As Hurricane Irma made its way across the Atlantic this week, the U.S. Army Corps of Engineers Jacksonville District began a pre-storm drawdown in canals and waterways to make room for the water expected from the storm,
While the “drawdown” includes water releases from Lake Okeechobee, there wasn’t enough time to make a significant difference in the level of the Big Lake.
At a press briefing on Wednesday, Sept. 6, corps officials estimated the pulse releases to the Caloosahatchee and the St. Lucie before the winds force them to close the locks could reduce the lake level by one-tenth of a foot — around 1.2 inches.
Roughly 12 billion gallons of water equates to one inch on Lake Okeechobee.
The change in lake level could be less than the target, since rainfall directly in the lake and in the drainage basin north of the Big O could continue to flow water into the lake.
In addition, reaching the target flows for lake releases to the east and west may not be possible, since these gravity flow releases depend on the level of water in the canals. On Tuesday, the corps set the target flow for the Caloosahatchee Estuary at 4,000 cubic feet per second (cfs) as measured at Moore Haven Lock (S-77) located in the southwest part of the lake. The flow for Sept. 5 averaged 932 cfs, according to the data on the Corps website.
The flow for Sept. 6 was 3,464 cfs from the Lake into the Calooshatchee River at Moore Haven. The flow at the Franklin Lock, which was primarily local basin flow on Sept. 6 was 8,382 cfs.
Prior to Sept. 5, no lake water had flowed through that lock since the start of the wet season.
During the dry season, some flow from the lake was sent to the Caloosahatchee in order to prevent saltwater intrusion.
The target flow for the St. Lucie Estuary was set at 1,800 cfs on Tuesday as measured at St. Lucie Lock (S-80) near Stuart.
Prior to Sept. 6, no water from the lake flowed into the St. Lucie this year. Water from the C-44 canal has been backflowing into the lake since the start of the rainy season. More than 24 billion gallons of water from the St. Lucie basin has backflowed into the lake since June 1.
On Sept. 5 water was still backflowing into the lake at Port Mayaca at 224 cfs. Flow into the estuaries at the St. Lucie lock was 468 cfs — all of that water was basin runoff.
In a press conference on Wednesday, Sept. 6, corps spokesperson John Campbell said because the lake is relatively low, it limits how much water can flow out of the lake into the C-44 canal at Port Mayaca.
“The water in the canal itself has to get lower before water can gravity flow from the lake,” he said. With the St. Lucie lock open, they expect the water level in the C-44 canal to fall enough to allow flow from the lake.
On Sept. 6, the flow from the lake into the C-44 at Port Mayaca was 897 cfs. The flow at the St. Lucie Lock, which includes basin runoff, was 1,748 cfs.
“We want to be ready for the heavy precipitation from Irma,” said Col. Jason Kirk, Jacksonville District Commander. “We anticipate direct rain over the lake could add a foot to the water level. We’ve seen basin runoff from past events cause the lake to rise 3 feet over the span of a month. We want to do all we can to ensure we have as much storage as possible for Irma.
The corps anticipates the releases will last a short time, with future water management decisions dependent on the water level in the lake after the storm passes.
“We will only be able to release water for about three days at these rates,” said Col. Kirk on Sept. 4. “As the storm gets closer, we’ll have to close all the gates around the lake to reduce the risk from potential storm surge that may develop from high winds on the lake.”
The corps continues to closely coordinate its water management activities with the South Florida Water Management District. For more information on water level and flows data for Lake Okeechobee, visit the corps’ water management website at http://www.saj.usace.army.mil/Missions/CivilWorks/WaterManagement.aspx.