Sending more water south may not be enough to offset sea level rise

OKEECHOBEE — Sending the excess freshwater from Lake Okeechobee south, rather than east and west, may not be enough to save Florida Bay from rising sea levels.

Florida Senate Bill 10, passed in 2017, directed the South Florida Water Management District to come up with a plan for a reservoir south of Lake Okeechobee to reduce harmful discharges to the coastal estuaries east and west, and to provide more freshwater flow to Everglades National Park and Florida Bay.

Plans for the Everglades Agricultural Area reservoir have gone to the U.S. Army Corps of Engineers for review. The plan is considered a change or addition to the already approved Central Everglades Planning Project, which will increase flow to the water conservation areas south of the lake and increase flow under the Tamiami Trail to Everglades National Park and Florida Bay.

The reservoir will be designed to hold at least 240,000 acre-feet of water and include water quality features necessary to meet state and federal water quality standards. Together with the CEPP projects, the reservoir will send approximately 370,000 additional acre-feet of clean water south to the Everglades each year. That’s 120.5 billion gallons of water. It may not be enough to offset the damage from rising sea levels.

A new study asks water managers to consider sea level rise when making plans to increase freshwater flow south from the big lake.

Environmentalists have long pleaded for more freshwater to be sent to the Everglades. For decades, flow has been limited by the size of the canals from the lake to the water conservation areas, and blocked by the Tamiami Trail, which acts as a dam dissecting the Everglades from Tampa to Miami. One mile of raised bridging was completed in 2013. A project to raise another 2.5 miles of the road started in 2016. Another 3 miles of bridging are planned.

Now that plans are finally moving forward to send more water south, some researchers warn that due to the rising sea level, additional year-round freshwater flow is needed to prevent the environmental damage from excess salinity of Florida Bay.

“Effects of sea-level rise and freshwater management on long-term water levels and water quality in the Florida Coastal Everglades,” by Florida International University researchers Shimelis B. Dessu, Rene M. Pirce, Tiffany G. Toxler and John S. Kominoski, was published in the April edition of the Journal of Environmental Management.

The authors state that, “Since the 1880s, hydrological modification of the Greater Florida Everglades has reduced water levels and flows in Everglades National Park (ENP). The Comprehensive Everglades Restoration Program (CERP) began in 2000 to restore pre-drainage flows and preserve the natural landscape of the Everglades. However, sea-level rise (SLR) was not considered in the development of CERP. We used long-term data (2001-2016) from the Florida Coastal Everglades-Long Term Ecological Research Program to quantify and model the spatial dynamics of water levels, salinity and nutrients in response to changes in climate, freshwater management and SLR in the Shark River Slough (SRS), ENP. Results indicate that fresh-to-marine head difference (FMHD) was the single most important factor affecting marine-to-freshwater hydrologic connectivity and transport of salinity and phosphorus upstream from the Gulf of Mexico. Sea level has increasingly exceeded ground surface elevation at the most downstream freshwater site in SRS, thereby reducing the FMHD. We showed a higher impact of SLR in the dry season when there was practically no freshwater inflow to raise FMHD. We also demonstrated effectiveness of inflow depends more on the monthly distribution than the total annual volume. Hence, the impact per unit volume of inflow is significantly higher in the dry season in preventing high salinity and marine-derived nutrient levels. We advocate that FMHD needs to be factored into water management decisions to reduce adverse and likely irreversible effects of SLR throughout the Everglades landscape.”

Will there be enough freshwater to stave off the effects of sea level rise in South Florida? The freshwater available is not enough to restore historic flow – and restoring historic flow may not be enough in the face of rising sea levels.

According to “Lake Okeechobee: A Synthesis of Information and Recommendations for its Restoration,” published by Audubon of Florida in 2005, before Lake Okeechobee was contained by the Herbert Hoover Dike an estimated 866,000 acre-feet of water flowed south each year. The system of dikes, ditches and levees constructed in South Florida for flood control reduced the flow south to approximately 664,000 acre-feet of water.

Those numbers seem to indicate that the additional flow from CEPP and the EAA reservoir plan could restore the natural flow. However, other factors are in play. According to the 2005 Audubon report, about 65 percent of the 664,000 acre-feet of water flowing south annually is taken for human use. Human use includes crop irrigation as well as lawn and golf course watering, drinking, cooking, washing and flushing.

According to the United States Geological Survey data, each American uses an average of 80 to 100 gallons of water a day. According to USGS, the largest use of household water is to flush the toilet, followed by showers and baths. A full bathtub uses about 36 gallons of water. An old-style shower uses 5 gallons of water per minute; water-saving showerheads cut that to 2 gallons per minute.

The census shows that more than 6 million people live in South Florida, with 1.9 million in Broward County alone. And the population of the Sunshine State keeps growing.

Another factor to consider: The water flowing overland was only part of the historic flow.

According to the 2005 Audubon report, before the lake was diked, water also flowed through the soil.

The authors explain: “A common perception of the pre-drainage lake is that when the water got deep enough, ‘it overflowed into the Everglades.’ While it is true that the lake did overflow into the Everglades when deep enough, that was not the only time Okeechobee served as the Everglades’ wellspring.

“Organic soils, limestone and sand deposits can be extremely porous; even when Lake Okeechobee was not deep enough to flow above the muck soils, it probably always flowed through and under them.”

The diking, dredging and drainage projects stopped that flow.

The hydrological changes didn’t start with the 1948 flood control project. The changes to the lake started in the 1880s, according to the Audubon report. “The lake was forever lowered by a series of major drainage projects starting in the late 1880s. The first project connected the lake to the Caloosahatchee River near Moore Haven, sending water to the Gulf Coast. The North New River, Palm Beach, Hillsboro, Miami, and St. Lucie canals were completed by 1924 and drained water toward the Atlantic Coast. These drainage projects reduced the lake’s average high-water level from around 20 feet to a current average in the 16- to 18-foot range,” the report states.

The current footprint of Lake Okeechobee is about one-third smaller than the historic footprint.

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