Heavy rain falls for days and days, building up too much water for groundwater systems to handle. Slowly, the water level begins to rise, creeping higher and higher until it floods into your home, submerging your furniture and belongings. In extremely bad cases, flash floods, with their rushing torrents of muddy water, sweep away houses, cars, trees, and people with ease. How can we, as humans, protect ourselves against this phenomenon? How can we learn, recover, and rebuild? Explore this section to find the answers to these questions.
the legend
Most people have heard the Biblical story of Noah and the Ark. God became upset with the people of the world and sent a flood to wash away everything and start anew. Choosing a good man named Noah, God told him to build an ark and take aboard a male and female of every type of creature. After this was done, it rained for 40 days and 40 nights, so heavily that the entire earth was covered with water. When the rains finally stopped, Noah sent out a dove. After sending it out twice, the dove did not reappear, letting Noah know that the waters had finally receded enough for the bird to find land. God then said that he would never flood the earth again to destroy mankind. He told Noah to remember what he said every time he saw a rainbow. In fact, evidence of a massive flood 5,000 years ago has been found in Iraq, between the Tigris and Euphrates rivers. It spread over 30,000 square miles, drowning villages in river valleys. Many people believe this was “Noah’s flood.”
what ' s a flood ?
A flood is the submerging with water of a normally dry area. Floods are caused by many things, including rainstorms, slow water run-off, earthquakes, broken dams, underwater volcanic eruptions and tsunamis, or hurricanes. People need water to live. The oceans cover three-fourths of Earth, making it the only blue planet in our solar system that can sustain life. We drink it, we bathe in it, we swim in it. 65% of our bodies are composed of water. But when it floods, water has the ability to kill. In fact, most floods cause a lot of damage because they happen so unexpectedly.
flash floods
Flash floods are much more dangerous and flow much faster than regular floods. They result from tropical storms, dam failures, or excessive rain and snow. The Big Thompson River flows through the Big Thompson Canyon on its way down from the Rocky Mountains to the South Platte River. On July 31, 1976, the canyon was filled with vacationers in motels and campgrounds. As thunderstorms began to form late in the afternoon, no one was greatly worried. However, this line of storms began to weaken as it reached Big Thompson, leaving 12 miles of intense storms over the upper third of the canyon. It soon began to sprinkle, and then to pour. By 6:30 in the evening, rain crashed down from the sky and continued for nearly five hours. A state trooper claimed raindrops were half an inch wide and dropped straight down. At 8:00 p.m., news came that U.S. Highway 34 had washed out. When a trooper went to investigate, he realized that the situation was becoming extremely dangerous. He radioed for everyone to evacuate as quickly as possible. As his own patrol car began to wash away, he had to abandon his position and swim to higher ground. In some areas of the canyon, the river had risen more than 20 feet. The water tore down the banks, sweeping away everything in its path. Campers, tents, trailers, trucks, and cars were washed away. Buildings were torn apart, and many people disappeared. The narrow walls of big Thompson Canyon forced the river and rainwater to flow dangerously fast down to the sea. Because of the strength and speed of the water, this flash flood was extremely hazardous.
causes of floods
Tropical Storms
In 1969, a tropical storm caused extensive floods in Virginia. Hurricane Camille had spent most of its initial energy over the Gulf of Mexico. As it neared Virginia, it met a line of large thunderstorms coming down from the north. The warm, moist air carried by Hurricane Camille clashed with the cold northern storms, meeting over the Tye and Rockfish river valleys. Together, the two weather systems caused huge sheets of rainfall. In some areas, hard rain accumulated 31 inches in only six hours. The Rockfish River rose 30 feet. Rivers and creeks in the region’s valleys could not hold all the water, so it spilled over the banks and covered the lowlands. Water then cascaded into the valleys. Rain softened a large chunk of a nearby mountain, causing it to slide into the valley, carrying boulders, mud, and thousands of giant trees. When the landslide ended, farms were buried under 30 feet of soil and earth. More than 125 lives were lost in the flood, while many others were left without homes. Camille was an example of one of the most common causes of floods - heavy rains that come with tropical storms. Such storms form over the warm waters of the tropics, so they are full of moisture. When the right conditions form, bringing these giant storms toward land, many inches of rain usually fall. The heavy precipitation is too much for the streams and rivers to handle, causing water to overflow and produce inland floods. Many of these tropical storms form over the Gulf of Mexico.
Seasonal Flooding
In May 1990, heavy rainstorms caused floods in Texas, flooding more than 200 square miles along the Trinity River. A record 100,800 cubic feet of water passed through Lake Livingston Dam, destroying crops far east. More than 700,000 acres of Louisiana farmland were covered with water. According to some scientists, the record floods of 1990 were caused by global warming, the theory that the earth’s atmosphere traps heat near the earth, slowly warming the earth. This greenhouse effect may have heated the water in the Gulf of Mexico, causing it to evaporate faster. With more water vapor in the lower atmosphere, small storms escalated into large systems with lots of moisture. These storms moved over the southern United States and released a torrent of rain that led to massive flooding. If the global warming theory is correct, sea levels will rise three to five feet in 60 years. Coastal areas may be partially submerged underwater or easily flooded. Other seasonal weather conditions can also cause floods. In September of 1982, large amounts of rain fell in Utah, followed by heavy winter snows. In May, an unexpected heat wave melted the snow, causing water to cascade down the mountain slopes. The water raced into Salt Lake City and through much of the plains. The next winter brought another heavy snowfall. Now that they were prepared for what was to come, residents filled over one million sandbags and placed them on riverbanks to keep them from overflowing in the spring. In May, a heavy thunderstorm hit, starting a new series of floods. The water swelled the Great Salt Lake, bursting through dikes and flooding wetlands, marshes, roads, parks, and homes. Fortunately, very few people or livestock died because of the warning people had beforehand. The flood had also built slowly, giving people lots of time to prepare.
Coastal Flooding
The earth has seven oceans that cover almost three-fourths of its surface. Naturally, wind and other events caused ocean water to sometimes overflow. When this happens, flooding on the shores occurs. Ocean storms can dump lots of water on a coast, raising the sea level in that area. These are known as storm surges, and cause coastal flooding. Coastal flooding usually occurs as a result of severe storms, either tropical or winter storms. Ocean waves intensify on the open ocean, and these storms make surface water much choppier and fierce than normal. Raging winds can create huge waves that crash on unprotected beaches. In the winter of 1978, the northeastern United States coast saw severe flooding that resulted from high winds, high tides, and a storm surge. High winds coincided with unusually high waves. South of Boston, Massachusetts, waves came over a seawall built to protect houses in the region. These wild waves destroyed the houses into rubble. Low-lying towns were also flooded with ocean water and ice.
Tsunamis Coastal flooding can also be caused by long, low sea waves caused by volcanoes, landslides, earthquakes, or explosions. These waves are tsunamis. These giant tidal waves are difficult to detect on the open sea, so seismologists must keep track of seafloor movements that warn of possible tsunamis. These waves are extremely dangerous because of their high speeds. Deeper water means faster-traveling waves. When the sea floor is several miles below, waves can travel more than 600 miles an hour. As they near shallow water, they slow down, but build in height. Some tsunamis can be 50 to 100 feet high when they hit shore. Tsunamis can travel thousands of miles without weakening, their initial energy being transferred along the open ocean to coastlines. They can pass through islands or coral reefs without stopping, but usually die or bounce back after hitting a large land mass.
Earthen dams and levees
People living in valleys or on flood plains next to earthen dams or levees are susceptible to sudden flooding. Earthen dams are usually built across rivers to dam them up. Levees, on the other hand, are made alongside rivers to keep them from spilling over. However, both can fail in the event of an earthquake, internal erosion, poor engineering and construction, or avalanches. The most common cause of failure, however, is too much rainfall. When a region experiences heavy rainfall, the water behind a dam or levee builds up and flows over the top. This will wash away the upper portion of the barrier and carves out deep grooves. The dam or levee will eventually weaken so that the water behind it destroys the structure, suddenly releasing tons of water.
concrete dams Concrete dams are often made to seal off deep river valleys and create hydroelectric power. Water will build up behind the barriers, creating artificial lakes known as reservoirs. The deep water created by their construction improves fishing and boating in the area. However, the real purpose is to have the water turn giant turbines and create inexpensive electrical power. These dams also help control floods. They are built across rivers that have a history of flooding, holding back waters in a man-made lake. The water is then released through floodgates at a regular pace so it doesn’t overflow the rivers below the dams. The idea is for the lakes behind the dams to never overflow. However, this does not always happen. The Vaiont Dam was a structure built in the Italian Alps in northeastern Italy. A concrete barrier 873 feet above the river valley below, it kept back water from a number of small villages that dotted the waterway. In the fall of 1963, the area experienced much rainfall. The extra water eroded earth and rocks on the mountain slopes surrounding the reservoir. On October 9 of that year, part of 6,000-foot-high Mount Toc fell into the lake. A downpouring of earth, mud, rocks, and trees crashed into the water, causing tons of water to overflow the dam into the valley below. The next morning was a scene of chaos. There were almost no buildings left standing in the village of Langarone, only piles of stones. Most of the populations of other small villages had perished. Eventually, a study showed that the dam was situated in a poor location. If the area had been investigated beforehand, it would have been found that the area experiences frequent earth movements. Once the inquiry ended, the dam was shut down. The nearby villages have since then been rebuilt.
Souces: Materials of this page is obtained from thinkquest.org
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