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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bosley Wash Drainage Maste~f:lan. . .. ".." " ... Phase~' Watershed Floodplain Delineation ~ 11 I: lft~1~~~H?tla_A~~~~:~~~o;b~;.~":':/:c\,'\;Z~ttffi;'~.~~'4"/- ,,-">>:;, 5.5 Sedimentation and Erosion The potential for large amounts of sediment movement exists within the basin, The upper portions of the watershed can generate significant debris flows, however the storage along 1-70 effectively limits the impact on the downstream portions of the basin, As such, sediment which might be generated above 1-70 was ignored in this study, The lower portion of the basin is characterized by relatively flat slopes and well defined small channels, While erosion may occur within the channels, it is generally localized and not likely to profoundly affect the floodplain, The same can be said for overbank flooding, These flows are slow and will not transport great quantities of sediment that may affect floodplain limits, As such, the analysis in this study assumed all discharges were relatively clear water flows, No bulking was assumed, 6. CONCLUSIONS AND RESULTS This report has been prepared by HDR as the primary deliverable for Phase 1 of the Bosley Wash Drainage Master Plan as defined in HDR's Scope of Work Submittal dated December 12, 2000. It presents a Watershed Floodplain Delineation for the Bosley Wash Basin, 6.1 Conclusions Figure 7 presents the delineation of the 100-year floodplain for the Bosley Wash Watershed, This report not only establishes the floodplain for Bosley Wash but also provides the starting point and the tools to be used for the development of flood control measures during the subsequent phases of this project. Some of the key observations and conclusions will come as no surprise to many of those with previous experience within this watershed, . The basin can generate a surprisingly large amount of discharge but basin characteristics limit the free flow of those waters; . Culverts are undersized at key locations within the watershed; .. ,~~ !~~~'i'i,r,.".. -~.... . (~\ ~ lilt ,(:""';;-, ,{~:";; :::~~'~'~L~~.;'-;; ~'" ~ 26