Atmosphere And Weather

Assessing the Effect of Sea Walls against a Storm Surge



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Seawalls can be very effective in protecting areas from the effects of damaging storm surges. They are not infallible; any seawall could conceivably be defeated by the forces of nature, and their effectiveness is determined by where and how well they are built and maintained. But the stories of two cities half a world apart illustrate that seawalls are indeed a worthwhile investment that can save lives.

Before 1900, the Gulf coast city of Galveston, Texas was a thriving port and commercial center, one of the wealthiest cities in the Southwest. Built on a low barrier island only eight feet above sea level at its highest point, the city was in a risky location for storms. In 1875, the city of Indianola on Matagorda Bay about 100 miles south of Galveston had been nearly destroyed by a hurricane. The city, an important Gulf port like Galveston, was rebuilt only to be destroyed again in 1886. After that, the people of Indianola gave up and moved away. Some of Galveston's citizens took note of the plight of Indianola and called for the construction of a seawall, but history was not on their side. Galveston had faced storms from the Gulf before and had always come through with little damage, and most of the people in the city saw no reason for such an expensive project. That would, of course, turn out to be the wrong answer.

Weather forecasting in the last year of the 19th century was primitive by today's standards, but not nonexistent. Observers with basic measuring instruments formed a network linked by telegraph to the main Weather Bureau office in Washington, and their reports combined with reports brought in by ships at sea were passed along to areas that might be affected. Galveston received the first notices of the storm in the Gulf of Mexico on September 4 and updates over the next three days, but forecasters were unsure of its intensity or direction. It passed over Cuba on September 6; Cuban forecasters felt it would continue westward, while the American weathermen expected it to turn north and east toward Florida. But by the morning of September 8, even though the hurricane was not yet obvious, the chief of the local Galveston weather office was sufficiently concerned by the reports and the unusually high tide and swells in the Gulf to raise the hurricane warning flags.

It was a prescient move, even if it did not help much to avert the coming disaster. By mid-morning the first rain showers started, and grew increasingly frequent and stronger along with the rising wind throughout the day. His last report at about 3:30 pm described half the city as being flooded, with water still rising from the Gulf. The hurricane made landfall, probably just to the south of Galveston, at around 7:30 pm, bringing with it winds of 120 miles per hour or more and, most destructively, a storm surge15 to 20 feet high. Galveston, the city whose people had decided it couldn't happen to them, was completely washed over by the sea and almost wiped off the map. Over 3,600 buildings were completely destroyed and at least 6,000 people were killed; many victims were never found, and there is reliable evidence the total might have been as high as 8,000 to 10,000 deaths. In terms of loss of life, it remains the worst natural disaster in U.S. history.

To their credit, the survivors of Galveston learned their lesson. In 1902, construction of a massive, 17-foot high seawall was begun and the land behind the wall was filled in to raise the elevation of the entire city to a similar level. In 1915, the new seawall got its first real test when Galveston was struck again by a hurricane almost identical to the 1900 storm. Even though the city suffered some damage and about 275 were killed, the seawall held back the storm surge. In the years since, Galveston has been affected by several more hurricanes, and while parts of the island not protected by the seawall are usually seriously affected the city itself has, so far anyway, not seen anything even remotely approaching the scale of the 1900 disaster.

More than a century after the destruction of Galveston, a city on the other side of the world was saved by the foresight of its founders nearly three centuries earlier, in conditions very different from a hurricane. Pondicherry is a city on the southeast coast of India, until 1954 a French colony. In the 18th century, the French began constructing a huge seawall, which grew over time to nine meters in height and stretched the length of the central waterfront of the city. On December 26, 2004, Pondicherry was struck by the catastrophic Indian Ocean tsunami. The highest of the waves was nearly nine meters high, and wrought terrible destruction along the coast on either side of Pondicherry. But the city itself was unscathed thanks to the seawall, and only 25 people who had been reportedly walking along the promenade atop the wall at the time were killed.

Both Galveston and Pondicherry demonstrated the effectiveness of seawalls against storm surges and massive waves. But both cities do not take their defenses for granted, and spend considerable amounts of money and effort in maintaining the seawalls. Wave action at the base, or "toe," of the seawall constantly erodes the seabed, requiring constant filling, usually with large, heavy boulders. Seawalls also block natural dune movement and alter currents that deposit new sand on beaches. The beaches must be continually replenished, not only for cosmetic and recreational purposes, but because they help absorb some of the impact of surges and waves against the seawall. And seawalls do have one negative side effect in some storm conditions when heavy rains and runoff cause flooding on the landward side, trapping the water and actually making the floods worse.

Finally, no matter how well they are maintained, seawalls do not guarantee permanent protection against nature's wrath. There have been storm surges higher than 17 feet, and tsunami higher than nine meters. It seems axiomatic that the highest or strongest wall Man can build against the raging sea will, someday, be defeated. Even so, for the people whose lives depend on the seawalls, the effort to keep them in good order and the odd chance that they won't work anyway are probably reasonable prices to pay for feeling secure most of the time.




Firsthand report of the 1900 Galveston Hurricane:
http://www.history.noaa.gov/stories_tales/cline2.html

Brief description of the effect of Pondicherry's seawall and other information about the tsunami:
http://www.iitk.ac.in/nicee/RP/2006_Effect_EQSpectra.pdf

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