The greatest threat to the Herbert Hoover Dike during a hurricane is storm surge. Sustained hurricane force winds can push the lake’s water from one side to the other, sending a wall of water crashing into, or even over, the dike.
On Sept. 11, as winds from Hurricane Irma pushed Lake Okeechobee’s water north, the lake level at the north end of the lake was 19.5 ft. (above sea level), according to the report given by U.S. Army Corps of Engineers Col. Jennifer Reynolds at the Nov. 3 meeting of the County Coalition.
“At that same time, near Pahokee on the southeast side of the lake, the lake level was 9.5 feet,” she said.
“That’s what surge impact looks like on Lake Okeechobee,” she said.
She said the corps must take into account how the dike would respond “if you saw that surge on every reach.”
For planning purposes, the corps divides the Herbert Hoover Dike into “reaches.” The reaches on the south end of the dike, where land elevations are lowest have been deemed most at risk in a storm event.
Big Lake area residents were lucky this time. The land on the north side of the dike is the highest elevation, so the storm surge to the north poses the least risk.“We did not see a significant amount of erosion or over wash during Hurricane Irma,” said Col. Reynolds.
Had that surge hit a different area of the dike, the results could have been different. “Due to Hurricane Irma, the lake rose tremendously,” she said. “There is still a lot of water in the system and it’s a challenge moving forward.”
Col. Reynolds says the storm caused a rapid rise in water levels in Lake Okeechobee and in water conservation areas west of the Miami and Fort Lauderdale metro areas.
“We have been using all tools available to us since the beginning of the summer to address the high water in the conservation areas in south Florida,” said Col. Reynolds. “Irma has increased the challenge by dumping a lot of water in areas that were feeling impacts from heavy rains at the beginning of wet season. It also took Lake Okeechobee from a level that was in the middle of our preferred range to the highest stages we’ve seen in more than a decade.”
Following heavy rains in June, the corps implemented deviations to allow more water to flow from Water Conservation Area 3A into Everglades National Park and Big Cypress National Preserve. In August, the corps implemented a deviation to allow high stages in Water Conservation Area 2A to reduce some of the flows going into Water Conservation Area 3A.
“The high water levels in the conservation areas took away any possibility that we could send water from Lake Okeechobee south when it rose after Irma,” said Col. Reynolds. Before Irma, the stage at Lake Okeechobee was elevation 13.67 feet, well within the Corps’ preferred range of 12.5-15.5 feet. Rain from the hurricane took the water level to 17.2 feet, its highest stage since 2004. In mid-September, the Corps started releasing water from the lake to the Caloosahatchee River and St. Lucie Canal.
“We’ve been releasing as much water as we can from Lake Okeechobee since late September,” said Col. Reynolds. “We’ve had to slow the rate of discharges at various times due to high tides and heavy precipitation. We will continue to monitor downstream conditions and adjust accordingly.”
From Sept. 15 to Oct. 29, more than 477 billion gallons of water has found its way into the lake from the watershed to its north and west. The corps has released 170 billion gallons of water to the Caloosahatchee and 105 billion gallons to the St. Lucie.
The Corps resumed daily inspections of the Herbert Hoover Dike that surrounds the lake this week as precipitation from Tropical Storm Phillipe caused the stage to rise above 17.0 feet again. The Corps conducted five daily inspections on the southern half of the dike between Moore Haven and Port Mayaca. A weekly inspection was also conducted on the northern half of the dike. No issues of concern were identified.