(By Kimberly Miller – Palm Beach Post Staff Writer) South Florida’s water managers watched the rain pile up in southeast Texas with the same horror as the rest of the nation, but with a heightened anxiety that comes in knowing the limitations of the state’s own flood control system.

Despite decades of environmental manipulation to corral, channel and pump water away from developed areas, if 30-plus inches of rain fell steadily over several thousand square miles of South Florida, it would be under water — several feet under water.

“You get three feet of rain in that magnitude over that amount of area, you are going to have three feet of water everywhere,” said Tommy Strowd, director of operations for the Lake Worth Drainage District. “Houston is hard to look at. It’s the kind of thing that keeps you up at night.”

Areas of southeast Texas received a record 52 inches of rain over a roughly 72-hour span during Hurricane Harvey’s extended stall over the state.

If South Florida experienced something similar, at best, water would invade homes and turn roads into canals.

In a worst-case scenario, the deluge would fall into Lake Okeechobee, or just north of it.

John Campbell, a spokesman for the Army Corps of Engineers, which manages Lake Okeechobee, discussed Harvey-like scenarios with an engineer last week. He said 20 inches of rain could increase lake levels seven to eight feet.

“It would likely exceed the design capacity of the dike,” Campbell said. “We’ve seen performance issues at 17 feet (above sea level).”

Lake Okeechobee is surrounded by the vulnerable Herbert Hoover Dike, which protects Glades-area communities from flooding. On Thursday, the lake was at 13.53 feet above sea level, meaning 20 inches of rain could quickly swell it to 21 feet. The Corps likes to keep the lake between 12.5 feet and 15.5 feet above sea level. The highest the lake has been was 18.7 feet in 1947.

“I don’t know that any flood-control structure is designed to handle every imaginable flood,” Campbell said.

We have it better than Houston

“It would likely exceed the design capacity of the dike,” Campbell said. “We’ve seen performance issues at 17 feet (above sea level).”

Lake Okeechobee is surrounded by the vulnerable Herbert Hoover Dike, which protects Glades-area communities from flooding. On Thursday, the lake was at 13.53 feet above sea level, meaning 20 inches of rain could quickly swell it to 21 feet. The Corps likes to keep the lake between 12.5 feet and 15.5 feet above sea level. The highest the lake has been was 18.7 feet in 1947.

“I don’t know that any flood-control structure is designed to handle every imaginable flood,” Campbell said.

We have it better than Houston

Another advantage in South Florida is the network of canals and pumping systems that can be put into overdrive before and during a storm to mechanically move water from inland areas to the open ocean — a factor that would further reduce the amount of time flood waters would stick around.

Houston’s drainage is by gravity through a set of slow-moving bayous that have been enlarged into flood-control ditches, said Texas climatologist John Nielsen-Gammon. Once the bayous flood, water moves outward onto freeways, then residential streets and eventually into homes.

Both South Florida and Houston can have problems with too much concrete and not enough green space to soak up rainfall. Runoff from streets and parking lots and driveways can quickly fill up drainage ponds or run into lower lying areas.

While building codes in Palm Beach County require newer developments to account for this runoff by building retention ponds to collect the water, older communities don’t have that or were built at lower elevations.

South Florida Water Management District permitting requires new developments to build floors in homes at or above a level to stay dry during a 100-year, three-day rain event, said district spokesman Randy Smith.

Smith said last week he didn’t want to speculate on what would happen if a Harvey-style rainfall hit South Florida.

“I don’t want it to look like we’re trying to do fear-mongering,” Smith said. “But logically, if you look at 40 to 45 inches in a short amount of time, it would be widespread flooding.”

Strowd, of the Lake Worth Drainage District, said when nearly two feet of rain fell in January 2014 over a 24-hour period, flooding was exacerbated in 70’s-era neighborhoods surrounded by newer communities “designed to move more water.”

In the 40-year-old Kings Point subdivision west of Delray Beach, a 90-year-old man walked through waist-deep flood waters into a canal and drowned during the 2014 soaking.

“Newer developments learn from the lessons of the past so they build higher and have more infrastructure,” Strowd said. “That can leave older developments at higher risks.”

In August 2012, Tropical Storm Isaac sputtered by South Florida with little wind damage, but dumped more than a foot of rain when a training band of moisture stalled over the county. Some residents in western Palm Beach County were stranded for five days in floodwaters that lapped at their front doors. Firefighters drove trucks through four feet of water to help get people to doctor’s appointments or to buy supplies. Some schools were closed for a week.

But Isaac was considered a success story because few, if any, homes had water push over their thresholds.

“The drainage system worked like it was supposed to,” Strowd said.

When would flooding start?

How much rain can fall before flooding starts depends on many factors. Has it been dry in the weeks preceding a deluge? Is it a neighborhood or an entire county getting the rain? Is it inland or near the Intracoastal?

Most experts said, in general, 20 inches is about what South Florida can manage before widespread flooding occurs.

But the precedent for rainfall in Florida is 45.20 carried by a storm similar in strength and saturation as Harvey.

In September 1950, Hurricane Easy — born when storms were named phonetically — lingered off Florida’s west coast north of Tampa for two days with 125-mph winds. It did two loop-de-loops on its path, and made two landfalls in Florida, before taking an unusual path to the northwest as the larger and stronger Hurricane Dog spun east of Florida.

Over 24 hours, Easy dropped 38.7 inches on Levy County’s Yankeetown, with a 72-hour total of 45.2 inches.

Meteorologists used Easy as a benchmark to determine how much rain the atmosphere could muster in Florida and coastal areas through Texas in the heaviest storm possible. They came up with 55.7 inches.

“Fortunately, we haven’t had a rainfall event the size of Easy until a few days ago,” said Louisiana State Climatologist Barry Keim, referring to Harvey. “Basically, 67 years ago we had a storm like Harvey in Yankeetown. Where the next one hits, we don’t know.”

http://www.mypalmbeachpost.com/weather/hurricanes/hurricane-harvey-could-happen-palm-beach-county/dAXPVk6r1PYjyO2XRtgLFK/

By staff

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