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<br />IDD8 - Colorado Rood Hazard Mitigation Plan (4DD)
<br />
<br />taken at this subdivison in 1984 is displayed on the
<br />previous page_ This site has a history of severe dam-
<br />age and resulting costs to local, state, and federal
<br />govemments_ -
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<br />2.2.1.5 Ice Jam Floods
<br />
<br />Ice jam floods can occur by two phenomena_ In the
<br />mountain floodplains during extended cold periods of
<br />20 to 40 degrees below zero the streams ice over_ The
<br />channels are frozen solid and overbank flow occurs
<br />which results in ice inundation in the floodplains_ Ice
<br />jam floods can occur when frozen water in the upper
<br />reaches of a stream abruptly begins to melt due to
<br />warm Chinook winds_ Blocks of ice floating down-
<br />stream can become lodged at constrictions and form a
<br />jam_ The jam can force water to be diverted from the
<br />stream channel causing a flood_ An ice jam can also
<br />break up; suddenly causing a surge of water as the
<br />"reserwir" that was formed behind it_ is sudden_ly
<br />released_ 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
<br />in Meeker.
<br />
<br />2.2.1.6 Dam Failure Floods
<br />
<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_
<br />
<br />Dam failure by hydrologic deficiency is a result of
<br />inadequate spillway capacity, which can cause a dam
<br />to be overtopped during large flows into the reserwir.
<br />Dam failure by hydrologic deficiency occurs from ex-
<br />cessive runoff after unusually heavy precipitation in
<br />the basin_ Large waves generated from landslides into
<br />a reservoir or the sudden inflow from upstream dam
<br />failures are other causes of dam failure by overtop-
<br />ping_ Overtopping is especially dangerous for an earth
<br />dam because the downrush of water over the crest will
<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 f100dplain_
<br />
<br />Examples of structural deficiencies include seepage
<br />through the embankmen~ piping along internal con-
<br />duits, erosion, cracking, sliding, overturning rodent
<br />tunneling, or other weakness in the structure_ Old age
<br />is often at the root of structural deficiencies_ Seismic
<br />activity 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_
<br />
<br />The mechanics of a structural failure depend on the
<br />type of dam and the mode of failure_ Dam failure
<br />floods due to structural deficiencies are characterized
<br />
<br />by a sudden rise in stream Iel/el and relatively short
<br />duration similar to a thunderstorm flood_ They can
<br />occur at any time, but earthen dams appear to be
<br />most susceptible to structural failure during the fall
<br />and spring freezing and thawing cycles_
<br />
<br />There are approximately 27,000 dams in the Stale of
<br />Colorado_ This includes 2,292 dams,which are under
<br />the jurisdiction of the state engineer, sel/eral thousand
<br />low dams for small capacity reserwlrs known as
<br />"Livestock Water Tanks" (which are not normally in-
<br />spected), and potential artifICial impoundments cre-
<br />ated by highway embankments constructed across
<br />drainage ways. A dam must be at least 10 feel high or
<br />its reservoir must have a surface area of at least 20
<br />acres or a storage capacity of at least 100 acre-feet to
<br />fall under the review of the state's dam safety pro-
<br />gram_ Of the 2,292 inspected dams, 120 are federally
<br />owned, and 2,172 are nonfederally owned_
<br />
<br />Although few lives hB\le been lost from dam failures,
<br />property damage has been high_ There hB\le been at
<br />least 130 known dam failures and incidents in Col-
<br />orado since 1890_ The failure of the Lower Latham
<br />Reservoir Dam in 1973 and subsequent flooding in the
<br />town of Kersey, Weld County, Colorado, resulted in a
<br />Presidential Major Disaster Declaration_
<br />
<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_
<br />
<br />In June 1965, a flood occurred on Clay Creek in
<br />Prowers County, which overtopped an earthen dam
<br />being constructed by the _Colorado Game, Fish, and
<br />Parks Commission_ Although the dam did not fail, it
<br />did divert floodwater into an adjacent drainage_ The
<br />subsequent damage and death from this flood resulted
<br />in an important legal controversy known as the Barr
<br />Case_ This case was finally decided in 1972 by the
<br />Colorado Supreme Court, which recognized the con-
<br />cept of probable maximum flood as a predictable and
<br />foreseeable standard for spillway design purposes_
<br />
<br />In 1982, the failure of the Lawn Lake Dam, a pri'lalely
<br />owned dam on National Park property, caused $31
<br />million In damages in larimer County and Estes Park.
<br />A lawsuit awarded $480,000 to the family of one of the
<br />four persons killed in the disaster_ The most unusual
<br />flood from the failure of a man-man structure in
<br />Colorado is probably the complete draining of Lake
<br />Emma, a natural lake located high in the San Juan
<br />Mountains above Silverton, CoIorado_ On June 4,
<br />1979, floodwater flowed through a network of tunnels
<br />in an abandoned mine, which extended under the lake_
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<br />FiIe:FH/IIP.QJapler-2-pub
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<br />1121/98 6:14 AM
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<br />ChaptBr 2 - 8
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