For ease of classification, different types of floods are often identified by their cause. Floods occur for many reasons, from the torrential but predictable rains of monsoon season to the sudden massive water wall of a tsunami. Unfortunately, almost any body of water has the potential to flood. Heavy rains can overfill rivers and streams, leading to flash flooding. Coastal communities can find themselves swamped by ocean waters from hurricanes.
Flash flooding is sudden, severe, and often potentially life threatening. When sudden rains or spring snowfalls occur in higher elevations, the water runs down, gathering speed as it travels. Often, smaller flows of water combine with larger flows repeatedly until they converge on a river or streambed. By then, the massive amount of water is flowing so quickly that it can destroy almost anything in its path. Trees, cars, and sometimes homes can be swept away by the rushing water.
Like flash floods, monsoon floods are often caused by large amounts of rain falling in a short period of time. Flash floods, however, are generally localized. Monsoon rains tend to affect larger areas, causing more damage and higher loss of life. Monsoon rainfall is generally repeated several times within a season. Each rainfall poses more potential for flooding as large amounts of rain are dumped on ground that is saturated and rivers that are already overflowing their banks.
Those in warm-weather coastal areas can find themselves at the mercy of flooding caused by hurricanes. Along with the drenching rains that are typical with hurricanes, residents often have to deal with seawater driven into their communities by gale-force winds. Compounding the risk, many ocean-side settlements are built on land that is near to or in part below sea level. These communities often rely heavily on levees to keep ocean waters at bay.
Unlike most types of floods, tsunamis are largely unaffected by weather patterns. These gigantic waves are instead caused by earthquakes or volcanic eruptions in or near large bodies of water. Tsunamis are among the most catastrophic types of floods both in property damage and loss of life.
One of the deadliest tsunamis on record is the 2004 Indian Ocean Tsunami. This tsunami was started by an earthquake in the Indian Ocean and caused massive damage in several coastal countries including Indonesia, Sri Lanka, India, and Thailand. The death toll was estimated at over 220,000.
Sometimes, human construction and inclement weather combine to form deadly floods. Such a disaster occurred on 8 August 1975 in the Henan province of China. On that day, typhoon-force rains overwhelmed the Banquito Dam on the Ru River. The resulting wall of water and debris swept down the Ru at speeds in excess of 30 miles (approximately 50 kilometers) per hour. It was estimated that 26,000 people died in the flood, including the entire 9,600 population of the town of Daowencheng.