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<br /> <br />CHAPTER 1: THE HIGH RISK AREAS <br /> <br />Any town, USA n The sun sinks behind the hills. glVl1lg way to a sky <br />full of stars. As the lights of town darken, so does the sky. Thunder <br />rumbles faintly in the nearby mountains. The rumble becomes a roar. <br />Blinding rain pelts the landscape for an hour; then a second hour. Dry <br />land becomes a rushing river, carrying trees, boulders and mud. The <br />nexl morning's sunrise reveals that homes several blocks from the river <br />are undercut by deep gullies. Countless basements and first floors are <br />filled with mud and debris. Not a store window remains intact on Main <br />Street. <br /> <br />We all know that flooding can have dramatically different impacts in different ar- <br />eas. Our example illustrates some of the characteristics of high risk flooding. There are <br />four in all: <br /> <br />High velocity <br /> <br />Debris in the flood water <br /> <br /> <br />Suddenness <br /> <br /> <br />Long duration <br /> <br />Under one or a combination of these conditions, buildings seldom just get wet, <br /> <br /> <br />They are severely damaged or destroyed, Reuse of the flooded land may be difficult. <br /> <br /> <br />All fifty states have high risk flood hazard areas. Preliminary estimates show that <br /> <br /> <br />20 to 25 percent of the nations's floodplains are high risk areas. Much of the current <br /> <br /> <br />floodplain development is concentrated in these areas. The total flood risk in the U.S. is <br /> <br /> <br />increasing as more people build on and live in these areas. Development also increases the <br /> <br /> <br />level of risk on adjacent lands by altering natural hydrologic conditions (e.g., restricting <br /> <br /> <br />stream channels, increasing runoff or changing slopes and stream gradients). <br /> <br /> <br />In this guidebook, we explain the risks - and suggest possible risk reduction tech- <br /> <br /> <br />niques - for nine types of high risk flood hazard areas not presently addressed in most <br /> <br />state and local programs. Floodways and coastal velocity zones are not considered in this <br /> <br /> <br />guidebook because they are already regulated by many states and local governments. <br /> <br />ALLUVIAL FANS <br /> <br />An estimated 20 to 30 percent of the land in the Southwest consists of deposits of <br />soil washed down from mountain slopes in fan-like deposits. These include many urban <br />areas such as Los Angeles County and Clark County (Las Vegas). Often the entire fan is <br /> <br />A-I <br />