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I. . . . i. . . I. . . . . . . . . . . . . . . . . . . . . . . . . . . !. . . . . I. . . I. . . Bosley Wash Drainage Master Plan Phase I: Watershed Floodplain Delineation __ .'~ II JIll<~~,i""""4ifJ -~;>;-3'C4J'iW"""~4!"'~';""'" ,','I:'" ,. ~'1'mi __ \>Ie ~~~"",",,"~:";":~Z~~,I,!..,;;V;~,:+-i,,'" 1..-' 5.2.2 Cross-Section Processing Hydraulic cross-sections were developed using a combination of the field survey and the GIS mapping, Overbank areas of each cross-section were obtained from the 2 foot contour mapping included within the GIS coverages provided by the County, The main channel for all reaches was determined using points from the field survey combined with field observations, 5.2.3 Hydraulic Control The downstream control used for all flood evaluations in the hydraulic model was the FEMA lOO'year water surface elevation for the Colorado River. Other intermediate control sections were derived from the backwater profile of the Bosley Wash main stem, 5.2.4 Highline Canal Siphon Crossing The Government Highline Canal presented a unique condition, HDR was directed to assume that no flow would be intercepted by the canal. While some anecdotal evidence suggests this may not always be the case, any possible weir flow over the canal is assumed to be negligible relative to flows through the culvert, It was assumed that there is sufficient available storage behind the Government Highline Canal to prevent weir flow, A further assumption is that all flow stored behind the siphon was metered out through the siphon over time and that no flow would extend west past 34 Road, The siphon through the Government Highline Canal was modeled as a culvert, A rating curve was developed for this structure and a stage-storage-discharge relationship was input into HMS, The peak flow from the model was input into the HEC-RAS, 5.2,5 Stub Ditch The Stub Ditch flows were limited by the calculated capacity. These calculated capacities were used in the development of the HMS model. While this reduced the computed discharges, the Stub Ditch still overtops along much of its route during large events, The computed flows that remain in the ditch were used in the hydraulic model. ~~1t41~~1~:;~;;i:';;,-,."-~ ~ /---" ~ ~ lil"1 Qt''''"'''''. " . ...,,~ 'ff ..~' '''$O'::~J?-~'\~~/' Page 20