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I . . . I I I . . . . . I '. . I I I 'I All of these items were considered in the evaluation of the total cost of improvements for the selected alternative. The construction costs as defined represent approximate costs for a structure over a usable life of 50 years. Maintenance costs will not lengthen this life appreciably, if at all, but will guarantee the operation of the improvement at the time of need. An annual interest rate of 6-7/8% was used for economic calculations. 1. CONSTRUCTION COST Prel iminary costs estimates were made for the improvements listed. The costs were based on prel iminary size requirements and unit costs associated with large volume projects of this type. A summary of Some of the major unit costs is presented in Table VII 1-1. These unit costs represent items complete in place and were obtained from source data based on estimated 1978 dollar values. Estimating construction costs at this time in the metropol itan area is very difficult because of the unstable conditions due to inflation and shortages. The costs provided are intended to be relative approximations of the actual cost. Their primary use is to evaluate the economic merit of the proposed improvements. 2. RIGHT-OF-WAY COSTS The costs associated with land acquisition within the study area were based on data suppl ied by the United States Army Corps of Engineers. 3. OPERATION AND ~INTENANCE COSTS Maintenance costs have been analyzed for the masterplan improve- ments in the study area. Historical data on maintenance costs for this area are not readily available for grass-lined channels, underground facilities and floodways. However, past information available through the City of Denver Parks Department, City of Boulder Parks Department, the Denver Water Board and various other sources were considered and used for an approximation of the main- tenance costs associated with the study area. An important part of floodway maintenance is the restoration of the channel to its former dimensions after each major flood to assure the proper future operation of the floodway. Drs. Simons and Li have estimated that between 24,000 cubic yards to 29,000 cubic yards of sediment will be deposited within the study area after the 100-year flood event and that 30% of this sediment volume will be deposited above 6th Street. Operation and maintenance costs include the replacement of pedes- trian bridges and landscaped trees and shrubs at 25, 50, and 75 yea r s. Operation and maintenance costs associated with the study area are tabulated in Table VIII-3. 4. ENGINEERING COSTS A varying amount of cost was assigned to the design and preparation of construction deocuments for each improvement based on a 6 1