Nationwide, only 20% of American homes at risk for floods are actually covered by flood insurance. Most private insurers do not insure against the peril of flood due to the prevalence of adverse selection, which is the purchase of insurance by persons most affected by the specific peril of flood. 

Flooding is defined by the National Flood Insurance Program as a general and temporary condition of partial or complete inundation of two or more acres of normally dry land area or two or more properties (at least one of which is your property) from: Overflow of inland waters, unusual and rapid accumulation or runoff of surface waters from any source, and mudflows

This can be brought on by landslides, a hurricane, earthquakes, or other natural disasters that influence flooding, but while a homeowner may, for example, have earthquake coverage, that coverage may not cover floods as a result of earthquakes.