|Storm Surge: A Rising Danger
by Andrew M. Dzambo
Citizens of the United States are migrating towards the nation’s coasts; according to the U.S. Census Bureau, the U.S. coastal watershed county population has increased by 50.9 million from 1970 to 2010. Those living near the US east coast and Gulf of Mexico enjoy majestic views of the beach and open waters year-round. Unfortunately, this time also falls during hurricane season, where anything from weak tropical storms to destructive hurricanes can ruin vacation plans. While powerful winds and heavy rains threaten lives and do considerable damage, nothing kills more people, defaces beaches faster, or closes beachside resorts longer than storm surge.
Storm surge, according to the National Hurricane Center (NHC), is an abnormal rise of water generated by a storm over and above the predicted astronomical tides. Hurricanes generate storm surge primarily through strong onshore winds: the wind literally “pushes” water from the sea onto the land (Figure 1). Conversely, offshore winds cause water to be pushed away from the shore. Another factor that affects the maximum possible storm surge is the hurricane’s movement: the hurricane’s motion determines where the strongest winds will occur. A counterclockwise rotating hurricane moving towards the east, such as the storm depicted in Figure 1, will have its strongest winds on the south side of the storm since the storm movement and storm rotation reinforce there; the westward winds to the north are partially canceled by the eastward motion there, so we see an asymmetry in the winds associated with the storm. A faster moving storm would further increase this asymmetry in the storm’s wind speed, so a faster moving storm can have a larger storm surge than an identical storm moving more slowly.
Figure 1: A schematic explaining where wind and pressure components of hurricane storm surge. Credit: The COMET Program.
A major non-hurricane related factor that affects the height of a hurricane’s storm surge is the slope of the continental shelf (i.e. the bathymetry of the ocean floor near the coast). Recall from Figure 1 how water “piles up” near land to create storm surge. Water piling up on a shallow continental shelf forces incoming water upwards, creating higher storm surge. Likewise, a steep continental shelf will not force incoming water upwards as high compared to the shallow continental shelf; therefore, the height of this storm surge would be less. Louisiana coastline has a shallow continental shelf compared to Miami Beach, which has a steep continental shelf. According to the National Hurricane Center, storm surge from a Category 4 hurricane could produce a 20-foot storm surge on the Louisiana coastline while the same storm could create only an 8–9 foot storm surge for Miami Beach (Figure 2). On top of it all is your location above mean sea level – how high must the ocean rise before it comes in your front door?
Figure 2: A shallow continental shelf (top) compared to a steep continental shelf (bottom). Credit: The COMET Program.
To answer this question, we must also consider the shape of the coastline. Consider a coastline inside a large bay and a coastline along a narrow bay, where both bays are affected by the same hurricane. Water forced into this large bay would be spread out, thereby resulting in a lower storm surge. Alternatively, water funneled into the narrow bay would result in much higher storm surge. If your front door is near the coastline of the narrow bay, the odds of storm surge damaging or destroying your home are much greater. Cities located along a narrow bay or lake face the greatest danger of a hurricane’s storm surge.
New York City and New Orleans, both cities in proximity to a small lake or bay, experienced multi-billion dollar disasters over the last ten years due to a hurricane’s storm surge. In late October 2012, Hurricane Sandy ravaged the mid-Atlantic coast, especially New York City. As Sandy moved up the eastern seaboard and towards the New Jersey coast, it generated surge as high as 15 feet into New York City and northern New Jersey. In New York, boats docked in the Great South Bay were converted to piles of scrap along the coastline, the Battery Park tunnel and surrounding subway stations completely flooded, and New York City’s Bellevue Hospital was forced to close for 99 days. Sandy’s powerful surge even pulled a large roller coaster out into the Atlantic (Figure 3). Considering that the northern New Jersey and New York City coastlines lie only a few feet above sea-level, it should be no surprise this extensive damage occurred.
Figure 3: A collage of the damage caused by Sandy’s storm surge. Boats were piled up onshore (left), Battery Park tunnel flooded, completely submerging a large truck (center), and a roller coaster collapsed off the New Jersey Boardwalk and was swept out to sea (right). Credit: The Associated Press.
Hurricane Katrina’s demolition of New Orleans overshadows Sandy’s epic destruction. The nearby coast’s shallow continental shelf and the city’s average elevation of six feet below sea-level combined to create a storm surge as high as 25 feet that ravaged the city – and this was compounded by the failures in the levees that were intended to protect the city. Approximately 80% of the city flooded with some places submerged as much as 15 feet underwater (Figure 4). This made Katrina the most expensive natural disaster in US history and the third costliest natural disaster of all time. The unabated damages caused by the failed levee system show how destructive storm surge can be if a city’s levee system is poorly maintained.
Figure 4: Hurricane Katrina’s widespread damage of New Orleans. Note the flood heights and the number of nearly submerged houses. Credit: The Associated Press.
Current levee systems and sea walls, unfortunately, are becoming more susceptible to a long-term factor that will increase storm surge’s deadliness and destructiveness over the next century: rising sea-levels due to climate change. As the climate has warmed, we have observed increases in sea level of as much as 7.7 inches around the US coast since 1870. Much of this rise in sea-level is a result of melting polar ice (Figure 5). Large masses of polar ice can be thought of as ice cubes in a warm drink: a warmer drink melts ice cubes faster compared to a cooler drink. Since ice is less dense than water, the drink will have more volume once it completely melts. If all the polar ice melted into the ocean, sea levels would rise approximately 70 centimeters (~2.3 feet) across the globe. This amount of sea-level rise on its own could submerge several beaches and flood coastal towns and cities that do not properly build up and maintain their levee systems. Rising sea-surface heights will essentially “add” more water that can potentially “pile up” with a hurricane’s storm surge, which will become a grave danger for US citizens near the east coast and Gulf of Mexico.
Figure 5: Arctic sea-ice coverage during September 1979 (top) and September 2007 (bottom). Notice the dramatic change in sea-ice coverage over this 28-year period. Credit: EPA.
Leading climate scientists are also fearful of what rising sea-levels and storm surge could do over the next several decades. In their recent publication, climate scientists Ning Lin, Kerry Emanuel, Michael Oppenheimer and Erik Vanmarcke (Lin et al., 2012) suggested that storm surge levels near the Battery in New York City could increase as much as 1.24 meters (~4 feet) in 50 years, 5.7 feet in 100 years and 9.1 feet in 500 years. Dr. Jenni Evans, a Professor of Meteorology and the Acting Director of Penn State’s Earth and Environmental System’s Institute, added “As sea level rises, the danger of storm surge would change dramatically due to the complicated interactions between the storm and ocean bathymetry, along with the continued migration of our population.”
The evidence is clear: the increasing threat of storm surge is a grave danger along the US east coast and Gulf of Mexico. Destructive storm surges, such as those produced by Hurricanes Sandy and Katrina, possessed enough power to make the top-5 all-time most costly US natural disasters. With the amplifying effects of climate change, a direct hit from a powerful hurricane’s storm surge on a major US city 50 or more years from now will easily surpass Katrina’s US damage record of $100+ billion dollars. The increasing danger of storm surge, however, can be mitigated with a strong commitment to improving our current coastal defenses. If Americans along our coastlines work together to build higher sea walls and strengthen existing levee systems, our coastal cities and beach homes will be protected from the rising danger of storm surge.
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