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MONUMENT DAM <br />- A CASE STUDY - <br />INSTRUMENTATION OF LANDSLIDE ENABLES <br />CONTINUED OPERATION OF RESERVOIIZ <br />WITHOUT COMPROMISING SAFETY <br />AUTHORS: Jim NorIleet, P.E., Dam Safety Engineer, Colorado Division of Water <br />Resources, Montrose, CO <br />and <br />Bruce Marvin, P.E., President, Western Engineers, Inc., Grand Junction, CO <br />The Mo»ent Dam is owned and operated by the Minnesota Ditch and Reservoir Company of Paoma <br />Cotorado. The dam is located on Dry Fork of Mimmesota Creek within the boundary of the (s rmison Natia~nal <br />Forest at 7340 feet ekvalioa Dry Fodr is a trrbutary to Mimv'sota Creek which flows through the northeast <br />section of Paoma before the confluence with the North Fodr of the G~mnison River. Headwaters of Dry Fork <br />extend to 8200 feet elevation, encorrTpassing a drainage area of 5.6 squaze m$es above the reservoir. Water is <br />also iro;rorted into the tipper part of 9re basin from the drainage to tbe east of 6~e Dry For)c. <br />The stmctme is a 56 foot high, homogeneous, earthfill embankment with a reservon• impou~ment vohtme of <br />about 450 acre-feet Faihne of the dam world cause significant property damage, hot loss of life is not <br />envlsia~ned. The dam is eginpped with a 12 inch diameter, casEiron outlet pipe with ~ discharge <br />capacity of about IS cfs. A topographic phm of the dam is shown on Figure 1. <br />• Conshuction history of the dam is very sketchy. As best caa be determined, rnnstrnction of ~ dam began <br />abort 1899, accmdmg to the water right decree. Forest Service records indicate, work colleted from 1901 to <br />1908 consisted of ertargement of an e~sting dam. Records also show a Departtmeent of hrteucr easement for <br />the dam was granted in 1905. Detar7ed design drawings m reports of the original cons(xuction do not e%ist <br />Records of dam safety inspections and related coaespondettce ~ file with the Division of Water Reso~s date <br />back to 1970. <br />1980 <br />Phase I inspection report was completed in 7amrary. At ire t~ of the Phase I program, the dam was classified <br />as %righ hazrrd°. the irspx[ibe team identi5ed signs of poss~bk unstable oondifions on &e downstream <br />surface of the ®haokmeM induced by seepage. The signs were described as "slides or sloughs with seepage <br />exiting at the base" of the feature. The spiIlway opacity was-also determined inadequate for a high hazard <br />stcochme. The questunabk stabtlr!' y of the dowroheam stops, due to seepage high as the embao and the <br />inadequate spiIIway resulted in a storage resUdction of 101bet below tbe lowest point in the dams crest, 5 feet <br />below the spiIlway crest Based on the Phase I results, the dam was placed on the Army Cotp of Engineers list <br />of unsafe: "high bar.¢d" strucbnes <br />The Please I report also described signs of potential or existing slides arotmd the reservoir rim. Slide scazps <br />were observed on canyon walls in several locations upstream and downstream of the dam site. Based on <br />reco~enda6oro of ~ Phase I report the owngs initiated a study to evaluate the stabBity of the embattlement <br />and determine the feasrln7ity of repair, inchuliog possible enffigemem of storage capacity. Sari saraphx of fbe <br />embankmeffi matetials and the fo>mdation tvere obtained from hole baaingc. Two open tube piezometers <br />installed from the dam crest during the investigation contimre to provide vahtable data far the ctment <br />iastr>~nffitiom program.. <br />• <br />