How floods form A flood occurs when water inundates land that's normally dry, which can happen in a multitude of ways. Excessive rain, a ruptured dam or levee, rapid melting of snow or ice, or even an unfortunately placed beaver dam can overwhelm a river, spreading over the adjacent land, called a flood plain.
Where do floods occur?
Flood/ By EarthProfessor Floods typically occur in lands that are adjacent to rivers or at coastal areas. Land adjacent to rivers, known as floodplains, is susceptible to floods when there is excessive rain. Coastal areas face flooding only when a tsunami or large storm forces the sea to surge inland.
What happens during a flood?
A flood happens when water overflows or soaks land that is normally dry. There are few places on Earth where people don’t need to be concern ed about flooding. Generally, floods take hours or even days to develop, giving residents time to prepare or evacuate.
Why do flash floods occur?
Flash floods occur within a few minutes or hours of excessive rainfall, a dam or levee failure, or a sudden release of water held by an ice jam. Flash floods can roll boulders, tear out trees, destroy buildings and bridges, and scour out new channels. ... Water held back by the ice jam or debris dam can cause flooding upstream.
What causes flooding upstream and downstream?
Water held back by the ice jam or debris dam can cause flooding upstream. Subsequent flash flooding can occur downstream if the obstruction should suddenly release. Coastal flooding occurs when strong onshore winds push water from an ocean, bay or inlet onto land.
What causes river flooding?
Winter or spring rains, coupled with melting snows, can fill river basins too quickly. Torrential rains from decaying hurricanes or other tropical systems can also produce river flooding. Repeated heavy rain from thunderstorms over a period of weeks contributed to the Mississippi River Flood of 1993.
How dangerous is flash flood?
Heavy rains from showers and thunderstorms can quickly fill dry stream and river beds, sending torrents of water downstream. Fast-moving water is extremely powerful. The result can be deadly to anyone in the water's path. The force of flash flood waters can be extremely dangerous to motorists who unwittingly or unknowingly drive over water-covered ...
What is the most frightening thing about rain?
Most frightening is the rapidity with which the water rises. Torrential rains associated with land falling tropical systems sometimes produce flash floods. Advertisement. In the western United States, the soil is generally dry, sandy and unable to absorb large amounts of water. Heavy rains from showers and thunderstorms can quickly fill dry stream ...
How much water is needed to sweep away a car?
The force of flash flood waters can be extremely dangerous to motorists who unwittingly or unknowingly drive over water-covered roads - only two feet of running water are needed to sweep away a car. Floods There are various types of flooding based on where they occur.
What is a flash flood?
Flash Floods. A flash flood is typically caused by sudden, excessive rainfall that sends a river, stream or other body of water rapidly out of its banks. Often this occurs in a short amount of time, only several hours or even less.
What causes a treacherous overflow?
The constant influx of water finally causes a treacherous overflow to begin, powerful enough to sweep vehicles away, roll boulders into roadways, uproot trees, level buildings, and drag bridges off their piers. Most frightening is the rapidity with which the water rises.
What is coastal flooding?
Coastal flooding occurs when strong onshore winds push water from an ocean, bay or inlet onto land. This can take the form of surges associated with tropical storms and hurricanes, or can be associated with non-tropical storms such asnor'easters.
Why do hydrologists study flood patterns?
Today, hydrologists study past flood patterns to help predict where and when floods will happen in the future. The predictions are only estimates, however. Weather, land, and climate can all change.
What river was Johnstown on?
Johnstown, Pennsylvania, was on a flood plain at the meeting of the Stony Creek and Little Conemaugh rivers. As more people moved to the city, the banks of the rivers were paved and narrowed, causing yearly flooding. Residents were prepared for this. They watched the river and moved their belongings upstairs or onto rooftops as the city flooded.
What can floods do to a landscape?
Floods can cause even more damage when their waters recede. The water and landscape can be contaminate d with hazard ous materials, such as sharp debris, pesticide s, fuel, and untreated sewage. Potentially dangerous mold can quickly overwhelm water-soaked structures. As flood water spreads, it carries disease.
What dam was Lake Conemaugh in?
Located in nearby mountains, Lake Conemaugh was a reservoir created by the South Fork Dam. The lake was an exclusive retreat for members of the South Fork Fishing and Hunting Club, which owned the dam. Lake Conemaugh contained 20 million tons of water.
How many people died in the Johnstown flood?
The most famous flood in American history, the Johnstown Flood, was a man-made disaster. The tragedy killed 2,209 people and made headlines around the country.
What is the chance of a 100 year flood?
But this is only an estimate. What “100-year flood” actually means is that there is a 1 percent chance that such a flood could happen in any given year. In recent decades, 100-year floods have occurred more frequently. This may be due to global warming, the current period of climate change.
What is the term for the thick vegetation between a river and a flood plain?
The thick vegetation between a river and a flood plain is called a riparian zone.
How does snowmelt affect flash floods?
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 flash flooding. Melting snowpack may also contribute to 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.
What is NSSL system?
NSSL has developed the state-of-the-science system for estimating heavy rainfall and resultant flash flooding. The Multi-Radar Multi-Sensor system provides rainfall rate estimates across the U.S. every two minutes, and these are input to a suite of hydrologic models and forecasting tools within the Flooded Locations and Simulated Hydrographs (FLASH) system.
What causes flash floods?
Flash floods occur when heavy rainfall exceeds the ability of the ground to absorb it. They also occur when water fills normally dry creeks or streams or enough water accumulates for streams to overtop their banks, causing rapid rises of water in a short amount of time.
Why do rivers and streams jam in winter?
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.
Why are floods the most common natural disaster?
Flash floods occur when heavy rainfall exceeds the ability of the ground to absorb it.
Why are levees important?
Areas near rivers are at risk from 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 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.
What is the term for the overflowing of water onto land that is normally dry?
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 quickly, or when dams or levees break. Damaging flooding may happen with only a few inches of water, or it may cover a house to the rooftop.
How do flash floods occur?
Several factors contribute to flash flooding. The two key elements are rainfall intensity and duration. Intensity is the rate of rainfall, and duration is how long the rain lasts. Topography, soil conditions, and ground cover also play an important role.
What causes flash flooding?
Most flash flooding is caused by slow-moving thunderstorms, thunderstorms repeatedly moving over the same area, or heavy rains from hurricanes and tropical storms.
How high can a flash flood be?
Flash floods can roll boulders, tear out trees, destroy buildings and bridges, and scour out new channels. Rapidly rising water can reach heights of 30 feet or more. Furthermore, flash flood-producing rains can also trigger catastrophic mud slides.
Why do rivers flood?
Some floods occur seasonally when winter or spring rains, coupled with melting snows, fill river basins with too much water, too quickly. Torrential rains from decaying hurricanes or tropical systems can also produce river flooding.
What is an arroyo in urban flooding?
FLASH FLOODING IN ARROYOS/WASHES. An arroyo is a water-carved gully or norma lly dry creek bed. Arroyos can fill with fast-moving water very quickly.
How to find out if your area is flood risk?
Know your area's flood risk. For information, call your local National Weather Service office, Red Cross chapter, or local emergency management agency. Check your homeowner's or renter's insurance. Homeowners' policies do not cover flooding. Contact your insurance agent to find out how to get flood insurance.
How much water does a car carry?
For each foot the water rises up the side of the car, the car displaces 1,500 lbs. of water. In effect, the car weighs 1,500 lbs. less for each foot the water rises. Two feet of water will carry away most automobiles.