FLOOD07152

• County: Delta
• Community: Orchard City
• Stream Name: Sruface Creek, Cedar Run, Tongue Creek
• Basin: Gunnison
• Title: Orchard City, Colorado 100-Year Floodplain Delineation
• Date: 2/20/2003
• Prepared For: CWCB
• Prepared By: Gared Grube, Thomas Gilman
• Floodplain - Doc Type: Floodplain Report/Masterplan

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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 rising 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
storm-surge occurs when strong winds push water onto normally dry land, A hurricane is the
most common cause of a storm-surge flood, Debris, landslide, and mudflow 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, 2000).

In mountainous areas, 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 storm 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 is indeed real, Even when the community is not located along a river, such as
Las 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 as readily as in natural
environments, runoff occurs very rapidly and can cause a flash flood (perry, 2000), "While some
conununities in Colorado are more prone to flooding than others, there is no place that is totally

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