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I I I I I I I I I I I I I I I I I I I Sec t i on V I I I RECOMMENDED PLAN DRAWINGS Master Plan drawings have been prepared showing the location and extent of improvements associated with the selected alternative. These drawings are presented in this report at a reduced scale. Other information on these drawings includes the existing floodplain as del ineated by the U.S. Army Corps of Engineers, the residual floodplain associated with the selected alternative, water surface and channel profiles, cross- section locations. The modified Plan I I I approach downstream of l7th Street to 55th Street together with channelization between 55th Street and 28th Street has a benefit cost ratio of 1.71. It is recommended that an analysis of the impacts of sediment and debris in this area, both with and without the proposed improvement be conducted. If the sediment and debris analysis does not reveal any major weaknesses, it is recommended that the modi- fied Plan II I approach downstream of 17th Street to 55th Street together with channelization between 24th Street and 28th Street be implemented. The annualized improvement cost associated with the recommended plan between l7th Street and 55th Street is $305,000 with an average ailnual damage reduction of $520,000 based on a 50-year life and an interest rate of 6-7/8%. The Plan I I approach from the canyon mouth to 17th Street has a benefit cost ratio of 0.55. It is recommended that a modified Plan III approach including floodproofing, improved crossings over Boulder Creek, and minor channel work be studied for the reach upstream of l7th Street to the canyon mouth. PRIORITY Master plan improvements would start downstream and proceed upstream to prevent an increase of existing flood hazards caused by the premature confinement of the estimated 100-year discharge of 12,000 c.f.s. to the main channel area before downstream flow obstructions and constrictions are removed. 75