Floods that occur in the United States and abroad can be classified in multiple ways. There's no steadfast rule for categorizing flooding along a floodplain or after a tropical cyclone. Instead, broad types of flood labels are applied to any type of water inundation that results in damages. Flooding is one of the most dangerous types of all natural disasters.
Floods can be most broadly classified as either river floods or flash floods. The main difference is in the onset of the flooding. With flash floods, there is often little warning that flooding will occur. With river flooding, communities can prepare as a river nears its flood stage.
Flash floods are usually the most lethal. Heavy downpours, often in mountainous highlands, can lead to surges of water that turn dry river beds or flood plains into raging torrents within minutes.
Local communities usually have little time to flee to higher land, and homes and other property in the water's path can be totally destroyed. Vehicles crossing roadways that are dry or barely wet in one moment can be swept away in the next. When roads and railways are made impassable, delivery of aid can become much more difficult.
Slow Onset Floods
Slow onset floods, such as those that hit Bangladesh almost every year, can also be lethal but they tend to give people much more time to move to higher ground. These floods are a result of surface water runoff.
Flash floods can also be the result of surface water runoff, but the terrain is a larger factor in the severity of the flood. They often occur when the ground is already saturated and simply cannot absorb any more water.
When deaths occur during slow onset floods, they are much more likely to come about due to disease, malnutrition or snakebites. Floods in China displaced tens of thousands of snakes into neighboring areas in 2007, increasing the risk of attacks. Slower floods are also less likely to sweep away property, although it may still be damaged or destroyed. Areas are likely to remain under water for months at a time.
Storms, tropical cyclones, and other maritime extreme weather can also produce deadly storm surges, as happened in New Orleans in 2005 after Hurricane Katrina, Cyclone Sidr in November 2007, and Cyclone Nargis in Myanmar in May 2008. These are most prevalent and dangerous along the coasts and near large bodies of water.
Detailed Flood Types
There are numerous other ways to classify floods. Many types of floods are a result of the location of the rising waters or other environmental factors. FEMA has a broad classification of flood types as follows:
- Riverine floods
- Urban floods
- Ground failures, such as dam breaks
- Fluctuating lake levels
- Coastal flooding and erosion
In addition, flooding can result from ice jams, mine accidents, and tsunamis. Remember that there are no steadfast rules for determining exactly what type of flood may be associated with any given area. Getting flood insurance and following the guidelines for flood safety are critical to keeping yourself, your family, and your property safe during a flood event.