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ORDINANCE NO. 2022-18 <br />Page 19 <br />(6) Curve numbers for the pre -development condition shall <br />reflect the average type of land use over the past 10 years <br />and not only the current land use. <br />Pre -development Curve Numbers — For wooded or <br />brushy areas, use listed values from TR -55 NRCS <br />USDA Urban Hydrology for Small Watersheds, <br />1986 in good hydrologic condition. For meadows, <br />use listed values. For all other areas (including all <br />types of agriculture), use pasture, grassland, or <br />range in good hydrologic condition. <br />ii. Post -development Curve Numbers - Open space <br />areas shall use post -construction hydrologic soil <br />groups from the Rainwater and Land Development <br />Manual unless the soil is amended using the soil <br />profile restoration design criteria in the Rainwater <br />and Land Development Manual. All undisturbed <br />areas or open space with amended soils shall be <br />treated as "open space in good condition." <br />(7) Time of Concentration - Use velocity -based methods from <br />(TR -55 NRCS USDA Urban Hydrology in Small <br />Watersheds, 1986) to estimate travel time (Tt) for overland <br />(sheet) flow, shallow concentrated flow, and channel flow. <br />i. Maximum sheet flow length is 100 ft. <br />ii. Use the appropriate "unpaved" velocity equation <br />for shallow concentrated flow from Soil <br />Conservation Service National Engineering <br />Handbook Section 4 — Hydrology (NEH-4). <br />(8) The volume reduction provided by runoff reduction SCMs <br />maybe subtracted from the post -development stormwater <br />volume. Volume reductions for these SCMs may be <br />demonstrated using methods outlined in the Rainwater and <br />Land Development Manual or a hydrologic model <br />acceptable to the Village Engineer. <br />b. 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 <br />determined in Section 1128.09(d)(3)(A), determine the percent <br />increase in volume of runoff due to development. Using the <br />