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Section 2 Floods I- life along our rivers brings along with it the danger of floods. Some floods are seasonal, as when winter or spring rains and melting snow drain down narrow tributaries filling river basins with too much water, too quickly. Others are flash floods - the result of heavy precipitation - raging torrents that rip through river beds and surge over their banks sweeping everything before them. A special river and reporting network is maintained by the National Oceanic and Atmospheric Administration (NOAA) through its National Weather Service. See Figure 2.1. Rainfall and river data are analyzed continually to provide river forecasts and flood warnings. The flood warning service is part of NOAA's environmental monitoring and prediction program. Timely warnings are provided of atmospheric hazards like hurricanes, tornadoes and other severe storms and of earthquake-generated seismic sea waves. These flood warnings give residents time to evacuate low- lying areas, to move property and livestock to higher ground, and to take necessary emergency action. Gauges mark the height of water in the river basin. The area Weather Service Office can tell you where the nearest river gauge is located in relation to your property. Every resident in an area subject to flooding should know the elevation of his property and how it relates to the river gauge. Flood warning broadcasts may make reference to a flood stage level at a location other than the nearest river gauge. You should know before the flood the relative elevation of your property in relation to flood stage level broadcasts. The city or county engineer probably can help to provide this information. Factors that affect the critical flood stage level for a specific property include: . Elevation of your property in relation to the reference flood stage levels . Flood history of your area . Effects of flood control projects or other man-made changes . Potential flooding conditions of evacuation routes, particularly bridges and low level roadways Forecast procedures are designed by hydrologists, who study the history of each storm. Relationships between precipitation, melting snow, soil conditions, and stream-flow are analyzed: and river forecasting procedures are developed. The hydrologist predicts the amount of water that will flow into rivers and streams - and the time it will take to reach them - under different conditions of temperature, soil moisture, and precipitation. Flood Warnings Residents in low~lying areas given early flood warnings have time to get to higher ground with their personal property, mobile equipment, and livestock. Crops may be harvested before the flood in some cases. Preparations for handling refugees and the impending health hazards caused by the flood also can be made by emergency relief teams. Flood warnings are broadcast over radio and television, and they are made by emergency agencies. Such warnings tell whether the flood is expected to be minor, moderate, or severe, and they give the name of the river and the time and place of flooding. Careful Introduction preparation and prompt response will reduce property loss and ensure personal safety. Flash Floods Flash floods are treacherous. They follow heavy precipitation, move quickly and carry with them boulders, uprooted trees, remains of buildings washed away, and any other mud and debris that has been picked up farther upstream. Persons living in low-lying areas should know the elevation of their property and that of streams and waterways near them. Know ahead of time areas that are safe and what to do in a flash flood emergency. See page 5-1. No stages can be forecast in flash floods. Severity of expected flash flooding may be indicated by terms such as minor, moderate, or severe. Communities are alerted to flash floods by a "flash-flood watch" and a "flash-flood warning:' In the event of a flash-flood watch, listen to radio and television stations for a possible flash-flood warning and reports of flooding in progress. 8e prepared to move out of danger's way at a moment's notice. If on a road, watch for flooding at bridges, dips, and low areas. Watch for signs (thunder, lightning) of distant rainfall. See Section 3, Emergency Actions, for steps to take in the event, of a flash flood warning. See also Section 6, Community Resources and Planning, for preparedness actions communities can take before a flood emergency arises. How Floods Damage Properly Few people realize the force of flood waters unless they have witnessed them. A 1 OO-mile-per-hour wind blowing on the side of a house would be equal to about 12 inches of water flowing at 10 miles per hour against the same wall even though the distribution of forces would be different. Water 1-foot deep flowing at 20 miles per hour against the wall would be 4 times as strong as a 10-mile-per- hour water current at the same 1-foot.depth. None of this considers water erosion around the house, suction behind it, or the battering it receives from objects inside and out. Possible damage resulting from these is considerable. Several factors determine how much damage flood waters will cause to homes. For instance, how long was it submerged? If water rises and falls rapidly, relatively less damage can be expected because some materials absorb water slowly. Property exposed to water for days or weeks will become thoroughly soaked and damage will be extensive. The following pages (2-3 through 2-5) illustrate some of the ways damage to the structure can occur. Vulnerability to damage can often be decreased by preventive measures such as those described starting on page 5-4. 2-1