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EXHIBIT A: CHAPTER 928 <br />(3) Model pervious, directly connected impervious and disconnected <br />impervious areas as separate subwatersheds. <br />(4) Drainage area maps shall include area, curve number, time of <br />concentrations. Time of concentration shall also show the flow <br />path and the separation in flow type. <br />(5) Rainfall Depth - For the most accurate, up-to-date, location - <br />specific rainfall data for stormwater design, use the Precipitation - <br />Frequency Atlas of the United States, NOAA Atlas 14, Vol 2(3). <br />[available online: http:/ihdsc.nws.noaa.gov/hdsc/pfds/.] <br />(6) Temporal Distribution — Use the SCS Type II rainfall distribution <br />for all design events with a recurrence interval greater than 1 year. <br />Include lot coverage assumptions used for full build out of the <br />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 <br />areas (including all types of agriculture), use pasture, <br />grassland, or range in good hydrologic condition. <br />ii. Post -development Curve Numbers - Open space areas shall <br />use post -construction HSGs from Rainwater and Land <br />Development unless the soil is amended after development <br />according to the following protocol: till the subsoil to 15-18 <br />inches, then till using a chisel, spader, or rotary tillage and <br />incorporate compost through top 12 inches, replace topsoil <br />to a minimum depth of 4 inches. All undisturbed areas or <br />open space with amended soils shall be treated as "open <br />space in good 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 <br />shallow concentrated flow from Soil Conservation Service <br />National Engineer Handbook Section 4 — Hydrology <br />(NEH-4). <br />(9) The volume reduction provided by permeable pavement, <br />bioretention, or other LID SCMs may be subtracted from the post <br />development stormwater volume. Volume reductions for these <br />practices may be demonstrated using methods outlined in <br />