South Florida came under another flood watch Wednesday evening as more rain threatened to drench already saturated ground.
The National Weather Service’s Miami office warned that the rain could prompt more flooding in Miami-Dade, Broward and Palm Beach counties as parts of Miami-Dade dug out from a Tuesday deluge.
The days-long rain that started over the weekend left roads underwater, overwhelmed storm systems and caused a district wastewater plant to back up and dump about a million gallons of sewage.
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At Miami International Airport, more than 14.6 inches fell in just three days — an amount equal to a rare, once in 40-year flood event, said Brian McNoldy, a researcher at the University of Miami's Rosenstiel School of Marine and Atmospheric Science.
The unending rain left canals gushing — water discharging from the Hillsboro Canal tripled between Monday and Tuesday — and saturated the ground. In Hialeah, groundwater levels jumped about 2.5 feet in just hours, according to the U.S. Geological Survey.
The rain, coupled with the high groundwater, also helped doubled the amount of wastewater flowing in the Miami-Dade’s North District treatment plant. Flows doubled to 200 million gallons a day, causing sewage to overflow, said Miami-Dade deputy water and sewer director Doug Yoder. Two pipes that pump sewage also broke.
“So it’s been a mess for sure,” he said.
The South Florida Water Management District said in a statement that it was operating the flood control system “at full capacity.” The district did not provide a request for more information by Wednesday evening.
The sewage spill largely flowed into mangroves near the plant. While the county has been testing sewage for the COVID-19 coronavirus, Yoder said the virus doesn’t increase the risk of coming into contact with raw sewage.
“There are enough other pathogens there that you want to steer clear of raw sewage,” he said.
So far, while sewage can be tested to detect evidence of the virus in waste, the virus itself does not appear to survive, he said. Ultraviolet light used to treat sewage can also destroy the virus, he said.
While so much rain is rare, sea rise is making South Florida’s drainage canals less efficient, Yoder said. Most of the coastal gates that release floodwater have gates that rely on gravity. But with sea rising driving up water levels, the water no longer flows out.
“If those canals which discharge into the bay can't drain the water fast enough, then the water from the secondary canals can't get into the primary canals,” he said.
That leaves no place for the groundwater to go.
“So the water has to pile up someplace, and in some cases it's in your front yard,” he said.
The district is currently taking a look at coastal infrastructure, he said. But replacing structures can be expensive. Last year, the district’s hydrology chief said 26 of 36 coastal pumps were likely to fail and replacing them could cost $70 million a year.
The heavy rain also exposed weaknesses in the county’s wastewater system. While building a system to handle so much capacity on a daily basis — with bigger pipes and more treatment plants — would be inefficient, storage could be added to hold untreated sewage until storms pass, Yoder said.
And expanding the system may become unavoidable if extreme events worsen. This year, the United Nations warned that in Miami and Key West, 100-year storm events could occur yearly by 2050.
“That may become something that has to be considered,” he said, “If what used to be a 50-year storm becomes a 10-year storm which, if you have enough, it will.”
