Floods, Tsunamis

Water covers 70% of the Earth's surface and is vital for human civilization. However, natural disasters involving water, such as floods and tsunamis, can devastate communities and cause environmental, financial, and human loss. Global studies play an increasingly important role in these events: For example, the 2004 Indian Ocean tsunami, which affected a dozen countries on two continents, provoked an international humanitarian response. Another example is worldwide climate change related to global warming, which has led to increased frequency and intensity of some natural disasters. This entry provides an overview of floods and tsunamis, including their characteristics and their impact on global society. There is also discussion of cooperative initiatives across countries to mitigate their negative impact, including the eventual goal of a global detection and warning system.

A tsunami is a series of large waves in the ocean or large body of water, caused by an earthquake, landslide, volcanic eruption, or other mass movement. Tsunamis are frequent in the Pacific Ocean because of tectonic plate activity. They begin when small waves on the ocean surface travel outward in all directions at the site of disturbance; these waves can gradually reach speeds of higher than 800 kilometers per hour, extending over hundreds of miles. Waves increase in height, sometimes 30 to 50 meters, as they approach shallow water and land; they hit the shore with immense energy and water volume, often devastating coastal regions. The first wave is followed by subsequent waves, over a period of several hours. Some waves travel over land, flooding buildings and homes, carrying debris such as boulders, buildings, trees, and vehicles. Most tsunami-related deaths are the result of drowning.

On December 26, 2004, a series of tsunamis occurred in the Indian Ocean, caused by an undersea earthquake near Indonesia. It was a global disaster that killed approximately 230,000 people and devastated communities in a dozen countries bordering the Indian Ocean. Indonesia, Sri Lanka, India, and Thailand were the worst affected areas. A global humanitarian response ensued, with aid agencies and international organizations involved in response efforts. Citizens, corporate donors, and agencies around the world donated more than US$7 billion in aid. In March 2011, an earthquake followed by a tsunami devastated the eastern coast of Japan, damaging a nuclear energy plant and creating a major ecological and social crisis.

The impact of tsunamis can be reduced by artificial barriers, channels redirecting water, floodgates, and trees and forests along coastlines. Tsunami warning systems can predict the occurrence of tsunamis and are based on detection of earthquakes, seismic waves, and sea level data. There are international and regional warning systems, coordinated by the United Nations, such as the Pacific Tsunami Warning System, Indian Ocean Tsunami Warning System, and the Caribbean Sea and Adjacent Regions Tsunami Warning System. The eventual goal is to form a global warning system, alongside other cooperative initiatives.

Floods are another phenomenon involving water that devastate human settlements across the globe. They occur when water accumulates during heavy rainfall, or when water level in a river or other water body overflows onto adjacent land. Riverine floods are common and are caused by either heavy rain or snow melt; obstructions in rivers, such as debris and landslides; or failure of flood control structures. Coastal and estuarine flooding is caused by severe weather conditions, such as sea storms, strong winds, tidal surges, and cyclones. Flash floods are periods of sudden, intense flooding, usually within several hours of rainfall; they have a high debris load and water velocity. The frequency and intensity of floods are expected to increase in many world regions as a result of global climate change, such as changing rainfall patterns and sea level.

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