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Colorado Flood Hazard Mitigation Plan -1999 1998 CollI'Ido 0111 F1dIare UlIIn 2J1 The Cart Sm~h Reservoir failed on the evening of May 2, 1998, Cart Sm~h Dam is an 850 acre-foot, Class 1 off- channel reservoir In Leroux Creek Basin north of Hotchkiss, Colorado. The failure was a result of a large slide on the down- stream slope that extended across the crest and into the upstream slope, The releasing water swiftly eroded down through the top half of the remaining em- bankment and quickly released about 500 acre-feet of storage, The peak dis- charge just below the dam was deter- mined to be around 3,300 cts, Several residences were evacuated, The only loss of life was livestock, The high water washed out numerous bridges. and di- version structures were quickly rebuin to restore water to inigators. stream channel causing a flood, An ice jam can also break up, suddenly causing a surge of waler as the "reservoi(' that was formed behind it is suddenly re- leased, Ice jamming occurs in slow moving streams where prolonged periods ~ cold weather are experi- enced, Sometimes the ice jams are dynam~ed, allow- ing a controlled release of the backed up waler to flow downstream, In 1955, 1962, and 1983, flooding in Rangely resuned from ice jams, as did 1973 flooding in Meeker. 2.2.1,6 Dam Failure Floods Dam failure floods are primarily a resun of hydrologic or structural deficiencies, The operation 01 a reservoir can also influence the safety of the structure. Dam failure by hydrologic defICiency is a resun ~ in- adequate spillway capacity, which can cause a dam to be overtopped during large flows into the reservoir. Dam failure by hydrologic defICiency occurs from ex- cessive runoff after unusually heavy precipi1ation in the basin, Large waves generated from landslides into a reservoir or the sudden inflow from upstream dam fail- ures are other causes of dam failure by overtopping, Overtopping is especially dangerous for an earth dam because the down-rush ~ water over the crest will Chapter 2 - 8 I I I erode the dam face and, d continued long enough, will breach the dam embankment and release all the stored waler suddenly into the downstream floodplain. Examples ~ structural defICiencies include seepage through the embankment, piping along internal con- durts, erosion, cracking, sliding, overturning rodent tun- neling, or other weakness in the structure, Old age is often at the root ~ structural deficiencies, Seismic ac- tivity in Colorado has recently been recognized as a potential source of structural problems due to liquefac- tion ~ sand layers in the embankment ~ a dam, The mechanics ~ a structural failure depends on the type of dam and the mode of failure, Dam failure floods due to structural deficiencies are characterized by a sudden rise in stream level and relatively short duration similar to a thunderstorm flood, They can 0c- cur at any time, but earthen dams appear to be most susceptible to structural failure during the fall and spring freezing and thawing cycles, There are about 27,000 dams in Colorado, the majority of them being livestock water tanks, which are small, low hazard dams located in rural areas, This number includes 1,829 jurisdictional-sized reservoir dams that are greater than 10 feet in vertical heigh~ or have a reservoir whose surface area exceeds 20 acres, or its capacity exceeds 100 ecre-feet, In addition there are several non-jurisdictional sized (NJ) reservoir dams that have been rated as SignifICant Hazard because of their potential impact on improved properties. The con- struction and repair ~ these non-jurisdictionai sized dams must be approved by the State Engineer, and all the reservoir dams, including the Significant Hazard NJ dams, receive safety inspections periodically to assure they are being operated and maintained in a safe man- ner. Although ffNI lives have been lost from dam failures, property damage has been high, There have been at least 130 known dam failures and incidents in Colo- rado since 1890, The failure ~ the lower Latham Res- ervoir Dam in 1973 and subsequent flooding in the Town of Kersey, Weld County, Colorado, resutted in a Presidential Major Disaster Declaration, The earliest recorded dam failure flood in the Estes Park region occurred on May 25, 1951, when Ully Lake Dam failed, sending flood walers down Fish Creek and into lake Estes, In June 1965, a flood occurred on Clay Creek in Prow- ers County, which overtopped an earthen dam baing constructed by the Colorado Game, Fish, and Parks Commission, Although the dam did not fail, rt did di- vert flood water into an adjacent drainage. The subse- quent damage and death from this flood resutted in an important legal controversy known as the Barr Case, I I I I I I I I I I I I I I I I lIUXl5:30 PM