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<br />5-502 AA <br />Drainage <br />~, I <br />The primary objective of drainage design is protection <br />of County roads and property while minimizing possible <br />flood damage to surrounding propertiee~ and structures. <br />It should be emphasized that good drainage is one of <br />the most important factors in road de~;ign. It <br />preserves appearance as well as the level of road <br />service, while minimizing maintenance costs. Culverts <br />under all roads shall be designed to accommodate <br />25-year-frequency-storm run-off utilizing the maximum <br />head, as determined by the upper-most ponding elevation <br />chosen to prevent flood damage to upstream properties. <br />Inlets and other facilities draining the road surface <br />shall be designed to accommodate 10-year frequency <br />storm run-off. All roads shall be designed to remain <br />free of ponding. 'i <br />All drainage installations shall be designed to permit <br />free, unobstructed passage of debris and silt, or to <br />provide for their deflection and/or collection at a <br />point upstream in a manner that will not create an <br />expensive maintenance problem. Settlement basins shall <br />be provided when a silting problem may exist <br />downstream. Modification of natural channels or <br />transfer of run-off from one basin to another shall be <br />prohibited, except where no reasonable alternative <br />exists as determined by the County Engineer. The <br />following methods may be used for estimating peak <br />flows: <br />Run-off from stream records; <br />II. Soil Conservation Service Method, applicable to <br />watersheds smaller than 1,000 acr~as, expresses <br />run-off in terms of geographical position, <br />drainage areas and land use (See CDOT Roadway <br />Design Manual); <br />III. Rational Method, applicable to watersheds smaller <br />than 200 acres, uses the formula: <br />Q = CiAd, <br />where Q = run-off, ft~/sec, <br />C = a "run-off" coeffic;ient, expressing <br />the ratio of rate of run-off to <br />rate of rainfall, <br />i = intensity of rainfall, in/hr, for a <br />duration equal to t:he time of <br />concentration, and <br />Ad = drainage area in ac:ces. <br />Page 5-5-24 i <br />