When a hurricane makes landfall, the greatest threat to life and property is sometimes storm surge rather than wind. Storm surge is defined by the National Oceanic and Atmospheric Administration (NOAA) as, "an abnormal rise of water generated by a storm, over and above the predicted astronomical tide.
It is difficult to forecast accurately because of the many factors that influence its height and impact. Even a minor change in one of those factors, particularly the storm's trajectory, can have a major bearing on the damage it inflicts.
Storm surge forms primarily on the right side of the storm track (in the Northern hemisphere) because the hurricane's cyclonic and steering winds move in the same direction there and push water onshore.
On the left side of the track, the winds generally blow offshore, thereby pushing water away from the coast and generating little in the way of a surge.
'Nowhere to go but up'
The storm's circling winds create a vertical circulation of water in the sea beneath it that is disrupted by the ocean bottom as it approaches the shore. The water then has nowhere to go but up and inland.
The larger the storm and the stronger its wind, the higher the mound of water pushed ahead of it. Wind piling up water typically accounts for at least 85 percent of the storm surge.
A further 5-10 percent of the storm surge is caused by the low pressure of a hurricane lifting the surface of the ocean in the eye of the storm, and another 10 percent or so is the result of wave set-up (i.e., waves pushing water inland faster than it can drain off). The state of the tide at the time of the storm's landfall is also extremely important; the higher the tide, the more water will be carried onshore.
20 feet or more above tide
Although usually much lower, storm surge can be as much as 20 feet or more over and above the tide. It moves ashore with the forward speed of the hurricane—often 10–15 mph. Even at these speeds, water velocity, in addition to water depth, can be a significant source of damage: A 1-foot storm surge is enough to sweep cars off roads. The damage done by the water's velocity is increased if it is carrying debris, as it often is. Breaking waves can exacerbate damage by extending the height and hydrodynamic force of the water, and their velocity is even greater than that of the current on which they ride.
The physical characteristics of the seabed and coastline impacted play a major role, too. A storm impacting a coast with a shallow and gently sloping seabed will create a higher surge if its approach is perpendicular to the shore than if its approach is at a more oblique angle and/or the seabed is steeper. Coastal orientation also matters , as a large volume of water forced into a narrow bay or inlet has nowhere to go but up.
Once it has come ashore, local terrain impedes the inland progress of storm surge. Steeper slopes lead to more rapid attenuation, but unimpeded surge can reach far inland; during Hurricane Ike in 2008, it penetrated nearly 30 miles into Texas in some places.
In addition to damage from water velocity and depth, storm surge can also cause damage through saltwater corroding machinery or contaminating agricultural land. Finally, the heavy rainfall associated with tropical storms can cause inland rivers and streams to swell; their levels will rise further and exacerbate flooding if their drainage into the ocean is inhibited by already elevated water levels at the coast.
High insured losses
Storm surge from hurricanes can be devastating, resulting in substantial damage and high insured losses. The steady growth in the value and density of property on the Gulf and East coasts of the United States is increasing the need for reliable and detailed information on storm surge risk.
AIR's new hydrodynamic storm surge module for the U.S. hurricane model was developed to more accurately assess this risk at very high resolution.
Jonathan Kinghorn is the editor of the AIR In Focus blog and a writer and editor with Corporate Communications at AIR.
This article was produced by AIR Worldwide and first published in AIR's In Focus blog.