A. Purpose: The purpose of this section is to diminish threats to the public health, safety and welfare caused by runoff of excessive stormwater from new development and redevelopment. This excessive stormwater could result in the inundation of damageable properties, the erosion and destabilization of downstream channels, and the pollution of valuable stream and lake resources.
B. Compliance With Other Rules And Regulations: All drainage design shall be in compliance with applicable and current statutes of the state of Illinois and with the applicable and current ordinances of the county. Where this section is more restrictive than those references, the requirements of this section shall govern.
C. Storm Sewer Design: The storm sewer system shall be separate and independent of the sanitary sewer system. All storm sewers shall be designed by the rational method on the basis of a minimum of a ten (10) year recurrence frequency storm and shall be sized to adequately carry flows from the tributary areas. Inlets shall be provided so that surface water is not carried across or around any intersection in the gutter. Surface water drainage patterns shall be shown for each and every lot and block. Copies of the design computations, together with drawings showing the storm sewer layout and drainage areas, shall be submitted to the village engineer for review.
D. Detention Pond Design; Inflow Hydrograph: For tributary areas of less than or equal to fifty (50) acres, the rational method may be used. The soil conservation services "Urban Hydrology For Small Watersheds (TR-55)" may be used for areas up to fifty (50) acres and shall be used for areas greater than fifty (50) acres. Rainfall data shall be taken from the Illinois state water survey, bulletin 70, latest edition.
E. Stormwater Runoff Release Rate:
1. The stormwater release rate from all on site land development for the 2-year 24-hour, 10-year 24-hour, and 100-year 24-hour storm events shall be limited to the calculated peak discharge for the undeveloped site condition for the 2-year 24-hour storm event.
2. The pond outlet structure shall be designed to meet the criteria outlined above. A detailed drawing of the structure shall be provided. Routing calculations shall be provided to confirm the basis of design. The structure shall include a weir (or similar device) to allow flows up to and including the 100-year storm to be directed to the main outlet pipe to ensure that the pond berm is not overtopped. The structure shall include orifices, weirs or similar devices to achieve the release rate requirements for the 2-year, 10-year, and 100-year storms as required above.
3. The pond berm shall have an emergency spillway designed to allow storm events in excess of the 100-year storm to be discharged without causing erosion to any part of the pond berm.
F. Off Site Tributary Areas:
1. The stormwater drainage system for all off site land development and improvements shall be designed with adequate bypass capacity to convey the stormwater runoff flow from all tributary watershed areas through the land development area to the existing drainage outlet. When possible, stormwater runoff from upstream tributary areas shall be routed around the detention facilities for the site being developed. The bypass transport systems shall be designed to carry the peak flow from the 100-year storm. An allowance will be made for upstream detention storage when such upstream storage and runoff release rate has previously been approved by the village and has been constructed.
2. If it is necessary to route off site storm flows into a detention pond, it is acceptable to design the outlet structure to pass this incremental portion of the flow through the pond without reducing the calculated peak. That is, this peak flow can be added to the allowable release rates described above.
G. Detention Pond Design Details:
1. Concrete low flows channels shall be provided from each inlet point to the outlet structure point.
2. The minimum pond bottom slope shall be one percent (1%) unless the village engineer determines that a lesser value is acceptable.
3. The emergency spillway and the downstream end of the outlet pipes and the inlet pipes shall have riprap or concrete aprons to prevent erosion.
H. Stormwater Drainage Report: A stormwater drainage report shall be submitted showing the details of the overall stormwater design as detailed above. The report shall be neat, typed, and signed by a registered professional engineer. (Ord., 12-27-2007)
Notes
| 1 | 1. See also section 9-1-11 of this title. |
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