Water can be a nightmarish foe. Inside your home, less than one inch of standing water can cause thousands of dollars in damage by destroying wood-products and sheet rock, and providing an environment for mold. Outside in motion, water is incredibly powerful. One cubic yard weighs over 1,700 pounds. A mere 6 inches (that's 31.21 pounds) of water can knock an adult off their feet. Flood waters can plow through earthen dams, pull homes off their foundations and destroy them, sweep cars swiftly downstream, knock out power lines, and contaminate drinking water for weeks. Floods are the number one widespread natural disaster in the US and they often happen due to heavy rain fall from multiple storms or hurricanes.
|Saturated Soil: When the soil in your area is so saturated with water that it can no longer hold any more. The ground will feel soft and squishy beneath your feet. Water will be forced out of the soil where you step. Look for standing water in ditches or in other low-lying areas long after rains have passed.|
|Drought: When droughts or intense heating has dried the soil so thoroughly that it repels water (making it "hydrophobic") instead of absorbing it. In such arid environments, flash floods are a serious danger.|
|Constant, Heavy Rain: When torrential downpours produce too much rain too fast so that the soil can't absorb (or is too frozen to absorb) water fast enough.|
The National Weather Service issues a flood watch when conditions are favorable for flooding to occur.
A flood warning means flooding will occur or is already happening.
According to FEMA, everyone lives in a flood zone. The difference comes in the level of risk. However, there are specific qualities that enhance the odds for flooding:
|Lack of vegetation. Plants absorb water to grow. In areas where vegetation is sparse due to natural forces or human action (such as construction), soil will be more prone to drying out or saturating quickly.|
|Urban and suburban (to a degree) environments are heavily paved and so produce large volumes of rain run off during storms.|
|The local topography. The physical shape of the land surface channels run off into draws and gullies which feed creeks and streams and ultimately rivers. Small multiple feeder streams that together usually produce a tranquil 3 inch deep trickle can easily swell during a down pour to a raging 5 foot deep torrent with enough power to carry off a car. Homes and communities built on floodplains face danger from river flooding and in some locations from flash flooding.|
|Ice jams cause partial blockages of a stream or river near bends or narrow sections with fragments of ice. More common during late winter, ice jams can cause sudden flooding and destruction of bridges.|
A floodplain is a low-lying area adjacent to streams and river channels. This low ground periodically floods when the streams and rivers overflow. Floodplains are often flanked by geologic terraces or "benches" that are slightly higher than the floodplain surface. Benches are usually the remnants of much older banks. Flooding beyond these benches can also occur. Broad river valleys, such as the Mississippi, Missouri, Ohio, and the Platte tend to have broad, flat flood plains due to the river channel's tendency to meander across the entire floodplain. Many rivers in the eastern US, particularly those in the mountains, run through steep sided river valleys and have narrow floodplains, making for deeper and faster flooding events.
The concept of a 100 year flood comes from geologist shorthand to describe the probability for flooding to occur within a particular geographic area. "One hundred year flood" refers to the one in one hundred chance for flooding to occur every year for a particular level — say the "100 hundred year floodplain". In other words, buildings on the one hundred year flood plain face a 1 chance in 100 of being flooded every year. A five hundred year flood means a 1 in 500 chance of being flooded every year. So, while there is always a chance for flooding every year, the risk drops the further away from the stream or river you go.
A flood stage is the stage at which a stream overflows its natural banks and begins to cause damage in the stream's channel area as defined by its measured elevation. The term is very localized and measured in "feet above flood stage". A measurement of 23 feet above flood stage might mean an inch of water in the streets of one town, while in another it drowns Main Street under three feet. When the National Weather service announces an action stage for a section of river, it means there could be limited flooding of low lying fields or roads near the river. Due to variations in the topography, streams and rivers don't always come out of their banks at the same flood stage height. In narrow places with steep banks, the flood stage can be read a very high but produce little damage, at other that are broad and wide, the flood stage can be much lower but produce lots of damage. Flood stage is also used to determine where the crest or high point of flood waters.
A flash flood occurs when powerful storms drop several inches of rain in one hour, the ground will not be able to absorb water fast enough. This produces heavy runoff that can quickly flood low-lying areas in less than 6 hours. Just one inch of rain over one acre of land can produce 27,154 gallons. That's over 100 tons of water suddenly on the move, carrying mud, boulders, and other debris. Flash flooding in urbanized and other paved areas can produce extreme flooding quickly, turning highway underpasses, underground parking third highest weather-related killer in the US.
|Listen to the news and weather reports. NOAA Weather Radio All Hazards (NWR) broadcasts continuous weather information directly from the nearest National Weather Service office.|
|Listen for flood watches/warnings in your area. If your home is located in a flood plain or low-lying area prone to flooding, consider packing an emergency supply kit and personal items and evacuating to higher ground.|
|Protect your possessions and gather records:
|If your area is flooding or is flooded, stay out of flood waters as much as possible.
Floods." Centers for Disease Control and Prevention, U.S. Department of Health & Human Services, 28 Aug. 2017, www.cdc.gov/disasters/floods/index.html.
"Flood Warning vs. Watch." National Weather Service, National Oceanic and Atmospheric Administration, www.floodsafety.noaa.gov/watch_warning.shtml.
Holmes, Robert R., and Karen Dinicola. "100-Year Flood-It's All About Chance." USGS Publications Warehouse, U.S. Department of the Interior, 30 Nov. 2016, pubs.usgs.gov/gip/106/.