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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />Some removal of these pollutants will occur naturally on DFA 0054. The natural <br />forces of purification are many and varied, but are never fast. The presence of acquatic <br />plants and animals (such as would be present in open grass-lined channels) and drop <br />structures all help to aerate the water and maintain the dissolved oxygen concentrations <br />necessary for the elimination of organic compounds. Drop structures and channel linings <br />help to reduce erosion and consequently the suspended solids load. Detention ponds <br />provide the stilling effect necessary for suspended solids to settle out of stormwater. The <br />general effect that each of the alternative plans will have on the water quality of DFA <br />0054 can be judged by the provisions for the natural purification processes present in each <br />plan. These are listed in Table V-l. In no case will constructing any of the alternatives <br />have a detrimental impact on the water quality of DFA 0054, except possibly for brief <br />periods during any channel improvement construction. <br /> <br />Control of pollution beyond the capacity of the natural purification processes <br />present in DF A 0054 will require supplemental study and is beyond the scope of this <br />report. Each jurisdictional entity should be involved in setting objectives and strategies <br />for embarking on a water quality management program. The tools for reducing <br />storm water pollution, from enforcement of litter laws to installation of complex water <br />treatment facilities, do exist and are constantly being increased in number and improved <br />upon. A detailed discussion of present stormwater quality technology is contained in <br />reference O. <br /> <br />Preliminary Desig-n Criteria <br /> <br />For comparison purposes, a preliminary design of each structural alternative plan <br />based in part 011 the major drainage criteria contained in the Urban Drainage and Flood <br />Control District's Urban Storm Drainag-e Criteria Manual, Volumes I and II, was done to <br />determine economic feasibility. For the structural alternative plans for DFA 0054, the <br />preliminary design criteria utilized include: <br /> <br />(1) Maximum velocities of 7.5 feet per second and 15 feet per second for grass <br />and concrete-lined open channels respectively for the laO-year peak flow. <br /> <br />(2) Side slopes of 4:1 and 2:1 horizontal to vertical for grass-lined and concrete- <br />lined open channels respectively. <br /> <br />(3) A minimum of one foot of freeboard on all open channels. <br /> <br />(4) Hydraulic roughness coefficients of .035 and .012 for grass-lined and <br />concrete-lined channels, respectively. <br /> <br />(5) All channel designs have included a trickle channel for conveyance of low <br />flows, except in Adams County only when design flows are less than 50 cfs. <br /> <br />(6) Drop structures consisting of concrete and riprap with a vertical drop of not <br />more than three feet to control average flow velocities in open channels. . <br /> <br />(7) Headwalls and, 450 wingwalls on all new and upgraded culverts to reduce <br />erosion potential and increase culvert efficiency. <br /> <br />(8) A 10 foot wide paved road has been considered as part of the open channel <br />to provide access for maintenance. <br /> <br />(9) Utilities other than sanitary sewers will be relocated as required. Sanitary <br />sewer conflicts (if any) will be resolved during later design stages. <br /> <br />, (10) Detention dam improvements shall meet State Engineer's Standards, if <br />applicable. <br /> <br />(11) Multiple use considerations shall be given to flood control plans. <br /> <br />V-8 <br />