Coastal flooding is a significant natural hazard that impacts shorelines and low-lying coastal communities globally. It involves the temporary inundation of land by seawater, causing widespread disruption to infrastructure and ecosystems. Understanding the mechanisms behind this increase in ocean water level is necessary for coastal resilience and disaster preparedness. This analysis explores what defines coastal flooding and the physical forces that drive these destructive events.
Defining Coastal Flooding
Coastal flooding is defined as the inundation of dry, low-lying land areas bordering the ocean, caused by seawater rising above the normal predicted astronomical tide level. This is distinct from riverine flooding or flash flooding. The source of the water is always the ocean, and the location is restricted to the immediate coastline, estuaries, and tidal waterways.
The severity of coastal inundation depends heavily on the local topography, including the elevation of the land and the shape of the coastline. Low-lying deltas and coastal plains are particularly susceptible to widespread flooding when water levels temporarily spike. Although coastal flooding is a natural process that affects environments like salt marshes, human development in these vulnerable areas transforms the event into a disaster that threatens property and life.
The Primary Mechanism: Storm Surge
The greatest cause of severe coastal flooding is the storm surge—the abnormal rise of water generated by a severe weather system, such as a hurricane or an extratropical cyclone. This rise in sea level is superimposed upon the normal astronomical tide, creating the combined height known as the storm tide. Two main physical forces within the storm work together to generate this dangerous mound of water.
The first and most dominant factor is the force of high-speed, sustained winds pushing water toward the shore, known as wind stress or wind setup. Powerful onshore winds drag the surface water mass toward the coast. This continuous piling up of water against the shoreline can create a massive dome of water, especially in shallow coastal areas where the force is amplified by the sloping seabed.
The second factor is the inverse barometer effect, caused by the extreme low atmospheric pressure at the center of the storm. Under normal conditions, the weight of the atmosphere presses down on the ocean surface. However, a deep low-pressure system releases some of that pressure, allowing the sea surface to bulge upward beneath the storm’s eye.
For every drop of one millibar in atmospheric pressure below the norm, the sea level rises approximately one centimeter. For a major storm with a pressure drop of 100 millibars, this can account for a one-meter rise in water level. When the wind stress and the inverse barometer effect combine, particularly at the time of high astronomical tide, the resulting storm surge becomes highly destructive, pushing seawater far inland.
Influence of Tides and Persistent Weather
Coastal flooding also results from the combination of normal astronomical tides and persistent weather patterns. Astronomical tides, which are the regular, predictable rise and fall of sea level caused by the gravitational pull of the moon and sun, establish the baseline water level. Flooding is significantly more likely when a weather event coincides with a period of high tide, such as a spring tide or a king tide.
Moderate, non-cyclonic weather systems can contribute to temporary sea level rise through persistent onshore winds. Even without a major storm, winds blowing consistently toward the land can produce a measurable water setup similar to the wind stress component of a storm surge. This effect is often enough to cause minor or “nuisance” flooding in very low-lying areas, sometimes referred to as high tide flooding.
Another contributing factor is wave setup, which occurs when breaking waves transport water toward the shore and push the mean water level higher. As waves break in the shallow nearshore zone, their momentum forces an additional, localized rise in the water level on the coastline. This wave setup, combined with high tides and moderate onshore winds, elevates the total water level and increases the risk of inundation.
Long-Term Drivers of Increased Risk
Beyond the immediate meteorological and tidal triggers, long-term environmental changes are raising the risk of coastal flooding globally. The most significant driver is global mean sea level rise, which is primarily caused by two factors related to a warming climate. One factor is the thermal expansion of seawater as it warms, and the other is the addition of water from melting land-based ice sheets and glaciers.
This continuous rise in the baseline water level means that every flood-causing event, whether it is a storm surge or a high tide, starts from a higher elevation on the shore. Consequently, smaller storm surges that might have been harmless in the past can now breach coastal defenses and cause significant flooding. The permanently higher ocean surface allows the same weather events to cause more frequent and deeper inundation.
Local factors, such as land subsidence, further amplify this risk in specific regions. Subsidence is the sinking of land, which can be caused by natural geological processes or human activities like the extraction of groundwater. When the land sinks while the sea rises, the area experiences an accelerated rate of relative sea level rise, making it vulnerable to both acute and chronic flooding.