Credit: Martin County Times
It's been a quiet hurricane season so far, despite a long-term forecast consensus among hurricane experts, who say conditions this year favor an active season with many storms. But that could be getting ready to change, writes hurricane expert Jeff Masters in his Weather Underground blog ("Fernand Hits Veracruz, Mexico; Active Atlantic Hurricane Pattern Setting Up").
"It's been an unusually quiet August for hurricane activity in the Atlantic, and if we finish the month without a hurricane, it will mark the first year since 2002 without an August hurricane," Masters notes. "However, the quiet weather pattern we've been blessed with is about to come to an end, as conditions favorable for hurricane formation move into place for the last few days of August and the first week of September. The big guns of the African Monsoon are firing off a salvo of African tropical waves over the next two weeks that will find the most favorable conditions for development that we've seen this year."
If a big, drenching storm should happen to cross Florida, there's an unusual risk, Masters observed in an earlier post: an overflowing Lake Okeechobee could rupture an old Corps of Engineers dike and flood nearby communities ("The Battle to Draw Down Lake Okeechobee").
"After the wettest July ever recorded in Florida, the Army Corps of Engineers is battling to draw down the level of Lake Okeechobee before the September peak of the rainy season," Masters writes. "The huge lake represents an important source of fresh water to South Florida, but also poses a grave danger. The 25 - 30'-tall, 143-mile long Herbert Hoover Dike surrounding the lake was built in the 1930s out of gravel, rock, limestone, sand, and shell using old engineering methods. The dike is tall enough that it cannot be overtopped by a storm surge from anything but an extreme hurricane, but the dike is vulnerable to leaking and failure when heavy rains bring high water levels to the lake. The Army Corps of Engineers is scrambling to complete a $300 million upgrade to the dike to reduce the chances of such a failure. However, those repairs are not scheduled to be completed until 2018, and the Corps is warning that the Lake Okeechobee dike is in danger of failure this year should heavy rains from a tropical storm or hurricane raise the lake level and put high stresses on the old dike."
A 2008 Army Corps of Engineers risk study of the lake ("Lake Okeechobee and the Herbert Hoover Dike") said, "There is limited potential for a dike failure with lake levels as low as 18.5 feet. The likelihood of a failure increases at higher lake levels. At a lake level of 21 feet — a 1-in-100 year flood event — a dike failure would be likely at one or more locations. In the event of a dike failure, waters from Lake Okeechobee would pass through the breach — uncontrollably — and flood adjacent land. Flooding would be severe and warning time would be limited. And with 40,000 people living in the communities protected by the Herbert Hoover Dike, the potential for human suffering and loss of life is significant. Our engineering studies indicate the southern and eastern portions of the dike system are more likely to fail than the northern and western portions of the dike. In general, we would expect a warning time of 24 to 48 hours prior to a dike failure that releases water from the lake; however, under some conditions the warning time might be longer, and under others, a dike failure could occur with no warning."
"A breach of the Herbert Hoover Dike would almost certainly rank as a catastrophe — but the scope and scale of loss would vary widely depending on where and when it burst and how much water was in Lake Okeechobee," the Miami Herald reports ("Computer simulation shows flooding threat from Lake O dike failure," by Curtis Morgan).
Officials think that efforts to strengthen the dike have helped. But they're reluctant to go public with details, the Herald reports: "While federal engineers openly discuss dike deficiencies the Corps remains reluctant to provide details, maps or modeling of the potential consequences of a failure, citing security concerns heightened since the 9/11 terrorist attack. 'We try and balance the risk of making sure the public is informed, and keeping the public safe with operational security of not allowing others to know what our vulnerabilities are,' said Laureen Borochaner, engineering division chief of the Corps' Jacksonville office, which monitors and maintains the dike."
Palm Beach County maintains a public website ("Emergency Management - Herbert Hoover Dike Information") with readiness plans to be implemented in case the dike is breached. The county warns residents of low-lying towns near the lake to plan out their escape route in advance: " If a mandatory evacuation is ordered, you and everyone else will be on the road. Plan ahead and anticipate supplies you might need for the drive. Know where you're going in advance of an evacuation and map out several routes in case some roads are blocked by flood waters or other hazards."