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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bosley~V!ash. Dr(l!:na[?e Master Pl(ln Phase 2: Alternative Drainage Facility Evaluation ,n; ('';:>.__; may cause the downstream channel sections to be undersized and may result in an increase in the flood hazard. These impacts will be evaluated when entire systems are considered. 3.3.1 G Road Culvert The culvert at G Road is one of the most critical structures in the entire Bosley Wash system. The current structure is vastly undersized. As a result, it causes a major reduction in the flow reaching the lower portions of the drainageway. It also causes considerable flooding upstream that impacts several structures. After evaluation, it was concluded that the impact of enlarging this structure; while reducing flood hazard upstream, would cause such a negative impact on the downstream channel that it should not be considered further. 3.3.2 Siphon under Government Highline Canal The siphon under the Government Highline Canal is similar in size to the one under G Road. As such, improvements to one, but not the other, would be ineffective. Additionally, for the reasons mentioned when evaluating the G Road culvert, any enlargement would have a dramatic and negative effect on the downstream flood hazard and should not be considered further. It has been noted that this structure, being a siphon, is subject to extraordinary maintenance demands. Since the siphon may not always be free flowing, the potential for deposition is great. This structure must be maintained with great diligence so that the anticipated conveyance is maintained and no increased hazard is created upstream. 3.3.3 Bosley Wash along F % Road The culverts along F % Road create backups that flow out of the banks and to the south. This creates a significant flood hazard. Culvert upgrades along this reach, including all the undersized driveway crossing culverts, in combination with minor channel improvements, can confine the flows likely to be conveyed to this location. Any improvements in this reach must be done in conjunction with affirmative controls of flow upstream as it is impractical to control major flows along F % Road. We envision the ~ZJJ-E~~~ ?z}EJ:ri27}iB~r-s;rr~/:i~-~;;~.Z~~.L;_ _,~__- ~',""""~'" "'-~- "'"'. _~ '_n _. _ _ " _ , ~"".~.~_,-,:,_ ._-""- ~-":.>...-~._ - __~ .. n' ~'_ >< lj~St.~::::l!.~t'f1;.. '\" A '~~ ~; k! .\~h,~-~it;~~'; Q) Page 13 c;;. ~~ @l.