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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 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)