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<br />Floods
<br />
<br />Floodplains are, by definition, lands that are formed by and continually sub-
<br />ject to inundation by water. Depending on the location, topography, soils, and
<br />weather conditions, that flooding can take a variety of forms. Riverine floods
<br />can result not only from heavy rainfall and rapid snowmelt but also from dam
<br />and levee failure, ice jams, and channel migration. Coastal flooding can be
<br />caused by hurricanes, winter storms, tsunamis, and rising sea leveL Indi-
<br />vidual storms and long.term climate variations cause flooding around lakes.
<br />Other flood prone areas include alluvial fans, unstable and meandering chan-
<br />nels, and areas affected by land subsidence and ground failure. In addition,
<br />flooding due to surface runoff and locally inadequate drainage can be a major
<br />problem, particularly in rapidly urbanizing areas.
<br />
<br />Riverine Flooding
<br />
<br />Riverine flooding-overflow of water from the channel onto the adjacent
<br />floodplain-is the most common type of flood. Hundreds occur each year in
<br />the United States.
<br />
<br />. Flash fWoding occurs in all 50 states: in narro\'.', steep valleys, on alluvial
<br />fans, on denuded areas, and along urban drainage courses, usually as a
<br />result of high intensity, shan duration storms occurring on steep gradient
<br />streams. Flash floods can be more dangerous than other floods because
<br />of their suddenness, the velocity of the water, and the large amount of
<br />debris carried by the flood waters,
<br />
<br />Examples of Recent Flash Floods Causing Serious Loss of Life
<br />
<br />February 1972, Buffalo Creek, West Virginia-125 killed and hundreds of
<br />homes washed away when a dam made of coal mine waste material gave way
<br />after heavy rains.
<br />
<br />June 1972, Rapid City, South Dakota and adjacent areas-236 dead and
<br />$100 million in property damage after a large, slow~moving thunderstorm
<br />unleashed heavy rain on the slopes of the Black Hills.
<br />
<br />July 1976, Big Thompson Canyon, Colorado-139 killed and millions of dol~
<br />lars in property damage after a thunderstorm inundated the ","'eStern third of the
<br />canyon with 12 inches of rain in less than six hours.
<br />
<br />July 1977, Johnstown, Pennsylvania-77 dead and more than $200 million in
<br />property damage when violent thunderstorms pnxiuced 11 inches of rain over a
<br />seven~county area in nine hours. Several dams failed, compounding the stream
<br />flooding and causing 40 of the deaths.
<br />
<br />September 1977, Kansas City. Missouri, and adjacent areas-25 killed and
<br />$90 million in property damage when thunderstonns turned several streams into
<br />raging torrents, including "gentle" Brush Creek, which flows through the heart
<br />of Kansas City.
<br />
<br />Suwu: Fedl'nl Emergl'OI;y Management Agency
<br />
<br />. AlluviolfanfWoding can cause great damage because of the high veloci~
<br />ties, large amounts of sediment and debris, and wide area covered by
<br />the flood waters. Alluvial fans occur mostly along the base of moun~
<br />tains in the \....estern states. An estimated 15-25% of the arid \Vest,
<br />including Los Angeles and Las Vegas, is covered by alluvial fans.
<br />
<br />. Unstahle and meandering stream channels are also frequently flooded. I\t1anyof
<br />them are the product of several decades of human activities, particularly
<br />in the arid and semi~arid vVest. Overgrazing, mining, forestry, urbani-
<br />zation, gravel and sand extraction, and the construction of railroads,
<br />
<br />
<br />
<br />Big Tfwrnps()11 CaI!J;a7l, Colorado, faUowing flash flood,
<br />J"'y 1976.
<br />
<br />
<br />AlluL'ial}an flooding at t~ mfJuth aJ ravines (IT the foat aJ
<br />mfJuntains rxcurs thraugJwut the United SIat&5, but is /TWst
<br />jJrrvaknt and poses tM greaJest hazard in tM arid West.
<br />
<br />lv/obik homt plUk, Ca{orado riva, near Parker, Arizona.
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