Flooding is a natural hazard that impacts communities across the United States, but the risk and type of flood vary significantly by region and local geography. Flooding occurs when water inundates normally dry land, driven by factors ranging from intense local rainfall to massive storm systems originating far out at sea. Understanding the specific mechanisms and locations where water commonly overwhelms systems is important for assessing risk.
Coastal and Storm Surge Vulnerability
The Gulf and Atlantic Coasts face the nation’s most persistent threat from coastal inundation, primarily driven by tropical storms and hurricanes. Storm surge, an abnormal rise of water generated by a storm over and above the predicted astronomical tide, is the primary destructive force in these areas. This surge is amplified by the wide, shallow continental shelf extending from the Gulf of Mexico and the Southeast Atlantic coastline. This allows water to pile up and push far inland, especially in low-lying states like Louisiana and Florida.
Specific regions, including the Gulf Coast of Texas and Louisiana, and the Mid-Atlantic states of New Jersey and North Carolina, are repeatedly exposed to these events. Cities like New Orleans and Miami Beach are highly vulnerable due to their low elevation; parts of New Orleans lie below sea level and are protected by extensive levee systems. Beyond major storm events, many coastal areas also experience frequent “nuisance” or high-tide flooding, which is increasing in frequency due to rising sea levels. This type of flooding often disrupts daily life by inundating low-lying roads and overtaxing storm drainage systems.
Major River Basin Flood Risks
Inland areas of the US are most susceptible to large-scale, prolonged flooding associated with major river systems. This type of flooding, often called riverine or fluvial flooding, develops slowly over days or weeks. The cause is usually sustained, heavy rainfall over a large area, often combined with rapid snowmelt in the spring.
The entire Mississippi River Basin, which drains water from over 41% of the contiguous United States, represents the country’s most expansive area of risk. States in the Midwest and Central US, such as Missouri, Illinois, and Kentucky, are frequently impacted when the Mississippi, Missouri, and Ohio Rivers overflow their banks. The Ohio River Basin, in particular, has seen increasing trends in the frequency of large floods, often caused by complex systems of heavy rainfall. These floods cover vast agricultural and populated floodplains, and their severity can be compounded by the failure of aging levee systems designed to protect communities.
Flash Flood Hotspots
Flash floods, defined by their rapid onset, are the most dangerous type of flood in terms of fatalities, striking within minutes to a few hours of the causative event. These floods are common in regions with specific topographical or climatic features that prevent water from soaking into the ground quickly.
The arid and semi-arid Southwest, encompassing states like Arizona, Utah, and Nevada, is a major hotspot. In these desert environments, the ground is often hardened and dry, leading to limited water absorption and extremely rapid runoff into normally dry washes or canyons. Mountainous regions, including the Appalachians and the Rockies, are also prone to flash flooding because steep slopes accelerate water flow, quickly overwhelming stream capacity. Furthermore, urban areas across the country, such as parts of New York, Chicago, and Houston, are susceptible to localized flash flooding. The widespread presence of impervious surfaces like concrete and asphalt prevents rainfall infiltration, quickly channeling massive amounts of water into streets and drainage systems that cannot cope with the volume.