Sinking Cities

by Conroy

Baltimore flooded by Hurricane Isabel

Are America’s coastal cities facing a losing battle against the sea? Consider these facts from cities all along the U.S. East Coast: In Galveston, Texas, a long sea wall shields the city from the Gulf of Mexico. The sea wall worked for a century, but in 2008 the storm surge from Hurricane Ike overtopped the wall and flooded the adjacent streets. In the early 1980s Miami-Dade County spent $50 million to rebuild the eroding shore line of Miami Beach. Similar projects, officially termed “beach nourishment”, are common all along the Atlantic seaboard¹. In some precariously narrow sections of Hatteras Island, North Carolina, far off the mainland in the state’s Outer Banks, sand bags line both sides of the state route 12 to keep the Atlantic on one side and Pamlico Sound on the other from splitting the island. Back in 2003 when Hurricane Isabel roared up the East Coast, the island was split and an emergency project was required to stop the erosion and prevent the split from becoming permanent. Hurricane Isabel continued its movement up the East Coast flooding coastal areas of Virginia and as it moved inland its powerful storm surge funneled water up the Chesapeake Bay causing widespread flooding in coastal Maryland, and putting Baltimore, including downtown, under several feet of water. The flooding even extended far up the tidal Potomac to Washington, D.C. In 2011, the storm surge from Hurricane Irene flooded parts of Brooklyn and lower Manhattan. Further north, Nantucket Island off of Cape Cod is literally eroding into the sea, taking pricey homes with it.

As these examples above demonstrate, being next to the sea has its dangers, and fighting the long-term trend of beach erosion and flooding are never-ending expensive battles. This situation is only going to get worse if a planet-wide trend continues: rising sea levels. Consider that just along the Atlantic and Gulf coasts Boston, New York, Philadelphia, Baltimore, Washington, Jacksonville, Miami and Southeast Florida, Tampa Bay, and New Orleans are situated at least in part at sea level. And it’s not just major metropolitan areas that are at risk, the low-lying shores of the Houston shipping channel, just inland from Galveston, are home to one of the nation’s largest ports and lined with the nation’s largest concentration of petroleum refining plants. Are all of these areas destined to be inundated over the next century?

Sea Levels Rising
Of course the first thing that has to be answered is: Are sea levels actually rising? And to answer that, we need to understand what sea level is. Sea level is harder to define than it may seem because of course the flowing, undulating sea never seems to have one level. However, the most basic definition is the average surface elevation of the oceans when surface fluctuations, like high and low tides are averaged out. This seems like a reasonable definition but it nevertheless can be hard to pin down. Note that the average sea level of the Atlantic end of the Panama Canal is nearly eight inches lower than at the Pacific end. Still, it’s fair to say that there is a local “sea level” for all places where land meets tidal water. Historically these levels have been measured from set gauges on land, and more recently satellites have been used to observe global sea levels.

The results of these measures seem to confirm that sea levels are indeed rising all over the world. According to the Intergovernmental Panel on Climate Change (IPCC)², global sea levels have been rising by about 1.8 millimeters per year (mm/yr.). If this estimate is in any way accurate, and indicative of a continuing trend (and not a rate that may slow or even reverse in the near future), there’s probably no need for immediate panic. At this rate it would take about 555 years for global sea levels to rise by one meter or about 170 years to rise one foot. This is hardly a catastrophic rate of increase. However, rising sea levels, whatever the rate, do pose an obvious risk to coastal areas. And many estimates, including those by the National Oceanic and Atmospheric Administration (NOAA), put the annual sea level rise much higher than the IPCC.

But what causes sea level to change? On geologic timescales, ocean bottom elevations and plate tectonics play major roles. These occur so slowly that they don’t affect us. More important are short term causes like tides, onshore winds, atmospheric pressure, surface water temperatures, evaporation, precipitation, river runoff, and in extreme cases major storm surges and tsunamis. On the longer term sea levels can change because glaciers and ice caps shrink or expand. In the first case, melt water is freed from the land and adds to the volume in the sea. In the latter case, water is trapped in ice and global sea levels drop. Another long term trend is changes in land elevation due to groundwater depletion, erosion, or general geological subsidence. But it’s the melting glaciers and ice caps that have gathered most attention in recent decades, the result, many will claim, of rising global temperatures, which is in turn the result of man-made effects to the environment.

Melt water on the Greenland Ice Sheet

Man-made or natural, rising temperatures will melt snow and ice. If the entire Greenland Ice Cap were to melt, global sea levels could rise by as much as 24 feet. If the entire Antarctic Ice Cap were to melt global sea levels would rise by 200 feet. These are extremely remote possibilities, Greenland and Antarctica are really cold places and it would take many thousands of years for them to melt. And Arctic sea ice melting would have a negligible effect on sea levels because these ice packs are already floating on the sea. More likely is increased melting of ice on the periphery of Greenland and Antarctica, which could still result in several feet of sea level rise. Also, if sea surface temperatures increase, water density decreases and its volume increases. So higher sea temperatures result in higher sea levels irrespective of the contribution of more water (melted ice). It’s worth noting that global temperatures haven’t increased over the last dozen or so years, which is directly contrary to the predictions of climate models. If global temperatures do stabilize then sea water temperatures and ice cap/glacier melting may not be as significant an issue as many fear.

Now I don’t know if or how much global sea levels may be rising. Like all earth systems, the various components that affect sea level are complicated and combine in unexpected ways. For example, increasing global temperatures will lead to increased precipitation, which, if it falls in the polar regions like Antarctica, will actually increase the thickness of the ice caps and the amount of impounded water, which would lower global sea levels. However, higher temperatures should result in greater calving and icebergs breaking away from the subpolar regions, which would raise global sea levels. How might these two trends interact and which is greater?³ When you consider all of these components there has to be question of how measurements of these global trends can even be made and how accurate models of these global systems can be. Let alone what predictions for future developments are possible.

It’s also worth noting that many of the flooding examples cited at the beginning of this post were the result of major storms, which are not in themselves indicative of sea level rise. Never in my memory had Baltimore flooded from the sea like in 2003, and the Galveston sea wall had stopped all storms, including hurricanes, for a century. There’s plenty of room for debate about whether American cities face an impending crisis from rising sea levels.

Nevertheless, even if the global climate doesn’t warm appreciably over the next century, America’s coastal cities would be negligent to not prepare for rising sea levels and ever more frequent flooding. Sea levels appear to be rising slowing but consistently and this can’t be ignored. And as we consider coastal flooding, we should first turn from the U.S. East Coast to an ancient city on the Mediterranean.




Cities and the Sea

Familiar Venice floods

No city in the world is married to the sea like Venice. Built upon a series of small flat islands inside of a large lagoon off the far northwest corner of the Adriatic Sea, Venice has suffered from flooding for centuries. Tides and storms from the sea have washed over the quays and low-lying public places forcing the population to trudge through inches (or more) of water. These floods have given Venice a distinctive sea-damp odor that is as unpleasant as its unique setting is captivating. In the 20^th century the situation got worse. Wells were sunk to draw groundwater from under the lagoon and surrounding mainland. The groundwater drain caused the underlying aquifer to contract and the land above it, including the Venetian islands, to sink. Flooding became more frequent. Policy changes have stopped the sinking⁴ and the Italian government is sponsoring a massive project to float gates into the strait that connects the Venetian lagoon to the Adriatic. Once completed, the gates can be closed when high tides are expected, blocking high water from entering the lagoon⁵.

This isn’t the only example of gates being built to stop the sea. London is vulnerable to flooding when storm surges and high tides flow up the Thames estuary from the North Sea. Even routine tidal flows are famously extreme along the Thames as the water literally rushes in a wave to and from the sea. When weather systems cause these tides to rise unusually high, damage and deaths can result, as happened many times throughout the 20^th century. So in the early 1980s the Thames Barrier was constructed at the cost of hundreds of millions of pounds. The barrier consists of a system of movable gates placed across the Thames south of the city center. During very high tides or storm surges, the gates can be deployed to stop the incoming flow of water.

In America, the city that is probably most associated with flooding vulnerability is New Orleans. No one could ever forget the terrible scenes as large parts of the city were literally submerged as the levees that blocked the city from the sea were breached in the aftermath of Hurricane Katrina. About half of New Orleans is below sea level, in fact in many parts of the city residents look up at the levees that hold back the Mississippi River and Lake Pontchartrain. It’s this damming of the Mississippi within and upstream of New Orleans that has resulted in the river “rising” and the land sinking⁶. The “high” half of the city is still no more than a few feet above sea level. New Orleans remains vulnerable to flooding even as major flood control projects continue in and around the city.

New Orleans will always be vulnerable to flooding, but it isn’t the only major urban area that is dealing with coastal flooding as a present threat rather than a future concern.

America’s Most Vulnerable Urban Area

Satellite view of Hampton Roads

One of the world’s great natural harbors lies in southeast Virginia where the wide tidal mouth of the James River flows into the Chesapeake Bay as it empties into the Atlantic Ocean. This mixing of waters is known as Hampton Roads and it’s surrounded by low-lying coastal plain carved by tidal rivers and streams. This land is heavily built-up and includes the large cities of Norfolk, Portsmouth, Chesapeake, and Virginia Beach on the south; Hampton and Newport News to the north.

This area is awash with history. It was only a short distance up the James where the first permanent British settlement on mainland North America was established at Jamestown. It was at Yorktown (just to the north) where General Cornwallis surrendered to George Washington, effectively ending the American Revolutionary War. Later, during the American Civil War, Hampton Roads was the scene of several important battles including the first ever clash of ironclad ships as the Confederate Merrimack⁷ traded cannon fire with the Union Monitor. Naval warfare would never be the same. In a way this is very fitting because Hampton Roads is synonymous with naval power. Norfolk has long been home to the U.S. Atlantic Fleet, and Norfolk Naval Base is the largest in the world. It’s deep harbor and proximity to the ocean has also made the cities of Hampton Roads major commercial ports, collectively they are second only to New York among Atlantic ports in total tonnage⁸.

In many ways Hampton Roads reminds me of my hometown of Baltimore: a large urban area surrounding a deep natural harbor and a culture that embraces this coastal proximity; although Hampton Roads is closer to the sea – both in the literal and figurative senses. This proximity to the ocean may have major drawbacks as Hampton Roads could be the first urban area in the U.S. to be inundated by rising sea levels. Norfolk has seen a lot of flooding in recent years, from storm surges, to extreme high tides, to rain water that can’t drain from the low land. It doesn’t help that Hampton Roads is built on land that is sinking naturally by seven inches per century.  Leaders of coastal cities all across the country are keeping a wary eye on Hampton Roads to see how the region adapts to rising seas.

Solutions
As the routine flooding of Venice or the catastrophic Katrina-caused flooding of New Orleans demonstrate, it’s unlikely that geographic vulnerability will result in the abandonment of coastal cities. Hampton Roads will stay where it is, but solutions to hold back rising water will have to be found. These could include flood walls and levies like in New Orleans or the Netherlands, tidal gates like in Venice and London, and new powerful pumping stations to keep the lowest-lying terrain dry. It could also include common sense policies like avoidance of groundwater depletion (as enacted in Houston for instance), restrictions on future development in the most vulnerable flood areas, and even abandonment of some areas that are unlikely to be saved from rising water. And there are other environmental solutions, like reestablishment of coastal wetlands that can blunt the effects of high tides and storm surges.

The rub in all of these solutions is that they’re expensive. Estimates for Norfolk range from many hundreds of million dollars to several billion dollars. For one location this may not seem excessive, but multiplied by several dozen, and saving the American urban coastline could be one of the largest public works projects in history. Fortunately, even the most pessimistic estimates of sea level rise give cities, states, and the federal government decades to plan for the right solutions and dedicate needed funding.

As Venice and London solved their flooding challenges, so can American cities. Addressing natural challenges is never easy but it’s a challenge we must take on. Assuming the right steps are taken, Norfolk won’t be washed away and neither will the rest of America’s coastal cities, or for that matter, the many global urban areas that front the sea.

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NOTES:

1. I remember just such a project in the late 1980s to save the beach at Ocean City, Maryland, the long, thin barrier island and major East Coast summer resort on Maryland’s Atlantic coast. Today the island of Ocean City remains locked in place where it’s been since development began early last century. Compare that to gradual westward migration of Assateague Island immediately to the south, a state and national park where development is prohibited. Today the two islands are visibly offset with the southern end of Ocean City jutting out into the open ocean.

2. An organization that has suffered its share of criticism over the last several years.

3. And this is only one example of how changing global temperatures could cause rising or falling sea levels. When you consider the effects of tides, current, winds, storms, pressure, sea temperature, precipitation, global weather patterns like El Nino, it seems like it’s hard to pin down the specific reasons for changes to sea level or any other earth system. It certainly makes intuitive sense that rising global temperatures will lead to rising sea levels, but Earth systems seek equilibrium and changes to one system may not result in the expected changes to another system.

4. Or at least greatly reduced it. The policy changes include eliminating wells that draw from the underlying aquifer.

5. A close friend of mine is working on the mechanical system that will manipulate the gates.

6. The lower Mississippi River is crucial to the economy of New Orleans, and hence the Army Corps of Engineers has spent the last 100 years building levees along its banks to keep it flowing on its current course. The river would naturally like to shift to take a more direct route to the Gulf of Mexico. This shift would have naturally occurred decades ago at the Atchafalaya River north of Baton Rouge, a major distributary that already carries 30 percent of the Mississippi’s flow. Instead, flood control projects have kept the Mississippi where it is and prevented flood waters from depositing sediment along its shores. The result is the rising of the river relative to the land, which ultimately has put New Orleans at a greater risk for flooding.

7. This was the original name of the ship, which was sunk early in war, captured and raised by the Confederacy and renamed the C.S.S. (Confederate States Ship) Virginia.

8. During just one day on a recent visit I saw no less than a dozen large freighters, an amphibious assault ship (like a small aircraft carrier used by the Marines for amphibious assaults), and an Arleigh Burke destroyer plying the waters of Hampton Roads.I also saw (and heard) six F-18s either flying to or from an offshore aircraft carrier or conducting exercises.