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.
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.
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.
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.