A. Public Works and Engineering Design Guidelines. All design and construction of storm sewers, detention facilities, stormwater management facilities, and land grading, and other stormwater management or runoff control activities shall be in conformance with the Public Works and Engineering Design Guidelines.
B. Drainage of Surface Water. To diminish or remove any adverse impact of surface water drainage and run-off on an adjacent property, no new building, other structure or addition shall be constructed which will result in the surface water run-off, during and following construction of any such improvement, at a rate greater than the water run-off immediately prior to such construction and no building permit shall be issued unless and until the Village Engineer determines that the construction complies with the applicable requirements of this chapter, the Public Works and Engineering Design Guidelines, and the Watershed Management Ordinance.
C. Land Grading. No permit shall be issued for any land grading that will permanently alter the existing land elevation or grade of a parcel of property so as to cause surface water runoff to be diverted onto or detained on abutting or nearby property, significantly alter existing drainage patterns, or increase or concentrate stormwater runoff onto abutting or nearby property.
D. Stormwater Detention Required. Developments required to provide storm water detention on site, include, but are not limited to multiple lot single family residential subdivisions, single family residential subdivisions of an individual lot, multi-family residential development and commercial developments.
E. Stormwater Detention Design Standards.
1. New home construction on a previously developed lot shall provide storm water detention for the volume difference between using the runoff coefficient based upon the maximum impermeable lot coverage, per the Village of Winnetka's Zoning Code, and the runoff coefficient based upon the existing condition, for a 100-year storm event. The allowable release rate for both conditions will be determined by using a runoff coefficient of 0.15 and the rainfall intensity for a 3-year storm event. New home construction on a previously undeveloped site, or the redevelopment of a site for a different use (i.e. single family to multi-family, or commercial redevelopment) shall provide storm water detention for the total required detention volume based upon a 100-year storm event, using a runoff coefficient based upon the maximum impermeable lot coverage, and the allowable release rate using a runoff coefficient of 0.15 and a rainfall intensity for a 3-year storm event.
2. Improvements to an existing home and/or lot, causing an increase in impermeable lot coverage greater or equal to 25%, shall provide storm water detention for the difference between the proposed and existing condition, for a 100-year storm event and an allowable release rate based upon a 3-year storm event and a runoff coefficient of 0.15. The actual proposed lot coverage may be used to calculate the proposed runoff coefficient.
3. The storm water detention facilities shall be designed in accordance with the requirements of the Metropolitan Water Reclamation District of Greater Chicago for storm water detention as modified by the rainfall frequencies set forth in Bulletin 70 "Frequency Distribution and Hydroclimatic Characteristics of Heavy Rainstorms in Illinois" prepared by the Illinois State Water Survey, 1989. Design high water level (HWL) will consist of the elevation of the storm water in a 100-year storm event.
4. Stormwater storage volume located within best management practices, such as a rain garden or the aggregate base of permeable pavement, that are not otherwise required for site development shall be credited toward the required detention volume.
5. For projects required to provide storm water detention by the WMO, the detention design shall meet the requirements of Article 5 of the Watershed Management Ordinance.
(MC-5-2014 § 5, 04/17/2014)