Breached dike is tragedy link
Earle Kimel Sarasota Herald-Tribune | USA TODAY NETWORK
Abreach in the dike separating the areas drained by Phillippi Creek and Cow Pen Slough in central Sarasota likely played a key role in August’s catastrophic flooding of the Laurel Meadows subdivision after Hurricane Debby, according to a new study. ● “It should have never gotten into their homes; that’s astounding,” said Stephen Suau, an engineer and hydrologist who founded Sarasota County’s stormwater division 25 years ago and conducted the independent study.
The flooding after Debby’s unprecedented rainfall dumped more than a foot of rain in 24 hours got into 84 of 86 homes in Laurel Meadows, where the “streets are designed to flood but not their homes,” Suau said.
Laurel Meadows was designed for families and built between 2000 and 2005.
The dike was built decades ago to facilitate farming. An unnoticed breach sometime in the last few years created an opening for Debby’s floodwaters to flow through, inundating a subdivision miles away.
The dike failure was only one cause of some of the most severe inland flooding Sarasota County has seen in years during a hurricane season like no other for the area, which saw three hurricanes batter the area in less than two months.
Unlike Debby, the two storms that came after, Hurricanes Helene and Milton, were not big rainmakers but record storm surge caused unprecedented coastal flooding.
Sarasota’s Pinecraft neighborhood was also devastated by Debby’s cascading rains, but county officials are reviewing
how they managed the runoff and whether anything could have been differently to mitigate the disaster.
More detailed public discussions are expected early next year.
Suau detailed his findings on the Laurel Meadows flooding to more than 100 people at a presentation last month before the Sarasota Citizen Action Network. A recording of that meeting is on YouTube at https://bit.ly/3Z5UNLL.
Since then Suau, who studied the flooding that resulted from Debby – a tropical storm well offshore when it impacted Sarasota County – has forwarded findings from his review and recommendations to avoid future problems to Sarasota County officials.
Sarasota County to discuss Debby flooding in January workshop
Sarasota County has been working on its own computer modeling of Debby’s flooding, including how much water crossed over from the Cow Pen Slough watershed to the Phillippi Creek watershed, said Sarasota County Public Works Director Spencer Anderson.
“While Steve was doing his thing, we got hit by a couple more hurricanes,” Anderson said.
That modeling is anticipated to be complete by the end of November and should be reviewed at a Sarasota County Commission in a workshop now set for Jan. 21.
Officials will present recommendations for possible regulations to reduce future flooding.
“All of those things are topics we’ll bring back to the board and then they may give us some policy direction on moving forward with new regulation or new policies that the board wants to see, to manage the stormwater regulation all through the floodplain and mitigation of future flooding,” Anderson said.
Discussion of the current stormwater drainage system and development standards, which are designed to handle 10 inches of rainfall in 24 hours, and whether they should be modified will likely be up for debate, too.
Public criticism of development regulations and local government growth management mounted after Debby, as people questioned whether more pavement and construction contributed to the flooding.
Prior to that January meeting, members of the Sarasota County Planning Commission will likely get an earful from members of SCAN, Sarasota Audubon members and others concerned about overdevelopment in flood-prone areas when it meets at 5 p.m., Nov. 21, in the Robert L Anderson Administration Center, 4000 S. Tamiami Trail, Venice.
That’s when the panel will host a public hearing on a proposal to increase zoning from one unit per 10 acres to 3.5 units per acre on 49.13 acres east of Raymond Road – essentially across the street from the Celery Fields in northeast Sarasota County.
That zoning change would allow D.R. Horton to build 170 homes on the property, north of where the Pinecraft and Phillippi Creek flooding occurred.
Evidence points to dike breach for Laurel Meadows flooding
Suau – who was asked by Sarasota County to review the flooding but not paid – started with a working hypothesis that a breach in the dike between Cow Pen Slough, which is at a higher elevation than the Phillippi Creek basin, would explain the volume of water that flooded Laurel Meadows.
“If that dike breached it would explain to me why so much water gushed into Laurel Meadows, just because of the topography,” Suau said.
During that time, Sarasota county was checking for blockages in the drainage system that would cause a backup.
Suau’s theory was confirmed by a YouTube video posted by a driver heading west on Delft Road, which is south of Laurel Meadows. From 35 seconds into the video until the 50 second mark, water can be seen flowing from south to north.
Suau was able to see the exact location of the breach – east of Rothenbach Park – on LIDAR aerial imagery taken by the United States Geological Survey between 2018 and 2019 but only made available to Sarasota County through the state of Florida in 2023.
LIDAR is short for light detection and ranging. While it is also used in vehicle driver assistance applications, LIDAR can create high resolution models of ground elevation accurate to within four
inches, according to the USGS.
The aerial LIDAR surveys are refreshed roughly every decade.
While overgrowth of a variety of vegetation, including Brazilian pepper trees, obscured the break from aerial photography, the LIDAR images produced by the state of Florida, clearly showed the breach.
While the dike’s break had been there for several years, it only became a problem after Debby dumped record-breaking rainfall across Sarasota and Manatee counties, with some areas getting as much as 15 inches in less than 24 hours.
“If that dike’s in place, it doesn’t matter,” Suau said of the rainfall. “If it gets breached you’ve got elevation for water to gush through.”
The void in the dike acted as a release valve for water from Cow Pen Slough – which would normally flow south into Sarasota Bay – to essentially make a Uturn to flow north along a lower elevation toward Laurel Meadows with the rest of the water in the Phillippi Creek basin.
Once the breach was found, it was filled by Sarasota County before Hurricane Helene.
“We found the breach after we did the survey work; we went in and cleared that area and regraded,” Anderson said. “There was obviously movement of sediment from the Cowpen Slough side into the Phillippi Creek side.
“If there was a low spot there initially, it was exacerbated by the flooding from Hurricane Debby,” he added, mindful that Debby dumped record rainfall on Sarasota County, including the Phillippi Creek watershed.
“We can’t forget it was just an enormous amount of rain that occurred at a high intensity over a long period of time,” Anderson said. “We’re typically dealing with high intensity over short durations but we had rain from Debby for nearly 24 hours of constant high intensity,” he added. “It hasn’t happened for a long time.”
Wetlands drained for farming, repurposed for development
The dike separating the Phillippi Creek watershed from Cow Pen Slough was one of several built in the early days of Sarasota County, when the Fruitville Drainage District was established in the decade after the county was established in May 1921.
“That area out there was almost another Celery Fields. It was identical in size, shape and area as the Celery Fields and it was drained to grow vegetables back in the 1920s,” Suau said, referring to the agricultural area near Fruitville Road and Interstate 75 that is now a vast water retention, wildlife and park area. “They cut a canal across the ridge – Tatum Ridge.”
Suau speculates that the dike was built to make it easier to grow crops and limit the water flow from the east to allow the land in the Phillippi Creek watershed be drained and farmed.
Debby home flooding had other causes
Not all inland flooding was directly related to the dike breach east of Rothenbach Park. Areas by the Celery Fields were flooded because of how public works employees chose to release water from it into Phillippi Creek.
While the 300-plus acre Celery Fields has become a wildlife haven and an attraction for birders, it was bought in the early 1990s as one method to control flooding in low-lying areas such as Pinecraft.
Phillippi Creek was severely impacted by storms as far back as 1962, flooding Pinecraft, both Suau and Anderson said, as well as the no-name storm in 1992 and others that year that prompted Sarasota County to establish a countywide stormwater management fee to pay for flood control.
At that same time, Sarasota County adopted the 100-year flood model for stormwater systems, which calls for the 10 inches of rain in 24 hours – which was a 1% chance of happening in any given year – and is more stringent than the 50-year flood model – 8 inches of water in a 24-hour period – used elsewhere in the state. The 100 year mode During Debby, Sarasota County workers were manipulating the flow within the Celery Fields, “managing a bubble” of water in an attempt to avoid making problems worse in one area over another by changing the system, Anderson said.
“We did things based on standard protocol for our stormwater system, especially around the Celery Fields,” he added. “In this one there was so much rainfall, everything was flooded.”
As part of the preparation for that Jan. 21, workshop, Anderson’s team is using computer models to see if they could have done something differently to decrease flooding in some areas, without increasing it elsewhere.
What are the recommendations?
Suau’s recommendations to Sarasota County fall in two categories – operations and policy.
Many involve countering public perceptions that Sarasota County neglects its stormwater management duties.
“The most common thing I hear from the public is the county’s not maintaining the drainage system. Whether they are or not, that’s the perception I’m hearing from people,” said Suau, who added that Sarasota County now collects about $27 million annually in stormwater management fees, with more than half of that used to maintain the system.
Most new stormwater systems.are built by developers for individual subdivisions.
“The county has no obligation to do that; they’re just obligated to maintain the ditches where the stormwater systems eventually dump into,” Suau said. “That perception that they’re not maintaining the system is a big deal that they need to overcome.”
He also suggests that databases the county can use to track stormwater system improvements be made available to the public, “so they can see that ditch that’s a problem” near them, when work is planned and when it has been done.
Sarasota County – which actively tracks roughly stormwater hot spots – should also host annual open houses where residents can learn about the process and perhaps advise of other problem areas, Suau said.
He also suggested subcontracting with USGS to take over monitoring and maintaining rainfall collection and stream stage gauges.
Suau noted that during Debby, several Phillippi Creek gauges failed.
He also suggested Sarasota County inventory all dikes in Sarasota County – including privately owned ones like the Hidden River levee that breached in October 2022 after Hurricane Ian – and establish responsibility to maintain them.
“There’s not a lot but we’ve seen how at risk to flood damage properties that are relying on a dike,” Suau said.
Is the 100-year flood standard still adequate?
On the policy standpoint, the most obvious questions surround the use of the current 100-year storm criteria for stormwater drainage capacity.
As part of that criteria, no rainfall accumulation is supposed to occur on a hurricane evacuation route, while there can be six inches on an arterial road, nine inches on a collector road and 12 inches on a neighborhood road.
“Is the 100-year storm different, has it changed?” Suau said. “I think that’s absolutely a worthwhile discussion to pursue – both how much rainfall and how frequently are they happening.”
Suau, then added that the criteria used to develop Sarasota’s guidelines were based on an analysis of rainfall patterns done through 1996.
He noted that more recent analyses, such as one from the University of Central Florida in 2011, the National Oceanic and Atmospheric Administration in 2013 and the USGS in 2022 indicate that rainfall is trending up.
Climatologists, including Bob Bunting, CEO and chairman of the Sarasotabased Climate Adaptation Center, suggest that one aspect of global warming is an increase in the frequency of large rainfall events.
Suau would also like to see updated maps from Sarasota County reflected in maps for the Federal Emergency Management Agency sooner.
“That has happened painfully slowly,” Suau said. ”To date, the Cow Pen Slough and Myakka River watersheds are still not reflected on the FEMA maps but the county has them.”
He would also like to see the county preserve more floodplains through its Environmentally Sensitive Lands Protection Program, and noted that when 100-year floodplains are preserved, FEMA gives a credit that is reflected in lower flood insurance premiums.
Anderson agreed that the stormwater criteria merits a second look.
“Things have been dry, they’re starting to get more wet and people have seen conditions on property that they haven’t seen for a while,” Anderson said.
