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<br />.~ <br /> <br />~ <br /> <br />Spa~se existino development p~ovides maximum <br />flexibility and a wlde choice of feasible <br />management tools. As shown in Figu~e 6.4, <br />whole-fan tools (e.g., deb~is basin and channel <br />system) a~e needed only fo~ the case whe~e high <br />development densities a~e expected in the <br />channelized zone. Elevation on a~mo~ed fill is <br />~ecommended only in the sheet flow zone, whe~e <br />velocities and erosion are relatively low. <br />Elevation on piles or on armored fill ~ppears <br />to be cost-effective only in a~eas of spa~se o~ <br />medium density development. Local levees and <br />street conveyance for flood waters appear to be <br />the best tools for a variety of development and <br />hyd~aulic conditions, because they a~e <br />~elatively lnexpensive, can be implemented by <br />developers, and work effectively for all but <br />channelized flows. <br /> <br />. <br /> <br />. When mode~ate density development has already <br />taken place without proper planning for flood <br />conveyance, the options for flood management <br />are d~astically reduced. Whole-fan measu~es <br />'. <br />are the best choice in the channelized flow <br />zone, since other tools a~e ineffective. <br />Adequate flood plain zoning is not possible <br />when development has al~eady occu~red. If <br />existing development is left unp~otected, <br />management tools used to protect new <br />development must be carefully designed to avoid <br />agg~avating existing flood p~oblems. Fo~ <br />ex~mple, if streets a~e used to convey flood <br />flows in new developments but down-fan existing <br />streets do not have sufficient capacity or do <br />not connect smoothly to the new streets, severe <br />flOOding of the existing homes could result. <br /> <br />91 <br />