Depending on their type, floods can develop very slowly over time after extensive rains or in just a few minutes, very quickly, without any sign of rain.
There are many types and ways floods can occur, including, due to overflowing rivers, due to extreme coastal events, by natural or artificial ground saturation from excess rainfall, or by catastrophic failure in infrastructure.
Flash floods are extreme versions of a river flooding event. They can occur very quickly, often without warning and with little or no excessive rainfall. Flash floods are the result of a river blockage either natural or artificial (such as a landslide, glacier, or dam) giving way and releasing a massive amount of built up water.
Areal or urban flooding occurs when low-lying impenetrable ground becomes saturated as rainfall cannot run off as quickly as the accumulation of water. For example, on natural drought hardened or frozen farmland, or on concrete paving.
Floods can damage bridges, roads and other transport links. Infrastructure such as buildings, cars and houses can be left saturated or completely taken by the waters. While sewage systems and power grids can be destroyed.
After floodwaters recede, land can be contaminated with hazardous material, such as building debris, fuel and untreated sewage. Residents are often left without power or clean drinking water which can lead to outbreaks of diseases.