Despite all the focus on salt water covering coastal streets during seasonal king tides in South Florida, most of the problem floods in Miami-Dade come from well-known sources: rainfall and the vast pool of fresh water under our feet.
In low-lying inland areas, floodwater quickly fills up South Florida’s Biscayne aquifer, the freshwater drinking supply just under the ground’s surface. One solid rainstorm — the kind Florida is famous for — and water ponds up in streets and yards. When the aquifer is brimming full, there’s no place for that groundwater to go but up and onto land. At least until it flows into drainage canals and sewer systems.
But, thanks to sea level rise, that common kind of flooding could be more frequent and worse in the future, a new study from South Florida researchers shows.
“We just can’t tolerate the rainfall anymore,” said Florida International University hydrologist Michael Sukop, a lead researcher on the study.
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The team focused on one particular flood-prone neighborhood near North Miami — Arch Creek. The area could see double the number of damaging floods and triple the number of severe floods by 2060, the FIU analysis found. A companion study found, without a major overhaul of the drainage system, that could cause up to $8 million of damage countywide per month.
The researchers analyzed the neighborhood and found that the groundwater — already extremely close to the surface — is rising at the same rate as the ocean a few miles away, leaving water managers stuck pulling off a balancing act.
Officials can — and do — lower the groundwater and canal levels before big storms and higher-than-normal tides, both of which are expected to become more common as the planet warms. But lowering the freshwater level can open the gates for sea water to push further inland underground, which can taint the aquifer and eventually cut into the underground supply of fresh drinking water. Rising seas will only add more pressure for salt water to wedge inland.
When that happens, the salt water taints freshwater wells in a process known as saltwater intrusion. It’s been a problem in coastal Florida for decades, and some wells have already been abandoned. To keep that from happening, officials have to keep the groundwater level high — and may have to push it even higher in the future to protect the drinking water supply.
Water managers have to walk a tightrope between flooding a city or contaminating water.
“That’s really the crux of the problem,” Sukop said. “It’s about this trade-off. They have these opposing things they’re trying to achieve.”
These low-lying areas are the natural spots where water used to flow from the Everglades to the ocean. Now developed, they experience worse flooding than surrounding areas — over and over again. This includes the neighborhoods near Little River, the Miami River, “and that pattern keeps going all the way south,” said Katie Hagemann, Miami-Dade County’s sea level rise adaptation manager and another researcher on the study.
The closer to the coast, the trickier the problem gets, Hagemann said. There, the groundwater and sea water levels are even more interconnected. Simply changing the water level won’t cut it.
“Over the longer term it’s harder to manipulate that,” she said.
Last year, the county partnered with the Urban Land Institute, a Washington, D.C.-based think tank, to research solutions. One proposal involved slowly buying and demolishing the hardest hit homes and returning them to nature, but that would require homeowners to willingly sell their land, and the government to find the money to buy it.
The county declared the Arch Creek area a pilot project for innovative solutions, and held a contest for new ideas in June. Three winners were selected for a second round of conversations, but without a source of funding, that’s as far as the projects went.
“In this case we were sort of testing the idea,” said James Murley, the county’s Chief Resilience Officer. “We don’t have a pot of money set aside to fund the concepts that were presented.”
Furthering the problem is how little of the land in the area the county owns, most of which is residential. Murley said any major action in the area would come from the city of North Miami, and he hopes the studies the county commissioned will influence future projects.
“Perhaps they’ll go about it in a different way than they would have in the past,” he said. “In the future, pilot projects will likely be tied to potential funding sources.”