Storm surge barriers work—up to a point; that point being the surge height
 for which they are designed.

But even when overtopped, experts are banking on the barriers mitigating the
 kind of run-away disaster that befell New Orleans during Hurricane Katrina
 in 2005.

One thing is for sure, though. The absence of storm surge barriers can
 amount to no protections worthy of the name, as New York City learned Oct.
 29 as Hurricane Sandy rolled ashore.

Exposed towns, cities and even nations, such as The Netherlands, have slowly
 and quietly been building up storm surge defenses to protect themselves for
 decades, averting millions of dollars in damages as a result.

For example, Superstorm Sandy had swelled water levels to 9.5 ft as it
 approached Providence, R.I., but thanks to the Fox Point Hurricane Barrier,
 standing 26.7 ft high, that city avoided potentially millions in dollars 
in damages.

“I believe even if Providence had been hit directly, it would have been fine,” says John MacPherson, deputy manager for the U.S. Army Corps
 Engineers’ Cape Cod Canal Field Office. “When the Corps designed the project, they took meteorological readings [and planned for a] worst-case
 storm. The 1944 storm had the most energy off shore. They modeled that storm hitting Providence coming up the bay as a direct hit, and generated a water
 level to design for.”

The 700-ft-long concrete barrier extends west across the Providence River. It includes three 40-ft-high, 40-ft-wide tainter gate openings that prevent
 floodwaters from entering the bay when closed. Two 10- to 15-ft high
earth-filled dikes with stone-protected slopes flank each side. The eastern
 dike is 780 ft long and the western dike is 1,400 ft long.

Built in 1966 at about $15 million, the barrier “has prevented loss of life
 and property time and again, says New England district spokesman Timothy Dugan. “In fiscal year 2011, the Corps of Engineers staff operated the 
barrier for flood control on 12 occasions during coastal storms.

Devastating events, including a 1938 storm with a surge of about 17 ft, 
prompted the city to ask the Corps to build the barrier, says MacDugan.

Because of a series of storms between 1849 and 1936, Congress passed a
 series of Flood Control Acts. The act of 1955 authorized construction of the
 Providence barrier.

MacPherson says the city of Providence is responsible for operations and
 maintenance of components located outside the river banks, such as the dikes
 that flank each side and five vehicular street gates and five sewer gates.
 Five large pumps pass accumulated water back over the dam and back out to 
Narragansett Bay. The 10-ft diameter, 55-ft-long pumps can pass 630,000
 gallons per minute.

Jurisdictions that are blessed with natural landforms that limit areas of 
vulnerability to relatively short gaps between headlands have accomplished a
lot of protection with a relatively modest bit of engineering and 
construction. But other jurisdictions, like New Orleans and the Netherlands,
with long reaches of high exposure, and flat coasts riddled with bays, lakes and lowlands, have been forced to build sprawling defenses to protect their
 economies and populations.

In most, if not all cases, the will to build storm surge defenses arose from
 the mud, death and heartbreak of disasters, such as the flooding of the
 Dutch lowlands in 1953 or the catastrophe in New Orleans in 2005.

The events in New Orleans spawned a new discipline for designing systems to protect large geographic areas from storm surge. Using now available supercomputing capabilities, scientists and engineers have, for the first time, begun not to rely on historic records to estimate worst-case scenarios, but have embraced science and statistical-probability risk modeling of the storm surge-generating potential of the hurricane environment. They are factoring for uncertainty at the component calculation level and arriving at a reach-by-reach, feature-by-feature evaluation of risk and fragility to
 design defensive systems with a high degree of reliability for blocking 1:100, 1:500, 1:1000 or 1:10,000 year storms.

The accompanying slide show is a rundown of some of the already built storm surge defenses around the world.