Unwelcome Rains Will Put Stress on Lake Okeechobee’s Dike

A steadily organizing Invest 99L was bringing heavy rains and strong wind gusts to the northern Lesser Antilles on Wednesday morning, and is an increasing threat to develop into a tropical storm. Even if 99L never develops into a tropical cyclone, it has the potential to dump a large amount of rain on a place that doesn’t need it—the catchment basin of Lake Okeechobee in Central Florida. The huge lake represents an important source of fresh water to South Florida, but also poses a grave danger. The 25 - 30'-tall, 143-mile long Herbert Hoover Dike surrounding the lake was built in the 1930s out of gravel, rock, limestone, sand, and shell using old engineering methods. The dike is tall enough that it is very unlikely to be overtopped by a storm surge from the waters inside the lake, but the dike is vulnerable to leaking and failure when heavy rains bring high water levels to the lake. Torrential rains of 7+ inches from a tropical storm or hurricane are capable of raising the lake level by over three feet in a few weeks; this occurred in 2008, when Tropical Storm Fay took a leisurely romp across Florida, and again in 2012, when Tropical Storm Isaac lumbered past. At a lake water elevation of 15.5’, the U.S. Army Corps of Engineers dumps water out of the lake as fast as it can in order to keep high stresses on the dike from causing a failure. The lake reached this level early in 2016, after unusually heavy winter rains. The Corps was forced to do emergency dumping for most of February, and dumping has continued into August—though at a slower rate.

Currently, the lake level stands at 14.7’; the Wednesday morning Quantitative Precipitation Forecast from the National Weather Service predicted that the Lake Okeechobee region would receive 3 - 6” of rain over the next week, which would likely be enough to raise the lake level by 1 - 2 feet and bring it near the 15.5’ level where maximum dumping must occur. If 99L develops into a hurricane whose core passes over Lake Ockeechobee, the lake could easily received 10 - 15” of rain, enough to raise the lake level to near the 18.5’ level, where failure is possible. Under ideal conditions, the Army Corps can only lower the lake at a rate of about 0.4" per day, so they will have to do a lot of emergency dumping if 99L brings heavy rains to Florida. And 99L may only be setting the stage for the next storm—Florida often experiences multiple tropical storms or hurricanes in one year, and a wet storm later in September could really cause a serious situation with the dike.


Figure 1. Water level of Florida's Lake Okeechobee between January 2015 and August 23, 2016. Heavy winter El Niño rains forced emergency dumping in February, and dumping at a slower rate has continued all year. The Army Corps tries to keep the lake level below 15.5'; the dike surrounding the lake is in danger of failure when the lake level hits 18.5'. As of August 23, 2016, the lake level was 14.7’. Lake Okeechobee reached an elevation of 18.6' and 18.5'--both 1-in-30-year events--in 1995 and 1998. Image credit: U.S> Army Corps of Engineers.

Probable failure rate: once every 14 years
The Army Corps of Engineers has spent $500 million since 2007 to upgrade to the Herbert Hoover Dike. Replacement of 32 culverts has been partially completed, and has been funded through 2021. However, an additional $800 million in unfunded repairs is needed. A 2011 risk assessment estimated the dike's probable failure rate at every fourteen years. A 2008 Army Corps of Engineers study said this about the vulnerable dike:

"There is limited potential for a dike failure with lake levels as low as 18.5 feet. The likelihood of a failure increases at higher lake levels. At a lake level of 21 feet--a 1-in-100 year flood event--a dike failure would be likely at one or more locations. In the event of a dike failure, waters from Lake Okeechobee would pass through the breach--uncontrollably--and flood adjacent land. Flooding would be severe and warning time would be limited. And with 40,000 people living in the communities protected by the Herbert Hoover Dike, the potential for human suffering and loss of life is significant. Our engineering studies indicate the southern and eastern portions of the dike system are more likely to fail than the northern and western portions of the dike. In general, we would expect a warning time of 24 to 48 hours prior to a dike failure that releases water from the lake; however, under some conditions the warning time might be longer, and under others, a dike failure could occur with no warning."

The city most at risk from a dike failure may be Belle Glade (population 18,000) on the southeast shore. Belle Glade is at 16' elevation. If Lake Okeechobee is at 20' above mean sea level when the dike fails, this implies that at least three feet of water could flood Belle Glade. If a wide section of the dike breaks and there is a Cat 3+ hurricane driving a massive storm surge at the time, then the flood could be much higher. During the 1928 hurricane, which had 130 mph winds while over the lake, the water from the storm surge reached seven feet above ground level in Belle Glade.


Figure 2. Aftermath of the 1928 Okeechobee Hurricane, showing damage to a cluster of Everglades scientific work stations in Belle Glade. The hurricane killed 2,500 people, mostly near Belle Glade. Image credit: University of Florida, via the historicpalmbeach.com.

The Great 1928 Lake Okeechobee Hurricane
The shores of Lake Okeechobee are the site of the second deadliest natural disaster in U.S. history--the 1928 Okeechobee Hurricane. This mighty hurricane caused catastrophic damage where it struck the Florida coast as a Category 4 storm near Palm Beach, and weakened only slightly to Category 3 strength with 130 mph winds when it passed over Lake Okeechobee. The powerful winds of the hurricane brought a 12' storm surge to the south end of the lake, which overwhelmed the 6' high levees protecting the farm lands to the south. The resulting flood covered an area of hundreds of square miles with water up to 20' deep, and killed at least 2,500 people--mostly black migrant farm workers. A mass grave at the Port Mayaca Cemetery east of Port Mayaca contains the bodies of 1,600 victims of the hurricane. The Herbert Hoover Dike was built in the 1930s around most of Lake Okeechobee in response to this disaster.
 

Figure 3. Tom Wippick, from the Florida Department of Environmental Protection, takes a sample for testing of the awful smelling algae near the Central Marine boat dock along the St. Lucie River on July 11, 2016 in Stuart, Florida. The algae bloom, due to polluted water from Lake Okeechobee, created angry communities, closed beaches and has had an economic impact as tourists and others are driven away by the smell and inability to enjoy some of the waterways. (Photo by Joe Raedle/Getty Images.)

Lake Okeechobee runoff contributing to toxic algae blooms
In May, a 33-square-mile algal bloom blossomed over Lake Okeechobee, due to heavy agricultural pollution. The Corps has been dumping water out of the lake all year to keep the lake below 15.5’, and most of this excess water was sent out Lake Okeechobee's western drainage canal into the Caloosahtchee Estuary, which empties into the Gulf of Mexico at Fort Myers. A lesser amount of Lake Okeechobee water has been sent eastwards into the St. Lucie River, where it drains into the Atlantic Ocean near Stuart through the Indian River Lagoon. A similar level of discharge goes down the C-51 canal in into the estuary by West Palm Beach, the Lake Worth Lagoon. The polluted Lake Okeechobee water, combined with large amounts of polluted local runoff water from heavy rains, has caused havoc in these coastal waters this summer, shutting down businesses and closing beaches during the critical summer tourism season. A state of emergency was declared last month in the coastal counties of St. Lucie, Martin and Palm Beach because of  algae blooms caused by the polluted Lake Okeechobee water.

We'll have a full update on 99L late this morning (as well as on Tropical Storm Gaston).

Jeff Masters