Taking Safety Littorally

PASADENA – In 2011, the Tohoku earthquake and tsunami devastated Japan’s coastal region. Last year, Hurricane Sandy caused a wall of water to engulf low-lying coastal areas on the East Coast of the United States, particularly in New York and New Jersey. Such catastrophic events underscore the vulnerability of coastal regions worldwide to extreme weather events that produce intense storm surges (increased water depth at the coast) and large, powerful waves.

Although Sandy, at its peak, was only a post-tropical cyclone when it hit the US, its winds spanned an area of 1,800 kilometers (1,100 miles), leading to extreme storm surges and waves that decimated the Jersey Shore, flattening communities and destroying the casinos and boardwalks on which the local economy largely depends. At Battery Park, on Manhattan’s south end, the surge height reached 4.2 meters, flooding homes and businesses and plunging millions into darkness. Waves also reached extreme heights, with a buoy near the entrance to New York Harbor measuring a peak wave ten meters high, from crest to trough.

Seven years earlier, Hurricane Katrina struck the coast of the Gulf of Mexico in the US as a Category 3 storm. With a surge height of 7-10 meters, and flooding at some locations extending 20 kilometers inland, Katrina caused catastrophic damage to the Gulf Coast, which has yet to be fully repaired. In 1989, Hurricane Hugo struck near Charleston, South Carolina, with a surge height of nearly four meters. The list goes on.

When coastal areas were not heavily inhabited, such storms, while violent, did not cause significant, lasting damage to people’s livelihoods and lifestyles. But now, with commerce and recreation dominating coastlines worldwide, the “let it be” approach of the past is no longer practical.

Moreover, as climate change causes sea levels to rise, extreme storms’ destructive potential is being significantly enhanced. Although this rise may seem small in the short run, especially when compared to the immediate increase caused by major storms, its long-term effects must not be ignored.

Clearly, the combination of storm surges and large waves causes major devastation in coastal areas. But these challenges are not insurmountable. In fact, engineering approaches have been developed that can protect coastal residents from the consequences of extreme storms.

Previous efforts to enhance coastal protection can provide a roadmap to reconstruction that will help to avert future damage. One suggested approach is to leave an uninhabited buffer zone extending shoreward from the water. In Hilo, Hawaii, after devastating tsunamis struck in 1946, 1960, and 1964, the vulnerable area near the city was declared a municipal park, where no structures were allowed to be built.

By contrast, Japan relied almost exclusively on an extensive series of seawalls and offshore breakwaters along the east coast of the island of Honshu. But, in many areas, the Tohoku tsunami overflowed and even destroyed these protective structures – including the seawall at the Fukushima Daiichi nuclear power plant in northern Honshu. As a result, the plant’s three active reactors suffered near-meltdowns, leading to a buildup of hydrogen gas, a series of explosions and fires, and leakage of radioactive material into the atmosphere.

To be sure, such coastal defenses could help to protect crucial structures. But they should be combined with shoreward buffer zones, where homes, schools, and hospitals would not be permitted. Defensive barriers would not necessarily require heavy investment; they could be mounds of sand several meters high located along and near the shoreline. In fact, in some areas of the US and other countries, only extensive dune fields and vegetated areas are used to separate buildings from the seashore.

Sand mounds are particularly advantageous, given coastal areas’ economic value. They provide immediate protection during storm season, but can be removed at times of year when extreme storms are unlikely and recreational beach use is most important. In order to protect the local economy further, beaches should be repaired after major storms through “beach nourishment” (replacement of lost sand from external sources).

Protecting urban areas that are located near the ocean, but lack buffer beaches, requires a different approach. One option would be to construct seawalls and/or rock revetments high enough to prevent shoreward inundation. But local residents may protest such structures, given that they could make the area less attractive. And, as Japan’s recent experience demonstrated, protecting against the most powerful threats – such as the 9.0-magnitude Tohoku earthquake and the subsequent three-meter tsunami – would require massive, costly construction projects.

For one such area, New York City, an alternative has been proposed: massive storm-surge barriers across the entrance to the harbor region that could be closed when a major storm approaches. Such structures have been built across the River Thames in London, and a similar barrier project is underway in Venice, Italy. But, in addition to requiring substantial investment, this approach raises serious questions, reflecting uncertainty about the effects that river flows can have on harbors, the environmental consequences of closing a bay, and the impact on shipping.

In any case, stricter building codes are crucial for structures built in coastal areas. This could include designing the ground levels of seaside buildings to permit storm-induced surges to pass through without flooding the lower floors, thus minimizing potential damage to offices and homes. In addition, construction in coastal areas damaged by extreme events could be prohibited, as has been done in Hilo, Hawaii.

Coastal regions are the most vulnerable to many kinds of extreme weather events. But measures can be taken to protect these communities. By considering safety, economics, and aesthetics, the right approach for a particular area can be developed – and local citizens, businesses, and the environment can be defended.

Fredric Raichlen is Professor Emeritus of Civil Engineering and Mechanical Engineering at the California Institute of Technology, and the author of Waves.

Copyright: Project Syndicate, 2013.

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