§ 62-67. Design criteria, standards, and methods.
(a)
Release rates. The drainage system for a property shall be designed to control the peak rate of discharge from the property for the two-year, 24-hour and 100-year, 24-hour events to levels which will not cause an increase in flooding or channel instability downstream when considered in aggregate with other developed properties and downstream drainage capacities. The park discharge from events less than or equal to the two-year event shall not be greater than four-hundredths cfs per acre of property drained. The peak 100-year discharge shall not be greater than one-tenth cfs per acre of property drained. The village reserves the right to require more restrictive release rates for any development within a watershed with either limited downstream conveyance or with observed historical flooding. For the detention basin outlet design, the backwater on the outlet structure from the downstream drainage system shall be evaluated when designing the outlet.
(b)
Detention storage requirements. The design maximum storage to be provided in a detention basin shall be based on the runoff from the 100-year, 24-hour event and reservoir (also called modified puls or level pool) routing or equal. Detention storage shall be computed using hydrograph methods as described in this section, for development equal to, or exceeding, five acres in size. For development less than or equal to five acres in size, use of the modified rational method in accordance with Exhibit A shall be acceptable.
(1)
Existing natural stormwater storage shall be maintained in accordance with subsection (i)(1) of this section.
(2)
For purposes of this section, a single development comprised of lots each smaller than such acreage shall be subject to the requirements of this article and no development or parcel shall be divided into lots of a smaller dimension to circumvent the requirements in this article.
(c)
Drainage system design and evaluation: The following criteria should be used in evaluating and designing the drainage system. The underlying objective is to provide capacity to pass the five-year peak flow in the minor drainage system and an overload flow path for flows in excess of the design capacity.
(1)
Design methodologies. Major and minor conveyance systems for areas up to ten acres may be designed using the rational formula. The rational formula may also be used in sizing the minor drainage system for larger sites. Runoff hydrograph methods as described in subsection (d) of this section must be used for major drainage system design for all systems with greater than ten acres of drainage area and for the design for all detention basins for development equal to, or in excess of, five acres.
(2)
Positive drainage. Whenever practicable, all areas of the property must be provided an overland flow path that will pass the 100-year flow at a stage at least one foot below the lowest foundation grade or exposed opening in the vicinity of the flow path. Overland flow paths designed to handle flows in excess of the minor drainage system capacity shall be provided drainage easements. Street ponding and flow depths shall not exceed curb heights by more than one inch.
(d)
Methods for generating runoff hydrographs. The following hydrologic design procedures are considered acceptable for generation of hydrographs-U. S. Army Corps of Engineers HEC-1 and Soil Conservation Services TR-20. Runoff hydrographs shall be developed incorporating the following assumptions of rainfall amounts and antecedent moisture:
(1)
Rainfall. All design rainfall events shall be based on the Illinois State Water Survey's Bulletin 70. The first quartile point rainfall distribution shall be used for the design and analysis of conveyance systems with critical durations less than or equal to 12 hours. The third quartile point rainfall distribution shall be used for the design and analysis of detention basins and conveyance system with critical durations greater than 12 and less than or equal to 24 hours. The fourth quartile distribution shall be used in the design and analysis of systems with durations greater than 24 hours. The first, third, and fourth quartile distributions described by Huff are presented in Tables 1, 3 and 4 of ISWS Circular 173.
(2)
Antecedent moisture. Computations of runoff hydrographs which do not rely on continuous accounting of antecedent moisture conditions shall assume a conservative wet antecedent moisture condition as a minimum. (E.g., SCS TR-20—antecedent moisture condition two.)
(e)
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.
(1)
Wet basin depths. Wet basins shall be at least six feet deep, excluding nearshore banks and safety ledges. If fish habitat is to be provided they shall be at least ten feet deep over 25 percent of the bottom area to prevent winter freeze-out.
(2)
Wet basin shoreline slopes. The side slopes of wet basins at the normal pool elevation shall not be steeper than five to one (horizontal to vertical).
(3)
Permanent pool volume. The permanent pool volume in a wet basin at normal depth shall be equal to the runoff volume from its watershed for the two-year event.
(4)
Wet basin aeration. The need for wet basin aeration shall be evaluated by the village engineer on a case-by-case basis.
(5)
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.
(f)
Wetland and dry detention basin design. In addition to the other requirements of this article wetland and 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.
(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 72 hours for all runoff events less than the 100-year event.
a.
Underdrains 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.
b.
Dry basins shall have a minimum bottom slope of two percent. Underdrains directed to the outlet control shall be used if the bottom slope requirement cannot be met.
(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. Any low flow bypass shall be disabled until the stilling/sedimentation basin is removed (subsection (f)(4) of this section).
(4)
Stilling/sedimentation basins. A temporary 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 cubic feet per acre of impervious surface in the drainage area. Side slopes of the basins shall be no steeper than three horizontal to one vertical and basin depths should be at least three feet to minimize resuspension of accumulated sediment. The stilling/sedimentation basin shall remain in place until the design tributary area achieves a built-out of 80 percent.
(g)
Minimum detention outlet size. Where a single pipe outlet or orifice plate is to be used to control discharge, it shall have a minimum diameter of four inches. If this minimum orifice size permits release rates greater than those specified in this section, and regional detention is not a practical alternative, alternative outlet designs shall be utilized which incorporate self cleaning flow restrictors. The orifice shall be constructed in the downstream invert of the catchbasin which shall be centered in the detention basin embankment to facilitate maintenance.
(h)
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 stormwater detention requirements of this article 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 1.5 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 streamflow 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 above the proposed ten-year flood elevation. All compensatory storage excavations shall be constructed to drain freely and openly to the watercourse.
(2)
Detention in floodways. Detention basins shall be placed in the floodway only in accordance with subsection (h)(3) of this section.
(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 this article with respect to water quality and control of the two-year and 100-year, 24-hour events from the property. Further criteria are presented in section 62-68. 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. 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;
d.
Shall not involve any stream channelization or the filling of wetlands;
e.
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 section 62-65; two-year detention/sedimentation basins for all development consistent with subsection (i)(4) of this section; and a program to control nonpoint sources at the source for prior developments constructed without appropriate stormwater BMPs.
f.
Shall not occur downstream of a wastewater discharge; and
g.
Shall comply with 92 Ill. Adm Code 700 and 702 and the floodplain ordinance (chapter 30 of this Code).
(i)
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 article, the following requirements shall be met for all developments which drainage flows into wetlands and depressional storage areas (as appropriate):
(1)
Detention in wetlands and depressional storage areas. Existing wetlands shall not be modified for the 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. 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.
(2)
Sediment control. The existing wetland shall be protected during construction by appropriate soil erosion and sediment control measures and shall not be filled.
(3)
Alteration of drainage patterns. Site drainage patterns shall not be altered to substantially decrease or increase the existing area tributary to the wetland.
(4)
Detention/sedimentation. All runoff from the development 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. This basin shall be constructed before property grading begins. In addition, the drainage hierarchy defined in section 62-65 should be followed to minimize runoff volumes and rates being discharged to the wetland.
(5)
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.
(j)
Street, parking lot, and culvert drainage.
(1)
Streets. If streets are to be used as part of the minor or major drainage system, ponding depths shall not exceed curb heights by more than one inch and shall not remain flooded for more than eight hours for any event less than or equal to the 100-year event.
(2)
Parking lots. The maximum stormwater ponding depth in any parking area shall not exceed 12 inches for more than four hours.
(3)
Culvert road and driveway crossings. Sizing of culvert crossings shall consider entrance and exit losses as well as tail water conditions on the culvert. The minimum size for roadway culverts shall be 18 inches and shall be sized for a minimum design recurrence interval of 25 years (an event having a four percent chance of occurring in any given year). The minimum size for driveway culverts shall be 15 inches.
(k)
Vegetated filter strips and swales. To effectively filter stormwater pollutants and promote infiltration of runoff, sites should be designed to maximize the use of vegetated filter strips and swales. Wherever practicable, runoff from impervious surfaces should be directed onto filter strips and swales before being routed to a storm sewer or detention basin.
(l)
Safety consideration. The drainage system components, especially all detention basins, shall be designed to protect the safety of any children or adults coming in contact with the system during runoff events.
(1)
Side slopes. The side slopes 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. Side slopes of detention basins and open channels shall not be steeper than four to one (horizontal to vertical).
(2)
Safety ledge. All wet detention basins shall have a level safety ledge at least four feet in width (2.5 to three feet) below the normal water depth.
(3)
Velocity. Velocities throughout the surface drainage system shall be controlled to safe levels taking into consideration rates and depths of flow.
(4)
Overflow structures. All stormwater detention basins shall be provided with an overflow structure capable of safely passing excess flows at a stage at least one foot below the lowest foundation grade or exposed opening in the vicinity of the detention basin. The design flow rate of the overflow structure shall be equivalent to the 100-year inflow rate.
(5)
Relationship to roadways. All stormwater detention basins shall conform to section 9-115.1 of Illinois Highway Code (605 ILCS 5/9-115.1).
(m)
Maintenance consideration. The stormwater drainage system shall be designed to minimize and facilitate maintenance. Turfed sideslopes shall be designed to allow lawn mowing equipment to easily negotiate them. Wet basins shall be provided with alternate outflows which can be used to completely drain the pool for sediment removal. Pumping may be considered if drainage by gravity is not feasible. Presedimentation basins shall be included, where feasible, for localizing sediment deposition and removal. Access for heavy equipment shall be provided.
(n)
Compensatory storage. Compensatory storage shall be provided for any development in the flood plain at a ratio of at least 1.1 to 1.0. If any provision of this section, or any other ordinance, statute, or regulatory provision shall specify a more restrictive ratio, then the more restrictive ratio shall apply.
(Code 1987, § 22.5-7; Ord. No. 07-02, 6-4-2007)
Editor's note
Exhibit A has not been set out herein but is available from the offices of the village.