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I I I I I I I I I I I I I I I I I I I Ice.jam and dam and levee failure floods can be considered different causes of flash floods. Ice- Jam floods result when a river becomes frozen in its higher elevations. The ice blocks the flow. causing pressure to build up upstream of the obstruction. Eventually the ice-jam will break. due to excessive pressure or ri",ing temperatures. causing a sudden surge of water release. This is similar to the catastrophe of a dam or levee failure. except that a dam failure is much worse. A stonn-surge occurs when strong winds push water onto normally dry land. A hurricane is the most common cause of a stonn-surge flood. Debris. landslide. and mud flow floods occur when debris. rocks, logs. or mud block the channel creating a temporary dam. Water then accumulates behind the obstruction causing upstream flooding. The dam eventually will then fail and cause a downstream surge of water (Perry, 2(00). In mountainous area~. excesSive precipitation from convective storms is a common cause of runoff leading to flooding and, therefore. is of concern. These convective storms precipitate a large amount of moisture into a small area. thus requiring a relatively small drainage basin to accommodate a large amount of water. There are many factors that affect the runoff from precipitation that lead to a flood event. Among these are meteorological factors that include the type of precipitation (rain, sleet. hail. or snow), the rainfall intensity. amount, duration. and distribution, the direction of the stonn movement, the antecedent soil moisture. and other factors that affect evapotranspiration such as temperature, wind. relative humidity. and season. The physical characteristics of the watershed also affect the runoff. These characteristics include land use, vegetation. soil type. drainage area, basin shape, elevation. slope, topography, orientation, drainage network problems, and retention areas such as ponds. lakes, and reservoirs (USGS. 2001). Since most communities must be located near a water source or along a waterway to prosper. the threat of a flood to; indeed real. Even when the community is not located along a river, such as La.. Vegas. Nevada. a flood can still occur if a large amount of precipitation falls over the large imperviousness of the community. Because a large area of an urban community is composed of asphalt and concrete. moisture does not infiltrate into the ground a'\ readily as in natural environments. runoff occurs very rapidly and can cause a flash flood (Perry, 2(00). "While some communities in Colorado are more prone to flooding than others. there is no place that is totally 7