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~ CHAPTER I INTRODUCTION This report describes a flood study of the Narrows Dam and Reservoir project on the South Platte River, near Fort Morgan; the location of the project is shown on the Basin Map, Plate I-I. The study and report were prepared by Dames & Moore under a contract with the Colorado Water Conserva- tion Board (CWCB) and the Lower South Platte Water Conservancy District (LSPWCD). The results of the studies are given in Chapters II, III, and IV of the report. Conclusions and recommendations are given at the end of Chapter IV. AUTHORIZATION The study was authorized by Contract No. 34-04-00, between Dames & Moore and the CWCB, dated dated February 13, 1975. February 4, 1975, and by a letter from the LSPWCD, The study was originally scheduled for completion on May 31, 1975, but was extended until June 30, 1975 by mutual agreement, documented in a letter from the CWCB to Dames & Moore, dated June 17, 1975. OBJECTIVES OF THE STUDY The objectives of the study, as set forth in the contract, are threefold: "(a) To evaluate the validity of the recent Bureau of Reclamation re-estimates of the probable maximum storm over the South Platte River Basin above the proposed Narrows damsite, and the Bureau's com- putations of the probable maximum inflow to the proposed Narrows Reservoir. In the event that the Bureau's estimates are not considered valid, the objective will be to develop an alternative estimate that can be supported from the viewpoint of sound engineering practice. (b) To give an opinion as to whether the resulting probable maximum inflow to the reservoir should be used for spillway design, or whether a very large flood of somewhat lesser magnitude, here- inafter called the inflow design flood (IDF), would be considered desirable from the combined viewpoints of economy, risk and safety. DAMES e MOORE