Tropical cyclones, known as hurricanes or typhoons, generate many hazards, but the most significant threat to life and property near coastal areas is storm surge. This abnormal rise of water rapidly floods low-lying regions. Understanding how this destructive wall of water forms is the first step in mitigating its risk. The sheer force and volume of the water moving inland cause immense erosion and structural damage, often extending miles from the shoreline.
Defining Storm Surge and Storm Tide
Storm surge is the abnormal rise of water generated by a storm, measured as the height of the water above the predicted astronomical tide. This measurement isolates the water rise caused solely by the meteorological forces of the cyclone, such as wind and pressure. It is a temporary, localized increase in sea level that occurs as the storm approaches and makes landfall.
It is important to distinguish this from the term “storm tide,” which represents the total observed sea level during the event. Storm tide is a composite measurement, calculated by adding the storm surge to the normal astronomical tide level at that specific time. For example, if a 10-foot storm surge coincides with a 2-foot high tide, the resulting storm tide is 12 feet above the normal tidal datum. This combined water level determines the extent of coastal inundation and widespread flooding.
The Mechanics of Storm Surge Formation
Storm surge is primarily driven by two physical forces working in tandem as a tropical cyclone moves across the ocean surface. The dominant factor is the sheer stress exerted by the storm’s powerful, cyclonically rotating winds. These sustained, intense winds act like a giant plow, pushing a vast volume of ocean water directly toward the coastline and piling it up against the shore.
This continuous push of water creates a dome or mound that travels with the storm system. The second force is the pressure effect, often called the inverted barometer effect. The extremely low atmospheric pressure at the center of a tropical cyclone lifts the sea surface beneath it.
For every drop of one millibar in atmospheric pressure, the sea level rises approximately one centimeter. While this pressure-driven rise accounts for only about 5% of the total storm surge height, it contributes to the overall mound of water that is then driven inland by the wind. When this dome of water reaches shallow coastal waters, its forward momentum forces it to build rapidly in height.
Factors Influencing Surge Severity
The ultimate height and reach of a storm surge are influenced by the local geography and the storm’s characteristics as it nears land. A primary geographic factor is the bathymetry, or the depth and slope of the continental shelf offshore. A wide, shallow, and gently sloping continental shelf is more conducive to producing a high surge than a narrow, steeply sloped shelf.
As the storm’s water mound moves over this shallow terrain, friction with the seafloor causes the water to slow down and stack up, amplifying the surge height before it hits the coast. Coastal geometry also plays a significant part in determining local severity. Bays, estuaries, and inlets can create a funneling effect, constricting the incoming water and forcing it to rise higher as it is squeezed into a smaller area.
The storm’s movement parameters are also relevant, particularly its speed and angle of approach. A storm that moves slowly or one that approaches the coast nearly perpendicular to the shoreline tends to generate a higher surge. This slow, direct movement allows the winds to push water toward the coast for a longer duration, maximizing the piling-up effect and coastal inundation.
Forecasting and Safety Measures
Accurately predicting storm surge is complex due to the many variables involved, but forecasters rely on sophisticated computational tools. The Sea, Lake, and Overland Surges from Hurricanes (SLOSH) model, developed by the National Oceanic and Atmospheric Administration (NOAA), is the primary tool used for this purpose. This model uses a storm’s predicted intensity, size, track, and speed to simulate thousands of potential scenarios for a given area.
The SLOSH model incorporates the unique bathymetry and topography of specific coastal regions to produce detailed inundation maps. These maps provide emergency managers with projections of how high the water could rise above ground level, which is important for planning evacuation routes and determining which zones are at risk.
Because storm surge is the most dangerous aspect of a tropical cyclone, the primary safety measure is mandatory evacuation of threatened coastal zones. For the public, the general safety directive during a hurricane threat is straightforward: run from the water, hide from the wind.