2008 005 Ordinance - Laserfiche WebLink

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(2) Calculations shall be based on the entire contributing watershed to the development area. (3) Curve numbers for the pre-development condition must reflect the average type of land use over the past 10 years and not only the current land use. (4) To account for future post-construction improvements to the site, calculations shall assume an impervious surface such as asphalt or concrete for all parking areas and driveways, regardless of the surface proposed in the site description. b. From the volume determined in Section 1128.09(D)(3)(a), determine the percent increase in volume of runoff due to development. Using the percentage, select the 24-hour Critical Storm from Table 3. Table 3: 24-Hour Critical Storm If the Percentage of Increase in Volume of Runoff is: The Critical Storm will be: Equal to or Greater Than: and Less Than: ---- 10 1 year 10 20 2 year 20 50 5 year 50 100 10 year 100 250 25 year 250 500 50 year 500 --- 100 year For example, if the percent increase between the pre- and post-development runoff volume fora 1-year storm is 35%, the Critical Storm is a 5-year storm. The peak discharge rate of runoff for all storms up to this frequency shall be controlled so as not to exceed the peak discharge rate from the 1-year frequency storm under pre-development conditions in the development drainage area. The post-development runoff from all less frequent storms need only be controlled to meet pre-development peak discharge rates for each of those same storms. E. Storm Water Management on Redevelopment Projects: Comprehensive Storm Water Management Plans for redevelopment projects shall reduce existing site impervious areas by at least 20 percent. Where site conditions prevent the reduction of impervious area, then stormwater management practices shall be implemented to provide storm water quality control facilities for at least 20 percent of the site's impervious area. When a combination of impervious area reduction and storm water quality control facilities is used, the combined area shall equal or exceed 20 percent of the site. Where conditions prevent impervious area reduction or on-site stormwater management for redevelopment projects, practical alternatives as detailed in Section 1128.10 may be approved by the Mayfield Village Engineer.