Floods are natural disasters that occur when water overflows from rivers, lakes or oceans and covers land that is usually dry. Floods can be caused by heavy rainfall, melting snow, hurricanes or storm surges. There are several types of floods, each with unique characteristics and causes, including:
Flash floods
Flash floods are the most dangerous type of flood because they can occur without warning and with great force. They are usually caused by heavy rainfall, which can quickly turn a dry stream bed or river into a raging torrent. Flash floods can also be caused by dam failures, ice jams, or sudden releases of water from reservoirs. They are characterized by rapidly rising water levels and can occur in any region, including urban areas.
River floods
These occur when water levels in rivers, creeks, and streams rise above their banks. This can be caused by heavy rainfall or snowmelt, lasting for several days or weeks. River floods can affect large areas and can cause significant damage to property and infrastructure. They are more common in areas with low-lying land and inadequate drainage systems.
Coastal floods
When water from the ocean or other bodies of water rises above its normal level and inundates coastal areas, it can cause coastal flooding. These are usually caused by storm surges, which are high waves that are pushed ashore by strong winds during hurricanes or other severe storms. Coastal floods can cause significant damage to buildings, infrastructure, and coastal ecosystems.
Urban floods
Urban floods occur when heavy rainfall or inadequate drainage systems cause water to accumulate on roads, sidewalks, and other urban surfaces. These types of floods are more common in urban areas where there is little natural vegetation to absorb rainfall. Urban floods can cause traffic disruptions, property damage, and health hazards, such as the spread of water-borne diseases.
Catastrophic floods
These are rare, but they can cause widespread devastation when they occur. Catastrophic floods are usually caused by a combination of factors, such as heavy rainfall, snowmelt, and dam failures. They can affect large areas, including entire cities or regions, and can cause significant loss of life and property damage.
10-year flood
This high streamflow that overflows the natural or artificial banks of a waterway has a 10 percent chance of occurring in any given year.
100-year flood
This term, which is sometimes labeled a “base flood” is used to define a flood that statistically has a 1-percent chance of occurring in any given year. A 100-year flood is less frequent than a 10-year flood, but it is deeper and far more destructive. The probability of a flood is not affected by the occurrence of past floods. Because each flood event is independent of previous ones, it is possible to have two 100-year floods very close in years or even in consecutive years.
