Flooding represents a significant natural hazard, and understanding its various forms is important for communities and individuals. One specific type, known as an areal flood, involves the widespread accumulation of water over broad geographical expanses, differing from other flood categories by its characteristic spread across land rather than being confined to a river channel or coastline.
What Defines an Areal Flood
An areal flood is characterized by the inundation of water over a wide, generally flat area, distinguishing it from flooding that is restricted to a specific watercourse. The term “areal” itself signifies “pertaining to an area,” indicating its expansive geographical coverage. This type of flooding occurs when excessive precipitation accumulates on the land, as the ground cannot absorb the water or existing drainage systems cannot effectively carry it away.
Unlike rapid onset floods, an areal flood develops gradually, over several hours or even days, as water slowly rises and spreads across low-lying, flood-prone regions. This slow progression allows water to pond or build up in small creeks and streams, eventually overflowing their banks and covering adjacent land. The National Weather Service issues areal flood warnings when such widespread, gradual water accumulation is expected, signifying a threat to life and property over a broad region.
Factors Leading to Areal Flooding
A primary cause of areal flooding is prolonged or persistent moderate-to-heavy rainfall over an extended duration. This sustained precipitation often exceeds the capacity of the ground to absorb additional moisture. When soil becomes saturated from previous rainfall, it loses its ability to infiltrate more water, forcing subsequent rain to become surface runoff.
Poor or overwhelmed drainage systems also contribute significantly to areal flooding, especially in developed areas. Urban infrastructure, composed of impervious surfaces like concrete and asphalt, prevents natural absorption, directing large volumes of water into storm drains that may become overloaded. Flat topography allows water to spread across the landscape rather than flowing quickly into defined channels, promoting accumulation and slow drainage. Rapid snowmelt, particularly when combined with rainfall, can further saturate the ground and contribute to increased surface water accumulation, exacerbating the conditions for areal flooding.
Distinguishing Areal Floods
Areal floods differ from riverine floods, which occur when water levels in rivers or streams rise above their banks, overflowing into adjacent floodplains. Riverine flooding is directly tied to the capacity of a specific water channel, whereas areal floods involve water spreading across land independent of a main river’s overflow.
Flash floods, in contrast to areal floods, are characterized by their rapid onset and intense, localized surge of water, occurring within six hours of heavy rainfall. They result from intense, short bursts of rain and can be dangerous due to their sudden nature and destructive force, frequently occurring in canyons, urban areas with poor drainage, or along small streams that rise quickly. Areal floods, by comparison, develop more slowly and cover a larger geographical footprint.
Coastal flooding is driven by oceanic factors such as storm surge, high tides, or sea level rise, which cause the inundation of land areas along the coast. Unlike precipitation-driven areal floods that occur inland over broad areas, coastal flooding results from the ocean encroaching upon land, often exacerbated by strong onshore winds.