Storm surges are high walls of water that cause significant inland flooding, especially along coastal regions. Typically, storm surges are the result of winds produced by tropical cyclones, severe weather systems characterized by heavy rains and exceptionally powerful winds. Some storm surges are also influenced by low pressure weather systems. Included in the category of tropical cyclones are hurricanes, typhoons, cyclones, tropical storms and tropical depressions. Another definition of a storm surge is simply water that is pushed toward the shore by the force of the winds swirling around the storm. This advancing surge combines with the normal tides to create the hurricane storm tide, which can increase the mean water level 15 feet or more.
Storm surges, most often associated with hurricanes, occur when strong winds push water into waves higher than sea level. Storm surges have been known to stretch over 50 miles wide. When combined with natural tides, they can produce water levels as high as 15 to 18 feet. Furthermore, storm surges can cause inland flooding as far as 25 miles away from the coastline.
Hammering rains and gusting winds typical of tropical cyclones begin their journey near large bodies of water such as the ocean. They dissipate as they travel inland. Residents and businesses located along or near the coast are in greater danger of storm surge flooding than inland areas. Nevertheless, the stronger the cyclone, the greater the storm surge and the more likely it is that it will cause damage in regions at significant distances from the coast.
Storm surges can be so powerful that they sweep away everything they pass over, causing extensive damage. Besides being responsible for nine out of ten hurricane deaths, storm surge floods have been known to erode beaches, wash away coastal roads and bridges, bury cars and their passengers and weaken and knock down houses and buildings.
When storm surges occur in combination with regular high tides, the resulting floods are even more catastrophic. Normal tides are the result of water’s natural ebb and flow, rising and falling predictably in accordance with the pulls of gravitybetween the earth, moon and sun. When storm surges are imposed on top of natural high tides, they produce a “storm tide.” Storm tides have an even greater reach than storm surges alone. Expressed another way, storm surges are elevated water levels over and above expected normal tide levels. Storm tides are extreme elevations of water levels resulting from the combination of storm surges and natural tides. For this reason, storm surge heights are often calculated by subtracting predicted regular tide heights.
Several other factors also influence the height of a storm surge and which regions are most likely to be hit. The angle at which a cyclone crosses the coast influences the altitude of a storm surge. That is, the storm surge power is affected by the shape and depth of the ocean floor. If the ocean bed slope at the coast is shallow, storm surges build up and overflow more easily. Conversely, if the ocean bed has a steep slope, the effect of the surge will be less. Faster and stronger cyclones produce higher surge waves. Also, the presence of bays, offshore islands and similar topography amplify the magnitude of a storm surge.
Emergency managers in coastal communities remain on high alert for the advent of cyclones and accompanying storm surges. One tool used for estimating winds and surge heights resulting from hypothetical (simulated) and predicted storms is a computer model run by the National Hurricane Center (NHC) known as SLOSH (Sea, Lake, and Overhead Surges from Hurricanes). With the data obtained from SLOSH, emergency personnel can determine which areas to evacuate in the event of a storm surge. The SLOSH model takes into account factors such as storm track, wind, forward speed, pressure and size. The model uses the National Geodetic Vertical Datum (NGVD) as the elevation reference level to which the SLOSH output is compared (measured in feet above the NGVD for the specified area). The SLOSH model is reported to be accurate within 20 percent up or down. It is considered the best means of predicting the potential maximum storm surge for a given locale.