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I I I I I I I I I I I I I I I I I I I Benefit/Cost Analysis During the Phase A study, the relative economic merits of each alternative were identified by comparing the benefits and costs of the respective alternatives. How well each al ternative satisfies the other objectives of the study is a matter of judgment, since no attempt was made to rate the alternatives on any quantifiable criteria other than economics. The primary purpose of the alternative plans considered in this study was flood control. Annual dollar amounts for flood induced damages were estimated for the existing condition, as previously described, and the purpose of the flood control plan is to reduce these damages. Thus, the benefits quantified in the analysis were the reductions in the average annual flood damages which would be realized after the construction of the project. For the alternative flood control plans, the costs for constructing the respective facilities were estimated using current real estate, materials, and labor prices. Also included in the cost for each alternative were the estimated annual operation and main- tenance expenditures associated with each type of flood control facility. An incremental benefit/cost analysis was performed to identify the channel design interval or pond size which would provide the greatest return in reduced flood damages for each construction dollar spent. The unit cost data and procedures utilized are described in Section VI of the Phase A Report. Selection of Recommended Plan Examination of the existing condition of the floodplains in Direct Flow Area 0054 reveals that the lack of a basin-wide, comprehensive plan has resulted in seemingly short- term and ad hoc solutions to the basin's drainage and flood control problems. For the long term solution to these problems, the selected plan was to be continuous, with the improvements of each Reach functioning together as a unit to produce an effective over- all flood control system. This dismissed the selection of a plan for implementation strict- ly on the basis of the benefit/cost analysis, as this might have resulted in an incoherent master plan and ignore many of the other study objectives. Continuity of the plan from reach to reach was therefore given principle consideration. There are also certain en- vironmental and political considerations which weigh heavily in the selection of a recom- mended plan. Al ternative flood control plans are identified by reach in an alternative matrix, Figure IV-I. Preliminary examination of each alternative plan led to the elimination of those shown in the shaded sectors of the matrix as being infeasible for practical appli- cation. Discussion of the feasibility of each of the alternatives is in the Phase A Report. The alternative flood control plans which appeared to be most promising are listed by reach in Table IV-2. Examination of the floodplains and benefit/cost data lead to the determination of those alternative plans for each reach which were the overall most economically beneficial. In Reaches 1 and 2 the incremental benefit/cost analysis indicated that the highest benefit per dollar invested would come from constructing a channel designed to accom- modate the 2-year design storm, with a grass-lined channel being the least expensive alternative. Increased upstream detention of runoff in Reaches 3 and 4 would reduce the cost of constructing this alternative by reducing the 2-year design flows as well as the peak discharges of storms of all magnitudes. Since no formal channel exists in Reach 1 to convey runoff of any magnitude, the construction of a graSS-lined channel to carry minor flows was the minimal action to be recommended. As development takes place, acquisition of right-of-way for 100-year flood control would be necessary regardless of the benefit/cost analysis, in order to prevent additional flood damages in new developments. A continuous channel is necessary in Reach 2 to convey flows efficiently from the 18 inch CMP that drains the Holly Street/Lower Medema Pond embankment. As a result of having the lowest total annual costs and the highest benefit/cost ratio, the 2-year grass channel also seemed to be the obvious choice. If future development were to occur in Reach 2, a 100-year floodway should be reserved to eliminate the possibility of additional flood damages. The selection of the 22 acre/feet capacity for the detention pond at East 112th Avenue, the enlargement of the existing Upper Medema Pond to 16 acre/feet and the decision to leave the Lower Medema Pond in its existing state was a direct result of the benefit/cost analysis, which indicated that this pond configuration is the most desirable economically. IlT-3