Flooding is the overflow of a large amount of water onto land that is normally dry. The sources of this excess water are far more diverse than many people realize, meaning floods can happen anywhere. While proximity to a major water body significantly increases the risk, even high-elevation or inland areas face growing threats from various mechanisms. The traditional understanding of flood risk is rapidly expanding due to changes in weather patterns and human development.
The Geography of Traditional Floodplains
The most commonly understood flood risk is associated with areas designated as floodplains, which are naturally low-lying zones near rivers, lakes, and coasts. Riverine, or fluvial, flooding occurs when prolonged heavy rainfall or significant snowmelt causes water volume to exceed the natural capacity of a river channel. This excess water spills out over the riverbanks, inundating the surrounding floodplain.
Coastal flooding is driven by different forces, primarily high astronomical tides, storm surges, and the long-term effect of sea-level rise. Storm surges, often associated with tropical cyclones, push large volumes of ocean water inland, overwhelming coastal defenses and low-lying communities. The combination of high tide and a storm surge significantly elevates the total water level, threatening areas that might otherwise seem protected.
Localized Flooding from Surface Water Runoff
Flooding can occur far from any river or coast due to a phenomenon called surface water runoff, also known as pluvial flooding. This localized hazard happens when intense rainfall overwhelms the ground’s ability to absorb water or the local drainage infrastructure’s capacity to transport it away. The resulting water pools in streets, basements, and low spots, even on relatively high ground.
A primary contributor to this risk is the prevalence of impervious surfaces, such as paved roads, sidewalks, and building rooftops, which are common in urban areas. These surfaces prevent rainwater from soaking into the soil, forcing it to flow rapidly over the surface. When the rate of rainfall exceeds the capacity of storm drains, flash flooding can occur quickly, with little warning.
Structural and Seasonal Causes of Flooding
Some devastating floods involve the failure of human-made infrastructure designed to control water, rather than natural events alone. Dams and levees, built to contain water or protect adjacent land, can fail due to structural, hydraulic, or mechanical issues. A levee failure can cause catastrophic flooding in downstream or landward areas that rely on the structure for protection, releasing a massive volume of water almost instantaneously.
Seasonal changes also create widespread flood risk, particularly significant snowmelt runoff in northern and mountainous regions. Warmer temperatures can cause a rapid thaw of accumulated snowpack, releasing a huge quantity of water that can overwhelm river systems over vast areas. This slow-onset flooding is often compounded if the melt coincides with additional heavy rainfall, leading to major regional inundation.
How Development and Climate Change Expand Risk
Two major factors are consistently expanding the boundaries of flood risk, making the threat more universal and less predictable: urbanization and a changing climate. Urban development significantly increases impervious surface area, which exacerbates localized runoff and urban flooding. This change causes cities to shed water much faster than natural landscapes, increasing the volume and speed of water entering drainage systems and making them more likely to be overwhelmed.
Climate change intensifies the hydrologic cycle, as a warmer atmosphere holds more moisture and leads to more extreme precipitation events. These intense “rain bombs” deliver massive amounts of water in short periods, increasing the frequency of flash floods in both inland and coastal communities. Rising global sea levels also directly increase the baseline elevation of coastal waters, making storm surges and even normal high tides more likely to cause inundation in vulnerable areas.