A. Release Rates: The drainage system for a property shall be designed to control the peak rate of discharge from the property for the 2-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 peak discharge from events less than or equal to the 2-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.
B. Detention Basin Outlet Design: Backwater on the outlet structure from the downstream drainage system shall be evaluated when designing the outlet.
C. Detention Storage Requirements: The design maximum storage to be provided in a detention basin shall be based on the runoff from the 100-year event of critical duration (requiring maximum storage). Detention storage shall be computed using methods as described in this section.
D. 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 10-year peak flow in the minor drainage system and an overland flow path for flows in excess of the design capacity.
1. Design Methodologies: Major and minor conveyance systems for areas up to forty (40) 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 E of this section must be used for major drainage system design for all systems with greater than forty (40) acres of tributary area and for the design of all detention basins designed to work in series. Runoff hydrograph methods may also be required if deemed necessary by the village engineer.
2. Positive Drainage: 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 (1') below the lowest foundation grade 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 street crown by more than three inches (3").
E. Methods For Generating Runoff Hydrographs: Runoff hydrographs shall be developed incorporating the following assumptions of rainfall amounts and antecedent moisture:
1. Rainfall: Unless a continuous simulation approach to drainage system hydrology is used, 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 twelve (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 twelve (12) and less than or equal to twenty four (24) hours. The fourth quartile distribution shall be used in the design and analysis of systems with durations greater than twenty four (24) hours. The first, third, and fourth quartile distributions described by Huff are presented in table 37 of bulletin 70. The SCS type II distribution may be used as an alternate to the Huff distributions.
2. Antecedent Moisture: Computations of runoff hydrographs which do not rely on a continuous accounting of antecedent moisture conditions shall assume a wet antecedent moisture condition of two (2) as a minimum.
F. 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.
1. Wet Basin Depths: 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. Wet Basin Shoreline Slopes: The side slopes of wet basins at the normal pool elevation shall not be steeper than five to one (5:1) (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 2-year event.
4. 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.
G. Dry Detention Basin Design: In addition to the other requirements of this chapter, 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. Dry Basin Drainage: Dry basins shall be designed so that eighty percent (80%) of their bottom area shall have standing water no longer than seventy two (72) hours for any runoff event less than the 100-year event. Under drains directed to the outlet control shall be used if necessary to accomplish this requirement. Grades shall not be less than one percent (1%) on basin floor.
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.
H. 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 (4"). 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 which incorporate self-cleaning flow restrictors shall be utilized.
I. 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 chapter 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 1.25 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 10-year flood elevation shall be replaced below the 10-year flood elevation. All floodplain storage lost above the existing 10-year flood elevation shall be replaced above the proposed 10-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 I3 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 chapter with respect to water quality and control of the 2-year and 100-year, 24-hour events from the property. 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 shall:
a. 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. Not cause or contribute to the degradation of water quality or stream aquatic habitat,
c. Include a design calling for gradual bank slopes, appropriate bank stabilization measures, and a presedimentation basin,
d. Involve any stream channelization or the filling of wetlands,
e. Not occur downstream of a wastewater discharge,
f. Require the implementation of an effective nonpoint source management program throughout the upstream watershed,
g. Comply with chapter 1 of this title.
h. Comply with chapter 3 of this title.
J. Drainage Into Wetlands: Wetlands shall be protected from damaging modifications and adverse changes in runoff quality and quantity associated with land developments. In addition to the other requirements herein, the following requirements shall be met for all developments whose drainage flows into wetlands:
1. Detention In Wetlands: 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 depressional storage in wetlands 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 2-year, 24-hour event and release over a twenty four (24) hour period to the wetland. This basin shall be constructed before property grading begins. In addition, the drainage hierarchy defined in section 11-2-7 of this chapter should be followed to minimize runoff volumes and rates being discharged to the wetland.
5. Vegetated Buffer Strip: A buffer strip of at least twenty five feet (25') in width, preferably vegetated with native plant species, shall be maintained or restored around the periphery of the wetland.
K. 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 street crown by more than three inches (3"). Streets shall not remain flooded for more than four (4) hours for any event less than or equal to the 100-year event.
2. Parking Lots: The maximum storm water ponding depth in any parking area shall not exceed six inches (6") for more than four (4) hours, or as directed by the village engineer.
3. Culvert Road And Driveway Crossings: Sizing of culvert crossings shall consider entrance and exit losses, as well as tailwater conditions on the culvert.
L. Infiltration Practices: To effectively reduce runoff volumes, infiltration practices including basins, trenches, and porous pavement should be located on soils in hydrologic soil groups "A" or "B" as designated by the U.S. soil conservation service. Infiltration basins and trenches designed to recharge ground water shall not be located within seventy five feet (75') of a water supply well or a building foundation. A sediment settling basin shall be provided to remove coarse sediment from storm water flows before they reach infiltration basins or trenches. Storm water shall not be allowed to stand more than seventy two (72) hours over eighty percent (80%) of a dry basin's bottom area for the maximum design event to be exfiltrated. The bottom of infiltration facilities shall be a minimum of four feet (4') above seasonally high ground water and bedrock.
M. Safety Considerations: The drainage system components, especially 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 (4:1) (horizontal to vertical) above water and three to one (3:1) underwater. At least one foot (1') of freeboard above the pond overflow elevation shall be provided around the pond.
2. Safety Ledge: All wet detention basins shall have a level safety ledge at least four feet (4') in width and two and one- half (2.5) to three feet (3') 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 storm water detention basins shall be provided with an overflow structure capable of safely passing excess flows at a stage at least one foot (1') below the lowest foundation grade in the vicinity of the detention basin. The design flow rate of the overflow structure shall be capable of passing at least the equivalent to the 100-year inflow rate.
N. Maintenance Considerations: The storm water 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. 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.) At the village engineer's discretion, presedimentation basins shall be included, where feasible, for localizing sediment deposition and removal. Access for heavy equipment shall be provided. On basin side slopes steeper than ten to one (10:1) (horizontal to vertical), excelsior blankets or sod shall be utilized. (Ord. 1630, 4-12-1999)