Architects understand gravity. To keep structure overhead is the first challenge of building. In coastal regions and earthquake-prone lands, it is also important to design for dynamic forces, with sudden impact and rapid changes in magnitude and location. Wind, water, and seismic loads are typically applied in a horizontal direction, very different from the vertical force of gravity.
The growing damage from intense storms and coastal flooding has modified building codes and standards in the years since 1992, when Hurricane Andrew devastated Miami-Dade County. Improvements to hurricane ties on frame structures and greater reinforcing in masonry structures have established buildings less likely to fall apart under storm conditions. For wind forces, buildings can be strapped into resistance; but in the wind versus water sweepstakes, water is more powerful by about 800:1. Bernoulli’s principle explains that pressure due to a moving fluid is proportional to the density of the fluid, and to the square of its velocity. Water is much more dense than air, and it doesn’t have to be moving very fast to knock a person off their feet, nor a building off its foundation. In coastal construction, the potential for water to escape its normal confines and spread across land should no longer be taken for granted, but accounted as part of the cost of building in this zone.
Storm surge builds as a hurricane approaches the coast. This phenomenon is especially pronounced when water is shallow, as it is in parts of the Gulf of Mexico. During Katrina, the Mississippi Sound, nominally 8-12’ deep for over 12 miles outward from the coastline, allowed the 250-mile wide hurricane system to propel a tremendous volume of water. There were no cliffs above or below the shoreline to break the waves, and no place for the water to go but up. The ground elevation along the gulf coast ranges from 5’ to 23’, low elevations resulting in flooding across the coastal counties. Beach houses, piers and boathouses were the first obstacles in the water’s path, and the spindly, stick-built structures couldn’t hold it back. Waves battered the tops of walls, overtopped roofs, and scoured roads, utilities, and foundations. The manmade levee of the railroad tracks one-quarter mile inland was the first massive and immovable object the waves encountered, and it halted the wave motion if not the water itself. The tide continued to lap over and around, through ditches and canals, insidiously finding every low point, every dip and pitch in which to deposit saltwater and all of the detritus picked up along its journey. And then it receded. The coastal water was gone within hours, pushed back to calmer seas by the force of hurricane winds blowing from the north once the eye had passed.
Location is critical, especially in proximity to coastlines, and to the zero elevation measure of the geodetic vertical datum. The denizens of New York City learned this harsh lesson in 2012, on the receiving end of the same damaging forces more typically experienced by coastal residents from Bangladesh to the Netherlands. Hurricane Sandy was unusual–late-season, upper Atlantic location–but the force of wind and water were the standard hurricane fare. In the northeast, storms like this happen so infrequently and unpredictably as to make designing a plan of resistance for every home and small building uneconomical. However, when the financial capital of the world is crippled by storms, it is useful to consider whether finding a way to keep water out of the city might be practical.
Water finds the cracks and basins in the urban system. Rain celebrates a flight of stairs leading down into the earth, an opening to below, an invitation to sweep objects along to return to the sands beneath the manmade crust. An open escalator, a ventilation grate, a bank of turnstiles – these offer no barrier to water.
New York City transportation systems failed as spectacularly as the electrical grid. Five million riders a day use the subways. On a fine day, 13 million gallons of water are pumped out of tunnels, both rainwater and the flow of sidewalk- and street-cleaning residue that enters through subway vents. On a wet day, the pumps can barely keep up; there is no extra capacity for storms or superstorms which bring in not only rain but introduce salt water, corrosive and conductive, into this electricity-dependent subterranean world. Hurricane Sandy closed stations, some for over seven months. “Because city officials are not in the business of advertising their concerns, most New Yorkers don’t realize that some have been imagining this scenario for a while. The culprit, some say, could be climate change. To be sure, New York faces unprecedented dangers in a warming world. Although the waters along the east coast of the United States have been inching up since the end of the last ice age, the rate of rise has accelerated in the last 150 years. This is particularly true in places like New York, where land is also subsiding as the Earth’s crust readjusts. If polar ice sheets continue to melt at their current rate, the water around Manhattan and Long Island could rise by five inches within the next eight years. By mid-century, local sea level could be up by a foot, and up by two feet by 2080.”[i]
The modern financial capital is becoming more like the world trade center of a thousand years ago. Venice defined global exchange in the twelfth century, but its susceptibility to encroaching waters affected the city’s stability as a financial powerhouse. If engineers can’t keep the water out, the beautiful palaces will continue to crumble, the floating hummocks sink, and the city will disappear beneath the waves, a modern Atlantis.
To resist water, there are only two options for structures: allow it to pass through freely by means of flood vents and breakaway walls, or create massive interceptors such as flood gates, barriers, seawalls, levees, and the like. In the face of current threats, cities including Rotterdam, Venice, and London are installing water barriers at the mouth of rivers and estuaries, in networks that close off the vulnerable and low-lying areas to the press of coastal flooding and sea level rise.
The stainless shells of the Thames Barrier present a brave face to the 60 kilometers of river estuary leading to the North Sea. Rotating arcs adjust the level of protection, or drainage required. Boat traffic is permitted or interdicted by the water level. This modern barrier is more useful today than fortifications on land, but it may not last as long as the city’s medieval walls; the Thames Barrier service life is expected to end in 2070, slightly less than 100 years from the beginning of construction. In contrast, the medieval walls lasted seventeen centuries, another example of the power of water to wear away and break down materials as seemingly impervious as the stone of the Grand Canyon or the stainless steel of the flood barrier.
As global weather patterns tip from the predictable to the extreme, coastal fortifications against hazards will increase. The force of water is impossible to resist forever. Venice learned this long ago, although elegant efforts valiantly extend the city’s lifeline. Will other centers of finance and trade along coastlines–New York City, Tokyo, Hong Kong–learn these lessons before they also succumb to the waves? Perhaps the future of New York City is a new harbor defence spanning across the Narrows, in place of Forts Wadsworth and Hamilton. The new enemy is not army, navy, or air force, but the water itself. A barrier to water can only hope to hold back the tide for a limited time.
[i] Jesse Newman, “For New York’s Subway, Sandy’s Devastation May Be Just the Beginning.” The Atlantic, Nov 1 2012.