The oceans are rising at an accelerating rate, and millions of people are in the way. Rising tides are already affecting cities along low-lying shorelines, such as the U.S. Atlantic and Gulf coasts, where sunny-day flooding has become common during high tides.
The U.S. Army Corps of Engineers has recently proposed building large and expensive seawalls to protect a number of U.S. cities from coastal storms and rising seas. Charleston, S.C., New York City and the Houston-Galveston metro area in Texas are considering proposals to build barriers, and the Corps recently published a draft proposal for a seawall for Miami.
I believe that large-scale seawall proposals raise long-term questions that residents, urban leaders and elected officials need to consider carefully before they invest billions of dollars. In my view, this approach is a short-term strategy that will protect only a few cities and will protect only selected portions of those cities.
The extent of high-tide flooding in low-elevation Atlantic coastal cities is well-documented, and so are future trends. In a 2017 study, the Union of Concerned Scientists assessed chronic flooding risks in 52 large coastal cities and found that by 2030, the 30 cities most at risk can expect at least two dozen tidal floods yearly on average. The study defined tidal flooding as seawater encroaching into at least 10% of a city.
This flooding won’t just become more frequent – it also will become deeper, extend farther inland and last longer as sea levels continue to rise.
The Army Corps of Engineers’ recent proposals include building an 8-mile seawall around Charleston at a cost of nearly $2 billion; a 1-mile wall for Miami-Dade County in Florida, with a price tag of nearly $4.6 billion; and a 6-mile barrier to shield portions of New York City and New Jersey, at an estimated cost of $119 billion.
The structures are intended to protect these areas from hurricanes and storm surges, and from future sea-level rise.
Here are some critical questions when projects like these are considered:
• Who and what will be protected by these large walls, and at what cost? With so many U.S. cities already experiencing coastal flooding, there are important questions about which portions of cities would be surrounded by walls and how much to spend.
• How many years of protection might these barriers provide? Flooding can result from short-term extreme events, such as hurricanes, and also from long-term sea-level rise. What time frame should these projects be designed to address?
• Who selects which cities or areas to protect? To date, proposals have come from the Army Corps of Engineers. Which officials and local, state or federal agencies should be involved in making these decisions and establishing policies that will guide responses to future sea-level rise?
• Do people really want to live behind walls? In New York, Miami and elsewhere, residents have objected to flood walls that would block views.
• Who will pay for the walls? Proposing multibillion-dollar walls is one thing, but where will the funds come from to construct and maintain these massive structures?
• Would these structures encourage additional development behind the walls? Typically, providing flood control encourages new construction in the now-protected area, which increases future liabilities and losses when walls are overtopped or fail.
• What other long-term options should be considered? Boston recently rejected building flood barriers in favor of softer options such as zoning with special requirements for new projects in flood-prone areas.
• Can cities that reject seawalls agree on thresholds or trigger points for taking other steps, such as using some combination of incentives and mandates to move people out of high-risk areas?
Such decisions will affect coastal communities, infrastructure and residents for decades. The sooner the science is understood and accepted, and everyone who is affected has an opportunity to get involved, the sooner cities can make plans.
Gary Griggs is the director of the Institute of Marine Sciences and distinguished professor of earth and planetary sciences at the University of California, Santa Cruz. Distributed by The Associated Press.
