TR-20, WinTR-20, HEC-1, HEC-HMS or a (municipality)-approved hydrograph producing a hydrologic model shall be used for the following:
1. To determine peak runoff rates for areas with a drainage area of 100 acres or greater; and
2. To confirm the stormwater storage requirements for stormwater facilities that have a drainage area of ten acres or more.
(b) TR-55 may be used to calculate discharges for areas that drain less than 100 acres. TR-55 with a pond routing program such as TR-20 or HEC-1 may be used to confirm stormwater storage requirements.
(c) The Rational Method may be used to calculate discharges for drainage areas of less than 20 acres and the storm sewer design. The Rational Method or the Modified Rational Formula shall not be used to determine detention storage requirements.
(d) Climatic sectional rainfall data with appropriate application of the areal to point ratio as presented in the State Water Survey Bulletin 70 shall be used for all hydrologic analysis.
(e) Rainfall should be distributed using the appropriate State Water Survey Circular 173 Huff rainfall distribution (Huff, 1990); except that, SCS Type II distribution is acceptable with TR-55.
(f) Runoff calculations for all off-site tributary land shall be based on either the anticipated future land use conditions or existing land use conditions. Anticipated future land use conditions will be based on future land use and existing off-site storage facilities. Existing land use conditions will be based on existing land use and existing off-site storage facilities.
(C) Detention facilities.
(1) Detention, along with green infrastructure, non-structural and structural BMPS. are required for projects involving more than one acre of new impervious area. Green infrastructure and non-structural BMPs are required to control stormwater runoff for projects with less than one acre of new impervious area.
(2) The area of development shall be used to calculate the detention volume and allowable release rate from a detention facility in accordance with the criteria stated herein in division (B)(1) above.
(3) All stormwater infiltration, retention and detention facilities shall be provided with an emergency overflow structure or path capable of passing the inflow from a critical duration 100-year storm without damages to any structures on adjacent land. The emergency overflow structure shall have an invert elevation at or above the design high water elevation. A minimum freeboard of one foot shall be provided above the design high water elevation. The freeboard should be measured from the design high water elevation to the top of the berm. The overflow path shall not be located on top of the outlet pipe.
(4) Where a single pipe outlet or orifice plate is to be used to control discharge, it shall have a minimum diameter of 12 inches. If this minimum size permits release rates greater than those specified in this section, alternative outlet designs shall be utilized which incorporate self-cleaning flow restrictors. The minimum area for the flow restrictor is 12.56 square inches (equivalent to a four-inch circular pipe). The outlet pipe and control devices shall be designed to minimize maintenance requirements and prevent tampering.
(5) Online detention facilities shall meet the following requirements.
(a) Online detention shall not be permissible on perennial streams. This shall include, but not be limited to, all streams designated as hydrographic category perennial in the NHD flowline feature class in the National Hydrography Dataset (NHD) database.
(b) Online detention shall not be permissible with an off-site to on-site drainage area ratio greater than 10:1.
(c) Online detention shall not be permissible if the drainage area is greater than 640 acres.
(d) The required online detention volume shall be calculated based on the hydrologically disturbed area of the ownership parcel and release rates. The control structure shall be designed based on the total tributary area (on-site and off-site) and release rates at the impoundment elevations established by the required detention volumes.
(e) A stable overflow structure capable of passing the 100-year, critical duration off-site flow rate shall be provided. The off-site flow rates shall be calculated assuming existing conditions or future conditions with detention required per this chapter, whichever is greater.
(f) All permitting requirements of USACE, IEPA and IDNR/OWR shall be met.
(g) Online detention volume shall be in addition to the existing floodplain storage.
(6) To the extent feasible, the distance between detention inlets and outlets should be maximized. If possible, they should be at opposite ends of the basin. There shall be no low flow bypass between the inlet and outlet, and paved low flow channels shall not be used.
(7) The side slopes at the shoreline of wet and wetland detention basins (from at least six inches below to at least six inches above normal water level) shall be no steeper than 10:1 to prevent shoreline erosion due to wave action and fluctuating water levels. Above shoreline areas, or in dry bottom portions, the maximum side slope shall be 4:1.
(8) A 20-foot minimum setback shall be required from all property lines and roadways to the normal pool elevation.
(9) A safety shelf with a minimum eight-foot width and no slope shall be constructed no more than one foot below normal water level.
(10) The shoreline of wet detention basins shall be protected from erosion. The preferred method of shoreline stabilization is native wetland and wet prairie vegetation with a deep root system to stabilize the soils.
(11) Off-site flow may be diverted around a proposed detention facility; provided that, the other applicable standards regarding regulatory floodplain or flood-prone areas are met.
(12) Any development involving the construction, modification, or removal of a dam as defined in 17 Ill. Admin. Code part 3702 (Rules for Construction of Dams) shall obtain an IDNR/OWR dam safety permit or a letter stating no permit is required prior to the start of such activity.
(13) Berms for stormwater infiltration, retention and detention facilities shall not be constructed in a regulatory floodplain or a flood-prone area unless approved by the Village Engineer. If a berm is constructed in a regulatory floodplain or a flood-prone area, the development must meet the requirements of the Floodplain Ordinance. The volume inside the stormwater facility shall not be considered available for compensatory storage unless the volume is in addition to the required detention volume and is available at the appropriate storage interval.
(14) Detention facilities may connect to existing drain tiles or storm sewers only if the applicant submits a maintenance agreement, recorded easement, and a report that indicates the existing system from the connection to the discharge point in an open channel has adequate hydraulic capacity and structural integrity. The recorded easement and maintenance agreement must extend from the connection to the discharge point in an open channel. The recorded easement and maintenance agreement must be approved by the Plat Officer prior to issuance of a stormwater management permit.
(15) Infiltration basins may be used as detention facilities subject to the following.
(a) The basin must be designed to dewater within 72 hours following the end of the 100-year critical duration storm event.
(b) The underlying soils must have an infiltration rate of at least one-half inch per hour as determined by an engineer.
(c) Pretreatment facilities must be provided to prevent obstruction.
(d) The basin must be at least 200 feet away from any water supply wells or maximum setback zone if established.
(e) Runoff from the areas that have water quality concerns or are subject to frequent winter deicing must not be routed to the infiltration facility.
(f) The bottom of the infiltration basin must be at least four feet above the seasonal high ground water elevation.
(D) Buffer areas. Buffer areas shall be required for all areas defined as waters of the United States (WOTUS). The buffer area for all WOTUS shall extend landward from the ordinary high water mark. The buffer area for jurisdictional or mitigated wetlands shall extend from the edge of the delineated wetland. A property may contain a buffer area that originates from WOTUS on another property. Buffer areas are divided into two types, linear buffers and water body buffers.
(1) Linear buffers shall be designated along both sides of all channels meeting the definition of WOTUS.
(a) When the channel has a watershed greater than 20 acres, the minimum buffer shall be 30 feet on each side of the channel.
(b) Channels with an Index of Biotic Integrity (IBI) greater than 35 shall have a minimum buffer width of 100 feet on each side of the channel. (Initial IBI based on IDNR, IEPA data or site specific assessment, whichever is most current.)
(2) Water body buffers shall encompass all non-linear bodies of water meeting the definition of WOTUS including wetlands, lakes and ponds.
(a) For all water bodies with a total surface area of one-tenth acre, but less than one acre, a minimum buffer width of 30 feet shall be established.
(b) For all water bodies with a total surface area greater than one acre, but less than two and one-half acres, a minimum buffer width of 40 feet shall be established.
(c) For all water bodies with a total surface area of two and one-half acres, a minimum buffer width of 50 feet shall be established.
(E) Soil erosion and sediment control.
(1) Subdividers shall be responsible for complying with state and local laws and regulations pertaining to the National Pollutant Discharge Elimination System (NPDES) stormwater permit program.
(2) All subdivision construction plan sets shall include a site-specific written document that identifies potential sources of stormwater pollution at the construction site, describes practices to reduce pollutants in stormwater discharges from the construction site (reduction of pollutants is often achieved by controlling the volume of stormwater runoff), and identifies procedures the operator will implement to comply with the terms and conditions of a construction NPDES permit.
(Ord. 16-102, passed 4-4-2016)