The Thames Barrier protects London. Designed by Rendel, Palmer and Tritton, the barrier consists of nine concrete piers and gates stretching 1,700 feet across the river. The piers house hydraulic machinery that can raise 60-foot-tall gates in 30 minutes to block the surge tide coming up the Thames Estuary. When not in use, the gates rest in concrete sills flush with the river bottom, allowing ships to pass. The sills exert no direct load on the riverbed but sit in ledges in the pier sides to prevent differential settlement. The specially designed barriers, called rising sector gates, are steel box spans stiffened by internal webs and diaphragms. There are six rising sector gates, four are 200 feet wide and two measure 103 feet. The remaining four gates are of conventional falling radial design. Construction began in 1975, was largely completed by 1982, and cost $700 million. The Barrier has been closed 119 times to date.

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 feet as it
 approached Providence, R.I., but thanks to the Fox Point Hurricane Barrier,
 standing 26.7 feet 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-foot-long concrete barrier extends west across the Providence River.
 It includes three 40-foot-high, 40-foot-wide tainter gate openings that prevent
 floodwaters from entering the bay when closed. Two 10- to 15-foot-high 
earth-filled dikes with stone-protected slopes flank each side. The eastern
 dike is 780 feet long and the western dike is 1,400 feet long.

Built in 1966, for 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 feet, 
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 55-foot-long pumps, 10 feet in diameter, 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 super
computing capabilities, scientists and engineers have, for the first time, 
begun not to rely on historic records to estimate worst-case scenarios, but
 on science and statistical-probability risk modeling of the storm surge
-generating potential of the hurricane environment.