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A-2 at high risk because of the high velocity of the water, erosion and drainage channels me- andering across the surface of the fan. AREAS BEHIND UNSAFE OR INADEQUATE LEVEES Twenty-five thousand miles of levees line streams and rivers throughout the Unitcd States. Breaching or overtopping of levees causes unexpected floods that are deep and have high velocity. When levees are overtopped, floodwaters are held back from re- turning to the river and inundation is prolonged. AREAS BELOW UNSAFE OR INADEQUATE DAMS More than 2000 communities are at risk from dams that have been identified as unsafe. Even dams classified as safe may be overtopped or breached by extraordinary floods, earthquakes, or improper maintenance. Flooding from breaching or overtopping is often deep, of high velocity and likely to occur with little or no warning, COAST AL FLOODING AND EROSION AREAS Erosion and flooding combine to increase flood damage along thousands of miles of coastline. The most serious problems are on barrier islands, along the Great Lakes shoreline and along the Gulf coast. Erosion removes natural protective barriers - beaches, dunes and bluffs - causing direct damage as well as exposing buildings to larger waves and storm surges. FLASH FLOOD AREAS Although they may occur in all fifty states, flash floods are most common in the arid and semi-arid west where there is steep topography, little vegetation and intense but short-duration rainfall. They rank first as a cause of flood-related deaths in the United States, Heavy rains, sometimes in combination with spring snowmelt, often lead to rapidly rising, fast moving water which can cause severe erosion as well as flood damage. Flash floods occur in both urban and rural settings, principally along smaller rivers and drainageways.