The following regulations shall apply to any development for which an application for a preliminary plat was filed on or after November 1, 1995, but prior to September 21, 2004.
(A) General regulations. A combination of storage and controlled release of stormwater runoff shall be required for all nonresidential developments of two or more acres in area, for all multiple-family developments of two or more acres in area, and all single-family developments of five acres or more in area, where the minimum individual lot area is less than one acre.
(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 an increase in flooding or channel instability downstream, when considered in aggregate with other developed properties and downstream drainage capacities.
(a) 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.
(b) The peak 100-year discharge shall not be greater then 0.15 cfs per acre of property drained.
(2) The rational method shall be used to determine the volume of detention to be constructed.
(3) The volume of detention storage shall be determined for the 100-year event of critical duration, which requires maximum storage.
(4) All design rainfall events shall be based on the Illinois State Water Survey's Bulletin 70.
(5) The Village Administrator or designee shall be authorized to require more stringent release rates where detailed regional studies indicate that the receiving stream does not have capacity to accept release at the rate allowed by § 53.02(A)(1).
(B) Hierarchy of site design features. The submittal of the stormwater drainage system shall include an evaluation of site design features that minimize the increase in runoff volumes and rates from the site. The submittal shall include evaluations of site design features that consider the following hierarchy:
(1) Minimize impervious surfaces on the property consistent with the needs of the project;
(2) Attenuate flows by use of open vegetated swales and natural depressions;
(3) Provide stormwater retention structures;
(4) Provide stormwater detention structures; and
(5) Construct storm sewers.
(C) Drainage system design features.
(1) The drainage system shall be designed to minimize adverse water quality impacts downstream and on the property itself.
(a) Detention basins shall incorporate design features to capture stormwater runoff pollutants.
(b) Retention of stormwater shall be promoted throughout the property's drainage system to reduce the volume of stormwater runoff and to reduce the quantity of runoff pollutants.
(2) The drainage system should incorporate multiple uses where practicable.
(a) Uses considered compatible with stormwater management include open spaces, aesthetics, aquatic habitat, recreation, wetlands and water quality mitigation.
(b) The applicant should attempt to avoid using portions of the property exclusively for stormwater management.
(D) Wet detention design features.
(1) Wet detention basin design. 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.
(2) Wet detention depths.
(a) Wet basins shall be at least three feet deep, excluding near shore banks and safety ledges.
(b) If fish habitat is to be provided, they shall be at least ten feet deep over 25% 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.
(E) Dry detention design features.
(1) Dry detention basin design. In addition to the other requirements of this section, dry basins shall be designed to remove stormwater pollutants, to be safe, to be aesthetically pleasing, and as much as feasible to be available for multiple uses.
(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.
(F) Minimum detention outlet size.
(1) Where a single pipe outlet or orifice plate is to be used to control the discharge, it shall have a minimum diameter of four inches.
(2) If this minimum orifice permits a release rate greater than those specified in § 53.02(A) and regional detention is not a practical alternative, alternative outlet designs shall be utilized that incorporate perforated risers or other self-cleaning flow restrictors.
(G) Detention in floodplains.
(1) The placement of detention basins within the floodplain is strongly discouraged because of questions about their reliable operation during flood events.
(2) However, the stormwater detention requirements of this chapter may be fulfilled by providing detention storage within flood fringe areas on the project site, provided the following provisions are met.
(a) Detention in flood fringe areas.
1. The placement of a detention basin in a flood fringe area shall require compensatory storage for 1.5 times the volume below the base flood elevation occupied by the detention basin, including any berms.
2. The release from the detention storage provided shall still be controlled consistent with the requirements of this section.
3. The applicant shall demonstrate its operation for all streamflow and floodplain backwater conditions.
4. Excavations for compen-satory storage along watercourses shall be opposite or adjacent to the area occupied by detention.
5. All floodplain storage lost below the ten-year flood elevation shall be replaced below the ten-year flood elevation.
6. All floodplain storage lost above the existing ten-year flood elevation shall be replaced above the proposed ten-year flood elevation.
7. All compensatory storage excavations shall be constructed to drain freely and openly to the watercourse.
(b) Detention in floodways. Detention basins shall be placed in the floodway only in accordance with § 53.02(G)(2)(c).
(c) On-stream detention.
1. 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.
2. If on-stream detention is used for watersheds larger than one square mile, it is recommended that the applicant use dynamic modeling to demonstrate that the design will not increase stage for any properties upstream or downstream of the property.
3. In addition, impoundment of the stream as part of on-stream detention:
a. Shall not prevent the migration of indigenous fish species that 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 non-point source management program throughout the upstream watershed.
(H) Drainage into wetlands.
(1) Wetlands shall be protected from damaging modifications and adverse changes in runoff quality and quantity associated with land developments.
(2) In addition to the other requirements of these regulations, the following requirements shall be met for all developments whose drainage flows into wetlands.
(a) Detention in wetlands.
1. Existing wetlands shall not be modified for purposes of stormwater 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.
2. Existing depressional storage in wetlands shall be maintained, and the volume of detention storage provided to meet the requirements of these regulations shall be in addition to this existing storage.
(b) Sediment control. The existing wetland shall be protected during construction by appropriate soil erosion and sediment control measures.
(c) Alteration of drainage patterns. Site drainage patterns shall not be altered to decrease or increase the existing area tributary to the wetland.
(d) Detention/sedimentation.
1. All runoff from the development tributary to a wetland shall be routed through a preliminary detention/sedimentation basin designed to capture the two-year, 24-hour event and hold it for at least 24 hours, before being discharged to the wetland.
2. This basin shall be con-structed before grading begins on the property.
(e) 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.
(I) Streets and parking lots.
(1) Streets. If streets are to be used as part of the drainage system, ponding depths shall not exceed curb heights by more than one inch.
(2) Parking lots: The maximum storm-water ponding depth in any parking area shall not exceed six inches.
(J) Infiltration practices. The use of infiltration practices, such as basins, drywells, trenches and porous pavement, shall not be allowed as a primary source of stormwater management.
(K) Maintenance considerations.
(1) The stormwater drainage system shall be designed to minimize and facilitate maintenance.
(2) Turfed side slopes shall be designed with a maximum slope of four horizontal to one vertical.
(3) Pre-sedimentation basins shall be provided with:
(a) Easy methods for removing sediment; and
(b) Access for heavy equipment.
(L) Accommodating flows from upstream tribu-tary areas.
(1) General principles.
(a) Stormwater runoff from area tributary to the property shall be considered in the design of the property's drainage system.
(b) Whenever practicable, flows from upstream areas that are not to be detained should be routed around the basin being provided for the site being developed.
(2) Upstream areas not meeting current requirements.
(a) When there are areas not meeting the storage and release rates of these regulations, tributary to the applicant's property, the applicant shall explore regionalized detention on that property.
(b) The following steps shall be followed:
1. The applicant shall compute the storage volume needed for his property using:
a. The release rates of § 53.02(A), General regulations;
b. The applicant's property area; and
c. The procedures described in § 53.02(B), Hierarchy of site design features.
2. Areas tributary to the appli-cant's property that do not meet the storage and release rate requirements of these regulations shall be identified.
3. Using the areas determined above in § 53.02(L)(2)(b)2, plus the applicant's property area, total storage needed for the combined properties shall be computed.
4. Allowable release rates shall be computed using the combined property areas.
a. Storage shall be computed as described in § 53.02(A).
b. If tributary areas are not developed, a reasonable fully developed land cover, based on local zoning, shall be assumed for purposes of computing storage.
5. Once the necessary combined storage is computed, the village may choose to require oversizing of the applicant's detention basin to accommodate the regional flows.
a. If such an oversizing determination is made:
(i) The applicant shall appropriately oversize the detention basin and associated facilities; and
(ii) The village may enter into a recapture agreement with the applicant to permit recovery of all oversizing expense.
b. If regional storage is selected by the village, then the design produced above in § 53.02(L)(2)(b)4. shall be implemented.
6. If regional storage is rejected by the village:
a. The applicant shall design the storage and release rates for the applicant's property only; and
b. Existing upstream flows shall be routed around the storage when practicable.
(3) Upstream areas meeting current requirements.
(a) When there are areas tributary to the applicant's property that meet the storage and release rate requirements of this chapter, the upstream flows shall be:
1. Bypassed around the applicant's detention basin; or
2. Routed through the applicant's detention basin, if this is the only practicable alternative.
(b) Storage needed for the applicant's property shall still be computed as described in § 53.02(L)(2).
(c) However, if the village decides to route tributary area flows through an applicant's basin, the final design stormwater releases shall be based on the combined total of the applicant's property plus tributary areas.
(d) It must be shown that at no time will the runoff from the applicant's property exceed the allowable release rate for that property alone.
(M) Use of detention, retention or depressional storage areas as part of drainage system.
(1) Where detention, retention or depres-sional storage areas are to be used as part of the drainage system for a property, they shall be constructed as the first part of the initial earthwork program.
(2) Temporary detention facilities can be provided where the schedule for the site does not include the permanent detention site as a part of the initial development.
(3) Bonding for the completion of adequate permanent facilities shall be provided with the first increment of the development approved and/or recorded.
(N) Maintenance responsibility.
(1) The maintenance of stormwater detention facilities shall be provided by the property owner(s) of the site.
(2) The owner(s) of the property shall grant an easement to the village in the event that it needs to enter the property to correct deficiencies in the maintenance provided by the owner(s), after acceptance of the facilities and expiration of the maintenance period as provided by ordinance.
(3) Pre-sedimentation facilities, inlet control structures and outlet structures shall be maintained by the village to assure adequate functioning of the facility.
(Ord. 869, passed 11-20-95; Am. Ord. 1358, passed 9-20-04)