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<br />Colorado Flood Hazard Mitigation Plan -1999
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<br />1998 Colorado Dam FaihlreIRUIR. UJ
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<br />The Carl Smith Reservoir failed on the
<br />evening of May 2, 1998, Carl Smith
<br />Dam is an 850 acre-foot, Class 1 off-
<br />channel reservoir in Leroux Creek Basin
<br />north of Hotchkiss, Colorado, The failure
<br />was a result of a large slide on the down-
<br />stream slope that extended across the
<br />crest and into the upstream slope, The
<br />releasing water swiftly eroded down
<br />through the top half of the remaining em-
<br />bankment and quickly released about
<br />500 acre-feet of storage, The peak dis-
<br />charge just below the dam was deter-
<br />mined to be around 3,300 cts, Several
<br />residences were evacuated. The only
<br />loss of life was livestock. The high water
<br />washed out numerous bridges, and di-
<br />version structures were quickly rebuilt to
<br />restore water to irrigators,
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<br />stream channel causing a flood, An ice jam can also
<br />break up, suddenly causing a surge of water as the
<br />"reservoi(' that was formed behind it is suddenly re-
<br />leased, Ice jamming occurs in slow moving streams
<br />where prolonged periods of cold weather are experi-
<br />enced, Sometimes the ice jams are dynamited, allow-
<br />ing a controlled release of the backed up water to flow
<br />downstream, In 1955, 1962, and 1983, flooding in
<br />Rangely resuited from ice jams, as did 1973 flooding in
<br />Meeker.
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<br />2.2.1.6 Dam Failure Floods
<br />Dam failure floods are primarily a result of hydrologic
<br />or structural deficiencies, The operation of a reservoir
<br />can also influence the safety of the structure,
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<br />Dam failure by hydrologic deficiency is a resuit of in-
<br />adequate spillway capacity, which can cause a dam to
<br />be overtopped during large flows into the reservoir.
<br />Dam failure by hydrologic deficiency occurs from ex-
<br />cessive runoff after unusually heavy precipitation in the
<br />basin, Large waves generated from landslides into a
<br />reservoir or the sudden inflow from upstream dam fail-
<br />ures are other causes of dam failure by overtopping,
<br />Overtopping is especially dangerous for an earth dam
<br />because the down-rush of water over the crest will
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<br />Chapter 2 - 8
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<br />erode the dam face and, if continued long enough, will
<br />breach the dam embankment and release all the
<br />stored water suddenly into the downstream floodplain,
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<br />Examples of structural deficiencies include seepage
<br />through the embankment, piping along internal con-
<br />duits, erosion, cracking, sliding, overturning rodent tun-
<br />neling, or other weakness in the structure, Old age is
<br />often at the root of structural deficiencies, Seismic ac-
<br />tivity In Colorado has recently been recognized as a
<br />potential source of structural problems due to liquefac-
<br />tion of sand layers in the embankment of a dam,
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<br />The mechanics of a structural failure depends on the
<br />type of dam and the mode of failure, Dam failure
<br />floods due to structural deficiencies are characterized
<br />by a sudden rise in stream level and relatively short
<br />duration similar to a thunderstorm flood, They can oc-
<br />cur at any time, but earthen dams appear to be most
<br />susceptible to structural failure during the fall and
<br />spring freezing and thawing cycles,
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<br />There are about 27,000 dams in Colorado, the majority
<br />of them being livestock water tanks, which are small,
<br />low hazard dams located in rural areas, This number
<br />includes 1.829 jurisdictional-sized reservoir dams that
<br />are greater than 10 feet in vertical height, or have a
<br />reservoir whose surface area exceeds 20 acres, or its
<br />capacity exceeds 100 acre-feet. In addition there are
<br />several non-jurisdictional sized (NJ) reservoir dams
<br />that have been rated as Significant Hazard because of
<br />their potential impact on improved properties, The con-
<br />struction and repair of these non-jurisdictional sized
<br />dams must be approved by the State Engineer, and all
<br />the reservoir dams, including the Significant Hazard NJ
<br />dams, receive safety inspections periodically to assure
<br />they are being operated and maintained in a safe man-
<br />ner.
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<br />Although few lives have been lost from dam failures,
<br />property damage has been high, There have been at
<br />least 130 known dam failures and incidents in Colo-
<br />rado since 1890, The failure of the Lower Latham Res-
<br />ervoir Dam in 1973 and subsequent flooding in the
<br />Town of Kersey, Weld County, Colorado, resuited in a
<br />Presidential Major Disaster Declaration,
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<br />The earliest recorded dam failure flood in the Estes
<br />Park region occurred on May 25, 1951, when Lilly
<br />Lake Dam failed, sending flood waters down Fish
<br />Creek and into Lake Estes,
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<br />In June 1965, a flood occurred on Clay Creek in Prow-
<br />ers County, which overtopped an earthen dam being
<br />constructed by the Coiorado Game, Fish, and Parks
<br />Commission, Aithough the dam did not fail, it did di-
<br />vert flood water into an adjacent drainage, The subse-
<br />quent damage and death from this flood resuited in an
<br />important legai controversy known as the Barr Case.
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<br />12/23/9910:00 PM
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