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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 existed 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 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.
(F) Design rainfall. Any design runoff rate calculation method for conveyance shall use State Water Survey Bulletin 70 based intensity-duration-frequency curves.
(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 § 151.021 (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.
(Ord. 18-08, passed 1-14-19)
(A) Release rate.
(1) If no release rate is specified in an adopted watershed plan in accordance with § 151.004, then sufficient flood storage shall be provided so that the site will not discharge at a rate greater than 0.15 cfs/acre of development during and after a rainfall event with a 100-year frequency except for sites exempted in this section. Unless exempted in this section, sites shall not discharge at a rate greater than 0.04 cfs/acre of development during and after a rainfall event with a two-year frequency.
(2) This area of hydrologic disturbance on the site shall be used to calculate the required site runoff storage volume. The on-site watershed area tributary to the point of discharge shall be used to calculate the allowable release rate for the site runoff storage facility, which shall be the maximum release rate allowed considering only the on-site watershed area runoff.
(B) Design methods.
(1) Event hydrograph routing methods or the modified rational method may be used to calculate design runoff volumes for site runoff facilities. The methods must be HEC-1, (SCS methodology), HEC-HMS, TR-20 or TR-55 tabular method. Event methods shall incorporate the following assumptions:
(a) Antecedent moisture condition equals two;
(b) Appropriate Huff rainfall distribution; and
(c) Twenty-four-hour duration storm with a 1% probability (100-year frequency) of occurrence in any one year as specified by the State Water Survey Bulletin 70 Northeast Sectional rainfall statistics.
(2) When the modified rational method is used, rainfall statistics shall be from the State Water Survey Bulletin 70 and the volume determined shall be multiplied by the following factor based on the date of final engineering approval:
(a) In the first year after the effective date of the chapter: 110%;
(b) In the second year after the effective date of the chapter: 120%; and
(c) After the third year following the effective date of the chapter: 130%.
(C) Existing release rate less than allowable. For sites where the undeveloped release rate is less than the maximum release rate in division (A) above, the developed release rate and corresponding site runoff storage volume shall be based on the existing undeveloped release rate for the development.
(D) Downstream water surface elevations. All hydrologic and hydraulic computations must utilize appropriate assumptions for downstream water surface elevations, from low flow through the base flood elevation, considering the likelihood of concurrent flood events.
(E) Extended detention requirement.
(1) The requirements of this section will apply only when an existing agricultural land use is downstream of and adjacent to a site runoff storage facility outlet, The runoff from a 0.75-inch rainfall event over the hydraulically connected impervious area of the new development shall be stored below the elevation of the primary gravity outlet (extended detention) of the site runoff storage facility. The facility may be designed to allow for evapotranspiration or infiltration of this volume into a subsurface drainage system and shall not be conveyed through a direct positive connection to downstream areas.
(2) The hydraulically connected impervious area used in the calculation of required extended detention volume may be reduced by the Administrator if the soils are prepared to maximize infiltration and deep-rooted grasses or other plants selected for their ability to promote infiltration or water absorption are planted in areas appropriately dedicated. The reduction in hydraulically connected impervious area used in the calculation shall be equal to the area of the development meeting the above soils/native planting requirement.
(3) Subsurface drainage systems may be designed as a component of the extended detention portion of the detention basin to assist in infiltration in accordance with the following criteria.
(a) The extended detention volume shall be discharged at a rate no greater than that required to empty the calculated extended detention volume within five days of the storm event.
(b) No subsurface drainage pipe shall be located within ten feet of drainage pipes directly connected to the detention basin.
(c) For purposes of meeting the maximum subsurface drainage discharge requirements, flow control orifices and weirs may be used.
(d) All design extended detention volume shall be provided above the seasonal high ground water table or the invert elevation of the ground water control system.
(e) Farm field tile shall not be considered a subsurface drainage system.
(F) Site runoff storage facility design requirements. Storage facilities shall be designed and
constructed with the following characteristics.
(1) The site runoff storage facility shall provide one foot of freeboard above the design high water elevation.
(2) The storage facilities shall be accessible and easily maintained.
(3) Storage facilities shall facilitate sedimentation and catchment of floating material. Unless specifically approved by the Administrator, concrete lined low-flow ditches shall not be used in detention basins.
(4) Storage facilities shall minimize impacts of stormwater runoff on water quality by incorporating best management practices.
(5) Storage facilities shall maximize the normal flow distance between detention inlets and outlets, to the extent possible.
(6) Storage facilities shall be designed so that the existing conditions pre-development peak runoff rate from the 100-year, critical duration rainfall will not be exceeded assuming the primary restrictor is blocked.
(7) Storage facilities with single pipe outlets shall have a minimum inside diameter of 12 inches. If design release rates necessitate a smaller outlet, structures such as perforated risers or flow control orifices shall be used.
(G) Site runoff storage facility requirements. Within the regulatory floodplain storage facilities located within the regulatory floodplain shall:
(2) Store the required amount of site runoff to meet the release rate requirement under all stream flow and backwater conditions in the receiving stream up to the ten-year flood elevation;
(3) Detention volume provided by enlarging existing regulatory floodplain storage without providing a structure controlling discharge (on-stream detention) will be allowed only as a variance. The applicant must demonstrate that flood damages are not increased and the development will not increase flood flows for both the two-year and 100-year floods on the stream with developed conditions on the site; and
(4) The Administrator may approve designs which can be shown by detailed hydrologic and hydraulic analysis to provide a net watershed benefit not otherwise realized by strict application of the requirements in (1) through (3) above.
(H) Site runoff storage facility requirements within the regulatory floodway. Storage facilities located within the regulatory floodway shall:
(1) Meet the requirements for locating storage facilities in the regulatory floodplain;
(2) Be evaluated by performing hydrologic and hydraulic analysis consistent with the standards and requirements for any adopted watershed plans; and
(3) Provide a net watershed benefit.
(I) Off-site facilities. Site runoff storage facilities may be located off-she if the following conditions are met.
(2) Adequate storage capacity in the off-site facility is dedicated to the development.
(3) The development includes means to convey stormwater to the off-site storage facility.
(J) Cross-stream structures for site runoff storage facilities. Structures constructed across the channel to impound water to meet detention requirements shall be prohibited on any perennial stream unless part of a public flood control project with a net watershed benefit. Those streams appearing as blue on a USGS quadrangle map shall be assumed perennial unless better data is obtained. All cross-stream structures for the purpose of impounding water to provide detention in all cases on perennial and intermittent streams must demonstrate that they will not cause short-term or long-term stream channel instability.
(Ord. 18-08, passed 1-14-19)
(A) Applicability. Regulations under this section apply only to croplands, pasturelands, farmsteads and outbuildings associated with those agricultural practices. Compliance with the requirements of this section shall be construed as compliance with the stormwater chapter for the above land uses and no further regulation under the chapter will apply. Any other land use, including greenhouses, nurseries, container grown plants, equestrian facilities, the sale of agricultural products to the public or where commercial activities involving the new construction of gravel or paved parking facilities or buildings whose aggregate area is 25,000 square feet or more are required to comply with all applicable sections of this chapter.
(B) Conservation planning and performance standards.
(1) To comply with this section, landowners shall practice conservation planning whose product shall be a management system, which addresses site runoff, soil erosion and sediment control, surface and subsurface drainage. Any acreage with a signed and approved NRCS conservation plan is exempt from the requirements of this section and the chapter.
(2) Applicable approved practices include:
(a) Vegetated grass waterways;
(b) Contour buffer strips;
(c) Critical area planting and cover crops;
(d) Terrace ridges and diversions;
(e) Contour strip cropping;
(f) Contour farming;
(g) Crop rotation;
(h) Conservation tillage and crop residue management; and
(i) Other standard practices for conservation planning in accordance with the NRCS Field Office Technical Guide (current edition) or as otherwise approved by the County NRCS District Conservationist or the County Agricultural Administrator.
(3) The performance standard for conservation planning shall be a management system which will develop a set of field practices which will reduce the calculated actual soil loss to the “tolerable soil loss” (T) as calculated by the revised universal soil loss equation for the actual site conditions. Cropland tillage and resource management methods shall be consistent with the Technical Guide Notice IL-108 and shall be considered evidence of compliance with the T performance standard.
(C) Drainage practices, requirements and design criteria.
(1) Drainage for agricultural purposes shall be consistent with those practices identified as appropriate for “good husbandry” given the soil types, slopes and crops. An agricultural drainage system may consist of both subsurface drainage systems and surface drainage systems. Where active drainage districts maintain drainage systems, they shall be consulted on surface and subsurface drainage within the district boundaries.
(2) Requirements applying to subsurface and surface drainage system shall be as follows.
(a) Subsurface drainage systems. Drain tile systems shall be maintained and constructed in accordance with subsurface drainage recommendations for the appropriate soil drainage group as specified by the State Drainage Guide, University of Illinois Extension Service Circular No. 1226. Surface inlets into the subsurface drainage system shall be allowed only to maintain good husbandry. Where their use cannot be practicably avoided due to topography, they shall be installed using flow controls such as orifices and perforated risers with gravel filters and/or vegetative filters.
(b) Surface drainage systems. Surface drainage systems shall be maintained and constructed in accordance with surface drainage recommendations for the appropriate soil drainage group as specified by the State Drainage Guide, University of Illinois Extension Service No. 1226. Surface drainage systems shall be built with geotechnically stable slopes and the surface when applicable shall be further stabilized utilizing the establishment of cool and warm season grass mixes as identified in Field Office Technical Guide (Illinois 108).
(c) Buffer strips. Open channels with a definable bed and banks shall use buffer strips in order to reduce the amount of erosion occurring from the conveyed flows as well as to help filter the runoff from the site into the waterway. Buffer strips shall be a minimum of 15 feet wide from the top of bank except where smaller widths are necessary due to site limitations and when approved by the Administrator.
(d) Agricultural drainage systems. Agricultural drainage systems shall also comply with all regulations regarding wetlands as enforced by Federal, state and local agencies.
(D) Sediment control for open channels.
(1) All open channel drainage systems shall maintain practices adjacent to the open outlet channel that will reduce the transportation of sediment off-site. Runoff from agricultural fields must pass through a sediment control system prior to discharge into the open channel conveyance system.
(2) Approved sediment control systems may consist of the following:
(a) Vegetated buffer zones planted with permanent grasses appropriate for soil stabilization and filtering;
(b) Grade control structures for over fall stabilization;
(c) Sediment traps adjacent to the stream channel; and
(d) Other standard practices for conservation planning in accordance with the NRCS Field Office Technical Guide (current edition) or as otherwise approved by the County NRCS District Conservationist or the Administrator.
(E) Maintenance and construction of drainage systems. Agricultural drainage systems shall be maintained so as to convey the expected flows for good drainage practices. The existing agricultural surface drainage systems shall not be enlarged unless the enlargement is consistent with all other sections of this section. Maintenance and construction of subsurface drainage systems will not be subject to the requirements of other sections of this chapter except as they are regulated by other agencies. Maintenance projects by legally functioning drainage districts on existing agricultural drainage systems will not be subject to further permitting requirements under this chapter except as they relate to the jurisdiction of other agencies.
(Ord. 18-08, passed 1-14-19)
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