Thirteen-foot-high floodwalls could line part of Miami's waterfront, under a proposed Army Corps of Engineers plan being developed to protect the area from storm surge. The $4.6 billion plan is one of several drafted by the Corps of Engineers to protect coastal areas in the U.S, which face increased flood risks stoked by climate change. Similar projects are already underway in Norfolk, VA and Charleston, SC.

The plan for Miami-Dade County, which is open for public comment, is intended to protect the 2.8 million people who live there from coastal flooding and storm surge during tropical storms and hurricanes. Many are concerned that the system of floodwalls, pumps and surge barriers doesn't directly address a threat many in South Florida are already dealing with more frequently than storms: rising sea levels. Chronic flooding is a problem for some neighborhoods in Miami-Dade County during heavy rain events and seasonal king tides.

Global warming is also making hurricanes more powerful, according to a recent study by the National Oceanic and Atmospheric Administration. Miami is among the cities most vulnerable to sea-level rise globally, which means that storm surges driven by hurricanes will be even higher and potentially more destructive than in the past.

In 1992, a 17 foot storm surge from Hurricane Andrew caused more than $500 million in losses to an area just south of Miami. Miami-Dade County's population has grown by a million people since then. The development of high-rise residential areas downtown and on Miami Beach expose many more people and structures to the risk of storm surge.

The Corps of Engineers plan calls for storm surge gates to be installed on three waterways that open onto Biscayne Bay, including the Miami River. That's a commercially important waterway, with cargo terminals and repair facilities.

Chief Resilience Officer for Miami-Dade County Jim Murley says: "You have navigation issues, we have drainage issues and water quality issues, all of which have to be factored into the ultimate design of any kind of gate."

The plan also calls for a series of pumps and floodwalls along Miami's waterfront. North of downtown, the floodwalls would be anywhere from one foot to 13 feet high, depending on the location. From the Miami River—in the heart of downtown—stretching south along the shoreline, a storm surge barrier as high as 36 feet would rise from the floor of Biscayne Bay.

The project manager for the Corps of Engineers, Holly Carpenter, says exact dimensions and design of the surge barrier haven't yet been determined. "It's definitely at a conceptual level," she says. "We're concerned more about the feasibility of the type of measures recommended."

Under the current proposal, that seawall would likely block views of the water and hinder access to the bay for people who live and work in Miami's Brickell neighborhood, a thriving residential and commercial district. Rachel Silverstein with Miami Waterkeeper says, "It's hard for me to imagine that this community is going to accept a massive seawall down the waterfront in Brickell."

The plan also calls for elevating and flood-proofing thousands of homes, businesses and public buildings in vulnerable neighborhoods.

Silverstein agrees that Miami needs infrastructure to protect it from storm surge, but believes the Army Corps of Engineers didn't give enough consideration to natural solutions, such as restoring coral reefs or "living shorelines."

"This project is really reflective of an agency that has a hammer and sees everything as a nail," Silverstein says. "We know there is really valuable protection from storm surge given by natural infrastructure like coral reefs, mangroves, dunes and things like that."

The Corps' plan calls for extensive planting of mangroves along the shoreline in a residential area in the southern part of the county. Susan Layton, who is overseeing the project, says the Army Corps of Engineers looked at natural solutions like oyster reefs and living shorelines for other parts of the county. But she says, "Many of these features, it's very difficult to get significant storm surge reduction or coastal storm benefits."

The storm surge initiative takes into account a projected three and a half foot rise in sea level in Miami over the next 60 years.

It doesn't offer any solutions to the increased flooding the city sees during high tides and big rain events. But Miami resilience officer Jim Murley says, during a storm, higher sea levels pose an even greater threat. "If you receive a storm surge, it's higher by virtue of the fact that the water itself has risen. Sea level rise plays an important aspect in everything we do."

Planning for the Miami storm surge project won't be complete until next year. At that point, the project is dependent on approval and funding from local officials and from Congress. A similar but much larger project was proposed after Hurricane Sandy to protect New York City and New Jersey from storm surge. It was put on hold by the Corps earlier this year after President Trump tweeted his opposition to it.

Copyright 2021 NPR. To see more, visit https://www.npr.org.