SEVERE WEATHER 101. Flood Basics

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1 SEVERE WEATHER 101 Flood Basics What is flooding? Flooding is an overflowing of water onto land that is normally dry. Floods can happen during heavy rains, when ocean waves come on shore, when snow melts too fast, or when dams or levees break. Flooding may happen with only a few inches of water, or it may cover a house to the rooftop. They can occur quickly or over a long period and may last days, weeks, or longer. Floods are the most common and widespread of all weather-related natural disasters. Flash floods are the most dangerous kind of floods, because they combine the destructive power of a flood with incredible speed and unpredictability. Flash floods occur when excessive water fills normally dry creeks or river beds along with currently flowing creeks and rivers, causing rapid rises of water in a short amount of time. They can happen with little or no warning. Where and when do floods occur? Flooding occurs in every U.S. state and territory, and is a threat experienced anywhere in the world that receives rain. In the U.S. floods kill more people each year than tornadoes, hurricanes or lightning. What areas are at risk from flash floods? Densely populated areas are at a high risk for flash floods. The construction of buildings, highways, driveways, and parking lots increases runoff by reducing the amount of rain absorbed by the ground. This runoff increases the flash flood potential. Sometimes, streams through cities and towns are routed underground into storm drains. During heavy rain, the storm drains can become overwhelmed and flood roads and buildings. Low spots, such as underpasses, underground parking garages, and basements can become death traps. Areas near rivers are at risk from flash floods. Embankments, known as levees, are often built along rivers and are used to prevent high water from flooding bordering land. In 1993, many levees failed along the Mississippi River, resulting in devastating flash floods. The city of New Orleans experienced massive devastating flooding days after Hurricane Katrina came onshore in 2005 due to the failure of levees designed to protect the city. Dam failures can send a sudden destructive wall of water downstream. In 1889 a dam break upstream from Johnstown, Pennsylvania, released a foot wall of water that killed 2200 people within minutes. Mountains and steep hills produce rapid runoff, which causes streams to rise

2 quickly. Rocks and clay soils do not allow much water to infiltrate the ground. Saturated soil also can lead rapidly to flash flooding. Vacationing or recreating along streams or rivers can be a risk if there are thunderstorms in the area. A creek only 6 inches deep in mountainous areas can swell to a 10-foot deep raging river in less than an hour if a thunderstorm lingers over an area for an extended period of time. Very intense rainfall can produce flooding even on dry soil. In the West, most canyons, small streams and dry arroyos are not easily recognizable as a source of danger. A wall of water feet high can scour a canyon suddenly. Additional high-risk locations include low water crossings, recent burn areas in mountains, and urban areas from pavement and roofs which concentrate rainfall runoff. Ice jams and snowmelt can help cause flash floods. A deep snowpack increases runoff produced by melting snow. Heavy spring rains falling on melting snowpack can produce disastrous flash flooding. Melting snowpack may also contribute to flash floods produced by ice jams on creeks and rivers. Thick layers of ice often form on streams and rivers during the winter. Melting snow and/or warm rain running into the streams may lift and break this ice, allowing large chunks of ice to jam against bridges or other structures. This causes the water to rapidly rise behind the ice jam. If the water is suddenly released, serious flash flooding could occur downstream. Huge chunks of ice can be pushed onto the shore and through houses and buildings. FLASH FLOOD or FLOOD WATCH: Flash flooding or flooding is possible within the designated watch area - be alert. FLASH FLOOD or FLOOD WARNING: Flash flooding or flooding has been reported or is imminent - take necessary precautions at once! Get to higher ground! URBAN and SMALL STREAM ADVISORY: Flooding of small streams, streets and low-lying areas, such as railroad underpasses and urban storm drains is occurring.

3 Flood Types River flooding occurs when water levels rise over the top of river banks due to excessive rain from tropical systems making landfall, persistent thunderstorms over the same area for extended periods of time, combined rainfall and snowmelt, or an ice jam. A river flood occurs when water levels rise over the top of river banks due to excessive rain from tropical systems making landfall, persistent thunderstorms over the same area for extended periods of time, combined rainfall and snowmelt, or an ice jam. A coastal flood, or the inundation of land areas along the coast, is caused by higher than average high tide and worsened by heavy rainfall and onshore winds (i.e., wind blowing landward from the ocean). Places like Charleston, South Carolina, and Savannah, Georgia, experience impacts from shallow coastal flooding several times a year because of coastal development and lower elevation.

4 Coastal flooding is caused by higher than average high tide and worsened by heavy rainfall and onshore winds (i.e., wind blowing landward from the ocean). Storm surge is an abnormal rise in water level in coastal areas, over and above the regular astronomical tide, caused by forces generated from a severe storm's wind, waves, and low atmospheric pressure. Storm surge is extremely dangerous, because it is capable of flooding large coastal areas. Extreme flooding can occur in coastal areas particularly when storm surge coincides with normal high tide, resulting in storm tides reaching up to 20 feet or more in some cases. Along the coast, storm surge is often the greatest threat to life and property from a hurricane. In the past, large death tolls have resulted from the rise of the ocean associated with many of the major hurricanes that have made landfall. Hurricane Katrina (2005) is a prime example of the damage and devastation that can be caused by surge. At least 1500 persons lost their lives during Katrina and many of those deaths occurred directly, or indirectly, as a result of storm surge. Inland flooding occurs when moderate precipitation accumulates over several days, intense precipitation falls over a short period, or a river overflows because of an ice or

5 debris jam or dam or levee failure. Hurricane Floyd (1999), aided by Tropical Storm Dennis (1999), caused widespread severe flooding that caused the majority of the $3 to 6 billion in damage reported after those storms. Areas damaged by wildfires are particularly susceptible to flash floods and debris flows during rainstorms. Rainfall that is normally absorbed by vegetation can run off almost instantly, causing creeks and drainage areas to flood much earlier and with higher magnitude than normal. A flash flood is caused by heavy or excessive rainfall in a short period of time, generally less than six hours. Flash floods are usually characterized by raging torrents after heavy rains that rip through river beds, urban streets, or mountain canyons sweeping everything before them. They can occur within minutes or a few hours of excessive rainfall. They can also occur even if no rain has fallen, for instance after a levee or dam has failed, or after a sudden release of water by a debris or ice jam. Areas damaged by wildfires are known as burn scars and are particularly susceptible to flash floods and debris flows during rainstorms. Rainfall that is normally absorbed by vegetation can run off almost instantly, causing creeks and drainage areas to flood much earlier and with higher magnitude than normal.

6 Frequently Asked Questions About Floods If I drive a pickup or a large SUV, why can't I drive through flood water? As little as 6 of fast moving water can sweep most any vehicle off a roadway. Besides, you can't determine the condition of the road bed under the water. The road could be washed out, or the water could be hiding a huge sinkhole. Does a 100-year flood occur once every 100 years on average? The 100-year flood is a climatic average, that is, there is a one percent chance a 100-year flood will happen in any given year. Is flooding really that big of a deal? Flooding causes more damage in the United States than any other severe weather related event, an average of $5 billion a year. Flooding can occur in any of the 50 states or U.S. territories at any time of the year. How can I find out if I am in danger from a flood? NOAA Weather Radio All Hazards is one of the best ways to receive warnings from the National Weather Service. NOAA Weather Radio All Hazards is a nationwide network of radio stations broadcasting continuous weather and river information direct from nearby NWS offices. Also, the NWS web site provides forecasts and warnings, and identifies where flooding is occurring. How do I know how severe a flood will be? Once a river reaches flood stage, the flood severity categories used by the NWS include minor flooding, moderate flooding, and major flooding. Each category has a definition based on property damage and public threat. Minor Flooding minimal or no property damage, but possibly some public threat or inconvenience Moderate Flooding some inundation of structures and roads near streams. Some evacuations of people and/or transfer of property to higher elevations are necessary. Major Flooding extensive inundation of structures and roads. Significant evacuations of people and/or transfer of property to higher elevations. What were some of the worst floods in U.S. history? The worst flood in U.S. history resulted from a dam break upstream from Johnstown, Pennsylvania, on May 31, Although ample warnings were given, they were disregarded by many townspeople, and 2,200 residents died. Hurricane Katrina, a Category 4 hurricane along the eastern LA-western MS coastlines in August 2005, resulted in severe storm surge damage (maximum surge probably exceeded 25 feet) along the LA-MS-AL coasts, wind damage, and the failure of parts of the levee system in New Orleans. Inland effects included high winds and some flooding in the states of AL, MS, FL, TN, KY, IN, OH, and GA. Preliminary estimate of well over $100 billion in damage/costs and

7 more than 1200 fatalities. On February 26, 1972, the Buffalo Creek Dam located in southern West Virginia collapsed sending a black wave of water through one coal mining town after another, and killing more than 100 people. 4,000 people were left homeless. On July 31, 1976, a storm in northern Colorado dumped a foot of rain into the Big Thompson Canyon producing a raging torrent of water 19 feet high and resulting in 145 deaths. Where is the water the highest during a flash flood, in the middle of the stream or along the edges? The water level is usually measured by either a staff gauge or water level gauge. These can be placed along the banks but in most cases they are mounted off the side of a bridge in the middle of the waterway. If water level gauges are mounted on the side of the stream, there is usually a pipe which goes out to the middle of the stream and measures the pressure of the water column above it to determine the height. Long story short, most times water level measurements are made from the center of the channel.