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<br />-5- <br /> <br />not be adequately attenuated by the existing <br />drainage system if these basins were not uti] ized. <br />In maKing its findings, the Commission and the <br />Director of Environment and Engineering shall <br />consider the following characteristics in relation <br />to the subdivision's impact on water qual ity: the <br />subdivision"s pollutant loading during a one-haif <br />inch storm event, slope and vegetation within and <br />without the subdivision along any surface on which <br />r'unoff from the subdil}ision I..<.liìl travel, the <br />subdivision"s proximity to a naturaJ 'Ñater <br />course, soi J type, recharge structur'es dol"mstream <br />from the subdivision, and the cummulative effect of <br />1 iKe subdivisions which would not utilize <br />filtration. <br /> <br />(2) Required water qual ity 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 />run-off waters in a peaK shaving basin up to the <br />capacity specified in the Austin Drainage Criteria <br />Manual which is hereby adopted and incorporated <br />herein by reference, and a cOPy of which shall be <br />maintained in the Office of the City Secretary of <br />the City of San Marcos. <br /> <br />(3) The design of all water qual ity basins shall allow <br />an average residence time of twenty-four (24) hours <br />for the first half-inch of run-off from all <br />impervious cover. <br /> <br />(4) The design of all water quality basins shall <br />incorporate efficient removai of such contaminants, <br />including but not limited to lead, zinc, iron, <br />tota 1 phosphorus, tota i nitrogen, tota i suspended <br />501 ids, and fecal col iform bacteria generated <br />as a consequence of the development for <br />which a basin is designated as approved by the <br />Director of Environment and Engineering in <br />accordance with the Contaminant Removal Guidei ines <br />of the City of San Marcos which is hereby <br />adopted and incorporated herein by reference, and a <br />coPy of which shali be maintained in the Office of <br />the City Secretary of the City of San Marcos. <br />These basins shall be maintained at all times <br />such that eff i c i en t removal of the can tarn i na.n ts <br />is continuous. <br /> <br />(5) Disposal of removed or filtered contaminants shall <br />be as approved by the Director of Environment and <br />Engineering in accordance with the Contaminant <br />Removal Guide] ines of the City of San Marcos. <br /> <br />(6) Input to and release from <br />sha.l1 utilize grass-lined <br />fJow dispersion measures. <br /> <br />water qual ity basins <br />swales and/or overland <br /> <br />(7) Contaminant removal and sedimentation basins shali <br />not be required in residential subdivisions which <br />have a density of six units per gross acre or less. <br /> <br />h. <br /> <br />Overland Flow and Natural Drainage. <br /> <br />(1) Natural dr'aìnage patterns sha,l1 be preser'\Jed <br />whenever possible and the loss of the pervious <br />character of the soil should be ]imited in order to <br />prevent erosion and attenuate the impact of the <br />contaminants collected and transported by <br />stormwater, Open surface drainage through <br />grass-i ¡ned swales shall be preferred. Drainage <br />objectives can best be accompl ¡shed by leaving <br />portions of a subdivision in an undeveloped and <br />natura] state. These natural, undisturbed drainage <br />areas should be located to receive run-off from the <br />developed areas for purposes of unchannel ized <br />overland flolJJ. <br />