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By Jeff Danner Jeff has worked in both the chemical and biotech industries and is the veteran of thousands of science debates at cocktail parties and holiday dinners across the nation. In his Common Science blog, Jeff aims to make technological and scientific concepts accessible to all.

Hurricanes Part II: Storm Surge

By Jeff Danner Posted June 10, 2013 at 12:01 am

Galveston I ended Part I of this series by noting that a category five hurricane can release up to 200 exajoules (1018) of energy per day, which is approximately 70 times greater than humanity’s global daily energy use. Winds, floods, and in particular storm surges driven by this phenomenal energy release, result in tremendous property damage and often, sadly, loss of life.

Winds from category five storms, which are at least 157 mph and sometimes over 200 mph, will knock down walls and rip off roofs of framed houses and can uproot large tress. Damage to less sturdy structures such as mobile homes, utility buildings, and boat docks can be nearly complete. Here in the triangle, we are far enough from the ocean that hurricane wind speeds tend to drop below 100 mph by the time they reach us, limiting the damage we suffer.

Although storms weaken by the time they reach the Triangle, they can bring prodigious amounts of rain, with hurricane Fran as perhaps the most noteworthy example. Fran made landfall at Cape Fear on September 9, 1996 and made her way up the Route 40 corridor toward Raleigh. Fran dropped 16 inches of rain in central North Carolina over the course of several hours, resulting in heavy floods. The picture below was taken next to the Crabtree Valley Mall during Fran.

Fran Flooding Crabtree Flooding also results in an indirect danger that you may not have considered. As waters rise, people and animals gather together on high ground, which often results in significantly increased rates of the transmission of rabies. When untreated rabies is fatal, so seek immediate medical attention whenever you are bitten by an animal, in flood or otherwise.

By a wide margin, the most dangerous and damaging element of a hurricane is storm surge, which is responsible for approximately 90% of hurricane-related deaths and a similar proportion of property damage. Storm surge is defined as the height to which the winds push the water levels above normal high tides. Maximum storm surge for hurricanes Katrina and Sandy were 27 and 18 feet, respectively. (27 feet!)

I have struggled a bit to find a way to adequately explain the awesome forces associated with storm surge. Storm surge waves move at about 15 mph, with each cubic yard of water weighing nearly a ton. Therefore, each successive wave is hitting objects on the shore like a fleet of trucks. Furthermore, the power of the waves is proportional to the square of their height. Therefore, a wave which is twice as high hits four times as hard. Successive waves of the storm surge have a cumulative impact as they batter structures on the way in and erode foundations on the way out.

The picture at the top of the page is from the worst storm surge incident ever in the United States during the Galveston Hurricane of 1900. (This storm predates the time when storms were given names.) Galveston is located on a barrier island on the Gulf Coast of Texas and its highest point is a mere nine feet above sea level. (For reference, the average elevation on Ocracoke Island on the North Carolina Outer Banks is five feet above sea level.) The surge from the storm was 15 feet high, allowing it to overrun the entire island, destroying all but a few buildings and killing up to 12,000 people in what continues to be the worst natural disaster in U.S. history.

In 2008 when Hurricane Ike was threatening Galveston, the local authorities were calling for a total evacuation of the island to avoid a reprise of 1900. As always seems to be the case, there was as small contingent of people who refused to evacuate. In what I found to be a creative move, the authorities told the holdouts that time to evacuate was running out and requested that those who chose to remain should write their social security numbers on their arms with a Sharpie® so that their bodies could be identified after the storm. This message had its intended effect and the holdouts left.

While hurricane preparedness is more critical in North Carolina along the coasts, we in the Triangle should prepare as well.
-A strong storm can result in power outages lasting up to a week, so ensuring that you have adequate reserves of food and water during hurricane season is advisable.
-When possible, avoid driving during times of flash flood warnings and never drive into running water.
-During times of strong winds, move to an interior part of your home away from windows.

As I am finishing the edits on this column, Tropical Storm Andrea is dropping quite a bit of rain here in Chapel Hill and a flash flood watch is in effect. The National Hurricane Center shows another system beginning to organize in the mid-Atlantic. Perhaps Barry is on the way.

Have a comment, question, or favorite hurricane story? Use the comment interface below or send me an email to commonscience@chapelboro.com.

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