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<br />. <br /> <br />. <br /> <br />. <br /> <br />Erosion <br /> <br />The problem of coastal flooding can be made worse by erosion. Along sandy <br />coastlines, the dunes and open beaches provide natural protection by causing the <br />waves to break close to shore, away from development. But these natural <br />protective features can be worn down by erosion, exposing areas farther inland to <br />stonnd~e. . <br />Erosion causes its own damage when it undercutS the foundations under <br />buildings. SlIUctures above the base floodplain can be d~ed during or after a <br />stonn when the underlying sand is washed away, removing the support for the <br />foundations. <br /> <br />Damage from coastal flooding and erosion <br /> <br />Under mirural conditions, coastal flooding and erosion cause little or no permanent <br />damage. Plant life restores itself after a storm. Sandy areas, such as barrier <br />islands, move back and forth as the water and wind move the sand. Coastal dunes <br />and barrier islands migrate naturally and rebuild themselves from stonn to storm. <br />The problem arises when people build on coastal floodplains, puuing <br />property and lives in the path of stann surges and the natural erosion process. <br />People want the sand to stay still so it will protect their buildings from damage. <br />SlIUctures, like seawalls, that are intended to protect buildings from water and <br />erosion, can make things worse by concentrating the water forces in front of the <br />wall or by transferring the problem to the properties at the end of the structure. <br />Damage from coastal flooding is. increasing just as riverine flood damage <br />is. This is primarily because of the great increase in coastal development over the <br />last 40 years: there are more and bigger structures built close to the shore. In <br />some areas, erosion has left these structures closer to the flood hazard,than they <br />used to be. . <br /> <br />Other Flood Hazards <br /> <br />Riverine and coastal flooding are the two most common flood hazards. Most <br />scientific srudies of a community's flood risk delineate the anticipated extent of a <br />flood of a certain frequency, and describe its hazards in terms of depth and <br />velocity of the flood water. Some types of flooding have additional hazards, such <br />as sediment loads that can reroute stream channels. These additional hazards <br />include: <br /> <br />. Sheet flow-In places where there are no defined channels, the flood <br />water spreads out over a large area at a uniform depth. <br /> <br />· Ponding-In flat areas, runoff collects or ponds in depressions and <br />cannot drain ouL Flood waters must infiltrate slowly into the soil, <br />evaporate, or be pumped out <br /> <br />. Closed basin lake Ilooding- This occurs on lakes with either no outlet <br />or a relatively small one. Seasonal increases in rainfall cause the lake <br />level to rise faster than it can drain. The water may stay at flood stage <br />for weeks, months, or years. <br /> <br />9 <br />