08 Jul 2013
Inside Coastal Protection
Hurricane Katrina hit New Orleans head-on in 2005. $50 billion in damages occurred as the storm rampaged along the coast. Why was it so much? For centuries the New Orleans coastline was protected by barrier islands and salt marshes that protected it from devastating storms. But the state has lost 4,900 km2 of coastal land in the last 75 years. These natural buffers disappeared, because dams on the Mississippi River trapped the sediment that would have nourished the islands and marshes. Following Katrina, the city constructed a 350-mile ring of levees, flood walls, gates and pumps to protect the city. It took 8 years and cost $12 billion to replace something we once enjoyed “for free”.
With the onslaught of extreme weather, the value of coastal protection - natural habitats that grow along the coastline and protect the land - has become clear. Many of the ocean's benefits seem to be "free" because we don't pay for them - but unfortunately, we do pay when we lose those benefits.
In late October 2012, Hurricane Sandy caused $19 billion in economic damage to the New York-New Jersey area, partly because development had consumed most of the salt marshes, vegetated islands and sand dunes that formerly protected the coast. In June 2013, New York City mayor Michael Bloomberg proposed constructing an integrated system of reinforced dunes, widened beaches, levees, floodwalls, bulkheads, tide gates and surge barriers to protect the city’s 520 miles of coastline. The plan blends artificial barriers with the natural protective capacity of restored dunes and more trees and vegetation along streets, on rooftops and elsewhere, that would absorb hurricane rainwater and channel it to new high-capacity sewers to reduce flooding in low lying areas. Building the system would be expensive---$20 billion---but the cost of not building could be even more costly, since the global reinsurance company Swiss Re Ltd., estimated that damage from a storm like Sandy in the 2050s would cost $90 billion in lost property, jobs, infrastructure and government expense.
If we knew how much of those costs came from replacing protection that marshes and vegetation-stabilized barrier islands had once provided, then we would begin to know what “free” actually means. That estimate would actually understate the true value of habitats that protect coastlines, since most of them also provide other benefits, including carbon storage, food, natural products, biodiversity, jobs, tourism and recreation, so their value is even greater and we should appreciate them all the more.
WHAT IS COASTAL PROTECTION?
As we become more aware of the damage that storm waves, tidal surges and tsunamis cause worldwide, particularly in combination with steadily rising global sea level, the importance of coastal protection becomes ever more obvious.
You might ask, ‘How can the ocean provide protection from itself?’ The answer: in addition to lots of water, the ocean also contains a diverse array of living communities, some floating or swimming, others attached to the sea bed or coastline. Some of those fringing communities---especially coral reefs, mangrove forests, seagrasses and salt marshes---buffer shores from storm waves, tsunamis and consequent flooding and coastal erosion. Sea ice has traditionally protected arctic shores in the same ways.
What else can stop large waves from pummeling a shore? Natural geological features such as slope of the coast, cliffs, points, shoals or offshore islands shelter shorelines fortunate enough to have them, but we can’t easily provide them for open coastlines that don’t. Clever engineering, such as systems of dikes in the Netherlands, tidal barriers on London’s Thames River, barriers proposed to protect Venice, Italy, or the systems built for New Orleans or planned for New York can also protect some areas.
HOW DOES THE OCEAN HEALTH INDEX EVALUATE COASTAL PROTECTION?
Natural habitats offer protection only where they are present and in good condition. As you can see below in Figure 1, the Ocean Health Index evaluates goals (or basic human needs from the ocean) by comparing the current status of each one to a selected reference point. In this case the reference point that we selected is the extent and condition of five key habitats that protect coastlines (mangrove forests, seagrass meadows, salt marshes, tropical coral reefs, and sea ice) that existed in 1980. The ~1980 reference point (target) was chosen because data are available and restoration of damaged habitats to their 1980 condition may be possible, but restoration to earlier more pristine historical extents and conditions is not realistic. In other words, we selected 1980 because we know the extent of each habitat for then, because satellite observations became available at that time. In most places there is less of all these habitats in 2013. So the 1980 extent (area) and condition of each habitat gives us an attainable target that is close to sustainable. For each habitat we take the extent of habitat today and divide it by the extent in 1980 (the reference point) and that forms the major part of current status (see Figure 1). The current status is 50% of the score.
The likely future status is made up of the trend (the average percent change for the most recent 5 years), the cumulative pressures that compromise future status, and the resilience factors that can reduce future pressures. Pressures are conditions that worsen the status of the habitat. Resilience is factors that promise to improve the status of the habitat. These three categories make up the other 50% of the score.
For coastal protection, pressures included subtidal and intertidal habitat destruction, destructive artisanal fishing, alien species, sea surface temperature, ocean acidification and ultraviolet (UV) radiation. Other known pressures like sediment from land erosion—known to be particularly damaging to coral reefs and seagrass beds--couldn’t be included because global data do not yet exist. Similarly, sea level rise (SLR) threatens mangrove forests, salt marshes and coral reefs, but even though its average global rate is known, oceanographic and geologic factors cause great variation locally, so SLR could not be evaluated in the global study.
Resilience factors included actions to mitigate pollution and protect biological habitats. Unfortunately, there is no quick or direct way to manage sea ice, though we can influence its long-term seasonal extent, duration and thickness through thoughtful policies on conserving, using and generating energy. Only well-governed societies can reduce pressures in these ways, so resilience calculations also include measures of social integrity contained in the World Bank’s Worldwide Governance Indicators.
Scores for current status and likely future status are averaged to produce the overall goal score.
Ideally this goal would evaluate the amount of protection provided to coastal areas that people particularly value, such as inhabited areas containing homes, businesses, schools, hospitals and other structures as well as uninhabited areas containing parks or other culturally or biologically special places. However, at the global scale it wasn’t possible to calculate how much protection features provided specifically to such areas, so the analysis assumed that all coastal areas are equally valuable and the total area and condition of protective features were evaluated without regard to their precise locations relative to coastal areas.
You can find details about the Coastal Protection goal and its components here.
COASTAL PROTECTION AROUND THE WORLD
New Orleans is vulnerable because it’s in the hurricane belt, and because it’s built on low, marshy land that compacts and settles as much as one inch every decade. Parts of the city lie as much as 17 feet below sea level. However, New Orleans is not unique: many cities worldwide are equally vulnerable. In a global survey of 136 port cities, about 50 million people and $3 trillion in assets (buildings, airports, subways and other infrastructure) were vulnerable to coastal flooding in 2005. The top ten cities for exposed population were Mumbai, Guangzhou, Shanghai, Miami, Ho Chi Minh City, Kolkata, Greater New York, Osaka-Kobe, Alexandria (Egypt) and New Orleans. The top ten cities for exposed assets were: Miami, Greater New York, New Orleans, Osaka-Kobe, Tokyo, Amsterdam, Rotterdam, Nagoya, Tampa-St Petersburg and Virginia Beach. By the 2070s, sea level rise and increased storminess caused by climate change, as well as subsidence, population growth and urbanization will expose 150 million people and $35 trillion in assets to coastal flooding---and that is just in port cities.
Several factors underlie the large projected increases in people and assets that will be exposed to coastal flooding in the future. Sea levels will continue to rise for some decades even if the rate of global warming slows. Increased global warming will probably cause more intense and frequent storms. The human population will grow by two billion or more people. And more people will inhabit coastal areas. About 40% of the global population live within 100 km of the coast and that percentage is rising as urbanization and migration to the coast occur everywhere. For example, in the continental U.S., shoreline counties make up less than 10% of the total land area, but hold 39% of the total population. Population in those counties increased nearly 40% from 1970 to 2010 and is expected to grow a further 8% by 2020.
Bangladesh is one of 9 countries that scored a perfect 100 for Coastal Protection, because its fringing habitats, especially the huge expanse of mangrove forest known as the Sunderlands, have largely been conserved. The mangroves absorb much of the force of storm surges caused by cyclones that make landfall in Bangladesh after sweeping up through the Bay of Bengal. Three-quarters of the nation is less than 10 m above current sea level and 80% of it experiences frequent cyclone- or monsoon-related flooding from the Ganges, Brahmaputra and Maghna Rivers, upon whose deltas the country is located. Unfortunately, Bangladesh’s mangroves can’t protect the country forever. Bank erosion caused by floods and increasing salinity caused by rising sea level can kill the trees. If the nations of the world do not quickly and significantly reduce their emissions of heat-trapping gases like CO2, natural habitats will be unable to save low lying countries like Bangladesh from floods and storm surges that will make large portions of their land uninhabitable.