STORM SURGE PAGE

What exactly is hurricane storm surge and why is it so dangerous?
Simply put, storm surge is an abnormal rise in sea level accompanying a hurricane or other intense storm, and whose height is the difference between the observed level of the sea surface and the level that would have occurred in the absence of the cyclone. Storm surge is usually estimated by subtracting the normal or astronomic high tide from the observed storm tide. Because of improvements in forecasting, the spread of information and timely evacuations (all coupled with a little luck), the number of fatalities from hurricane storm surge has been drastically reduced over the past 30 years. However, the element of risk is always there. It is quite simple: if you are caught in a storm surge while in your home or even worse, while in a vehicle, you will probably not survive. Therefore, with the enormous increase in coastal populations, it is more critical than ever to understand the risks from hurricane storm surge; and more importantly, heed the warnings and evacuation orders given by local emergency management officials. The rise in water, coupled with the battering waves on top of the elevated water level is the reason that sometimes millions of people are urged to evacuate their coastal homes.

Although coastal Georgia has been spared from the recent hurricanes that have affected the Carolina's, history tells us that there is a real danger from hurricane storm surge. The problem can be looked at in two categories: the geography of coastal Georgia and the perception of folks who live along the Georgia coast.

First, let's examine the geographic storm surge problem:

STORM SURGE: THE BEGINNINGS...
Hurricanes are born from some type of "seedling" in the tropics and subtropics. Whether it be a tropical wave or an old frontal boundary that leads to hurricane formation, hurricanes are the top of the food chain, so to speak, in the tropics

Tropical Systems

The first stage beyond a tropical disturbance is a tropical depression. A tropical depression has winds of less than 39 mph and a defined, closed circulation with wind spiraling in from all directions to the center. The air pressure is not as low as a tropical storm or hurricane. A tropical depression has no name, it is just called "tropical depression #1, #2 and so forth. There is typically heavy rain associated with a depression but no storm surge to speak of.

Moving up the scale, we get to a tropical storm. At this point the system gets a name, let's call our model storm "LISA". Lisa will be our model for examining how a tropical cyclone brings storm surge ashore and the devastating effects that come along with it. A tropical storm has winds from 39 mph to 73 mph and a better defined wind field and overall circulation pattern. There may be minor storm surge flooding in a strong tropical storm in the areas just to the right of where the center comes ashore. This is especially so if the storm has a fast forward speed. We'll look at why that is later on.

Lisa is now a minimal hurricane. With winds of 75 mph, Lisa does not quite have an eye feature yet, but that will come soon. The air pressure has dropped to 985 millibars placing Lisa in the category 1 area of the Saffir-Simpson Hurricane Damage Potential Scale. Storm surge will be from 4 to 5 feet depending upon the exact track and location of landfall of a category 1 hurricane. Our hurricane, Lisa, will continue to strengthen as we jump up to a category 3 hurricane.

The first sign of Lisa's impending threat are the massive swells that are generated in all directions from the hurricane's center. They move outward and arrive on our shores days after being generated by the powerful winds of our hurricane. As the swells encounter the coastline, dangerous rip-tides often result. So even though Lisa is still 9 days away, she is already creating a problem on the coast as the swells, or the forerunner waves, come rolling in. These waves typically add a foot or two to the overall height of the ocean at the beachfront. If Lisa were to pass harmlessly by and never get much closer, then these forerunner waves are all that we'd experience. Unfortunately, Lisa is going to threaten the Southeast Coast. Let's move ahead a few days.
As Lisa draws ever closer to the Southeast Coast, the swells intensify, heavy surf advisories are often issued and there is a looming sense that a hurricane lurks a few hundred miles off shore. Indeed, Lisa is now 36 hours away from making a direct hit somewhere along the Georgia coast. With the seas building and a constant onshore wind, small craft (and indeed all marine vessels) are urged to remain in port and prepare for the hurricane. At this point a Hurricane Watch has been issued. It means that hurricane conditions are possible within the watch area with 36 hours or less. It is time to get ready for hurricane Lisa and the destructive storm surge that she will bring. Now we are a mere 24 hours prior to landfall of what will be a destructive hurricane. This one will rival the infamous "1898 hurricane" in strength and in terms devastating storm surge. As evacuation takes place along the barrier islands and low-lying areas, weather forecasters across the country are focusing on the overall intensity of Lisa and where she might decide to end up. Let's take a look again at the stats regarding hurricane Lisa:

 THE RIGHT-FRONT QUADRANT 

The diagram at the TOP shows our hypothetical hurricane "Lisa" coming ashore in southeast Chatham county. This scenario puts the remainder of Chatham county (including the barrier islands and Savannah) within the dreaded "Right-Front Quadrant" (RFQ). This keeps the wind blowing onshore until the eye passes. The on-shore wind arrows indicate that Lisa struck the Georgia coast at a perpendicular angle, thus driving ashore massive amounts of sea water. Lisa also came in at high tide; almost a worst case scenario for a category 3 hurricane. Let's look at what the surge looks like at this point from the beach. Look at the diagrams below:

ON-SHORE WINDS

THE RESULT:

The illustrations below depict a category two hurricane and a category four hurricane at the beachfront. The illustrations show the beach in profile to demonstrate scale. Notice the differences in the two scenarios. The category two hurricane causes dune erosion and minor ocean overwash on well nourished barrier islands. Thus the damage is typically minor as compared to what will likely occur during a category four hurricane.