Three areas on the Herbert Hoover Dike could be at risk of overwash or overtopping when Hurricane Irma hits.
In a Friday morning news conference, Col. Jason Kirk, U.S. Army Corps of Engineers Jacksonville District Commander, said the current forecast will bring Hurricane Irma over Lake Okeechobee as a Category 4 storm, with winds up to 156 mph. The storm is also expected to dump 10-15 inches of rain on the area.
That means water could splash over or flow over three excavated areas in the dike which are currently protected by cofferdams.
“The corps risk assessment that we do on the lake is based on current lake stage, potential forecasted rainfall, the storm intensity, the duration the storm will sit on the lake and the wind direction,” said Col. Kirk.
He said the lake level was 13.7 ft. as of Friday morning. The corps attempted to lower the lake level with pulse releases to the east and west, but with additional water flowing in from the north, the lake level actually slightly rose since Wednesday. The locks will be closed at 4 p.m. Friday. No water will flow out of the lake until after the storm passes.
Col. Kirk said the current forecast is for 10-15 inches of rain and storm intensity of Cat 4 (130-156 mph winds).
The wind direction will be highly variable.
“The duration variable for hurricane force winds over the lake may be up to 7 hours,” he said.
“The fact that it is so unpredictable is a risk factor,” he added.
“We now believe we will see significant overwash and limited to moderate overtopping at three specific culvert construction sites in Glades, Hendry and Palm Beach Counties,” said Col. Kirk.
He said the dike itself is not at significant risk because the lake level is relatively low for this time of year.
The three spots at risk are places where culvert replacement projects are underway. These projects, which taken 18-24 months, involve installing a metal cofferdam to hold back the lake water and excavating part of the dike in order to remove the old culvert and install a new one. The cofferdams are about 10 feet shorter than the walls of the dike.
‘We expect to see water coming over the top of the cofferdams, which will aggravate any localized flooding,” said Col. Kirk.
He said corps officials inspected the dike on Thursday and there is no evidence of seepage.
“We are pre-positioning the dike experts to get back up on the dike as soon as they can, post storm, for assessment,” he said.
“We do not expect a significant risk of a dike breach condition,” he said.
The construction sites which may be at risk are near Harney Pond in Glades County, near Clewiston in Hendry County, and between Belle Glade and Pahokee.
During the storm, high winds could cause water to splash (oversplash) over the cofferdams, or it could fill up the “bathtub” area around the culvert and flow over (overwash) the cofferdam.
“The aggravation of localized flooding is going to be influenced by the rain that falls on the basin,” said Col. Kirk.
He said during the storm canals to the west and southeast of the lake could fill up with precipitation and possibly swell over their banks. Any water overtopping from these structures will add to that localized flooding.
Corps spokesman John Campbell said one foot of rainfall north of the basin will result in a 3 ft. rise inn the dike in the weeks following the storm as the water drains south. The lake currently has the capacity to hold that water.
The highest lake level on record was 18.77 feet.
“Today at 13.7 ft. we are in a low-risk condition for a breach scenario,” he said.