Flooding is one of the costliest natural disasters.
No matter where you live, chances are that you are vulnerable to flooding. A flood is a dangerous and frequent disaster all over the world. Floods happen when water goes places that it shouldn't, putting lives and property at risk. There are many varieties of flood events. Some occur with speed and violence, while others build slowly over the course of days. A flood may persist for weeks or be over in minutes. There are as many causes of flooding as there are types of flood. In any case, flooding should never be taken lightly.
Rain
Heavy rain can lead to flooding almost anywhere.
The most obvious cause of flooding is heavy rain falling over the same area for a long time. If the capacity of the drainage system is exceeded by the rainfall, then flooding is the result. The first waterways to be affected are small creeks and streams. In urban areas, streets may fill with water. Runoff eventually causes larger rivers to swell and leave their channels. Heavy rain also saturates the ground, making flooding even more of a threat if another storm system follows close behind.
Levee and Dam Failure
A secondary effect of prolonged heavy rain is dam and levee failure. Poorly built earthen dams can only tolerate so much pressure before giving way. Johnstown, Pennsylvania, experienced this in 1889 when a dam failed and killed more than 2,000 people. Levees are designed to control river and coastal flooding, but sometimes they also fail. Hurricane Katrina hit New Orleans in 2005 and demonstrated that no levee system is perfect. Water only needs one point of weakness in a dam or levee, and it can scour away the rest of the structure.
Snow-Melt
Rocky Mountain watersheds experience snow-melt flooding each spring.
In the mountains and colder climates, winter precipitation falls entirely in the form of snow. Rising temperatures in spring release all of this stored water into the creeks and streams. Often, the ground can't absorb the runoff because it's still frozen. If the melting is accompanied by heavy rain, then the flooding problems are compounded. Two factors that influence snow-melt flooding are how much liquid is stored in the snow and ice, and how fast the temperatures rise above freezing.
Ice Jam
In northern areas, wintertime leads to frozen rivers and lakes. As the spring thaw and sunshine warm things up, ice on the rivers breaks apart and moves downstream. If these ice chunks get clogged and restrict the flow of water, then the result is an ice-jam flood. Water quickly surges onto land, sometimes with little warning. Most areas that experience them are well prepared for ice-jam floods. However, the damage to bridges and erosion of the river channel can still be costly to repair.
Tsunami
A tsunami wave affects only the oceanfront coastline, but its human and economic costs can be staggering. An undersea earthquake or landslide is sufficient to cause a large and powerful tsunami wave. The shifting of land under water generates a pulse of energy that travels out in all directions. As the energy nears the coast, the water becomes shallower. The water first recedes, but then returns with amazing speed and force. A very large tsunami like the one that struck Japan in 2011 can travel for miles inland.
Lahar and Outburst
Lahar flooding can move enormous amounts of debirs.
A lahar is a sudden surge of glacial water and debris. It is typically caused by volcanic activity underneath of the ice. Lahars race downhill at great speed, carrying tons of mud and debris. Some of these flows have the consistency of wet cement, crushing anything in their path. A similar event called an outburst flood happens when a glacial lake is released from its basin. This release is unpredictable, but is often caused by an earthquake or landslide.
Tags: creeks streams, earthquake landslide, heavy rain, snow-melt flooding, then flooding, There many, tsunami wave