Tropical Storm Debby Tests City’s Flood Planning

We are again in the thick of the hurricane season and, while St. Petersburg has not had direct hit since the devastating Tarpon Springs Hurricane in October 1921, there have been many near misses: Hurricane Irma (Cat 5) in 2017, Hurricane Ian (Cat 5) in 2022, Hurricane Idalia (Cat 4) in 2023, and already this season Tropical Storm Debby, which evolved into a Category 1 after its center passed approximately 100 miles west of St. Pete. Even glancing blows from these storms caused significant damage to our city, particularly Hurricane Idalia, which flooded approximately 1400 homes though it made landfall 150 miles north. 

This hurricane season got off to another record-breaking start with Hurricane Beryl, the earliest Category 5 Atlantic hurricane on record. Recently the National Oceanic and Atmospheric Administration (NOAA) updated its seasonal hurricane outlook projecting between 17 and 24 named storms, of which 8 to 13 are expected to be hurricanes. Some four to seven hurricanes are expected to be major, with sustained winds of at least 111 mph (Category 3 hurricane). This projection includes Beryl and Debby.

Debby was nearly a 50-year storm event, dropping almost 12 inches of rain in parts of St. Petersburg. As measured at multiple locations throughout the city, over 10 inches fell within a 30-hour period. That level of precipitation had not occurred in a 48-hour period since 1997. Rainfall across the city of course was uneven, but some 429,000 gallons of untreated wastewater spilled into four neighborhoods, with 152,000 gallons reaching local waterways. This came from manhole sewer overflows, not the wastewater treatment facilities. Adjacent counties experienced as much as 22 inches of rainfall, triggering widespread significant flooding of homes, businesses, and other structures. That could just as easily have happened here.

A year ago, we wrote about the city’s new Stormwater Master Plan. That plan provided for approximately 76 major capital stormwater projects to be constructed over a 20-year period at a cost of $760 million. Projects include expansion of pipe systems, new box culverts, and street improvements to better handle stormwater such as streets with gutters, among other initiatives.

The plan also mandated that new construction would need to withstand updated “Level of Service” standards of 7.5 inches of rain in 24 hours (expected once in a 10-year period). Also, the state Southwest Water Management District is enforcing a new city 100-year/ 24-hour/ 12-inch storm standard and floodplain map. The previous standard was 3.2 inches of rain over one hour in a 10-year period. The plan also assumed a sea level rise of not more than 1.44 feet by 2050.  This is based on the NOAA Intermediate Sea Level Rise projections and is also the planning standard recommend by the Tampa Bay Regional Planning Council. In a recent post-Debby report provided the city council by Public Works Administrator, Claude Tankersley, it was noted that while NOAA projected the average high tide for the city this year at six inches, it has actually been running 1.13 feet.

A major part of the plan was development of a new floodplain map. The new map is geared to address the 10- and 100-year design storm intensities as well as the 2050 sea level rise standard. The plan also designated 12,682 subbasins throughout the city. (This new map is different from the FEMA flood maps used for evacuation and insurance purposes.) Based on the current stormwater utility rates that all water users pay, there will be sufficient funding to cover approximately 60% of the projects as originally proposed over the next 20 years.  

Stormwater Master Plan Updates

The Stormwater Master Plan has recently been updated, and now seeks a budget of $1 billion for the 76 projects; the current city budget for stormwater improvements is $17 million. While the city is proposing to increase this to $25 million for the upcoming year, the Public Works Administrator has said that to complete all the proposed projects within the 20-year timeframe, spending would need to be at the rate of $50 million a year.  

The new flood plain map locates flood hot spots throughout the city. While this map is used to designate the sites of needed flood control projects, it can also be used to dispatch police and traffic control units to shut down roadways impacted by flooding.

Other components of the revised plan include a stormwater project ranking methodology, compliance with recent state legislative mandates, and various code updates. One recent state legislative mandate requires new developments to reduce average annual phosphorous runoff from a storm by 80% and nitrogen runoff by 45%. Various code changes are proposed including providing the city with authority to levy fines against contractors and others for contributing to water pollution; clarifying that seawall owners are responsible for damage to adjacent properties when caused by failure to maintain their seawall; and fining contractors for construction sites that result in damage to the right of way and erosion. A related discussion at a recent council meeting involved an ordinance proscribing driving through flood waters in neighborhoods at high speeds causing waves resulting in water damage to adjacent homes.

While sewage discharge at any level is a serios concern, the recent 429,000-gallon discharge is well below the 200 million gallons of wastewater discharge that occurred in 2015-16. As a result of that discharge the state ordered the city to update its aging wastewater system, and the city committed to allocating $326 million to do so. Had that not been done, sewage discharge as a result of Debby would have been greater by far. Tropical Storm Debby was the first real stress test of the improved storm and wastewater system.

The Stormwater Master Plan is an element considered by the federal Community Rating System (CRS) that determines insurance rates for the National Flood Insurance Program. The city has a current rating of five, which equates to a 25% discount on federal insurance policies.  Given the new plan, the city has potential for achieving a rating of four, which would equate to a 30% discount for residents needing flood insurance.

Surge Planning

The Stormwater Master Plan does not include projects to prevent or mitigate flooding from storm surge, although that is frequently more a threat than flooding from intense rainfall. Peak surge from Debby in Tampa Bay was 3.4 feet. This compares with 4.6 feet for last year’s Hurricane Idalia. As a result of Debby approximately 40 homes reported flooding throughout the city, a far lower number than with Idalia, primarily due to Debby’s lower surge.

The city received a grant to undertake a seawall study, although this is limited to city-owned seawalls. The city is also pursuing a grant to undertake a “Comprehensive Vulnerability Assessment,” which will combine data from the seawall study, the Stormwater Master Plan, and other sources. However again, this study is limited to an assessment of which city-owned assets need protection from surge, including transportation, infrastructure, emergency facilities, schools, and cultural resources. Private residences and businesses are not included. These grant-funded studies represent very initial steps towards a comprehensive plan to mitigate flooding from storm surge throughout the city, which protects private as well as public property and enterprises.  

At a recent forum Mayor Ken Welch spoke of the growing need to shift development away from the Coastal High Hazard Area (generally near Tampa Bay). According to Whit Remer, Tampa’s sustainability and resiliency officer, “Mitigation, like preventative health care, is fiscally responsible. Every dollar invested in girding infrastructure for a major storm pays for itself four-fold.”

Miami-Dade offers one planning model for St. Pete to consider, although surge planning needs to be adapted to fit the geography and circumstances of each city. This $2.7 billion plan, developed in collaboration with the Corps of Engineers, aims to create multiple lines of defense and tackles the easier projects first. A major component is elevating nearly 2,100 residential structures, both single and multifamily. About 400 structures, mostly government buildings, would be floodproofed with barriers. Nature-based fixes such as planting of mangroves, humanmade island enhancements, and living shorelines are also emphasized.

All in all, Tropical Storm Debby was a huge rainfall event that our city weathered quite well.   Yet while the storm served as a good test of the extensive city storm and wastewater improvements to date, it also served as another warning of the urgent need to fully implement the Stormwater Master Plan with all deliberate speed, and to accelerate planning for mitigation of major storm surge.

Will Michaels is a former director of the St. Petersburg Museum of History and the author of The Making of St. Petersburg and The Hidden History of St. Petersburg. Will also serves as a commissioner on the city Community Planning and Preservation Commission. Reach him at wmichaels2222@gmail.com or 727-420-9195.