Overview

A flood is an overflow of water that causes a usually dry area of land to be submerged.  Floods cause property damage, when they penetrate buildings or structures, or damage equipment or property that is in the path of the flooding (ie. Roads, landscaping, cars, etc). 

There are essentially 5 recognized types of floods: Flash floods, Coastal floods, Urban flood, River (Fluvial) floods, and ponding (Pluvial) floods.

Flash Flood

Flash floods occur when excessive rainfall, over a short period of time, results in a rapid rise in the water height of a stream, dry riverbed, roads, canyon or channel.  Water typically overflows or overtakes land that is normally dry and exceeds the ability of the ground to absorb it.  This phenomenon can happen over the course of minutes or a few hours of excessive rainfall.  They can occur even with no rainfall, from an event further such as the failure of a dam, the release of an ice jam, or the freeing of debris.  Rainfall somewhere else can result in a rapid rise of river or channel levels downstream.

Flash floods often carry away trees and objects along a flowing river and these floods can destroy buildings, roads and other infrastructure.   As such, this type of flooding is one of the most dangerous types of flooding as it combines the destructive force of a flood with a strong velocity of water flow. 

River Flooding (Fluvial)

River flooding typically occurs when water level in a river or stream rises and overflows onto the surrounding banks, shores, and neighboring land.   The water level rise can be as a result of excessive rain, ice melt, or an excessive release of water (from a dam) upstream.    When  more water enters into the system than is able to be drained out into lakes, downstream, or otherwise, this results in rising water levels and ultimately to flooding. 

Recent wildfires make surrounding areas more prone to river flooding as the land becomes less absorbent, resulting in a greater flow of water down to streams and rivers.  

Coastal Flooding

Coastal floods typically occur when waves, tides a, storm surge, heavy rainfall, or a combination of all of all factors, results in flooding.  Coastal areas can also overflow due to overflowing rivers or streams, but typically coastal floods are considered to be as a result of extremely high tides or storm surges that cause seawater to spill into low lying area, until the tide recedes.   

Coastal flooding events have steadily increased since the 1950s, when flood events began to be tracked, resulting in more frequent coastal flooding days.  Rising sea levels, increases in severe storms and storm surges, and changes in precipitation patterns mean that the frequency of coastal flooding is likely to increase. 

Urban Floods

Urban flooding is flooding that occurs after periods of extended, high intensity rainfall, river overflow, and storm surge in heavily populated areas. Urban area flooding happens with the accumulation of floodwaters that result when storm water exceeds the capacity of a drainage system to carry it away.

Ponding (Pluvial Floods)

Urban flooding, can often cause pluvial floods, as water from impervious surfaces flows into rising rivers, lakes, or canals. 

Typically, Pluvial flooding is somewhat more gradual that flash floods or Urban flooding, allowing somewhat more time to prepare surrounding areas against flood damage.