What Constitutes a Flood?

The simple definition of what constitutes a flood is excess water temporarily inundating a normally dry area, according to the Federal Emergency Management Agency. Overflow sources range from inadequate drainage to mudslides and tidal waves. FEMA advises that floods usually fit into three categories: riverine, coastal or shallow.

Extent

• The National Flood Insurance Program clarifies that a flood designation requires the affected area to measure two or more acres, or cover two or more properties, as long as one of the properties belongs to the flood insurance policyholder.

Riverine

• Riverine flooding occurs when the rain fills the river basin faster than the drainage systems can carry it away. The river level rises until it overflows the banks. Erosion causes the breakdown of natural protective barriers as well, allowing the water to spill into surrounding areas.

Coastal

• According to the South Carolina Emergency Management Division, coastal flooding is usually the result of a tropical storm or hurricane. Storm conditions sometimes precipitate a tsunami, which contributes to flooding when it hits shore.

Shallow

• Shallow flooding occurs when groundwater and sewage seeps into underground areas such as basements, regularly or constantly. These floods can also affect low-lying roadways, reports the Suffolk County government on Long Island, New York.