2016 024 Ordinance - Laserfiche WebLink

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s ow~oa.t~a 4 '"°' ~.i~L~ ~~~ ~~efti.TC~,raheED~ r~e~a~-r~~~.s May 2016 (2) Calculations shall be based on the entire contributing watershed to the development area. (3) Drainage area maps shall include area, curve number, time of concentrations. Time of concentration shall also show the flow path and the separation in flow type. (4) Rainfall Depth -For the most accurate, up-to-date, location-specific rainfall data for stormwater design, use the Precipitation-Frequency Atlas of the United States, NOAA Atlas 14, Vo12(3). [Available online: http://hdsc.nws.noaa.gov/hdsc/pfds/] (5) Temporal Distribution -Use the SCS Type II rainfall distribution for all design events with a recurrence interval greater than 1 year. Include lot coverage assumptions used for full build out of the proposed condition. (6) Curve numbers for the pre-development condition shall reflect the average type of land use over the past 10 years and not only the current land use. i. Pre-development Curve Numbers -For wooded or brushy areas, use listed values from TR-55 NRCS USDA Urban Hydrology for Small Watersheds, 1986 in good hydrologic condition. For meadows, use listed values. For all other areas (including all types of agriculture), use pasture, grassland, or range in good hydrologic condition. (7) Time of Concentration -Use velocity based methods from (TR-55 NRCS USDA Urban Hydrology in Small Watersheds, 1986) to estimate travel time (Tt) for overland (sheet) flow, shallow concentrated flow and channel flow. i. Maximum sheet flow length is 100 ft. ii. Use the appropriate "unpaved" velocity equation for shallow concentrated flow from Soil Conservation Service National Engineer Handbook Section 4 -Hydrology (NEH-4). (8) The volume reduction provided by permeable pavement, bioretention, or other LID SCMs may be subtracted from the post development stormwater volume. Volume reductions for these practices maybe demonstrated using methods outlined in Rainwater and Land Development or a hydrologic model acceptable to the Village Engineer. b. 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 except in instances of engineered permeable pavement systems. 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. 25