Section IV. Stormwater Management
The City of Aurora Stormwater Ordinance (Stormwater Ordinance) is a modification of the Kane County Stormwater Management Ordinance and is enforceable throughout the entire City limits, even those areas outside Kane County. Refer to Sec. 18-201 through 18-204 of the Code of Ordinances for the City’s modifications to the Kane County Stormwater Management Ordinance. The City Engineer is the Administrator of the Stormwater Ordinance. In addition to the Stormwater Ordinance, the City has developed additional ordinances and standards, including these Standard Specifications, in order to provide additional criteria for minimum design standards. In the event that any conflict should arise between the Stormwater Ordinance and any other City ordinances or standards, the more stringent requirement shall apply, unless otherwise determined by the Administrator.
When a dormant Special Service Area (SSA) for a detention basin is required per the Stormwater Ordinance, the Special Service Area (SSA) documents shall be submitted to the City’s Law Department prior to approval of final engineering plans and prior to signing the plat of subdivision.
Properties within a five (5) mile radius of the Aurora Airport are subject to the planning recommendations of the Federal Aviation Administration’s Advisory Circular Number 150/5200-33A regarding “Hazardous Wildlife Attractants on or Near Airports”. In an effort to accomplish the FAA planning recommendations, all stormwater management facilities shall have a minimum bottom slope of 1.5% established with deep-rooted native plantings with no permanent open water feature, to deter waterfowl from utilizing the facility and to promote stormwater infiltration. In addition, the retention volume component of the Stormwater Ordinance will not be required, by utilizing the suggested design requirements of Section 9-107.C and 9-107.D of the Kane County Stormwater Technical Manual. It is understood that project design constraints are unique and therefore each project must be reviewed based upon site-specific conditions. Refer to Tables 7-10 in the Kane County Stormwater Technical Manual for approved native plants based on planting zone water depths.
B. RELEASE RATE
Release Rates shall be in accordance with the Stormwater Ordinance. The maximum release rate is 0.10 cfs/ac.
Tailwater conditions from the downstream drainage system shall be considered for the design of the outlet structure.
C. DETENTION REQUIREMENTS
- Volume Requirements
The detention storage shall be computed by using the Event Hydrograph Routing Method and by using the City of Aurora’s Modified Rational Method, as shown in Exhibit IV-C-1. Whichever method requires the greatest detention volume shall govern. The Event Hydrograph Routing Method shall be in accordance with the parameters set forth within the Kane County Stormwater Management Ordinance.
The existing and proposed conditions models shall be prepared and submitted for review. The peak discharge shall be based on the critical duration, considering storm events ranging between thirty (30) minutes and two hundred forty (240) hours. The results of the model shall be provided in a table in the Stormwater Report.
For projects within the Indian Creek and Blackberry Creek watersheds, the required stormwater storage volume shall be one hundred ten percent (110%) of the volume required by the foregoing provisions of this section. The depth associated with the additional 10% storage volume shall not be included in the total head calculation when sizing the restrictor.
Refer to Appendix A of the Kane County Stormwater Management Ordinance for a map showing the boundaries of major watersheds in Kane County.
- Fee-in-Lieu of BMPs
A fee-in-lieu of onsite BMPs may be paid wherever a BMP would be required by the Stormwater Ordinance and the criteria in Section 9-81.B.4 can be met. The fee shall be determined according to Section 18-204 of the Code of Ordinances.
- Design Rainfall
All design analysis for minor and major stormwater systems and detention shall incorporate point frequency rainfall intensities as listed in the Illinois State Water Survey’s Bulletin 75 publication ”Precipitation Frequency Study for Illinois,” specifically Table 7 “Rainfall (inches) for Given Recurrence Interval for Section 2 (Northeast).” The Huff Quartile Distribution tables shall be used for Unit Hydrograph Method. Duration 5 minutes to 6 hours shall use the 1St Huff Quartile, 12 hours shall be the 2nd Huff Quartile, 18 hours to 24 hours shall be the 3rd Huff Quartile, and 48 hours to 240 hours shall be the 4th Huff Quartile. The table below represents the rainfall depth (inches) for the 100-year frequency and the rainfall intensities for each time duration (see Section III-A for a table indicating the 10-year frequency).
Table 4: 100-Year Frequency Rain Event
- Positive Drainage/Overland Flood Route
An overland flood route shall be shown on the plan sheets and calculated to convey all the stormwater runoff for the 100-year storm event to the proposed detention basin assuming the minor stormwater system (storm sewer, inlets and catch basins) is not functioning. Calculations shall be submitted to the City for review. Local Streets may be used to convey the overland flood route and shall have a maximum flow depth at the crown of six inches (6”) and in no case shall the depth of flow be greater than twelve inches (12”). Culverts crossing Minor Collectors, Major Collectors, and Arterial streets shall be designed to convey runoff from the 100-year storm event without overtopping the road.
The overland flood route draining offsite properties shall be placed within an outlot, when required by the City Engineer. The width of the outlot shall be based on calculations with a minimum ten foot (10’) width.
All areas of the property must provide an overland flow path that will pass the 100-year flow. If the tributary area is less than 20 acres, the vertical distance between the 100 year high water level within the overland flow path and the lowest building opening shall be a minimum of one foot (1’). If the tributary area is greater than 20 acres, the 100-year high water level shall be at a stage at least two feet (2’) below the lowest opening of any building in the vicinity of the flow path, in accordance with the Kane County Stormwater Management Ordinance Table 9-139.B. When it is unavoidable, and an overland flow path is located in the side yard of a structure with a basement, there shall be no basement windows on the side of the house where the overland flow path is located.
The design flow rate for overland flow paths shall be a minimum of one cubic foot per second (1 cfs) per tributary acre.
If there is a stormwater detention basin adjacent to the residential building, the 100-year high water level of the detention basin shall be at a stage at least two (2) feet below the lowest building opening.
- Restrictor Sizing
Restrictors shall be sized for the allowable release rate of 0.10 cfs per tributary acre or the pre-developed release rate (whichever is the lesser rate) at the high water elevation of the required storage. For areas within the Blackberry Creek and Indian Creek watersheds, the restrictor shall be sized as stated above, however, the overflow weir shall be set to provide an additional ten percent (10%) of stormwater storage volume.
Calculated restrictor sizes shall be rounded to the nearest commercial pipe size.
- Minimum Restrictor Size
The minimum restrictor size shall be four inches (4”) in diameter. For restrictor sizes six inches (6”) in diameter or less, the outlet storm sewer shall be a minimum twelve inch (12”) diameter RCP. For restrictor sizes larger than six inches (6”) in diameter, the outlet storm sewer shall be a minimum size of twice the inside diameter of the restrictor pipe.
- Restrictor Type
All restrictors shall be Schedule 40 PVC pipe and shall be grouted-in-place. The restrictor shall not be the outlet storm sewer. Metal plates are not acceptable.
- Restrictor Catch Basin
Restrictor catch basins shall conform to Exhibits IV-C-4A and/or IV-C-4B and shall be located where they can be accessed by heavy vehicles. Preferably, the access shall be from a street. However, in the event that the restrictor is not accessible from a street, a ten foot (10’) wide paved path must be installed from the closest street to the restrictor catch basin to allow heavy vehicular access. The design and construction of said path shall be subject to the approval of the City Engineer.
- Equalizer Pipe
An equalizer pipe between stormwater management facilities shall be a minimum forty eight inches (48”) diameter RCP, or equivalent capacity. The City may request a hydraulic analysis to determine whether the size of the equalizer pipe needs to be increased.
- Underground Detention
Underground detention shall only be allowed within a concrete vault and shall be subject to the approval of the City Engineer. Storm sewers, inlet structures, catch basins and grate capacity (assuming 25% obstruction) shall be designed to convey the runoff from a 100-year storm event to the underground detention. The concrete vault may be constructed with an open bottom, provided that site specific soil borings demonstrate the underlying soils are conducive to infiltration and that the seasonal high groundwater elevation is lower than the open bottom. The bottom must also be at least two feet (2’) lower than the basement floor of adjacent buildings, existing and proposed.
- Parking Lots
The maximum stormwater ponding depth in any parking area shall not exceed one foot (1’) for more than four (4) hours.
- Safety Considerations
The drainage system components, especially all detention basins, shall be designed to protect the safety of any person coming in contact with the system, at any time. Where the detention basin will be detaining stormwater with a berm that is greater than three feet (3’) above the natural ground elevation, the berm shall be designed and inspected by a geotechnical engineer.
- Accomodating Flows from Upstream Tributary Areas
Stormwater runoff from areas tributary to the property shall be considered in the design of the property’s drainage system. Whenever practical, flows from upstream areas that are not to be detained should be routed around the proposed basin.
When stormwater runoff from an upstream area cannot be routed around the proposed basin, the proposed restrictor must be sized to pass the developed release rate (0.10 cfs per acre) from the on-site and off-site tributary areas.
Re-routed offsite flows shall be located within an outlot when required by the City Engineer.
- Overflow Structures
All stormwater detention basins shall be provided with an overflow concrete structure capable of safely passing excess flows and shall be in compliance with Exhibits IV-C-2 and IV-C-3. The overflow weir shall be designed to convey the proposed critical duration 100-year peak flow entering the basin, shall not exceed one foot (1’) of depth and shall have positive downstream conveyance. Once the width and elevation of the weir has been established, a minimum freeboard (grade surrounding the basin) shall be set one foot (1’) above the weir elevation. The top of berm shall have a minimum width of five feet (5’).
- Setback Requirements
- Distance from the Right-of-Way
The minimum setback from the right-of-way shall be as shown in Exhibit IV-C-5.
- Setback between the High Water Level of the Detention Basin and the Building/Lot
All residential development shall comply with the more restrictive of the following requirements:
- The high water level plus one foot (1’) of freeboard shall not encroach onto a residential lot, or
- The minimum distance between the residential building envelope and the high water level shall be fifteen feet (15’).
- Distance from the Right-of-Way
- Detention within the Floodplain
See Section 9-84.K of the Kane County Stormwater Management Ordinance.
- Early Completion of Detention Facilities
Where detention, retention, or depressional storage areas are to be used as part of the drainage system for a property, they shall be constructed as the first element of the initial earthwork program. This is to ensure that if soil erosion and sediment control practices are not adequately implemented, any eroded sediment will be captured in these areas. Captured sediment within detention, retention, or depressional storage areas shall be removed by the developer before project completion.
- Inlet and Outlet Orientation
To the extent feasible, the hydraulic distance between detention inlets and outlets shall be maximized. If possible, they should be at opposite ends of the basin.
- AASHTO Max Width
Tables 9-81 and 9-107 in the Kane County Stormwater Management Ordinance refer to the AASHTO max width for determining the detention and BMP requirements for linear projects, such as trails and roads. The recommended maximum lane widths from two AASHTO manuals are provided below for reference.
Lane Width per AASHTO “A Policy on Geometric Design of Highways and Streets,” 2018
- Local Residential Streets in Urban Areas = 11 ft
- Local Industrial Streets in Urban Areas = 12 ft
- Collector Streets in Urban Areas = 12 ft
- Arterial Streets in Urban Areas = 12 ft
- Most circumstances = 5 ft
- Adjacent to a narrow (7-8 ft) parking lane = 7 ft
- High bicycle use and without on-street parking = 8 ft
- High speed (> 45 mph) and high-volume roadways = not specified
Off-Street Shared-Use Path Width per AASHTO “Guide for the Development of Bicycle Facilities”, 2012
- Most circumstances = 14 ft
20. Volume Reduction
Per Section 9-107 of the Kane County Stormwater Management Ordinance, Category I and Category II BMPs shall provide volume reduction for one inch (1”) of rainfall over the new impervious area. This volume shall be retained below the primary gravity outlet and may be reduced or compensated for, by providing other measures on site. In cases where the retention component of the facility is not occupying the entire bottom of the basin, the minimum ground slope shall be two percent (2.0%).
The retention should be designed to allow for evapotranspiration or infiltration to a subsurface drainage system and shall not be conveyed through a direct positive connection to downstream areas.
Whenever the retention depth is less than three feet (3’) the retention area shall be constructed by over excavating the required retention volume by one foot (1’) and replacing such volume with one foot (1’) of topsoil suitable for growing wetland plants. The retention area must be planted with native wetland species that are tolerant to inundation. Refer to Tables 7-10 in the Kane County Stormwater Technical Manual for approved native plants based on planting zone water depths.
D. WET DETENTION BASIN DESIGN
Wet detention basins shall be designed to remove stormwater pollutants, so as to be safe and aesthetically pleasing.
A soil investigation by an Illinois licensed Professional Engineer and/or geotechnical engineer shall be performed to determine the ground water elevation to maintain the normal water elevation as proposed. The report shall be submitted to the City Engineer.
- Wet Basin Depth
Wet basins shall be at least five feet (5’) deep, excluding near shore banks and safety ledges and shall be at least ten feet (10’) deep over twenty-five percent (25%) of the normal water level surface area.
- Wet Basin Shoreline Slopes
The side slopes of wet basins from the safety ledge to the high water elevation shall not be steeper than 4 to 1 (horizontal to vertical), preferable 5 to 1 or flatter.
- Safety Ledges
All wet detention basins shall have a level safety ledge at least ten feet (10’) in width and three feet (3’) below the Normal Water Level.
- Normal Water Level
The normal water level of a wet detention basin shall be at least two feet (2’) lower than the basement floor of adjacent buildings, existing and proposed.
E. DRY DETENTION BASIN DESIGN
- Velocity Dissipation
Velocity dissipation measures shall be incorporated into dry basin designs to minimize erosion at inlets and outlets and to minimize the re-suspension of pollutants.
- Bottom Slope
The minimum ground slope shall be two percent (2.0%).
F. LEVEL SPREADERS
Level spreaders should be designed according to Illinois Urban Manual Standard Drawing No. IL-570. Level spreaders comprised of perforated pipe are not allowed.
G. LONG TERM MANAGEMENT
The property owner is responsible for the maintenance of the stormwater drainage system, including outfalls for stormwater storage facilities, except the portion of the system draining a public right-of-way. A long term maintenance plan shall be required for the stormwater drainage system. The long term maintenance plan shall identify the components of the stormwater drainage system and the party responsible for maintaining each component, and list the expected maintenance tasks. The long term maintenance plan shall be recorded as part of the Covenants, Conditions, and Restrictions (CC&R) for a subdivision, if applicable, or as a Stormwater Management Maintenance Covenant running with the land. A recorded copy of the document shall be submitted to the Engineering Division prior to occupancy. The CC&R or Maintenance Covenant shall reference the City of Aurora permit number or case file number.