Storm water management for all subdivisions shall comply with the following standards. Special attention shall be given to avoiding changes in storm water runoff onto adjacent properties. Final storm water management design shall not increase the runoff or peak discharge or decrease the time of concentration of storm water leaving the development.
(A) Runoff Calculations:
1. For areas, including all tributary areas, less than twenty- five (25) acres or less in size, the Rational Method may be used to calculate discharges but may not be used for determining detention storage requirements.
2. For areas, including all tributary areas, over twenty-five (25) acres in size and for areas less than twenty-five (25) acres but which require the determination of detention storage requirements, the TR-55 Method shall be used for determining discharges and detention storage requirements.
3. The determination of design rainfall shall use the "Frequency Distribution and Hydroclimatic Characteristics of Heavy Rainstorms in Illinois," Bulletin 70, 1989, prepared by the Illinois State Water Survey.
4. Runoff calculations for tributary areas shall be based on anticipated future land use conditions or existing land use conditions, whichever yields the greater runoff. Anticipated future land use of undeveloped areas shall be based on development of the area using the current zoning.
5. All existing or natural detention basins will be maintained within a development and their area specifically preserved for that use. The volume of detention storage required for the development shall be in addition to the existing storage.
(B) Release Rates and Discharges:
1. The release rates for the developed site shall be the lesser of the following:
a. Ninety percent (90%) of the storm water runoff for the site in its existing condition for storms of all durations;
b. The downstream storm water transport rate; or
c. The proportionate share for the site of the available downstream storm water transport rate based on the total tributary area.
2. All storm water discharges leaving a site must be conveyed into a well-defined receiving channel with adequate downstream capacity.
3. All areas of the development must be provided an emergency overflow path that will pass the 100-year flow without damage to structures or property.
(C) Detention Basins:
1. All detention basins shall be provided with an emergency overflow structure capable of passing the 100-year inflow rate without damage to structures or property.
2. All detention basins shall be developed to fit the site and provide a natural appearance. Restoration of the detention basin shall include the planting of trees and bushes and be in accordance with Section 6-4-9 of this Chapter.
3. Single pipe outlets shall have a minimum inside diameter of ten (10") inches. If design release rates require smaller outlets, structures such as perforated risers or flow control orifices shall be used.
(D) Ditches: Ditches shall be constructed on both sides of the roadway outside of the shoulder line. Ditches shall have a minimum depth of two feet (2') below the edge of the roadway surface with maximum side slopes of four to one (4:1) on the roadway side and two to one (2:1) on the property side.
Maximum ditch gradient without erosion protection shall be two percent (2%). If a ditch gradient exceeds two percent (2%), the ditch shall be protected from erosion by means of sodding, ditch checks, storm sewers or combinations thereof, as deemed necessary by the Village Engineer. Other creative means of erosion control will be considered on an individual basis. Minimum ditch gradient shall be three-tenths of one percent (0.3%). If this gradient cannot be maintained, storm sewers shall be installed.
(E) Driveway Culverts: Prior to acceptance of the street improvement or the issuance of any building permits, the applicant shall submit to the Village an overall map of the subdivision showing the proper size culvert for each lot, as determined from hydraulic calculations.
(F) Storm Sewers and Culverts:
1. Storm sewers shall be installed to drain all roads with curbs and gutters and on slopes where continuous ditches would be subject to erosion. The minimum internal diameter of a storm sewer shall be ten inches (10"). All storm sewers shall be concrete.
2. Minimum culvert sizes shall be twelve inches (12") for driveways and fifteen inches (15") for roadway crossings. At roadway crossings, a minimum depth of six inches (6") shall be maintained between the top of the pipe and the finished roadway subgrade.
3. All storm sewers and roadway culverts shall be fitted with end sections or headwalls and shall extend a sufficient distance beyond the shoulder line to fit the ditch side slope.
4. Special consideration shall be given to the end treatment of storm sewers and culverts thirty-six inches (36") in diameter or larger to assure compatibility with the surrounding environment.
5. Storm sewers, culverts and related structures shall be designed in accordance with current standards of the Illinois Department of Transportation for such drainage structures using the following design storms:
a. For developments of twenty-five (25) acres or less, including all tributary areas, a ten (10) year storm frequency shall be used.
b. For developments of more than twenty-five (25) acres but less than one hundred (100) acres, including all tributary areas, a twenty-five (25) year storm frequency shall be used.
c. For developments of one hundred (100) acres or more, including all tributary areas, a fifty (50) year storm frequency shall be used.
(G) Positive Drainage: All streets shall be designed so as to provide positive drainage that will alleviate any flooding in excess of six inches (6") over the street crown.
(H) Drainage Calculations: The applicant's engineer shall provide the Village Engineer with copies of all calculations required to design the storm drainage system for the proposed subdivision. The computations shall include data for the entire area tributary to the development. (Ord. 93-5, 5-24-93)