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<br />Major snowmelt floods are usually a result of an early spring <br />warming trend which lasts up to at least ten consecutive days. <br />Experience has shown that if a single cold day or cold front <br />interrupts the melting cycle, the rising waters will decline and <br />stabilize until the stair-step process is started allover <br />again. Major snowmelt floods occur along the streams just below <br />the winter snowpack in the high mountains areas along the <br />continental divide. <br /> <br />Floods in June 1957, which occurred in several western slope <br />communities, were a result of melting snow. <br /> <br />2.2.7 Rain on Snow Floods <br /> <br />During the spring months of May and June when rivers are <br />running high, there is a potential for flooding due to rain <br />falling on melting snow. usually such rain is over a small part <br />of a basin and the resulting flood is of short duration and may <br />often go unnoticed in the lower reaches of a large drainage <br />basin, <br /> <br />Flooding in June of 1965 at higher elevations and moderate <br />flooding on March of 1980 along the front range are examples of <br />flooding from rain on melting snow. <br /> <br />2,2.8 Dam Failure Floods <br /> <br />Dam failure floods are primarily a result of hydrologic or <br />structural deficiencies. The operation of a reservoir can also <br />influence the safety of the structure. <br /> <br />The most significant hydrologic deficiency is inadequate <br />spillway capacity which can cause a dam to be overtopped during <br />large flows into the reservoir, Dam failure by hydrologic <br />deficiency occurs from excessive runoff after unusually heavy <br />precipitation in the basin, Large waves generated from <br />landslides into a reservoir or the sudden inflow from upstream <br />dam failures are other causes of hydrologic dam failure. <br />Overtopping is especially dangerous for an earth dam because the <br />downrush of water over the crest will erode the dam face and, if <br />continued long enough, will breach the dam embankment and release <br />all the stored water suddenly into the downstream floodplain, <br /> <br />Examples of structural deficiencies include seepage through <br />the embankment, piping along internal conduits, erosion, <br />cracking, sliding, overturning, or other weakness in the <br />structure, Old age is often at the root of structural <br />deficiencies. Seismic activity in Colorado has recently been <br />recognized as a potential source of structural problems due to <br />liquifaction of sand layers in the embankment of a dam. <br /> <br />-313- <br />