The Rising Tide: Global Warming Accelerates Coastal Erosion

Sept. 1, 1999

For 8 mi. between Holly Beach and Constance Beach in southwestern Louisiana, State Highway 82 runs beside the Gulf of Mexico on a narrow ridge of sand. A primary hurricane evacuation route for five communities in the region, the two-lane highway originally was protected from the gulf by at least 300 ft. of beach and dunes when it was built in the 1930s. But severe erosion caused by a combination of storms, land subsidence, a deficit of sand in the coastal zone, and sea-level rise has required the state to move the road three times in the last 60 years.

Today, Highway 82 is making its last stand against the sea. The sand ridge on which the highway sits, a formation known to geologists as a chenier, is the only remaining barrier between the gulf and some 31,000 ac. of fragile marshes on the landward side. Erosion of the beach has progressed so far inland that waves now lap against the edge of the road when the wind blows from the south. “Even minor storm events overtop the highway,” says Steve Gammill, natural resources geoscience supervisor with the Louisiana Department of Natural Resources’ (DNR) Coastal Restoration Division.

Louisiana’s Department of Transportation and Development (DOTD) spends an average of $600,000 a year to repair and protect this 8-mi. stretch of Highway 82. Still, about 40% of the road’s seaward revetment needs repair or replacement, and large wave-scoured holes on the landward side threaten to undermine the westbound lane. Unless new protective measures are taken, it is only a matter of time before the sea starts cutting channels through the roadway embankment into the marshland beyond. The marshes are prime fish and wildlife habitat, a high-quality waterfowl wintering area, and the largest remaining buffer between the gulf and metropolitan Lake Charles to the north. “Any exposure to high-salinity gulf waters will result in severe salt burn and a loss of fresh and low-salinity brackish vegetation,” warns Gammill. “If the chenier is breached, there will be less storm protection and more loss of habitat in southwestern Louisiana.”

Global Warming and Coastal Lands

As rising seas threaten property, structures, and natural resources, the problems facing southwestern Louisiana will most likely affect much of the world’s other coastal regions. If predictions of future climate change and sea-level rise are accurate, communities and governments will grapple in the years ahead with a number of challenging questions about how to preserve and protect their coasts.

Scientists expect that global warming will raise sea levels by melting glaciers and ice sheets and by warming the upper ocean, causing it to expand. This sea-level rise will accelerate erosion, inundation, and the loss of beaches and wetlands. It could threaten aquifers and allow salt water to penetrate farther inland in wetlands and upstream in rivers. It will lead to higher storm surges, increasing the area subject to flooding from coastal storms and placing more land in the path of wave-driven erosion. Small islands, deltaic regions, coastal wetlands, and developed sandy shores are expected to be especially vulnerable to erosion and flooding.

According to the Intergovernmental Panel on Climate Change, a group of some 2,000 experts under the auspices of the United Nations Environment Programme and the World Meteorological Organization, the sea will rise globally by an average of 6-38 in. from today’s level by the year 2100. The panel’s best estimate is 20 in. In the United States, the sea currently is rising along the Atlantic coast by 0.08-0.10 in./yr., 0.10 in. along the gulf coast (except Louisiana, where it is rising by 0.40 in./yr.), and 0.04 in. along the Pacific coast. If those rates continue or accelerate over the next century, as scientists believe they will, the consequences will be serious.

A 1-ft. rise in sea level could erode beaches 50-200 ft. along most of the US’ Atlantic coastline, 200-400 ft. along the California coast, and perhaps several miles in low-lying Louisiana. A 20-in. sea-level rise could eliminate 17-42% of US coastal wetlands, with an even greater loss if shorelines are protected by bulkheads and other structures that prevent wetlands from migrating inland. A 3-ft. rise would flood 7,000 square mi. of dry land—an area the size of Massachusetts. Individual coastlines will experience varying degrees of sea-level rise depending on their geologic conditions, and the impact on coastlines will be affected by factors such as their relief and whether they are made up of rock or sand.

“We’re concerned about these predictions,” says Steve Underwood, a natural resources program manager with the Louisiana Department of Natural Resources. “We’re fighting natural subsidence all the time, and it doesn’t take much sea-level rise to inundate more marsh around here.”

EPA is concerned about the predictions too, and in 1982 it launched an outreach program to inform the public, state and local planners, and coastal community officials about the potential impacts of global warming on coastal resources.

The Erosion Threat

Lands below the tideline have long been subject to erosion from movement of tides and waves. But the eroded sediments, soils, and sands usually are redeposited so that shores or tidelands change in shape but do not completely disappear.

Stable lands that lie above the tideline in coastal areas, or fastlands, are vulnerable to erosion from storms and development. In the past few centuries, a gradual rise in sea level has moved the tide line inland, submerging many tidelands and subjecting some fastlands to greater erosion. Along undeveloped shores, grasses and other plants that trap eroded soil typically colonize the new tidelands created by sea-level rise. This allows tidal wetlands and beaches to gradually migrate inland along with the tideline—provided that the regional topography allows this migration to occur. If there is an abrupt change in elevation between the shore zone and the uplands, as in coastal Louisiana, this habitat transition cannot occur and tidal wetlands will be lost.

Shores in highly developed areas cannot move inland as the sea rises without affecting the neighboring community. It is here that sea-level rise is expected to have the most serious impacts. It also is where coastal communities will face many of their toughest choices. “If you’re making a decision to protect buildings and infrastructure that are up on the shoreline,” says Jeff Williams, coordinator of the coastal and marine geology program at the US Geological Survey, “you need to recognize that if you put in a seawall, a groin, or a revetment, eventually you will lose the beach.” When property is protected against the sea by bulkheads or other barriers that armor the coast, the natural inland retreat of the shoreline is blocked. Caught between rising sea level and these so-called “hard engineering” structures, many shores will have nowhere to go but under the sea.

The Soft Path

A growing number of coastal engineers and planners now recognize that it makes more sense to work with nature than to fight the coast’s powerful and dynamic forces of change. “The idea that engineers should build groins, jetties, seawalls, and revetments is very much going out of favor,” Williams observes. “What’s coming into favor are soft engineering practices, such as beach nourishment [adding sand to sustain the beach] and the construction of artificial dunes. We need to try to accommodate how we live in coastal areas with the natural processes that occur there.”

Louisiana’s long-term strategy to protect Highway 82 follows this softer path. Beginning in 1985, the state DOTD installed a series of six experimental breakwaters offshore of the highway to trap sand and reduce the energy of incoming waves. Based on the success of that initial effort, in 1991 Louisiana began building what is now the largest breakwater system in the US. Today, 85 breakwaters stand 200-500 ft. offshore to protect Highway 82 and the chenier from storms and wave-driven erosion. The 150-ft. rock structures rise 3 ft. above mean low tide, each breakwater separated from its neighbor by a gap of 300 ft.

So far, the project has been a qualified success. The structures are indeed diminishing wave energy and causing waves to dump their sediments, creating small hummocky sandbars called salients on the lee sides of more than half the breakwaters. But the gaps between the breakwaters are too large, allowing enough wave energy to pass through to erode the shoreline and the chenier. The shoreline in most areas behind the breakwaters is continuing its landward retreat-in some places by more than 17 ft. a year. The breakwaters also are not high enough to prevent storm waves from overtopping the highway.

The Louisiana DNR and DOTD are seeking funding to lengthen and raise the breakwaters and to manage the area’s sand resources. The state hopes to use dredged materials from a nearby ship channel or borrow sand from other nearby areas to create a line of “sacrificial dunes” that will protect the road from high tides and storm surges. The dunes could be maintained by mining sand from the salients that form behind the breakwaters, and new sand could be borrowed from other sites to create “feeder berms”—artificial beaches that would erode and supply material to areas farther down the shoreline, replacing sand lost to the system from sediment trapping by the breakwaters. “We think we can manage the sand so it stays in the system,” says Gammill. There is strong political and popular support for the project, which will cost an estimated $23 million-$30 million on top of the $8.5 million-$9 million spent to build the breakwaters. Gammill says he is optimistic that the project will be funded by a variety of state and federal sources.

Soft engineering makes sense in the effort to protect Highway 82, as building a seawall would cost $67 million and destroy the remaining beach in front of the chenier. But there is no perfect solution. “Soft engineering is a good approach in that it emulates the natural process, but it’s not permanent,” Williams points out. “You need to go into it with the understanding that it’s going to be expensive and that it’s an impermanent measure that will have to be revisited on a periodic basis.”

A Planning Issue

Addressing sea-level rise over the long term ultimately is a planning issue. “We ought to keep in mind that the coastal zone is a geologically hazardous place,” says Williams, “and we ought to be thinking very carefully about how much development we allow there.”

To this end, EPA is working with coastal communities, state coastal-zone managers, nongovernmental organizations, and other stakeholders to spread the word about the risks of sea-level rise to coastal areas. EPA’s goal is to increase public awareness so that people who live along coasts can make informed decisions or voice their opinions about risk-management strategies that address global warming.

The need for public awareness and planning is underscored by the fact that coastal areas in many parts of the world are experiencing rapid population growth. In the US, population densities on the coasts are five times the national average, and half the people in the nation live within 45 mi. of a shoreline. The population is growing faster in coastal areas than in any other region of the country.

For the most densely developed of these areas, holding back the sea or abandoning property might be the only options. But less-developed areas can avoid unnecessary costs and the loss of valuable natural resources by planning now to limit future development in the coastal zone and allowing the shoreline to migrate landward as the sea level rises.

State and local governments can limit coastal development by enacting regulations, purchasing land, or shifting economic risk from governments onto individual property owners.

Governments historically have played a strong role in the protection of shores and the control of erosion, with shore protection having the longest legal history. Under tenets of property law established at least as far back as the court of Roman Emperor Justinian, the state is supposed to hold in trust for the public all lands below the tideline. In the US, protection of public-trust tidelands is conveyed primarily through two points of property law: the “law of erosion” and the “public trust doctrine.” According to the former, public ownership migrates inland when shores erode; according to the latter, the state holds tidelands in trust for the people unless it decides otherwise.

Planners can use these laws to help allow the natural migration of shorelines. For example, rolling easements let property owners develop their land, providing that they do not hold back the sea with measures that eliminate publicly owned beaches and wetlands found below mean high tide. As the sea rises, the public’s easement “rolls” inland. In essence, this puts owners on notice that some of their property will be lost when and if the shore erodes enough to threaten it.

States that have enacted rolling easements include Texas, Maine, Rhode Island, South Carolina, and Massachusetts. The Texas Open Beaches Act puts builders of new houses on notice that a building must be torn down if it becomes seaward of the vegetation line in areas along the gulf coast where the public has an easement. Some states use setbacks to prevent coastal development. Setbacks are statutes or regulations that make it illegal for property owners to develop certain areas of their land, anywhere from a few feet to several hundred feet inland of their shoreward property line. Other planning tools include provisions to prohibit property owners from building bulkheads in the future and the outright purchase of land where wetlands or beaches can migrate as sea levels rise.

Shifting the economic risk of development onto landowners is another long-term strategy that may become available to governments. Federal, state, and local governments provide flood insurance, infrastructures such as sewage systems and bridges, and other public benefits. Although the flood insurance system as a whole is actuarially sound, current statutes provide people along the water’s edge in eroding areas with artificially low rates, subsidized by the flood insurance policies of people in relatively safe areas. States generally provide free land for the construction of revetments, bulkheads, and other erosion control structures along coastal areas.

But the idea that taxpayers should subsidize people who choose to live in high-risk areas is beginning to fall out of favor. “At the very least, there should be limits on insurance for areas that are subject to collapse within the next 10 to 20 years, based on historic erosion rates,” says Rutherford Platt, professor of geography at the University of Massachusetts at Amherst. “There should be limits on the availability of coverage and/or a surcharge on the cost of coverage in areas subject to imminent coastal erosion hazards, which are exacerbated by sea-level rise.”

Federal Climate Change Program

EPA is working with other federal agencies to help slow climate change through its voluntary partnerships under the federal Climate Change Action Plan. These programs directly reduce emissions of greenhouse gases from industry, transportation, office equipment, appliances, landfills, farms, and other sources. In 1997 alone, businesses and agencies participating in voluntary Climate Change Action Plan programs together reduced carbon-dioxide emissions by more than 40 million tons, saving more than $1 billion in the process.

Most of EPA’s efforts in the field of global warming have focused on reducing greenhouse gas emissions. But it is also active in developing policies to prepare for problems of global warming, such as rising seas. There is no choice: The earth is warming and the sea is rising.

If we plan now, we can save our natural wetlands and beaches without the harm to coastal property owners that has sometimes accompanied efforts to protect our environment. But to do it right, we will need the help of civil engineers, geologists, and other people who are on the front lines of addressing the challenges of coastal erosion.

Information on the coastal impacts of climate change and links to EPA publications on sea-level rise can be found on EPA’s global warming website.

About the Author

Julie Spyres

 Julie Spyres is the communication team leader for EPA's Climate Outreach and Innovation Division.