Last Updated 08/12/202
STORM WATER MANAGEMENT REGULATIONS
Storm water detention regulations
General regulations
(1) The peak discharge rate from a storm detention facility shall be controlled for the two year 24-hour storm event and the 100 year 24-hour storm event to levels that will not cause and increase in flooding or channel instability downstream when considered in aggregate with other developed properties and downstream drainage capacities. The peak discharge from events less than or equal to the two-year event shall not be greater than 0.04 cfs per acre of property drained. The peak 100 year discharge shall not be greater than 0.15 cfs per acre of property drained.
Design Objectives
(1) The drainage system should be designed to minimize adverse water quality impacts downstream and on the property itself. Detention basins shall incorporate design features to capture storm water runoff pollutants. In particular, designers shall give preference to wet bottom and wetland designs and all flows from the development shall be routed through the basin (i.e., low flows shall not be bypassed). Retention and infiltration of storm water shall be promoted throughout the property’s drainage system to reduce the volume of storm water runoff and to reduce the quantity of runoff pollutants.
(2) The drainage system should incorporate multiple uses where practicable. Uses considered compatible with storm water management include open space, aesthetics, aquatic habitat, recreation (boating, trails, playing fields), wetlands and water quality mitigation. The applicant should avoid using portions of the property exclusively for storm water management.
Wet Detention Basins
(1) Wet detention basin design. Wet detention basins shall be designed to remove storm water pollutants, to be safe, to be aesthetically pleasing, and as much as feasible to be available for recreational use.
(2) Wet detention depths. Wet basins shall be at least three feet deep, excluding near shore banks and safety ledges. If fish habitat is to be provided, they shall be at least ten feet deep over twenty-five percent of the bottom area to prevent winter freeze out.
(3) Inlet and outlet orientation. To the extent feasible the distance between detention inlets and outlets shall be maximized.
Wetland and Dry Detention Basin Design:
In addition to the other requirements of this ordinance, wet and dry basins shall be designed to remove storm water pollutants, to be safe, to be aesthetically pleasing and as much as feasible to be available for multiple uses.
(1) Wetland and Dry Basin Drainage: Wetland and dry basins shall be designed so that the portion of their bottom area which is intended to be dry shall have standing water no longer than seventy-two hours for all runoff events less than the 100-year event. Under drains directed to the outlet may be used to accomplish this requirement. Grading plans shall clearly distinguish the wet/wetland, portion of the basin bottom from the dry portion.
(2) Velocity dissipation: Velocity dissipation measures shall be incorporated into dry basin designs to minimize erosion at inlets and outlets and to minimize the resuspension of pollutants.
(3) Inlet and Outlet Orientation: To the extent feasible, the distance between detention inlets and outlets shall be maximized. If possible, they should be at opposite ends of the basin. There should be no low flow bypass between the inlet and outlet.
(4) Stilling/Sedimentation Basins: A stilling/sedimentation basin should be constructed at each major inlet to a wetland or dry basin. The volume of the basins should be at least 500 ft. per acre of impervious surface in the drainage area. Side slopes of the basins shall be no steeper than 3 horizontal to 1 vertical and basin depths should be at least 3 feet to minimize resuspension of accumulated sediment.
Detention in floodplains
The placement of detention basins within the floodplain is strongly discouraged because of questions about their reliable operation during flood events. However, the storm water detention requirements of this ordinance may be fulfilled by providing detention storage within flood fringe areas on the project site provided the following provisions are met:
(1) Detention in flood fringe areas: The placement of a detention basin in a flood fringe area shall require compensatory storage for 1.0 times the volume below the base flood elevation occupied by the detention basin including any berms. The release from the detention storage provided shall still be controlled consistent with the requirements of this section. The applicant shall demonstrate its operation for all stream flow and floodplain backwater conditions. Excavations for compensatory storage along watercourses shall be opposite or adjacent to the area occupied by detention. All floodplain storage lost below the ten-year flood elevation shall be replaced below the ten-year flood elevation. All floodplain storage lost above the existing ten-year flood elevation shall be replaced about the proposed ten-year flood elevation. All compensatory storage excavations shall be constructed to drain freely and openly to the watercourse.
(3) On-stream detention: On stream detention basins are discouraged but allowable if they provide regional public benefits and if they meet the other provisions of these regulations with respect to water quality and control of the two-year and 100 year, 24-hour events from this property. If on-stream detention is used for watersheds larger than one square mile, it is recommended that the applicant used dynamic modeling to demonstrate that the design will not increase stage for any properties upstream or downstream of the property. Also, impoundment of the stream as part of on-stream detention:
- (a) shall not prevent the migration of indigenous fish species, which require access to upstream areas as part of their life cycle, such as for spawning.
- (b) shall not cause or contribute to the degradation of water quality or stream aquatic habitat,
- (c) shall include a design calling for gradual bank slopes, appropriate bank stabilization measures, and a pre-sedimentation basin, and,
- (d) shall require the implementation of an effective nonpoint source management program throughout the upstream watershed which shall include at a minimum: runoff reduction BMPs consistent with this ordinance; 2-year detention/sedimentation basins for all development consistent with Section 709.4; and a program to control nonpoint sources at the source for prior developments constructed without appropriate storm water BMPs.
Protection of Wetlands and Depressional Storage Areas
Wetlands and other depressional storage areas shall be protected from damaging modifications and adverse changes in runoff quality and quantity associated with land developments. In addition to the other requirements of this ordinance, the following requirements shall be met for all developments whose drainage flows into wetlands and depressions] storage areas (as appropriate):
Detention in Wetlands and Depressional Storage Areas: Existing wetlands shall not be modified for the purposes of storm water detention unless it is demonstrated that the existing wetland is low in quality and the proposed modifications will maintain or improve its habitat and ability to perform beneficial functions. Existing storage and release rate characteristics of wetlands and other depressional storage areas shall be maintained and the volume of detention storage provided to meet the requirements of this section shall be in addition to this existing storage.
Alteration of drainage patterns: Site drainage patterns shall not be altered to substantially decrease or increase the existing area tributary to the wetland.
Vegetated Buffer Strip: A buffer strip of at least 25 feet in width, preferably vegetated with native plant species, shall be maintained or restored around the periphery of the wetland.
Streets and parking lots
- Streets: If streets are to be used as part of the drainage system, ponding depths shall not exceed curb heights by more than six inches and shall not remain flooded for more than eight hours for any event less than or equal to the 100-year event.
- Parking lots: The maximum storm water ponding depth in any parking area shall not exceed twelve inches.
Streets and parking lots
Infiltration practices including basins, trenches and porous pavement may be allowed if the following conditions are met:
- A soil survey indicates that the existing soil types are adequate for infiltration practice.
- The bottom of any proposed infiltration facility is located a minimum of four feet above the seasonally high groundwater and bedrock.
- A sediment settling basin is provided to remove sediment from storm water flows before they reach infiltration basins or trenches.
- Infiltration facilities are located more than 75 feet from any existing or proposed building foundation.
- Storm water shall not be allowed to stand more than 72 hours over eighty percent of a dry basin’s bottom area for the maximum design event.
- The infiltration facility is located so that a positive outfall could be installed and the basin could be converted to a dry detention basin if the bottom of the basin becomes clogged with sediment in the future.
Infiltration Practices
Maintenance Considerations
The stormwater drainage system shall be designed to minimize and facilitate maintenance. Turfed side slopes shall be designed to allow lawn mowing equipment to easily negotiate them. Pre-sedimentation basins shall be provided with easy methods for removing sediment. Access for heavy equipment shall be provided to the pre-sedimentation basins. In addition, all applicants shall be required to establish a restrictive covenant by notation on the record plat stating that the maintenance of the detention area is the responsibility of all property owners in the subdivision, including bank stabilization, bank maintenance, future sediment removal, or dredging, stabilization of water levels, outfall structures and storm sewer pipes within the detention easement.
Maintenance responsibility
The maintenance of storm water detention facilities shall be provided by the property owner(s) of the site. The owner(s) of the property shall grant an easement to the City of Joliet in the event that Joliet needs to enter the property to correct deficiencies in the maintenance provided by the owner(s). Presedimentation facilities, inlet control structures and outlet structures shall be maintained by the City of Joliet to assure adequate functioning of the facility.
View State Page
Frequently Asked Questions
How much does a Joliet detention basin have to control peak runoff?
The city sets specific numeric limits rather than a general performance standard: peak discharge for storms up to the two-year, 24-hour event can’t exceed 0.04 cfs per acre of property drained, and the 100-year, 24-hour peak can’t exceed 0.15 cfs per acre. Both limits apply in aggregate with other developed properties and downstream drainage capacity, so a basin’s design has to account for cumulative watershed impact, not just what happens on the individual site.
Does a Joliet detention pond have to hold water permanently, or can it drain dry between storms?
It depends on the basin type, and the city has different standards for each. Wet detention basins have to stay at least three feet deep, with at least ten feet of depth over a quarter of the bottom area if fish habitat is intended, to prevent winter freeze-out. Dry and wetland basins work differently: any portion designed to be dry can’t hold standing water for more than 72 hours after any storm smaller than the 100-year event, and grading plans have to clearly separate the wet and dry portions of the basin bottom.
Can a Joliet property put its detention basin in a floodplain?
The city strongly discourages it because of reliability concerns during actual flood events, but it’s allowed in flood fringe areas under specific conditions. The developer has to provide compensatory storage equal to the volume displaced by the basin below the base flood elevation, demonstrate the design works under all stream flow and backwater conditions, and ensure any lost floodplain storage gets replaced at the corresponding flood elevation. On-stream detention is even more restricted, requiring proof it won’t raise flood stages for neighboring properties and won’t block fish migration.
Who's responsible for maintaining a Joliet stormwater detention facility, the property owner or the city?
The information on this page is provided for general informational purposes only and is not legal advice. Regulations change frequently — AQUALIS makes no warranty as to the accuracy or completeness of this content, and any reliance on it is at your own risk. Consult a qualified professional or your local regulatory authority to confirm requirements applicable to your specific property.
