One of the most deadly and damaging effects of tropical cyclones is what is called the “storm surge”. The flooding associated with such a surge can carry for miles inland, flooding roads and buildings and trapping residents in extremely hazardous conditions.
The primary cause of a storm surge is the high winds of the storm pushing water along with the storm’s movement. As the storm winds blow across the surface of the water waves are created.
In the open ocean these waves don’t result in much actual water movement, or mass transport. However, as the waves come ashore they build and break like you see when ocean swells hit a beach. These breaking waves convert a rolling, open ocean wave into building surf carrying their momentum, and the associated water, forward.
A number of conditions can increase the effects of such breaking waves in creating a surge. The momentum of the waves carrying into a bay, estuary or other narrow inlet will be concentrated much like we see the effects of tides in places like the Bay of Fundy. A shallower, more gradually sloping, shoreline can allow the momentum to dissipate more gradually while a steeper shoreline can allow the force of the waves to carry more strongly ashore. Shallow shorelines will carry a greater surge with smaller waves while steeper shorelines are associated with stronger waves and lesser surge.
An additional factor causing storm surge is the low pressure associated with the storm. It is estimated that the surge is increased by just under one-half of an inch for every one millibar of pressure drop. This effect can add several feet of increased water height to the wind driven surge.
Another factor associated with the storm surge, although not technically part of the mechanics of the surge itself, is runoff meeting the incoming surge. A major hurricane can dump as much as twelve inches or more of rain over a wide area in 24 hours. As this runoff flows down streams and rivers it will eventually meet the incoming surge. Low-lying areas near such runoff will experience even greater water levels.
Recent experiences in Galveston, during Hurricane Ike, and New Orleans, Hurricane Katrina, have shown the damaging effects of storm surges. It is estimated that the surge associated with Ike at Galveston was well over 21 feet. With a highest point of just 20 feet, it is obvious that the entirety of Galveston Island was completely underwater during this storm. And, we are all familiar from the news stories with the enormous damage done in New Orleans by Katrina. While the surge of Katrina was nearly 30 feet at its highest points in the bays and estuaries of Southern Mississippi, even in New Orleans the surge was enough to top and undermine the levies protecting the city causing far more damage than the high winds and rain of the storm.
