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(A) The principal purpose of this chapter is to promote effective, equitable, acceptable, and legal water resource management measures by establishing reasonable rules and regulations for development.
(B) Other purposes of this chapter include:
(1) Managing and mitigating the effects of urbanization on stormwater drainage throughout the county through planning, appropriate engineering practices and proper maintenance;
(2) Protecting from, and reducing the existing potential for, loss of human life, health, safety and property from the hazards of flooding damages on a watershed basis;
(3) Preserving and enhancing the natural hydrologic and hydraulic functions and natural characteristics of watercourses and floodplains to protect water quality, protect aquatic habitats, reduce flood damages, reduce soil erosion, provide recreational and aesthetic benefits and enhance community and economic development;
(4) Controlling sediment and erosion in and from stormwater facilities, developments, agricultural fields and construction sites and reducing and repairing stream bank erosion;
(5) Requiring that planning for development provide for water resource management, taking into account natural features such as vegetation, wildlife, waterways, wetlands and topography in order to reduce the probability that new development will create unstable conditions susceptible to erosion;
(6) Protecting environmentally sensitive areas from deterioration or destruction by private or public actions;
(7) Requiring appropriate and adequate provision for site runoff control, especially when the land is developed with a large amount of impervious surface;
(8) Requiring the design and evaluation of each site development plan consistent with watershed capacities;
(9) Encouraging the use of stormwater storage and infiltration of stormwater in preference to stormwater conveyance;
(10) Lessening the taxpayers’ burden for flood-related disasters, repairs to flood-damaged public facilities and utilities and flood rescue and relief operations;
(11) Meeting the State Department of Natural Resources Office of Water Resources floodway permitting requirements delineated in ILCS Ch. 615, Act 5, § 18g (“An Act in Relation to the Regulation of the Rivers, Lakes and Streams of the State”), as amended from time to time;
(12) Making federally subsidized flood insurance available to property throughout the unincorporated county by fulfilling the requirements of the National Flood Insurance Program;
(13) Complying with the rules and regulations of the National Flood Insurance Program codified in C.F.R. Title 44;
(14) Minimizing conflicts and incompatibilities between agricultural and urban drainage systems and maintaining agriculture as a viable and productive land use;
(15) Encouraging cooperation and consistency in stormwater management activities within and between the units of government having floodplain and stormwater management jurisdiction;
(16) Restricting future development in the floodplain to facilities that will not adversely affect the potential for flood damage;
(17) Requiring regular, planned maintenance of stormwater management facilities;
(18) Allowing the use of simple technologies whenever appropriate and realistic, but requiring the use of more sophisticated techniques when necessary to ensure the adequacy of stormwater controls; and
(19) Requiring strict compliance with and enforcement of this chapter.
(C) The purposes of this chapter are consistent with and supercede the Plan.
(Ord. 10-164, passed 6-17-2010)
(A) This chapter recognizes the integrated nature of the watershed system and the need to study certain flood control alternatives and other stormwater management functions on a watershed-wide basis.
(B) (1) Individual watershed plans or interim watershed plans which recognize the unique attributes of each watershed may be prepared and periodically updated for the major watersheds, to identify management projects and establish criteria for development. These plans may also recommend changes to this chapter effective within the study boundary.
(2) Watershed plans or interim watershed plans may be adopted which recommend more or less stringent criteria than the criteria in this chapter. When adopted by the County Board, these watershed-specific criteria established in the watershed plans or interim watershed plans shall be set forth as Sections 105 through 117 of this ordinance.
(Ord. 10-164, passed 6-17-2010)
For the purposes of this chapter, the following definitions shall apply unless the context clearly indicates or requires a different meaning. Words and terms not defined shall have the meanings indicated by common dictionary definition.
ACCESSORY STRUCTURE. One which:
(1) Is subordinate to and serves a principal building or principal use;
(2) Is subordinate in area, extent or purpose to the principal building or principal use served;
(3) Contributes to the comfort, convenience or necessity of occupants of the principal building or principal use served; and
(4) Is located on the same zoning lot as the principal building or potential use served with the single exception of accessory off-street parking facilities as are permitted to locate elsewhere than on the same zoning lot with the building or use served.
ACOE. The United States Army Corps of Engineers.
ADMINISTRATIVE VIOLATION. Occurs when rules and procedures regarding permit applications and site development permits are not followed.
ADMINISTRATOR. The person designated by the County Executive to administer and enforce this chapter.
AGRICULTURAL PRACTICES. The use of land for agricultural purposes, including farming, dairying, pasturage, apiculture, horticulture, floriculture, viticulture and animal and poultry husbandry, and the necessary accessory uses for packing, treating or storing the produce; provided, however, that the operation of any accessory uses shall be secondary to that of the normal agricultural activities. AGRICULTURAL PRACTICES include truck farming, poultry farming, bee keeping, raising of fruit and berries, and the selling of agricultural products. AGRICULTURE PRACTICES shall not include the commercial feeding of garbage to swine or other animals. The minimum land area necessary to constitute an agricultural use is ten acres.
AGRICULTURAL SUBSURFACE DRAINAGE. A water management technique driven by economic and safety concerns, where the rate at which surplus groundwater should be removed is determined primarily by the moisture/air requirements of the vegetation (commonly called tiles, field tiles and the like).
APPLICABLE ENGINEERING PRACTICE. Procedures, methods or materials recommended in standard engineering textbooks or references as suitable for the intended purpose.
APPLICANT. Any person, firm or governmental agency who executes the necessary forms to procure official approval of a development or permit to carry out construction of a development from the county.
ARMORING. A form of channel modification which involves the placement of materials (e.g., concrete, riprap, bulkheads and the like) within a stream channel or along a shoreline to protect property above streams, lakes and ponds from erosion and wave damage caused by wave action and stream flow.
BASE FLOOD. The flood having a 1% probability of being equaled or exceeded in a given year. The BASE FLOOD is also known as the 100-year frequency flood event.
BASE FLOOD ELEVATION (BFE). The highest water surface elevation that can be expected during the base flood.
BENCHMARK. A permanent, stable object containing a marked point of known elevation (with respect to NAVD 88) which is used as a vertical reference point for topographic surveys.
BEST MANAGEMENT PRACTICES (BMP). A measure used to control the adverse stormwater-related effects of development. BMPs include structural devices (e.g., swales, filter strips, infiltration trenches and detention basins) designed to remove pollutants, reduce runoff rates and volumes, and protect aquatic habitats. BMPs also include nonstructural approaches, such as public education efforts to prevent the dumping of household chemicals into storm drains.
BLOCKED RESTRICTOR ELEVATION. The elevation to which stormwater would rise in a basin in a 100-year storm if the restrictor is blocked. See § 164.023(F).
BUILDING. A structure that is principally above ground and is enclosed by walls and a roof. The term includes a gas or liquid storage tank, a manufactured home, mobile home or a prefabricated building. This term also includes recreational vehicles and travel trailers to be installed on a site for more than 180 days, unless fully licensed and ready for highway use.
BUFFER. An area of predominantly vegetated land located adjacent to channels, wetlands, lakes or ponds for the purpose of reducing contaminants in stormwater that flows to those areas.
BULLETIN 70. Frequency Distributions of Heavy Precipitation in Illinois: Updated Bulletin 70 by James Angel and Momcilo Marcus of the Illinois State Water Survey, Prairie Research Institute (March 2019).
BYPASS FLOWS. Stormwater runoff or groundwater from upstream properties tributary to a property’s drainage system but not under its control.
CALCULATED HIGH WATER ELEVATION. The elevation to which stormwater would rise along an overland flow route, or similar conveyance system, in a 100-year storm.
CHANNEL. Any river, stream, creek, brook, branch, natural or artificial depression, ponded area, flowage, slough, ditch, conduit, culvert, gully, ravine, wash or natural or human-made drainage way, which has a definite bed and bank or shoreline, in or into which surface, groundwater, effluent or industrial discharges flow either perennially or intermittently.
CHANNEL MODIFICATION. Alteration of a channel by changing the physical dimensions or materials of its bed or banks. CHANNEL MODIFICATION includes damming, riprapping (or other armoring), widening, deepening, straightening, relocating, lining and significant removal of bottom or woody rooted vegetation but does not include the clearing of debris or removal of trash or dredging to previously documented thalwag elevations and side slopes.
CHANNELIZATION. A severe form of channel modification involving a significant change in the channel cross-section and typically involving relocation of the existing channel (e.g., straightening).
CHIEF SUBDIVISION ENGINEER. Stormwater Administrator for unincorporated Will County as outlined in § 164.162.
COMMERCIAL. Sale of goods to the public at large where the traffic generated warrants construction of site improvements.
COMMITTEE. The Land Use and Development Committee of the County Board.
COMMUNITY. The county or any city or village within the county.
COMPENSATORY STORAGE. An excavated, hydrologically and hydraulically equivalent volume of storage created to offset the loss of existing flood storage.
CONDITIONAL LETTER OF MAP AMENDMENT (CLOMA). A FEMA comment letter on a development proposed to be located in, and affecting only that portion of, the area of floodplain outside the regulatory floodway and having no impact on the existing regulatory floodway or base flood elevations.
CONDITIONAL LETTER OF MAP REVISION (CLOMR). A letter that indicates that FEMA will revise base flood elevations, flood insurance rate zones, flood boundaries or floodways as shown on an effective FIRM after the “as-built” record drawings are submitted and approved.
COE. The United States Army Corps of Engineers.
CONSERVATION DESIGN. A site design which incorporates the natural landscape, ecology, and/or historical significance of a site being developed, and which maintains the most valuable natural features and functions of the site. These natural and valuable features can be used for either passive or active recreational activity or preserved as naturally vegetated land.
CONSERVATION PLANNING. The practices and procedures associated with the management of soil, water, plants, plant nutrients and other elements of agricultural production. Documentation of the management system shall only be as required by the NRCS or in cases of a complaint, as requested by the Chief Subdivision Engineer in response to a notification of a complaint.
CONSERVATION PRACTICES. A listing of practices such as those identified in the USDA-NRCS Technical Guide § 4 that detail the standards and specifications for each practice listed. The electronic guide can be accessed at:
CONTROL STRUCTURE. A structure designed to control the rate of flow that passes through the structure, given a specific upstream and downstream water surface elevation.
COUNTY. Will County, Illinois.
CRITICAL DURATION. The duration of a storm event that results in the greatest peak runoff.
CRITICAL FACILITY. Any facility that is critical to the health and welfare of the population, and, if flooded, would create an added dimension to the disaster. Damage to these CRITICAL FACILITIES can impact the delivery of vital services, can cause greater damage to other sectors of the community, or can put special populations at risk. Examples of CRITICAL FACILITIES are: emergency services facilities (such as fire and police stations), schools, hospitals, retirement homes and senior care facilities, critical utility sites (telephone switching stations or electrical transformers), and hazardous material storage facilities (chemicals, petrochemicals, hazardous or toxic substances).
CULVERT. A structure designed to carry drainage water or small streams below barriers, such as roads, driveways or railroad embankments
D-FIRM. Digital flood insurance rate map published by FEMA showing special flood hazard areas in a digital format overlaid on an aerial photograph.
DAM. Any obstruction, wall embankment or barrier, together with any abutments and appurtenant works, constructed to store or divert water or to create a pool. DAMS may also include weirs, restrictive culverts or impoundment structures. Underground water storage tanks are not included.
DEPARTMENT. The County Land Use Department.
DEPRESSIONAL STORAGE. The volume contained below a closed contour on a one-foot contour interval topographic map, the upper elevation which is determined by the invert of a surface gravity outlet.
DESIGNATED FLOODWAY. See definition of FLOODWAY.
DETENTION BASIN (SITE RUNOFF STORAGE FACILITY). A constructed structure for the temporary storage of stormwater runoff with a controlled release rate.
DEVELOPER. A person who creates or causes a development.
DEVELOPMENT.
(1) Any constructed change to real estate including:
(a) Construction, reconstruction, repair or replacement of a building or an addition to a building;
(b) Installing a manufactured home on a site, preparing a site for a manufactured home, or installing a travel trailer or recreational vehicle on a site for more than 180 days. If the travel trailer or recreational vehicle is on-site for less than 180 days, it must be fully licensed and ready for highway use;
(c) Drilling, mining, installing utilities, construction of roads, bridges or similar projects;
(d) Construction or erection of levees, walls, fences, dams or culverts, channel modifications, filling, dredging, grading, excavating, paving or other nonagricultural alterations of the ground surface, storage materials, deposit of solids or liquid waste;
(e) Any other activity of humans that might change the direction, height or velocity of flood or surface water, including extensive vegetation removal; and/or
(f) Plowing and cultivation and other similar agricultural practices that do not involve filling, grading or construction of levees as regulated in § 164.024.
(2) The following are not considered
DEVELOPMENT: maintenance of existing buildings and facilities such as reroofing or resurfacing of roads with an impervious surface when there is no increase in elevation.
DIRECT DISCHARGE SITES. Parcels of land, or portions thereof, which are immediately adjacent and naturally drain directly to the banks of the Des Plaines River, Chicago Sanitary and Ship Canal, DuPage River and Kankakee River without crossing over other private or public property.
DIRECTOR. The County Executive or his or her designee charged with performing the duties specified in the County Stormwater Management Ordinance, as codified in Chapter 55.
DISCONNECTED IMPERVIOUS SURFACE. A disconnected impervious surface (DIS) is a built-upon area (usually a solar panel farm or a canopy) that discharges runoff to a vegetated area that is sized and graded to reduce runoff and pollutants. The structure shall have no impervious surface under it, other than supports required to support the structure, such as posts or piers. These foundations will count as impervious surface and be added to the total impervious surface on the site. The vegetated receiving area shall not include any impervious surface.
DRAINAGE AREA. The land area above a given point that may contribute runoff flow at that point from rainfall.
DRY BASIN. A detention basin designed to drain completely after temporary storage of stormwater flows and to normally be dry over the majority of its bottom area.
EFFECTIVE DATE. The date to be determined by the County Board.
ELEVATION CERTIFICATES. A form published by FEMA that is used to certify the base flood elevation and the lowest elevation of usable space to which a building has been constructed.
EPHEMERAL STREAM. A stream whose bed elevation does not intersect the groundwater table, it carries flow only during and immediately after a runoff producing rainfall event.
EROSION. The process whereby soil is detached by the action of water or wind.
EXISTING MANUFACTURED HOME PARK OR SUBDIVISION. A manufactured home park or subdivision for which the construction of facilities for servicing the lots on which the manufactured homes are to be affixed (including at a minimum the installation of utilities, the construction of streets, and either final site grading or the pouring of concrete pads) has been completed before the effective date of this chapter.
EXPANSION TO AN EXISTING MANUFACTURED HOME PARK OR SUBDIVISION. The preparation of additional sites by the construction of facilities for servicing the lots on which the manufactured homes are to be affixed (including the installation of utilities, the construction of streets, and either final site grading or the pouring of concrete pads).
EXTENDED DETENTION. A volume of runoff temporarily detained and released over a long period of time as specified in § 164.023.
FARMED WETLAND. As determined following the procedures outlined by the Food Security Act and Chicago District of the Corps of Engineers.
FEDERAL EMERGENCY MANAGEMENT AGENCY (FEMA). The federal agency and its regulations, at 44 C.F.R. §§ 59 through 79, effective as of September 29, 1989, or as amended.
FLOOD. A general and temporary condition of partial or complete inundation of normally dry land areas from overflow of inland or tidal ways or the unusual and rapid accumulation of runoff of surface waters from any source.
FLOOD FREQUENCY. Normally expressed as a period of years, based on a percent chance of occurrence in any given year from statistical analysis, during which a flood of a stated magnitude may be expected to be equaled or exceeded. For example, the two-year flood frequency has a 50% chance of occurrence in any given year. Similarly, the 100-year flood frequency has a 1% chance of occurrence in any given year.
FLOOD FRINGE. The portion of the floodplain outside of the designated floodway.
FLOOD INSURANCE RATE MAP (FIRM). A map issued by FEMA that is an official community map, on which map FEMA has delineated both the special flood hazard areas and the risk premium zones applicable to the community. This map may or may not depict floodways.
FLOOD INSURANCE STUDY (FIS). A study of flood discharges and flood profiles for a community, adopted and published by FEMA.
FLOOD PROTECTION ELEVATION (FPE). The elevation of the BFE plus two feet of freeboard for structures within the plan limits of the base flood elevation. Outside the floodplain limits, the blocked restrictor elevation of any adjacent stormwater facility plus one foot of freeboard.
FLOODPLAIN. The land typically adjacent to a body of water with ground surface elevations at or below the base flood or the 100-year frequency flood elevation including detached special flood hazard areas, ponding areas and the like. The FLOODPLAIN is also known as the special flood hazard areas (SFHA).
FLOODPLAIN ADMINISTRATOR. The person designated by the County Executive to administer and enforce this chapter (Chief Subdivision Engineer).
FLOODPROOF. Any combination of structural and nonstructural additions, changes or adjustments to structures or property which reduce or eliminate flood damage to real estate or improved real property, water and sanitary facilities, structures and their contents.
FLOODPROOFING CERTIFICATE. A form published by FEMA that is used to certify that a building has been designed and constructed to be structurally dry floodproofed to the FPE.
FLOODWAY. Includes the channel, on stream lakes, and that portion of the floodplain adjacent to a stream or channel which is needed to store and convey the critical duration 100-year frequency flood discharge with no more than a one-tenth-foot increase in flood stage due to the loss of flood conveyance or storage, and no more than a 10% increase in velocities.
FLOODWAY CONVEYANCE. The measure of the flow carrying capacity of the floodway section and is defined using Manning’s equation as:
K = 1.4863 AR
23
/n
where “n” is Manning’s roughness factor, “A” is the effective area of the cross-section, and “R” is ratio of the wetted area to the wetted perimeter.
FORESTED WETLAND. As defined by the Chicago District Corps of Engineers.
FREEBOARD. An increment of height added to the BFE or 100-year design water surface elevation to provide a factor of safety for uncertainties in calculations, unknown local conditions, wave actions and unpredictable effects such as those caused by ice or debris jams.
FUNCTIONAL. In the context of the usage in this chapter, FUNCTIONAL refers to stormwater facilities, which serve their primary purpose of meeting developed release rate requirements but do not meet all of the final design conditions. For example, a detention basin, which has been excavated but has not had the side slopes graded nor the final landscaping placed, may be considered FUNCTIONAL as a site runoff storage facility.
GOOD HUSBANDRY. The application of agricultural or conservation practices that minimize impacts to natural resources and neighboring landowners.
GROUND WATER. Water that is located within soil or rock below the surface of the earth. Same as SUBSURFACE WATER.
GROUND WATER CONTROL SYSTEM. A designed system which may consist of tiles, under drains, French drains or other appropriate stormwater facilities whose purpose is to lower the ground water table to a predictable elevation throughout the year.
HISTORIC STRUCTURE. Any structure that is:
(1) Listed individually in the National Register of Historic Places, or preliminarily determined by the Secretary of the Interior as meeting the requirements for individual listing on the National Register;
(2) Certified or preliminarily determined by the Secretary of the Interior as contributing to the historic district or a district preliminary determined by the Secretary to qualify as a registered historic district;
(3) Individually listed on the State Inventory of Historic Places by the State Historic Preservation Agency; and
(4) Individually listed on a local inventory of historic places that has been certified by the State Historic Preservation Agency.
HQAR. High quality aquatic resource as defined by the Chicago District Corps of Engineers. These include waters of the United States or isolated waters of the county that are determined to be critical due to their uniqueness, scarcity, function and/or value. Appendix E provides further descriptions of HQARs.
HYDRAULICS. The science and study of the mechanical behavior of water in physical systems and processes.
HYDRAULICALLY CONNECTED IMPERVIOUS AREA. Consists of those areas of concrete, asphalt and gravel surfaces along with roof tops which convey flows directly to an improved drainage system consisting of storm sewers or paved channels. Rooftops whose downspouts discharge to unpaved surfaces which are designed for the absorption and filtration of stormwater runoff shall not be considered as HYDRAULICALLY CONNECTED IMPERVIOUS SURFACES. Roadways whose primary conveyance is through open ditches and swales shall not be considered as HYDRAULICALLY CONNECTED IMPERVIOUS SURFACE. Roadways drained by curb and gutter and storm sewer, and driveways hydraulically connected to those roadways shall be considered as directly connected impervious surface.
HYDRAULICALLY EQUIVALENT COMPENSATORY STORAGE. Compensatory storage either adjacent to the floodplain fill or not located adjacent to the development but can be shown by hydrologic and hydraulic analysis to be equivalent to compensatory storage located adjacent to the development.
HYDRIC SOIL. A soil that is saturated, flooded or ponded long enough during the growing season to develop anaerobic conditions.
HYDROLOGICALLY DISTURBED/ HYDROLOGIC DISTURBANCE. An area where the land surface has been cleared, grubbed, compacted or otherwise modified that changes runoff, volumes, rates or direction.
HYDROLOGY. The science of the behavior of water, including its dynamics, composition and distribution in the atmosphere, on the surface of the earth, and underground.
HYDROPHYTIC VEGETATION. Plant life growing in water, soil or on a substrate that is at least periodically deficient in oxygen as a result of excessive water content.
IDNR-OWR. The State Department of Natural Resources, Office of Water Resources.
IMPERVIOUS. Surfaces that cause the majority of rainfall to be converted to direct runoff. Asphalt, concrete, roofing systems and gravel areas will be considered IMPERVIOUS.
INTERIM WATERSHED PLAN. A regional study of a watershed which does not address the entire range of purposes, goals and objectives outlined in the Countywide Stormwater Management Plan approved by the Committee and adopted by the county.
INTERMITTENT STREAM. A stream whose bed intersects the groundwater table for only a portion of the year on the average or any stream which flows continuously for at least one month out of the year but not the entire year.
ISOLATED WATERS OF WILL COUNTY. All waters such as lakes, ponds, streams, (including intermittent streams), farmed wetlands and wetlands that are located within unincorporated county but not necessarily under the United States Army Corps of Engineers jurisdiction. The limits of the ISOLATED WATERS OF WILL COUNTY extend to the ordinary high water mark or delineated wetland boundary. ISOLATED WATERS OF WILL COUNTY exclude permitted excavations created for purposes such as stormwater conveyance, detention/recreation areas as constructed as part of a stormwater management system, recreation, stock watering, irrigation, settling basins or wastewater treatment systems and roadside ditches. Compensatory wetland mitigation created to meet the requirements of this chapter or § 404 of the Clean Water Act is not excluded.
JURISDICTIONAL WETLAND. A wetland that is under the jurisdiction of the Army Corps of Engineers.
LETTER OF MAP AMENDMENT (LOMA). The official determination by FEMA that a specific structure is not in a regulatory floodplain. A LOMA amends the effective FIRM.
LETTER OF MAP CHANGE (LOMC). A letter from FEMA that revised base flood elevations, flood insurance rate zones, flood boundaries or floodway as shown on an effective FIRM.
LETTER OF MAP REVISION (LOMR). A letter from FEMA that revises base flood elevations, flood insurance rate zones, flood boundaries or floodway as shown on an effective FIRM.
LAKE. A natural or artificial body of water encompassing an area of two or more acres, which retains water throughout the year.
LOWEST FLOOR. The lowest floor of the lowest enclosed area (including basement). An unfinished or flood resistant enclosure usable solely for parking of vehicles, building access or storage, in an area other than a basement area is not considered a building’s LOWEST FLOOR provided that the enclosure is not built so as to render the structure in violation of the applicable non-elevation design requirements of this chapter.
MAINTENANCE SPECIAL SERVICE AREA. A backup funding mechanism which may be enacted by the County Board in the event that the person or entity charged with maintenance of the special service area fails to adequately carry out and/or provide maintenance and upkeep of the area as it relates to § 605 of this chapter.
MAJOR STORMWATER SYSTEM. The portion of a stormwater facility needed to store and convey flows beyond the capacity of the minor stormwater system.
MANUFACTURED HOME. A structure transportable in one or more sections, which is built on a permanent chassis and is designated for use with or without a permanent foundation when attached to the required utilities. The term MANUFACTURED HOME also includes park trailers, travel trailers and other similar vehicles placed on site for more than 180 consecutive days. The term MANUFACTURED HOME does not include a recreational vehicle.
MANUFACTURED HOME PARK OR SUBDIVISION. A parcel (or contiguous parcels) of land divided into two or more manufactured home lots for rent or sale.
MASS GRADING. Development in which the primary activity is a change in topography affected by the movement of earth materials.
MINOR STORMWATER SYSTEM. Consists of all infrastructure including curb, gutter, culverts, roadside ditches and swales, storm sewers and sub-surface drainage systems intended to convey stormwater runoff at less than a 100-year flood frequency. The MINOR STORMWATER SYSTEMS shall be designed to carry a ten-year storm event, or as required by the affected Highway Department jurisdiction.
MINOR SUBDIVISION. Any subdivision containing not more than five lots and not involving any new streets, roads or extension of municipal facilities, and not in conflict with any provision or portion of the zoning ordinance, subdivision regulations or official plan or map relating thereto.
MITIGATION. Measures taken to offset negative impacts from development in wetlands or the floodplain. MITIGATION includes those measures taken to eliminate or minimize damage from development activities by replacement of the resource, such as compensatory storage, soil erosion and sedimentation control, wetland replacement and channel restoration. MITIGATION may also include those activities taken to reduce a structure’s susceptibility to flooding.
NATURAL. In reference to watercourses, those stream channels, grassed waterways and swales means those stream channels, grassed waterways, and swales formed by the existing surface topography of the earth prior to changes made by man. A natural stream tends to follow a meandering path; its floodplain is not constrained by levees; the area near the bank has not been cleared, mowed or cultivated; the stream flows over soil and geologic materials typical of the area with no substantial alteration of the course or cross-section of the stream caused by filling or excavating. A modified stream channel, grassed waterway or swale which has regained natural characteristics over time as it meanders and re-establishes vegetation may be considered NATURAL.
NATURALIZED DETENTION BASIN. A constructed basin for the temporary storage of stormwater runoff with a controlled release rate which creates growing conditions suitable for emergent and riparian wetland plants, and which is explicitly designed to lessen the impacts of stormwater quality and quantity in urban and urbanizing areas.
NAVD 88 (NORTH AMERICAN VERTICAL DATUM OF 1988). The accepted standard orthometric data, referenced to the single control point at Father Point, Quebec, Canada. NAVD 88 supersedes the National Geodetic Vertical Datum of 1929 (NGVD).
NATIONAL FLOOD INSURANCE PROGRAM (NFIP). A federal program whose requirements are codified in C.F.R. Title 44.
NET BENEFIT IN WATER QUALITY. The institution of best management practices as part of a development that when compared to the pre-development condition can be judged to reduce downstream sediment loading or pollutant loadings.
NET WATERSHED BENEFIT.
(1) A finding that, when compared to the existing condition, the developed project will do one of the following:
(a) Substantially reduce (more than 10%) downstream peak discharges;
(b) Reduce downstream flood stages (more than one-tenth foot); or
(c) Reduce downstream damages to structures occurring in the pre-development condition.
(2) The demonstration of one of these conditions must be through detailed hydrologic and hydraulic analysis of watersheds on a regional scale as approved by the Stormwater Administrator.
NEW MANUFACTURED HOME PARK OR SUBDIVISION. Manufactured home park or subdivision for which the construction of facilities for servicing the lots on which the manufactured homes are to be affixed (including at a minimum the installation of utilities, the construction of streets and either final site grading or the pouring of concrete pads) has been completed after the effective date of this chapter.
NON-RIVERINE. Areas not associated with a stream or river such as isolated depressional storage areas, ponds and lakes.
NPDES. National pollutant discharge elimination system.
NPDES PERMIT. A permit from the State Environmental Protection Agency for discharge of stormwater associated with construction activities disturbing one acre or more.
NRCS. The United States Department of Agriculture, Natural Resources Conservation Service.
OBSERVATION STRUCTURES. Structures built on a field tile where the pipe inflow and outflow is visible upon removal of a lid.
ONE-HUNDRED YEAR EVENT. A rainfall, runoff, or flood event having a 1% chance of occurring in any given year.
OPEN CHANNEL. A conveyance system with a definable bed and banks carrying the discharge from field tiles and surface drainage. OPEN CHANNELS do not include grassed swales within farm fields under agricultural production, which are ephemeral in nature.
ORDINARY HIGH WATER MARK (OHWM). The point on the bank or shore up to which the presence and action of surface water is so continuous so as to leave a distinctive mark, such as by erosion, destruction or prevention of terrestrial vegetation, predominance of aquatic vegetation or other easily recognized characteristic.
OVERLAND FLOW PATH. A design feature of the major stormwater system which carries flows in excess of the minor stormwater system design capacity in an open channel or swale, or as sheet flow or weir flow over a feature designed to withstand the particular erosive forces involved.
OVERSIGHT COMMITTEE. The County Land Use and Development Committee. See § 164.165.
PARCEL. A contiguous area of land under unified ownership, separately owned and capable of being separately conveyed.
PEAK FLOW. The maximum rate of flow of water at a given point.
PERENNIAL STREAMS. Riverine watercourses whose thalwag generally intersects the ground water table elevation and flows throughout the year.
PERMITTING AUTHORITY. Will County.
PLAN. The County Comprehensive Countywide Stormwater Management Plan, adopted by the County Board on October 13, 1998, as amended from time to time.
PLANNING AND ZONING COMMISSION(PZC). See § 164.165.
POND. A body of water of less than two acres, which retains a normal water level year round.
PRIMARY GRAVITY OUTLET. The outlet structure designed to meet the release rate requirements of this chapter.
PROFESSIONAL ENGINEER. An engineer registered in the state, under the State Professional Engineering Practice Act. (ILCS Ch. 225, Act 325, §§ 1 et seq.), as amended.
PROFESSIONAL LAND SURVEYOR. A land surveyor registered in the state, under the State Land Surveyors Act. (ILCS Ch. 225, Act 330, §§ 1 et seq.), as amended.
PROPERTY. Contiguous land under single ownership or control.
PROPERTY OWNER’S ASSOCIATION. The legal entity created for the purpose of developing, selling, managing and maintaining a residential, commercial or industrial development which shall have the primary responsibility for providing for the care, maintenance, renewal and replacement of stormwater drainage facilities. May also known as HOMEOWNER’S ASSOCIATION.
PUBLIC BODIES OF WATER. All open public streams and lakes capable of being navigated by watercraft in whole or in part for commercial uses and purposes and all lakes, rivers and streams, which in their natural conditions were capable of being improved and made navigable, or that are connected with or discharge their waters into navigable lakes or rivers within, or upon the borders of the state, together with all bayous, sloughs, backwaters and submerged lands that are open to the main channel or body of water directly accessible thereto.
PUBLIC FLOOD CONTROL PROJECT. A flood control project, which will be operated and maintained by a public agency to reduce flood damages to existing buildings and structures, which includes a hydrologic and hydraulic study of the existing and proposed conditions of the watershed. Nothing in this definition shall preclude the design, engineering, construction or financing in whole or in part of a flood control project by persons or parties who are not public agencies.
PUBLIC FLOOD EASEMENT. An easement acceptable to the appropriate jurisdictional body that meets the regulations of the OWR, the Department, and the community, and that provides legal assurances that all areas subject to flooding in the created backwater of the development will remain open to allow flooding.
QUALIFIED WETLAND PROFESSIONAL. A person trained in natural and/or physical sciences (such as one or more of the disciplines of biology, geology, soil science, engineering or hydrology) whose training and experience ensure a competent analysis and assessment of stream, lake, pond and wetland conditions and impacts.
RECORD DRAWINGS. Drawings prepared, signed, and sealed by a registered professional engineer or registered land surveyor representing the final “as-built” record of the actual in-place elevations, location of structures and topography.
RECREATIONAL VEHICLE or TRAVEL TRAILER. A vehicle which is:
(1) Built on a single chassis;
(2) Four hundred square feet or less when measured at the largest horizontal projection;
(3) Designed to be self propelled or permanently towable by a light duty truck; and
(4) Designed primarily not for use as a permanent dwelling, but as a temporary living quarters for recreational camping travel or seasonal use.
REDEVELOPMENT. Development on a parcel upon which the existing condition is buildings, parking lots and/or infrastructure.
REGIONAL PERMITS. Offered for pre- approved projects that are considered minor projects that are permissible per IDNR/OWR Part 3708 rules for Northeastern Illinois regulatory floodways. A complete listing of the terms and conditions for specific project types can be obtained from the IDNR/OWR website. However, a regional permit from IDNR/OWR does not exempt a project from securing a site development permit.
REGISTERED LAND SURVEYOR. See PROFESSIONAL LAND SURVEYOR.
REGISTERED OR LICENSED PROFESSIONAL ENGINEER. See PROFESSIONAL ENGINEER.
REGISTERED STRUCTURAL ENGINEER. A person licensed under the laws of the state as a structural engineer.
REGULATORY FLOODPLAIN. The floodplain as depicted on maps recognized by FEMA as defining the limits of the SFHA.
REGULATORY FLOODWAY. Those portions of the floodplain depicted on maps as floodway and recognized by the IDNR-OWR for regulatory purposes.
REPAIR, REMODELING OR MAINTENANCE. Development activities that do not result in any increases in the outside dimensions of a building or any changes to the dimensions of a structure.
REPETITIVE LOSS. Flood-related damages sustained by a structure on two separate occasions during a ten-year period for which the cost of repairs at the time of each flood event, on the average, equals or exceeds 25% of the market value of the structure before the damage occurred.
RETENTION FACILITY. Stores stormwater runoff without a gravity release.
RIPRAP. Large size, coarse aggregate with angular edges normally placed at end sections of storm sewer inlets or outlets, culverts or along shorelines to reduce soil erosion.
RIVER FRONTAGE. The property that is immediately adjacent to and naturally drains directly to the Des Plaines River, Chicago Sanitary and Ship Canal, DuPage River or Kankakee River without crossing over other private or public property.
RIVERINE. Related to, formed by or resembling a channel (including creeks and rivers).
RIVERINE SFHA. Any SFHA subject to flooding from a river, creek, intermittent stream, ditch, on-stream lake system or any other identified channel. This term does not include areas subject to flooding from lakes, ponding areas, areas of sheet flow or other areas not subject to over bank flooding.
RUNOFF. The waters derived from melting snow or rain falling within a tributary drainage basin that exceeds the infiltration capacity of the soils of that basin.
RUNOFF POLLUTANTS. Sediment, heavy metals, petroleum-based organic compounds, nutrients, oxygen-demanding organics (BOD), pesticides, salt and pathogens which may be present in stormwater runoff.
SEASONAL HIGH GROUND WATER TABLE. The upper limits of the soil temporarily saturated with water.
SEDIMENTATION. The process that deposits hydraulically moved soils, debris and other materials either on other ground surfaces or in bodies of water, watercourses or stormwater drainage systems.
SEDIMENT TRAP. A structure or area that allows for the temporary deposit and removal or disposal of sediment materials from stormwater runoff.
SEEPAGE. The movement of drainable water through soil and rock.
SITE. A lot or a parcel of land where grading work is performed as a single unified operation.
SITE DEVELOPMENT PERMIT. A permit issued by the County Land Use Department for the construction or alteration of ground improvements and structures for the control of erosion, runoff and grading, or for the clearing, grading, stripping, excavating or filling of land.
SPECIAL FLOOD HAZARD AREA (SFHA). See FLOODPLAIN.
STORMWATER ADMINISTRATOR. The person designated by the County Executive to administer and enforce this chapter in the unincorporated areas of the county.
STORMWATER DRAINAGE SYSTEM. All means, natural or human-made, used for conducting stormwater to, through or from, a drainage area to the point of final outlet from a property. The STORMWATER DRAINAGE SYSTEM includes, but is not limited to, any of the following: conduits, appurtenance features, canals, channels, ditches, streams, culverts, streets, storm sewers, detention basins, swales and pumping stations.
STORMWATER FACILITY. All ditches, channels, conduits, bridges, culverts, levees, ponds, natural and human-made impoundments, wetlands, riparian environment, tile, swales, sewers or other natural or artificial structures or measures which serve as a means of draining surface and subsurface water from land.
STORMWATER RUNOFF. The waters derived from melting snow or rain falling within a tributary drainage basin which are in excess of the infiltration capacity of the soils of that basin, which flow over the surface of the ground or are collected in channels or conduits.
STORM SEWER. A closed conduit for conveying collected stormwater.
STREAM. Any river, creek, brook, branch, flowage, ravine or natural or human-made drainage way which has a definite bed and banks or shoreline, in or into which surface or groundwater flows, either perennially or intermittently.
STRUCTURE. The results of a built change to the land constructed on or below the ground, including the construction, reconstruction or placement of a building or any addition to a building; installing a manufactured home on a site; preparing a site for a manufactured home or installing a travel trailer on a site for more than 180 days unless they are fully licensed and ready for highway use.
SUBSTANTIAL IMPROVEMENT. Any reconstruction, rehabilitation, addition or improvement of a structure taking place during a ten-year period, in which the cumulative percentage of improvements equals or exceeds 50% of the market value of the structure before the improvement or repair is started. SUBSTANTIAL IMPROVEMENT is considered to occur when the first alteration of any wall, ceiling, floor or other structural part of the building commences, whether or not that alteration affects the external dimensions of the building. This term includes structures that have incurred repetitive loss or substantial damage, regardless of the actual work done. This term does not, however, include any project for improvement of a structure to comply with existing state or local health, sanitary or safety code specifications which are solely necessary to assure safe living conditions, or any alteration of a historic structure listed on the National Register of historic Places or the State Register of historic places, provided that the alteration will not preclude the structure’s continued designation as a historic structure.
SUBSURFACE DRAINAGE. The removal of excess soil water to control water table levels at predetermined elevations for structural, environmental or other reasons in areas already developed or being developed for agricultural, residential, industrial, commercial or recreational uses.
SUBSURFACE WATER. Water beneath the ground or pavement surface. Sometimes referred to as GROUND WATER or SOIL WATER.
T FACTOR. The soil loss tolerance. It is defined as the maximum amount of erosion at which the quality of a soil as a medium for plant growth can be maintained. Erosion losses are estimated by Universal Soil Loss Equation (USLE) and Revised Universal Soil Loss Equation (RUSLE).
TECHNICAL GUIDANCE MANUAL. The manual adopted by the County Board, which refers to the County Stormwater Management Ordinance, as codified in Chapter 55, and provides additional explanations and examples.
THALWAG. A line along the lowest point in a channel.
TRANSITION SECTION. Reaches of the stream or floodway where water flows from a narrow cross-section to a wide cross-section, or vice versa.
TRAVEL TRAILER OR RECREATIONAL VEHICLE. A vehicle which is:
(1) Built on a single chassis;
(2) Four hundred square feet or less when measured at the largest horizontal projection;
(3) Designed to be self propelled or permanently towable by a light duty truck; and
(4) Designed primarily not for use as a permanent dwelling, but as a temporary living quarters for recreational camping travel or seasonal use.
TWO-YEAR EVENT. A runoff, rainfall, or flood event having a 50% chance of occurring in any given year.
USABLE SPACE. Space used for dwelling, storage, utilities or other beneficial purposes, including, without limitation, basements.
VIOLATION. The failure of a structure or other development to be fully compliant with the community’s floodplain management regulations. A structure or other development without the elevation certificate, other certifications or other evidence of compliance is presumed to be in violation until a time as that documentation is provided.
WATER TABLE. The upper limit of a free water surface in a saturated soil or underlying material.
WATERS OF THE UNITED STATES. As defined by the United States Army Corps of Engineers in their Federal Methodology for the Regulation of Wetlands. For purposes of this chapter, WATERS OF THE UNITED STATES include wetlands, lakes, rivers, streams, creeks, bogs, fens and ponds. WATERS OF THE UNITED STATES do not include maintained stormwater facilities.
WATERSHED. All land area drained by, or contributing water to, the same stream, lake, stormwater facility or draining to a point.
WATERSHED BENEFIT. See NET WATERSHED BENEFIT.
WATERSHED CHARACTERISTICS. Includes land use, physiology, habitat, climate, drainage system and community profile.
WATERSHED PLAN. A study and evaluation of an individual drainage basin’s stormwater management, floodplain management, water quality and flood control needs and capabilities.
WET BASIN. A detention basin designed to maintain a permanent pool of water after the temporary storage of stormwater runoff.
WETLAND.
(1) As defined in current federal methodology recognized by the United States Army Corps of Engineers for regulatory purposes.
WETLANDS are areas that are inundated or saturated by surface or ground water at a frequency and duration sufficient to support, and under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions (known as hydrophytic vegetation).
(2) A
WETLAND is identified based upon the three attributes:
(a) Hydrology;
(b) Soils; and
(c) Vegetation as mandated by the current federal wetland determination methodology.
(3) Classification of areas shall follow the United States Army Corps of Engineers Wetlands Delineation Manual, Technical Report Y-87-1.
WETLAND BASIN. A detention basin designed with all or a portion of its bottom area as a wetland.
WETLAND IMPACT. Wetlands that are hydrologically disturbed or otherwise adversely affected by flooding, filling, excavation or drainage that results from implementation of a development activity.
WETLAND MITIGATION CREDITS. A credit is equal to one acre of wetland mitigation. The value of mitigation to generate credit is calculated differently depending the form of the mitigation.
WOODLANDS. An area with a contiguous tree canopy area, including younger understory trees, of at least one acre that contains healthy deciduous coniferous trees as determined by a qualified professional.
(Ord. 10-164, passed 6-17-2010; Ord. 19-238, passed 9-19-2019; Ord. 22-24, passed 2-25-2022)
REQUIREMENTS FOR STORMWATER MANAGEMENT
(B) Applicability of site runoff storage requirements (detention). All developments shall comply with the site runoff storage requirements provided in § 164.023 of this subchapter in which:
(1) Manufactured home parks are to be constructed on a parcel larger than one acre in size;
(2) Multi-family homes are to be constructed on a parcel larger than one acre in size;
(3) A parcel is being subdivided into single- family residential lots; and
(4) Nonresidential development.
(C) Exemptions from site runoff storage requirements (detention). Site runoff storage is not required under the following circumstances:
(1) Direct discharge industrial sites where appropriate BMPs are utilized;
(2) Non-industrial direct discharge sites 160 acres or less having the following minimum river frontage:
Site Area | Required Frontage |
0—2 acres | 50 feet |
Up to 5 acres | 100 feet |
Up to 10 acres | 150 feet |
Up to 40 acres | 200 feet |
Up to 80 acres | 350 feet |
Up to 160 acres | 500 feet |
(3) The development is of a nonresidential nature on an existing developed parcel, where new impervious surfaces do not exceed 25,000 square feet in aggregate provided that the design does not further concentrate stormwater flows from the existing condition, and the design provides appropriate best management practices on site;
(4) The development is of a nonresidential nature on an existing agricultural parcel, where new impervious surfaces do not exceed 25,000 square feet in aggregate, which are not exempted under § 164.024;
(5) Roadway developments in rights-of-way under the ownership or control of a unit of local governments when the contiguous area of new roadway construction (excluding previously paved areas) does not exceeds two acres;
(6) Minor subdivision of a single-family residential zoned parcel;
(7) Construction of a single-family residence, an addition to an existing single-family residence, and/or construction of an accessory structure where the cumulative impervious area does not exceed 25,000 square feet on an existing legal lot or parcel;
(8) Single-family structures on legally created agricultural parcels;
(9) Bike trails, pedestrian trails and multi-purpose trails on park district or forest preserve district property when the trail is designed so as to not further concentrate stormwater flows from the existing condition, and when it can be shown by calculations, as required under § 164.023(B), that there will be no net increase in stormwater discharge rates from the proposed construction during the two-year and 100- year rainfall events. BMPs can be utilized to help mitigate discharge rates. In addition, a trail shall not be designed, nor constructed in a manner that could result in impoundment of stormwater onto another property. The trails shall not exceed 12 feet in width to be exempt from the site runoff storage requirements; and/or
(10) A parcel being subdivided into single-family residential lots having a minimum lot size of two and one-half acres or greater. Provided that the exempt subdivision is designed so as to not further concentrate stormwater flows from the existing condition, and when it can be shown that there will be no net increase in stormwater discharge rates from the proposed construction during the two-year and 100- year rainfall events. BMPs can be utilized to help mitigate discharge rates (i.e., roadside ditches).
(11) In solar farm developments, the solar panels will be considered “disconnected” and therefore pervious, if the following criteria are met:
(a) Groundcover vegetation must be maintained in good condition in those areas receiving disconnected runoff, specifically the area under and between the rows of panels. Typically this maintenance is no different than other lawn or landscaped areas. However, care should be taken to avoid soil compaction in these areas.
(b) The total proposed impervious area on the surface of the ground must be less than 25,000 square feet. The total impervious must include any gravel, pavement, asphalt, concrete, panel posts, and so on.
(c) The open space between panels must be equal to or greater than the width of the row of panels. The runoff must sheet flow onto and across the vegetated area to maintain the disconnection. Sheet flow is optimal on sites with gradual slopes (<5%). Level spreaders, terraces, or berms may be used to maintain sheet flow conditions if the average slope is steeper than 5% and <10%. Installations on slopes >10% will require a detailed engineering study to demonstrate the safe and non-erosive conveyance of runoff to the property line or downstream storm water management practice.
(d) For permitting purposes the entire project area is counted toward disturbed area.
(Ord. 10-164, passed 6-17-2010; Ord. 22-24, passed 2-25-2022)
(A) Requirements applicable to all developments. No development shall:
(1) Result in any new or additional expense to any person other than the developer for flood protection; or
(2) Increase flood elevations or decrease flood conveyance capacity upstream or downstream of the area under the ownership or control of the developer. This requirement shall not prohibit the removal or reduction of built obstructions to flow, such as increasing culvert capacity or lowering roadway elevations.
(B) Building permits. Stormwater facilities shall be functional before building permits are issued for residential and nonresidential subdivision.
(C) Single parcel developments. Stormwater facilities shall be functional where practicable for single parcel developments before building construction begins.
(D) Overland flow paths. The development shall have an overland flow path at the downstream limit of the property that will pass the base flood flow at a stage at least one foot below the lowest adjacent foundation grade in adjacent structures without increasing damage to structures or property. If the upstream drainage area is less than 20 acres, the storm sewer pipe and inlet sized for the base flood can be constructed in lieu of providing an overland flow path. Overland flow paths internal to the site shall be considered as part of the major stormwater system and shall be designed for conveyance of the base flood (critical duration) and shall be a minimum of one cfs per tributary acre without damage to structures.
(E) Protection of buildings. All usable space in new buildings or added to existing buildings hydraulically connected to a major stormwater system, site runoff storage facility or overland flow path shall be elevated, flood proofed, or otherwise protected to at least one foot above the blocked restrictor elevation of the site runoff storage (detention) facility or the calculated high water elevation of the overland flow path, or two feet above the base flood elevation, whichever is greater.
(F) Depressional storage. The function of existing on-site depressional storage shall be preserved for both on-site and off-site tributary flows in addition to required detention. When depressional storage is removed, it must be compensated for in the site runoff storage facility at one and one-quarter to one ratio if the depression is identified as a regulatory floodplain and at a one to one ratio if the depression is not a regulatory floodplain, provided that offsite areas tributary to the existing depressional storage are routed through the site runoff storage facility. This requirement is in addition to the site runoff storage required in § 164.023. The Chief Subdivision Engineer may allow the function of depressional storage to be preserved if the applicant performs detailed pre- and post-project hydrologic and hydraulic modeling to identify the effect of the depressional storage on discharges over a range of rainfall frequencies.
(Ord. 10-164, passed 6-17-2010) Penalty, see § 164.999
(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 such that flows will not increase flood damage to adjacent property except when otherwise approved by the Chief Subdivision Engineer. 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. Storm sewers shall be sized to convey the ten-year storm in a full (non-surcharged) pipe condition unless otherwise directed by the road authority.
(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. Existing agricultural subsurface and surface drainage systems shall be evaluated with regard to their capacity, condition and capability to properly convey low flow groundwater and two-year site runoff to a surface outlet without damage to downstream structure and land use on the adjacent properties. 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.
(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 sub-surface 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 minor stormwater systems 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 (minimum ten feet wide) 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 commercial and/or industrial property, the stormwater system does not have to be located within an easement or public right-of-ways unless the system serves multiple properties or is constructed on an adjacent property. Instead, the owner or owner’s representative shall provide a signed agreement to the county 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 and ponding depths. Maximum flow depths for new transverse stream crossings shall not exceed six inches at the crown of the road during the 100-year storm condition. 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 100-year storm condition. The maximum flow depth on a roadway shall not exceed six inches at the crown for flow parallel to the roadway. The maximum stormwater ponding depth during the 100-year critical storm event in any yard or parking area shall not exceed nine inches, and the maximum storage elevation shall not be maintained for more than four hours. Inlets shall have capacity to allow the inflow, based on the design pipe capacity, with no more than three inches of ponding over street inlets.
(I) Diversion of flow to another watershed. Transfers of waters between watersheds (diversions) shall be prohibited except when such transfers will not violate the provisions of § 164.021(A) and are otherwise lawful. Watersheds for purpose of regulation under this section shall be the major watershed divides as defined in the County Stormwater Management Plan.
(J) Best management practices requirement.
(1) 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. Developments shall be designed to create a healthy aquatic ecology, provide for sustainability, minimize maintenance and human intervention, and treat stormwater as a multiple-use resource. The Chief Subdivision Engineer does reserve the right to require specific stormwater best management practices at a particular site if it discharges to a sensitive ecological area or if the intended use of the property produces a particularly detrimental water quality of the discharge.
(2) Listed below are examples of BMPs that meet the intent of the chapter:
(a) Minimize mass grading and disturbance of soils;
(b) Lay out streets and lots to conform to the natural topography of the site;
(c) Minimize new impervious surfaces by clustering of neighborhoods and homes, minimizing street widths and parking lots, and reducing lot sizes and building setbacks;
(d) Preserve and create natural landscaping, buffers and filter strips;
(e) Utilize permeable areas to maximize infiltration of runoff into the ground through the use of biofilters, filter strips, swales, infiltration trenches, permeable pavement and native vegetated open spaces;
(f) Direct runoff to permeable areas and/or utilize stormwater for reuse by:
1. Directing roof runoff towards permeable surfaces, drywells, French drains, vegetated swales or other BMPs instead of driveways or other non-permeable surfaces;
2. Grading impervious surfaces to direct runoff to permeable areas, utilizing level spreaders or other methods to distribute the impervious runoff onto pervious surfaces;
3. Using cisterns, retention structures or rooftops to store precipitation or runoff for reuse; and
4. Removing berms and designing pavement edges (e.g., curb cuts) in order to direct water to permeable landscaped areas.
(g) Improve water quality of stormwater leaving the site through the use of a naturalized detention basin designed to maximize the removal and transformation of runoff pollutants. Design should include:
1. Emergent vegetation in the bottoms of the wetland basins and along the periphery of wet bottom basins and side slopes vegetated in native prairie (traditional dry bottom basins are not approved BMPs);
2. Stilling basins at major detention basin inlets and maximizing the distance between major inlets and the basin outlet;
3. Installation of pre-settlement or mechanical stormwater treatment units prior to discharge of stormwater into primary detention basins; and
4. In locations where detention basin discharge to adjacent/downstream wetlands, designing detention basin outlet structures to spread and infiltrate runoff through the use of level spreader devices.
(Ord. 10-164, passed 6-17-2010; Ord. 19-238, passed 9-19-2019)
(A) Release rate.
(1) If no release rate is specified in an adopted watershed plan in accordance with § 164.003, 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 § 164.020(C). Unless exempted in § 164.020(C), 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.
(3) In the event the downstream creeks, streams, channels, conduits or other drainage facilities are inadequate to receive the release rate herein above provided and there is no adopted watershed plan available, then the allowable release rate shall be reduced to that rate permitted by the receiving downstream creeks, streams, channels, conduits or other drainage facilities; and additional detention volume shall be required to store that portion of the runoff exceeding the capacity of the receiving drainage facilities.
(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.
(2) 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 data from Illinois State Water Survey Updated Bulletin 70 (March 2019) Northeast Sectional Code.
(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 three-quarters-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 evapotransporation 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 Chief Subdivision Engineer 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 (no earlier than) that required to empty the calculated extended detention volume within five days of the storm event.
(b) For purposes of meeting the maximum subsurface drainage discharge requirements, flow control orifices and weirs may be used.
(c) 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.
(d) Farm field tile shall not be used as a component of the extended detention system, unless evaluated per § 164.022(D).
(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 calculated (blocked restrictor) 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 Chief Subdivision Engineer, 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. One foot of freeboard shall be provided over the blocked restrictor water level.
(7) Storage facilities with single pipe outlets shall have a minimum inside diameter of 12 inches. Where flow control orifices are used, the minimum diameter is four-inch. If design release rates necessitate a smaller outlet, structures such as perforated risers or other self-cleaning restrictors shall be used.
(8) Basin side slopes should not be steeper than four to one (horizontal to vertical); five to one side slopes are preferred. For wet-bottom basins, side slopes not steeper than two to one may be used below the safety ledge.
(9) Wet-bottom storage facilities must include a safety shelf, minimum six feet width, two and one-half feet to three feet below the normal water line.
(10) The permanent pool volume in wet-bottom basins shall be at least equal to the two-year, 24-hour runoff volume from the tributary watershed. The minimum permanent pool depth is three feet, excluding safety shelves. If fish habitat is to be provided, over 25% of the bottom area must be at least ten feet deep.
(11) Dry bottom detention basins shall be graded with not less than 1% slope across basin floor.
(12) All detention basins shall be provided with an overflow structure capable of passing pre- development peak runoff rate capable of meeting the requirements of division (F)(6) above.
(13) All detention basins discharging at grade shall discharge a minimum of 20 feet from any property line unless an agreement allowing discharge closer to the property line is secured from the adjacent property owner or road authority. Vegetative buffers, level spreaders or other appropriate BMPs shall be utilized at the outlet structure to promote spreading and infiltration of the discharge.
(14) Riprap or other approved method of dissipating energy shall be utilized at all end sections and point discharge locations.
(15) Standards for naturalized detention basins include: flat side slopes (eight to one from two- year water level to one-half foot below normal water level), shallow zones of emergent vegetation at water’s edge, combination of natural vegetation and open water area. Uses of aerators, cascades, water falls and the like are encouraged. A planting/ maintenance shall be submitted and approved by the Chief Subdivision Engineer.
(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; and
(3) The Chief Subdivision Engineer 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 division (G)(1) and (G)(2) above.
(H) Site runoff storage facility requirements within the regulatory floodway. Site runoff storage facilities shall not be located within the regulatory floodway.
(I) Off-site facilities. Site runoff storage facilities may be located off-site if the following conditions are met.
(2) Adequate storage capacity in the off-site facility is dedicated to the development and placed in an appropriate easement on the off-site property.
(3) The development includes means to convey stormwater to the off-site storage facility. The conveyance system shall be placed in an appropriate easement on the off-site property.
(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. 10-164, passed 6-17-2010; Ord. 19-238, passed 9-19-2019)
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