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Slide Cll. Factors contributing to flood damage Factors that determine how damaging a flood will be include its depth, velocity. duration, amount and kind of debris, the integrity of flood control structures. land uses in the watershed and floodplain, and the quality of building construction. The first three factors are primarily a function of the characteristics of the storm. Other things being equal, deep water, fast moving water, and water that stays for a long time cause extra damage. Slide C12. Debris in floodwaters, West Virginia The kind and amount of debris in flood waters is a critical damage factor. Here you see the impact of debris carried by floodwaters in West Virginia. Water-borne debris- trees, cars, parts of buildings-acts as battering rams, capable of demolishing bridges, levees, and buildings. Debris can dam up culverts and other restricted areas in a stream channel causing overflows to be deeper and more extensive than predicted on flood hazard maps. When hydrologists map 100-year floodplains for FEMA, they consider only clear-water flooding; however, floodwaters are rarely free of debris. Slide C13. Broken floodgate, Folsom Dam The integrity of flood control structures is also a critical damage factor. This is a 1995 photo of a broken floodgate at Folsom Dam above Sacramento, California. People in the highly urbanized area downstream were lucky-the dam did not fail completely and little damage was done. However, damage can be very severe when structures designed to prevent flooding fail or are overtopped by floods larger than they were designed to handle. The FIRM's are drawn assuming that flood control structures will work as intended. This is not always the case. During the 1993 Midwest floods, over 1,000 of the 1,500 levees along the upper Mississippi River and lower Missouri River broke or were overtopped. About one-third of damaging floods in the United States involve the failure or overtopping of a levee or floodwall intended to protect a community from flooding. Slide C14. New houses behind a dike along the Sacramento River Another problem with flood control structures is shown here. This man is standing on a dike along the Sacramento River. He lives in one of the new homes, shown on the right, that were built after the dike was constructed. The homes are in a frequently- flooded basin totally surrounded by rivers and, now, dikes. With the dikes in place, these houses and a 17,OOO-seat sports arena were built and developers are proposing to built another 33,000 houses in the basin. Property and lives are now at risk in this previously undeveloped floodplain. Another homeowner in this area, a city council woman, recognizes the inherent risk by storing a rowboat on her roof. Script-Task C, page 3