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I I I I I I I I I I I 'I I I I I I II I concrete pipe with emergency slide gates in addition to automatic flap gates. The basic reasons underlying the need for emergency slide gates on these structures are the long duration of relatively high stages of the Milk River for the levee design condition, high submergence depths for the outlet inverts of the structures, and the unavailability of adequate ponding areas within the protected area. Costs for the project include construction, or first costs, and investllent costs. Construction costs are the costs incurred in building the project. Total investment costs include interest during the construction period added to the construction costs. This is for the purpose of economic evaluation. Economic AnalTsis, Bene- fits are based on damages pre- vented to structures and their contents as well as external components. These benefits were assigned based on the level of protection provided by the levees. Benefits for each alignment are presented in table 6. .i .' .,.'...'...V~I~'.{i.,.,',....... 'loOcI Cclntr<lt......fftt lil~Leweo .' .t.SOOClt L...I <If A/nloL .,...." ...Iduel{.bl.tl"' ... Pr<lttctlOll leneflt......D...;ea.Arnlot D_ges . AII~ 1. lOO-Y..r 500- Y..r lllv->t IOCI-y..r . 5OG-Y_ AI.I~2 lOO-Y_, . 500-Y_ '-.:'- ..-'-. ',--,.'- ',' AII_n lOG-fur .. 500-f_ Both the lOO-year and 500-year levee designs for subarea 1 were found to be economically feasible. All other levee alternatives had costs that greatly exceeded the benefits which would be derived. Tables 7 and 8 pre- sent the benefit-cost analysis for the levee designs evaluated. An economnomic life of 100 years was used at the current interest rate of 8.875 percent. Operation and maintenance (0&1{) costs are based on current O&M expenditures at siailar projects. 28