The communities around Florida’s largest lake face a unique danger from Irma

One foot of rain means three feet of rise for Lake Okeechobee.

Lake Okeechobee at a calmer moment. CREDIT: AP Photo/Luis M. Alvarez
Lake Okeechobee at a calmer moment. CREDIT: AP Photo/Luis M. Alvarez

As coastal Florida scrambles to prepare for Hurricane Irma, Gov. Rick Scott (R) on Thursday ordered a mandatory evacuation of seven small inland towns: South Bay, Lake Harbor, Pahokee, Moore Haven, Clewiston, Belle Glade and Canal Point.

These towns are not subject to storm surge. Instead, they make up the communities south of Lake Okeechobee, the largest lake in Florida, located roughly 90 miles north of Miami and 40 miles inland from West Palm Beach. The 730-square-mile lake is surrounded by the 143-mile Herbert Hoover Dike. The dike has been under construction for over a decade, and it is in the “highest failure category” on the Army Corps of Engineers risk scale.

“This decision was made due to Gov. Scott’s sole focus on life safety as Hurricane Irma approaches Florida,” the governor’s office said in a statement, although the Corps have said that “the structural integrity of the dike will not be compromised.”

Army Corps officials told reporters the same thing Friday morning. “We now believe that we will see significant over-wash and limit to moderate over-topping at three specific sites,” Col. Jason Kirk said on a call. Over-wash is when water splashed over the top of the wall. Over-topping is when it runs over the wall. All three sites are construction sites for the ongoing culvert repairs.

“We would expect to see water coming over the top,” Kirk said, “which will aggravate any local flooding.” In other words, the Army Corps is not concerned about the dike collapsing. They are concerned about water being pushed into the nearby communities by record-strength winds. 

According to Kirk, the Army Corps changed its risk assessment Thursday, after the National Hurricane Center updated wind predictions. The intensity is now expected to be 130 to 156 mph (category 4) for up to seven hours.

At this point, there is “no evidence of any seepage” through the Herbert Hoover dike, he said. “We do not assess a significant risk of a dike breach.” 

The region is expected to get between 12 and 15 inches of rain from Hurricane Irma. For every foot of rain, the lake generally rises about three feet, according to officials. Last year, an Army Corps spokesperson told ThinkProgress that “concern” comes when the lake reaches 17 to 17.5 feet of elevation. On Friday, the lake’s elevation was 13.68 feet. If Irma’s rainfall proceeds as predicted, the dike should remain stable. If central Florida gets more rain than expected — or if another tropical storm passes through — that’s where the danger comes in.

The area has seen what happens when Okeechobee’s waters aren’t contained. During a 1928 hurricane, a five-foot dike around the lake collapsed, and some 2,500 people were killed in the ensuing flood. That storm prompted the Hoover administration to build the current dike.

Some of the culverts still date from the original construction, which is why the Army Corps has spent hundreds of millions of dollars on restoring the dike. President Donald Trump promised the project would be finished before 2020, but in his most recent budget proposal, Okeechobee only got $82 million. The estimated remaining cost to the federal government is $930 million.

Later this month, the Army Corps was scheduled to have a celebration of reaching the mid-point of construction. It remains to be seen if that will still happen, in the wake of Irma, Kirk said.