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<br /> -8- <br /> Sec. 4. Water qua1ity control. <br /> A. Any development between 50 and 200 feet of the <br /> bank will be required to follow the requirements of <br /> this section. Any development more than 200 feet <br /> from the bank that can achieve overland drainage of <br /> stormwater runoff is exempt from the requirement of <br /> this section. <br /> B. Water quality detention or sedimentation <br /> basins shall be situated and constructed to capture <br /> and hold at least the first one-half of an inch of <br /> runoff from all impervious cover. All subsequent <br /> runoff in excess of the design capacity of the <br /> basins shall bypass the basins and remain <br /> segregated from the contained runoff waters in a <br /> peak shaving basin up to the capacity specified in <br /> the Austin Drainage Criteria Manual. <br /> c. The design of all water quality basins shall <br /> allow an average residence time of twenty-four (24) <br /> hours for the first half-inch of runoff from all <br /> impervious cover. <br /> D. The design of all water quality basins shall <br /> incorporate efficient removal of such contaminants, <br /> including but not limited to lead, zinc, iron, <br /> total phosphorous, total nitrogen, total suspended <br /> solids, and fecal coliform bacteria generated as a <br /> consequence of the SMRC development for which a <br /> basin is designed as approved by the Director in <br /> accordance with the Contaminant Removal Guidelines <br /> of the City of San Marcos. These basins shall be <br /> maintained at all times such that efficient removal <br /> of the contaminants is continuous. <br /> E. Disposal of removed or filtered contaminants <br /> shall be as approved by the Director in accordance <br /> with the Contaminant Removal Guidelines of the City <br /> of San Marcos. <br /> F. Input to and release from water quality basins <br /> shall utilize grass-lined swales and/or overland <br /> flow dispersion measures. <br /> Sec. 5. Over1and f1ow and natura1 drainage. <br /> A. Natural drainage patterns shall be preserved <br /> whenever possible and the loss of the previous <br /> character of the soil should be limited in order to <br /> prevent erosion and attenuate the impact of <br /> contaminants collected and transported by <br /> stormwater. Open surface drainage through <br /> grass-lined swales shall be preferred. Drainage <br /> objectives can best be accomplished by leaving <br /> portions of a subdivision in an underdeveloped and <br /> natural, state and located to receive runoff from <br /> the developed areas for purposes of unchannelized, <br /> overland flow. The use of streets as the central <br /> drainage network is prohibited. <br /> B. Construction of enclosed storm sewers and <br /> impervious channel linings shall be permitted only <br /> when the Director, on the basis of competent <br /> engineering evidence, concludes that such storm <br /> sewers or impervious linings are the only <br /> justifiable option available. In the event that <br /> storm water drainage systems and/or culverts are <br /> necessary, such systems shall be designed to <br />