Withlacoochee River System Update - 9/3/2024

Effects of Hurricane Debby Linger

 

Rainfall:

• August brought times of heavy rainfall and times of dry weather, but Hurricane Debby ensured we ended the month with above average rainfall.
• Overall, the Withlacoochee region averaged 10.2“ of rainfall in August, higher than the historical average (8.1 inches).
• Some areas in the Green Swamp received more than 15 inches of rainfall last month.
• So far this year (January through August), our region has received 39.7 inches of rain, close to the historical average (39.8 inches) for the first 8 months of the year.
• Aquifer (groundwater) levels rose substantially in August and are currently in the 65^th percentile for this time of year, much higher than last year (47^th percentile).
• The figure below shows how much rainfall different areas received last month. Most areas received above average rainfall in August.

 

 

Withlacoochee River (from the Green Swamp downstream past Hwy 200):

• Hurricane Debby (Aug 4-5) brought big changes to the Withlacoochee River this summer.
• In the Green Swamp, the river peaked a few days after the storm, but water levels and flows downstream continued to rise for weeks.
• Near Trilby and Ridge Manor, where the river rose 9 feet after Debby, water levels peaked in mid-August and have dropped about 1 ½ feet over the past two weeks.
• The Withlacoochee crested near Nobleton and Hwy 48 nearly 2 ½ weeks after the storm and has only dropped 4-6 inches since then.
• At Hwy 44, the river peaked about 8 days ago and has dropped a couple inches over the past week.
• At Hwy 200, the river finally peaked last Wednesday, after rising 4 ½ feet from Debby.
• All along the Withlacoochee, river levels and flows remain high, but thankfully they were lower than the flooding we experienced after Hurricane Irma (2017) and the 2004 hurricanes.
• On average, river flows are nearly 6 times greater than they were in early August.
• The table below compares current river levels and flows to what they were last month and last year.

 

High water on the Withlacoochee River near Nobleton (August 2024)

 

 

Tsala Apopka Chain of Lakes:

• All three pools of the Tsala Apopka Chain of Lakes benefitted from above average rainfall in August.
• The Floral City Pool saw a net gain of 11 inches, while the Inverness and Hernando Pools rose 5 and 10 inches, respectively.
• The construction project on Sandpiper Drive (Golf Course structure) has affected our normal structure operations this summer.
• For about 12 days in August, we had to move water from the Inverness Pool into the Hernando Pool because of an emergency issue with one of the retaining walls that holds water out of the work area.
• Since August 20^th, we’ve been moving water from the Floral City Pool back into the Inverness Pool through the Moccasin Slough structure.
• Water is also entering the lake chain through the Leslie Heifner and Floral City structures.
• Structures were opened a couple weeks ago to help fill Lake Bradley and Lake Consuella.
• Next week, we’ll start mobilizing equipment to remove the retaining walls that currently prevent any water from flowing through the new Golf Course Structure.
• Once that process is complete, we’ll be able to move much more water into the lake chain and attempt to top off the pools while the river is still high.
• Additional rainfall on the lakes themselves, will also cause water levels to rise.
• Currently, all three pools are several inches higher than they were a year ago, due to higher rainfall so far in 2024.

 

Final concrete was poured at the Golf Course Structure last week (August, 29 2024)

 

 

Lake Panasoffkee and Wysong:

• Lake Panasoffkee rose 8 inches in early August from Hurricane Debby.
• In response to rising water on the Withlacoochee River, the Wysong Structure was fully lowered on August 12^th.
• Inflows from Shady Brook and Little Jones Creek have remained strong adding a healthy flow of water into Panasoffkee.
• The Withlacoochee River has now crested and is falling back down to its normal level just upstream of Wysong.
• Over the past two weeks, Lake Panasoffkee has declined about 6 inches.
• We anticipate re-installing the boat barriers this week and starting to slowly raise Wysong as river levels continue to naturally decline.
• Lake Panasoffkee is currently 4 inches higher than it was at the start of August and 7 inches higher than it was a year ago at this time.
• River flows at Wysong remain strong as our wet season continues.

 

Native eelgrass fills Little Jones Creek near Lake Panasoffkee (August 2024)

 

 

Lake Rousseau and the Lower Withlacoochee River (from Dunnellon to the Gulf of Mexico):

• Under normal conditions, water levels on Lake Rousseau remain relatively stable, even as areas upstream may rise or fall.
• When widespread rainfall events like Hurricane Debby impact the entire watershed, it can take weeks before downstream areas experience the full effect of the storm.
• For most of August, Lake Rousseau was lowered about 7 inches to help keep upstream river levels near Dunnellon below flood stage.
• The river upstream of Dunnellon finally peaked last Wednesday, allowing us to begin bringing Lake Rousseau back up to its normal level.
• Flows entering Lake Rousseau remain strong, with the Withlacoochee River is currently supplying more than 2/3 of the total inflow to the lake.
• Rainbow River flow (which reflects changes in aquifer levels) rose 15% in June and is 14% higher than it was a year ago.

 

• The Inglis Main Dam was opened in early August, for the first time in 18 months, in response to Hurricane Debby.
• The primary outlet from Lake Rousseau is through the Inglis Bypass Spillway to the Lower Withlacoochee River (see maps below).
• When the Bypass Spillway reaches its capacity (900 MGD), the Inglis Main Dam must be opened to discharge excess flows to the Barge Canal.
• Currently, the Bypass Spillway is flowing at 100% of its capacity, which is 28% more flow than a year ago.
• The Inglis Main Dam is discharging slightly more flow than the Bypass Spillway to keep up with increased inflows from the current high-water conditions upstream.

 

Inglis Main Dam discharging flood waters after Hurricane Debby (August 2024)

 

 

 

Have a wonderful weekend!

Mark

 

Mark Fulkerson, Ph.D., P.E.

Chief Professional Engineer

Southwest Florida Water Management District

 (352) 269-6073 (office)

(352) 279-4493 (cell)

Mark.Fulkerson@WaterMatters.org

 Withlacoochee River Watershed Initiative