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path and the separation in flow type. <br />(5) Use the Precipitation -Frequency Atlas of the United States, NOAA <br />Atlas 14, Vol 2(3). [http://hdsc.nws.noaa.uov/hdsc/pfds/] for <br />rainfall depth data for stormwater design. <br />(6) Use the SCS Type II rainfall distribution for all design events with <br />a recurrence interval greater than 1 year. Include lot coverage <br />assumptions used for full build out of the proposed condition. <br />(7) Curve numbers for the pre -development condition shall reflect the <br />average type of land use prior to any development and not only the <br />current land use. <br />i. Pre -development Curve Numbers — For wooded or brushy <br />areas, use listed values from TR -55 NRCS USDA Urban <br />Hydrology for Small Watersheds, 1986 in good hydrologic <br />condition. For meadows, use listed values. For all other areas <br />(including all types of agriculture), use pasture, grassland, or <br />range in good hydrologic condition. <br />ii. Post -development Curve Numbers - Open space areas shall use <br />post -construction hydrologic soil groups from Rainwater and <br />Land Development unless the soil is amended using the soil <br />profile restoration design criteria in Rainwater and Land <br />Development. All undisturbed areas or open space with <br />amended soils shall be treated as "open space in good <br />condition." <br />(8) Time of Concentration - Use velocity -based methods from (TR -55 <br />NRCS USDA Urban Hydrology in Small Watersheds, 1986) to <br />estimate travel time (Tt) for overland (sheet) flow, shallow <br />concentrated flow and channel flow. <br />i. Maximum sheet flow length is 100 ft. <br />ii. Use the appropriate "unpaved" velocity equation for shallow <br />concentrated flow from Soil Conservation Service National <br />Engineer Handbook Section 4 — Hydrology (NEH-4). <br />(9) The volume reduction provided by runoff reduction SCMs may be <br />subtracted from the post -development stormwater volume. <br />Volume reductions for these SCMs may be demonstrated using <br />methods outlined in Rainwater and Land Development or a <br />hydrologic model acceptable to the City Engineer. <br />To account for future post -construction improvements to the site, <br />calculations shall assume an impervious surface such as asphalt or <br />concrete for all parking areas and driveways except in instances of <br />engineered permeable pavement systems. From the volume determined in <br />