In its first detailed critique of a $4.6 billion plan to erect massive flood walls around Biscayne Bay, Miami-Dade County said the U.S. Army Corps of Engineers failed to fully consider South Florida’s complicated hydrology and warned the fixes could make flooding worse from routine rain.
The blueprint sometimes used faulty data and incorrect assumptions about the watershed, staff said.
“Significantly lacking data on local geology and hydrogeology - highly recommended that a geologist/hydrogeologist experienced in South Florida update the geology section,” one comment noted.
The Corps plan, known as the Back Bay study, focuses on seven of the county’s most vulnerable areas around Biscayne Bay. It calls for fortress-like flood walls along the downtown shorelines and flood gates across the Miami River, Little River and Biscayne Canal. Some buildings could be elevated and other property bought out to make way for structures.
The mile-long flood wall and gates drew the most scrutiny.
“The structures are really the hardest thing to get our arms around,” said Chief Resiliency Officer Jim Murley. “They're just not described in detail. They appear to be very, very much intrusive along our shoreline and we just don't see how they would operate.”
The Corps has said the design is only meant to be conceptual and that more expensive, detailed plans would come later. Murley said that complicates making decisions now. The county would be the local partner for the project, shouldering 35 percent of the cost.
“We have no experience with this kind of construction for the purposes of just storm surge, so I think that's the challenge,” Murley said.
The Corps is also planning flood projects for other cities and counties in South Florida and along the east coast. But Murley said the Miami-Dade plan is among the most complex because of the hydrology. Low-lying South Florida gets its water from the shallow Biscayne aquifer, which gets fed by rainwater. The porous limestone under the region means that the surface water and groundwater are often connected and readily flow into the bay and out to the Everglades off the high ridge occupied by much of South Florida’s neighborhoods and cities. Sinking flood walls could change that.
The Corps, which designed the region’s massive networks of canals and other flood control measures decades ago, has said flooding outside of storm surge can be addressed by operational plans.
But staff warned that the Corps needed to do a better job of ensuring the walls don't hasten saltwater intrusion as seas rise or worsen flooding from routine rain increases as the planet warms.
“The proposed alternative as presented may offer protection to surge risks with low probability of occurring while creating increased flooding during regular storms,” county staff wrote.
The plan also needs to better explain potential environmental damage and include more natural barriers, like mangroves that protect shorelines, staff said. The current plan only calls for restoring one small stretch of mangroves near the Deering Estate.
Biscayne Bay is already battling pollution that has wiped out more than 20 miles of seagrass meadows. Since the majority of freshwater entering the bay now comes from the rivers at the north end, environmentalists worry flood gates could worsen conditions.
“While we encourage investments in Miami-Dade that address storm surge risks, the proposed $4.6 billion plan is problematic,” stated the Everglades Coalition, an alliance of more than 60 environmental and conservation groups. “The study does not include costs to mitigate for the large-scale environmental impacts...including impacts to seagrass, corals, and other species.”
Over the next year, the Corps is expected to incorporate local concerns for a final version in the fall of 2021. The Corps could also agree to help pay for a local plan. Murley said the county is also in the midst of reworking its 30-year partnership with the Corps for maintaining beaches and is using that as a measure of how the two can work together.
“We've had a longtime experience and we understand working on the beach. Together, these are things that protect our community from storm surge,” he said. “And we may sometimes forget that from the federal government standpoint, they're willing to pay half because it's a storm protection investment.”
