A. General Information:
1. All developments shall meet the requirements specified for general stormwater and floodplain development, site runoff, sediment and erosion control, and performance security as outlined in this chapter, or as outlined in the DuPage County Countywide Stormwater and Floodplain Ordinance, whichever is more restrictive.
2. An increase of ten thousand (10,000) square feet or more of impervious area will require detention, rather than an increase of twenty five thousand (25,000) square feet or more as required by DuPage County.
3. Development shall also meet the more specific requirements of applicable adopted watershed plans or adopted interim watershed plans, set forth earlier in this chapter.
4. All developments within floodplain, floodway wetlands, buffers, and substantial improvements within a floodplain, shall also satisfy the requirements specified in section 10.106 of this article.
5. All developers shall submit the documents specified in section 10.107 of this article to verify compliance with these requirements.
6. Facilities constructed under the provisions of this chapter shall be maintained according to the criteria and guidelines established in the plan. Maintenance is the responsibility of the owner of the land on which the stormwater facilities are constructed unless the responsibility is assigned, pursuant to subsection 10.109H of this article, to an entity acceptable to the governmental unit that has jurisdiction over such land.
B. General Stormwater And Floodplain Requirements:
1. Development shall comply with the DuPage County Countywide Stormwater and Floodplain Ordinance sections 15-27 and 15-28 as well as the following requirements.
2. The analysis and design of all stormwater and floodplain facilities required for development shall:
a. Be consistent with the technical guidance for the DuPage Countywide Stormwater and Floodplain Ordinance for the plan;
b. Consider existing and ultimate watershed and land use conditions, with and without the proposed development.
3. Stormwater facilities shall be functional before building permits are issued for residential and nonresidential subdivisions. Stormwater facilities include site runoff storage and flood compensatory storage or building construction that increases total impervious area.
4. Stormwater facilities shall be functional where practicable for single parcel developments before general construction begins.
5. All usable space in new buildings or added to existing buildings adjacent to a major stormwater system, site runoff storage facility overflow path or site runoff storage facility, shall be elevated, floodproofed, or otherwise protected to at least one foot (1') above the design elevation to prevent the entry of surface stormwater. The design elevation is the elevation associated with the design rate as determined in subsection D8f of this section.
C. Site Runoff Requirements:
1. Development: Development shall comply with the DuPage County Countywide Stormwater and Floodplain Ordinance section 15-71 as well as the following requirements.
2. No Damage To Adjacent Property: Stormwater facilities shall be required and be designed so that runoff exits the site at a point where flows will not damage adjacent property.
3. Compliance With Illinois Law: Stormwater facilities shall be designed to conform with the requirements of Illinois law and the applicable watershed plans.
4. Design Runoff Rates Calculation: Design runoff rates shall be calculated by using continuous simulation models or by event hydrograph methods. If event hydrograph methods are used, they must be either HEC-1 SCS runoff method, TR-20, HEC-HMS, or TR-55 tabular method. Event methods must incorporate the following assumptions:
a. Antecedent moisture condition = 2; and
b. Huff or SCS type II rainfall distribution.
5. Stormwater System Location: Major and minor stormwater systems shall be located within easements or rights-of-way explicitly providing for public access for maintenance of such facilities.
6. Maximum Flow Depths: Maximum flow depths on any roadway shall not exceed six inches (6") during the base flood condition. This requirement does not apply to the at-grade repair, resurfacing or in-kind replacement of a roadway existing prior to the effective date of this chapter.
7. Incorporation Of BMPs: Developments shall, to the extent practicable and necessary, incorporate the following Best Management Practices (hereinafter referred to as "BMPs") into the design to minimize increases in runoff rates, volumes, and pollutant loads:
a. The following requirements for site runoff shall apply to industrial and commercial developments and also to residential developments to the extent not inconsistent with the requirements in subsection C7b of this section:
(1) All runoff from rooftops and parking lots, and discharge from sump pumps, that does not discharge into a site runoff storage facility shall be directed onto vegetated swales of filter strips, for a distance of at least fifty feet (50').
(2) Vegetated swales shall be utilized, where appropriate, as an alternative to storm sewers to promote the infiltration of stormwater and the filtration of stormwater pollutants.
(3) Effective impervious surface area should be limited by site designs which minimize the area of streets, parking lots, and rooftops and/or utilize permeable paving material such as concrete grids in low traffic areas.
(4) Other Best Management Practices (hereinafter referred to as BMPs) such as infiltration basins and trenches (where permeable soils are present) and filtration basins and sand filters (on highly impervious or industrial developments) shall be utilized where appropriate.
(5) The practices and procedures established in the technical guidance for the DuPage County Countywide Stormwater and Floodplain Ordinance for the plan and as described in the United States Environmental Protection Agency's "Guidance Specifying Management Measures for Sources of Nonpoint Pollution in Coastal Waters", dated January 1993.
b. The following requirements shall apply to all residential developments for which building permits are issued on and after March 1, 2009:
(1) For all teardown lots and all subdivisions less than one-half (1/2) acre in size:
(A) All gutters, downspouts and sump pump discharges shall have their flows directed in such a manner as to disperse the flow throughout the lot, to provide maximum infiltration, and reduce the concentration of flow which may otherwise have an adverse effect on neighboring properties. Where the property is tributary to a detention basin that was designed to accommodate the runoff from the lot or subdivision, downspouts and sump pump discharges shall be connected directly to the storm sewer. Where the property is not tributary to such detention basin, vegetated infiltration areas with subsurface drainage shall be used to provide stormwater quality benefits and reduce the concentration of runoff.
(B) Slopes on all lots shall be designed at a minimum of two percent (2%) grade, except that on lots existing prior to March 1, 2009, the existing slopes may be allowed to remain if they are a minimum of one and one-half percent (11/2%) grade. Following construction, an as built survey shall be submitted to the City in order to verify that the site was constructed to the elevations and slopes shown on the approved engineering plans. It shall be unlawful for the owner of any property which has been improved or graded in accordance with the provisions specified herein to alter the grading from that which is shown on the approved as built plan.
(C) Additionally, the entire runoff from the site shall be treated in accordance with the BMP requirements of the DuPage County Countywide Stormwater and Floodplain Ordinance, or a fee specified in the master fee schedule, shall be paid in lieu of providing such treatment. The fee in lieu rate shall be in accordance with the current County ordinance:
Use of the fee in lieu option shall satisfy the requirements with respect to the DuPage County Countywide Stormwater and Floodplain Ordinance BMP; however, all City of Wood Dale requirements must still be adhered to, including those in subsections C7b(1)(A) and C7b(1)(B) of this section. The fee in lieu funds collected shall be accounted for in a separate account and shall be used for the construction of new BMPs, or for the maintenance and repair of existing BMPs.
(2) For all subdivisions between one-half (1/2) and two (2) acres in size:
(A) All of the requirements in subsection C7b(1) of this section.
(B) The drainage system of all subdivisions whose final plats are approved on or after March 1, 2009, shall be required to connect to an existing storm sewer. If an existing storm sewer is not adjacent to the subdivision, an offsite storm sewer shall be constructed.
(3) For all subdivisions two (2) acres or greater in size:
(A) Site runoff storage (detention) is required in accordance with this Code.
(B) The detention facilities shall be designed in accordance with the provisions of the DuPage County Countywide Stormwater and Floodplain Ordinance and shall incorporate BMPs in accordance with the recommendations found in appendix E of the DuPage County Countywide Stormwater and Floodplain Ordinance: water quality Best Management Practices technical guidance. This shall be in addition to the requirements of this subsection C7 and this Code. There shall be no fee in lieu of providing water quality BMPs for residential subdivisions of two (2) acres or greater requiring runoff storage.
c. An alternative to providing water quality BMPs for other types of development, an owner or developer may pay a fee in lieu specified in the master fee schedule. No fee in lieu of providing water quality BMPs shall be granted for developments other than those expressly provided for in subsections C7a and C7b of this section and this subsection C7c.
d. Variances from the requirements of subsections C7a and C7b of this section shall conform to the procedures and requirements of section 10.112 of this article, except that, prior to submitting the petition for variance to the Community Development Director, the City Engineer shall submit such to the DuPage County Stormwater Management Planning Committee for review and the making of recommendation. The recommendations of the DuPage County Stormwater Management Planning Committee shall be submitted for consideration by the Community Development Commission at the same time as the petition for variance is considered. No variance requesting the construction of swales as the sole means of accommodating stormwater runoff in either new developments or on teardown lots shall be granted unless the appropriate drainage structures and storm sewers are provided at the receiving end of the swales to collect the runoff. No variance for a fee in lieu of providing water quality BMPs shall be granted where runoff storage (detention) is required.
8. Best Management Practices: Best Management Practices are required in accordance with the DuPage County Countywide Stormwater and Floodplain Ordinance sections 15-63, 15-64, and 15-65 as well as the following requirements.
a. Postconstruction Best Management Practices: PCBMPs, a term that also includes VCBMPs, are required onsite to treat stormwater runoff for pollutants of concern and reduce runoff volume for all developments with two thousand five hundred (2,500) square feet or more net new impervious area compared to predevelopment conditions, with the exceptions and exclusions noted below. Upon a documented finding by the City Engineer that providing either PCBMPs or VCBMPs onsite is impractical, then a PCBMP fee in lieu shall be paid by the applicant in lieu of providing onsite PCBMPs and VCBMPs.
(1) Onsite PCBMPs or VCBMPs are waived for the following developments in addition to those listed in the DuPage County Countywide Stormwater and Floodplain Ordinance:
(A) The development is limited to the resurfacing or reconstruction of an existing roadway or the replacement of an existing culvert or bridge that drains to an appropriately sized and functional PCBMP; or
(B) Linear utility developments. This exception does not include buildings, substations, pads, parking lots or other associated utility support facilities.
(2) DuPage County Countywide Stormwater and Floodplain Ordinance section 15-63.B prohibits onsite infiltration for certain types of projects. However, the City requires an equivalent PCBMP fee in lieu if specifically waived by the County:
b. Volume Control Best Management Practices Design Criteria: Refer to the DuPage County Countywide Stormwater and Floodplain Ordinance sections 15-63, 15-64, and 15-65 as well as the following requirements:
(1) The required VCBMP volume shall be calculated as the product of the new impervious area and the rainfall depth generated from a 1.25-inch rainfall event occurring over a two (2) hour period. No abstractions are taken on the rainfall depth.
(A) The VCBMP shall be designed with sufficient volume to store the calculated volume.
(B) The preferred method of discharge from the VCBMP facility is through an infiltration or evapotranspiration facility. Where soils are inappropriate for infiltration, a means of positive conveyance, such as a perforated drain, may be used provided that the VCBMP does not draw down in less than forty eight (48) hours. The underdrain may not be set with an invert below the midline of the trench or other excavation.
(C) A control structure or minimum four inch (4") perforated drain shall be used where volumes are very small and conveyance designs become impractical to implement, even if that causes drawdown times to be less than forty eight (48) hours.
(2) The design of the facility will be such that runoff from impervious surfaces is captured, with a preference for those impervious surfaces used by automobiles, if an element of the overall development.
c. Pollutants Of Concern: PCBMPs are required onsite to treat the identified pollutants of concern in runoff from newly constructed impervious surfaces in a development. Applicants shall identify the pollutants of concern that may be generated by the proposed development from the following list: total suspended soils (TSS), metals, oils, and nutrients consisting of nitrogen and phosphorous. Applicants shall design the combination of onsite VCBMPs and PCBMPs to store any required volume and treat the identified pollutants of concern. Proposed PCBMPs shall only be required to treat those pollutants identified and agreed to by the City Engineer.
d. VCBMPs Effective At Pollutant Removal: The following VCBMPs are considered to be effective at pollutant removal, provided that they are designed to also treat the pollutants of concern:
(1) Constructed wetlands, or
(2) Wet or wetland bottom site runoff storage basins, or
(3) Vegetated compensatory flood storage ponds, or
(4) Infiltration ponds or trenches, with vegetative surfaces, with or without an underdrain system.
(5) Vegetated swales with infiltration capability, with or without an underdrain system.
(6) Other VCBMPs that can be demonstrated to be effective at treating the pollutants of concern.
D. Site Runoff Storage Requirements (Detention):
1. Maximum site runoff storage requirements shall be consistent with the information, procedures, and requirements of the applicable Watershed Plans and Studies of this section, except as limited by the provisions of subsection D2 of this section.
2. If no release rate, or a greater release rate than identified below, is specified in the applicable Watershed Plans and Studies of this section, then sufficient storage shall be provided such that the probability of the post development release rate exceeding 0.1 cfs/acre of disturbed site area shall be less than one percent (1.0%) per year. Design runoff volumes shall be calculated using continuous simulation or event hydrograph methods.
3. If event hydrograph methods are used to calculate design runoff volumes, they must be either HEC-1 SCS runoff method with outlet routing option, TR-20 with outlet control routing option, or TR-55 tabular method. Events methods shall incorporate the following assumptions:
a. Antecedent moisture condition = 2; and
b. Huff or SCS type II rainfall distribution; and
c. Twenty four (24) hour duration storm with a one percent (1%) probability of occurrence in any one year as specified by Illinois State Water Survey Bulletin 70 Northeast Sectional Rainfall Statistics, as amended from time to time.
4. If continuous simulation methods are used, design runoff volumes shall be calculated using the Wheaton National Oceanic and Atmospheric Administration rainfall record from 1949 to present.
5. For sites where the undeveloped release rate is less than the maximum release rate in subsection D2 of this section, the developed release rate and corresponding site runoff storage volume shall be based on the existing undeveloped release rate for the development site.
6. All hydrologic and hydraulic computations must be verified under the full range of expected downstream water surface elevations, from low flow through the base flood elevation.
7. Responsibility for maintenance of all storage facilities shall be designated on the plat of subdivision or other recorded documents.
8. Storage facilities shall be designed and constructed with the following characteristics:
a. Water surface depths two feet (2') above the base flood elevation will not damage the storage facility.
b. The storage facilities shall be accessible and easily maintained.
c. All outlet works shall function without human intervention or outside power and shall operate with minimum maintenance.
d. Storage facilities shall facilitate sedimentation and catchment of floating material.
e. Storage facilities shall minimize impacts of stormwater runoff on water quality by incorporating Best Management Practices.
f. Storage facilities shall provide an overflow structure and overflow path that can safely pass excess flows through the development site. The minimum design rate shall be 1.0 cfs/acre of the entire upstream drainage area tributary to the storage facility without damaging any buildings onsite or surrounding the site with overbank flooding and shall discharge at the same location as where water leaves the site pre-development. This location may be modified by the Public Works Director or City Engineer. This conveyance shall begin operation at the elevation at which the site runoff storage volume is met. If one hundred (100) acres or greater drains to the emergency overflow, then the overflow is part of the major drainage system and it also falls under the floodplain management requirements. A minimum of one foot (1') freeboard shall be permitted around the detention basin above the design water level of the overflow structure.
g. Wet detention basins shall be designed to remove stormwater pollutants, to be safe, to be aesthetically pleasing, and as much as feasible to be available for recreational use.
(1) Wet basins shall be at least three feet (3') deep, excluding near shore banks and safety ledges. If fish habitat is to be provided, they shall be at least ten feet (10') deep over twenty five percent (25%) of the bottom area to prevent winter freeze out.
(2) The permanent pool volume in a wet basin at normal depth shall be equal to the runoff volume from its watershed for a twenty four (24) hour duration storm with a fifty percent (50%) probability of occurrence in any year.
(3) Wet basins shall be designed in a manner which will reduce as much as practicable nuisance problems such as algae. Aeration or other control methods may be required to minimize such nuisance problems.
h. The maximum stormwater storage depth in parking lots shall be not greater than six inches (6").
i. The shorelines of all detention basins at 100-year capacity shall be as level as practicable to prevent accidental falls into the basin and for stability and ease of maintenance. Shorelines shall not have a greater than four to one (4:1) horizontal to vertical slope. Shoreline protection shall be provided to prevent erosion from wave action.
j. The side of the banks of both wet and dry detention basins shall not have a horizontal to vertical slope steeper than four to one (4:1). Every retention or detention basin or other storage facility shall be constructed so that there is no encroachment of water at high water level upon adjacent lots or tracts.
k. All wet detention basins shall have a level safety ledge extending ten feet (10') into the basin from the shoreline and three feet (3') below the normal water depth. The ledge shall be pitched to the shoreline at four percent (4%).
l. Velocities throughout the drainage system shall be controlled to safe levels taking into consideration rates and depths of flow.
9. Developments with storage facilities that have off site flow tributary to the site either shall provide storage sufficient to accommodate runoff from the offsite tributary watershed and the site or shall store the site runoff and convey off site flow through the development while preserving the existing flow and storage of the site.
E. Detention Variance Fee Program: Refer to the DuPage County Countywide Stormwater and Floodplain Ordinance section 15-97.
F. Soil Erosion And Sediment Control Requirements: Refer to the DuPage County Countywide Stormwater and Floodplain Ordinance section 15-58.
G. Soil Erosion And Sediment Control Plan Design Criteria:
1. All items listed in section 15-59 of the DuPage County Countywide Stormwater and Floodplain Ordinance shall apply.
2. If the volume, velocity, sediment load, or peak flow rates of stormwater runoff are temporarily increased during construction, then properties and floodplain, floodway wetlands, and buffers downstream from such development sites shall be protected from erosion.
3. The surface of stripped areas shall be permanently or temporarily protected from soil erosion within fifteen (15) days after final grade is reached. Stripped areas that will remain undisturbed for more than fifteen (15) days after initial disturbance shall be protected from erosion.
4. All control measures necessary to meet the requirements of this chapter shall be kept operational and maintained continuously throughout the period of land disturbance until permanent sediment and erosion control measures are operational.
H. Modifications To Drainage Tiles: Refer to the DuPage County Countywide Stormwater and Floodplain Ordinance section 15-77. (Ord. O-19-023, 7-18-2019, eff. 7-18-2019)