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I I I I I I I I I I I I I I I I I I I INTRODUCTION Twi n Lakes Dam is an earth embankment wi th a hei ght of 35 feet and a crest length of 220 feet, ~t elevation 9300. It impounds a reservoir, when full, of 278 acre-feet at the normal water surface elevation of 9295. The dam and reservoi rare located in Larimer County on a tri butary to the South Fork of the Cache la Poudre &iver, about nine miles south of Rustic, Colorado. The dam was constructed in the early 1900' s by the Pingree Park Mining, Milling, and Irrigatjon Company, for irrigation supply. There is no record of the dam having been repaired since its original construction. The dam's operatin9 level has been restricted to zero storage since January 22, 1986 by the Colorado State Engineer's Office (SEO). This restriction was impo$ed due to the deteriorated condition of the outlet works and embankment seepBge. The reservoi r was vol untari ly restricted to zero storage by the City of Greeley in September, 1984 for the same reasons. No previ ous studies to rehabi 1 itate the dam or geotechni ca 1 i nvesti gati ons have been performed.. Morrison-Knudsen Engineers, Inc. (MKE) was contracted by the City of Greeley in June, 1986, to stUdy the feasibility of rehabilitating the dam and i ncreasi ng the acti ve storage to the decreed vol ume of 460 acre-feet. The study is partially financed by the Colorado Water Conservation Board (CWCB). The followi ng feasi bi 1 i ty report contai ns chapters of general i nformati on pertaining to the site, background information on the City of Greeley, benefits to agriculture, and the availability and utilization of water from the project. The plan of development, contained in Chapter 5, includes a di scussi on of the site i nspecti ons, geotechni cal i nvesti gati on, hydrol ogi cal analysis, alternatives for rehabilitating the dam and the selected feasibility design, as well as the recommended interim level of operation. -1-