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<br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br /> <br />Bosley.V(as~ Dra~na.ge Master PlanH. <br /> <br />Phase 2: Alter'native Drainage Facility Evaluation <br /> <br /> <br />that this structure was open during flood events. To assure this type of performance, a <br />new gate structure should be considered. <br /> <br />One citizen expressed concern over the handling of overflows into the Government <br />Highline Canal. Overflows into the Government Highline Canal were explicitly ignored <br />during this study at the direction of the sponsors. This assumption is based on the general <br />nature of canal structures and their operation during floods. In most cases, canals are <br />filled by runoff from upgradient areas and are at capacity after the very early parts of a <br />storm. Therefore, even when the banks of the canal may appear to be overtopped, the <br />canal generally does not divert water from the Watershed since it is already full. In the <br />case of the Government Highline Canal, the downstream embankment is higher than the <br /> <br />upstream embankment. Even if water exceeds the elevation of the upstr.;arn bank, it will <br />not spill over the downstream bank. This is the mode of operation assumed during this <br />study, and is one that appears to reflect actual performance during major flood events. <br /> <br />3.0 AL TERNA TIVE DEVELOPMENT <br /> <br />Based on the understanding of the nature of problems within the Bosley Wash <br />Watershed, alternative solutions were developed. These alternatives are intended to solve <br />specific problems, to address specific areas where problems have been identified, or to <br />provide benefits that extend across the entire Watershed. At this level of study, the <br />alternatives have been developed at a conceptual level. Preliminary sizing is based on the <br />hydrologic and hydraulic information used in the floodplain delineation. Alignment <br />information was developed based on available mapping and property ownership <br />information provided by the County. Cost information is based on unit cost estimates for <br />major work elements. The intent is to allow comparison between alternatives and to <br />establish broad conceptual level cost estimates. <br /> <br />3.1 Storage <br /> <br />The primary storage alternative is the formalization of the significant areas of inadvertent <br />storage throughout the Watershed. In this process, the area subject to periodic inundation <br />is designated for fee simple purchase or for the purchase of flood easements. In most <br /> <br />f~::,~ ~.; .~:'~':~2~:1J:,-';~-"; _;,~:-~':=:~7:~;:Y'~::;, ;Z.Z=;:jJ:J~:--~;~->~J~-:~:~~.-_ -"-~Q~~-2~~ - ~ ~~'~,.___'"- - _, ... <br /> <br />f;;~::'~i':t; <br />;;:~~~~ <br />~~{:1:~~kilt~~.'~' ' <br /> <br /> <br />Page 9 <br /> <br />C-,.~ ''\ <br />i~ <br /> <br />@l. <br />