Watershed Management Plan
The City received funding through the Southwest Florida Water Management District’s Cooperative Funding Initiative program to perform a City-wide evaluation of the City’s stormwater system through the development and utilization of a stormwater model. The evaluation identified where stormwater capital improvements are needed to improve the level of service and the quality of water discharged to the Gulf of Mexico and Boca Ciega Bay. The resulting Watershed Management Plan (WMP) was adopted early 2021 to replace the City’s former 1996 Master Drainage Plan and provide planning and management strategies to enhance the City’s resilience to sea level rise.
The WMP identifies resiliency measures to mitigate for the impacts of sea level rise to the barrier island. With some localized areas under elevation 2.0 feet (NAVD88), mean sea level projections in the Year 2100 would result in drastic impacts to quality of life within the community. Citywide terrain modifications, to achieve an average roadway elevation of 5.1 feet (NAVD88), will mitigate for this risk while facilitating the use of conventional stormwater collection and management techniques. Implementing this resiliency measure will fundamentally alter how development is designed, permitted and constructed within the City. As a “no fill” City, current regulations specifically restrict the use of fill material for public and private improvements. New development guidelines would require the use of fill material by establishing minimum low-floor elevations for structures and terrain modification goals for public infrastructure.
In the upcoming year, the City will begin the development of an Implementation Manual which will serve to guide the development community on design principles, establish review protocol for the City’s Community Development Department and recommend where code and ordinance modifications are needed. The draft guidance and recommendations will undergo an engineering peer review and an aggressive public outreach effort. The City is currently working through proposed amendments to its Comprehensive Plan to incorporate resiliency planning and the Watershed Management Plan. Once adopted, updates to the Land Development Regulations will promote the elevation of public and private property, including seawalls and further encourage the use of living shorelines.
As a barrier island, Treasure Island faces unique and imminent challenges. Successful implementation of the WMP provides a crucial path forward to help ensure that the City is given the opportunity to thrive into the future. While the Implementation Plan will provide an achievable location-specific tiered approach, a successful outcome relies on thorough and strategic consideration of the use of fill throughout the City.
WMP Key Recommendations:
- Elevation of low-lying sections of the City and roadway network to the highest achievable terrain modification goal: immediate, intermediate or long-term;
- Projects should be designed to begin meeting these goals in FY 2022;
- Expansion of water quality requirements within the Land Development Regulations;
- Pursuit of additional Community Rating System (CRS) creditable activities;
- A continuation of the City’s outfall maintenance program and the incorporation of existing outfalls into future projects;
- Strategic implementation of backflow prevention technology; and
- Promotion of low impact development (LID) projects.
- Adoption of amendments to the Comprehensive Plan;
- Updated Land Development Regulations pertaining to the use of fill, drainage and seawalls;
- Development of an Implementation Manual; and
- Public engagement.
Do the WMP and updates to the Land Development Regulations pertain to the elevation of private properties as well as roadways?
Yes. A goal of the Watershed Management Plan is to look at how the City would manage stormwater in the face of rising seas. Doing so required that the City be evaluated in holistic manner with a focus on publicly owned land and private property. Currently, a “typical” lot within the City slopes downward towards the road. This configuration allows for stormwater to drain off of private property, be collected by stormwater structures in the roadway and piped to the Bay. Drastic changes to this typical lot grading pattern are not proposed. The goal is to keep private property higher than the roadway network. Therefore, to elevate the roadways, elevation of the land on private property will also be needed.
How will the elevation of private property occur?
The elevation of each structure, both public and private, will be a gradual process. The elevation of a private residential structure, either with new construction or by substantial improvement, will be triggered by the actions of the property owner and permit issuance. Currently in Treasure Island this is done through the use of pilings (or through wet or dry flood proofing methods for commercial buildings). The elevation of property by use of fill material for terrain modification will be further encouraged through forthcoming updates to land development regulations.
Once the need to elevate a structure has been determined or otherwise desired by the property owner, FEMA’s Base Flood Elevation (BFE) will still be the basis of establishing the finished floor elevation. However, the elevation of the building’s “low floor” (or the structure's top of slab/attached garage) would be set based upon the requirements outlined within the upcoming code changes. Historically, this “low floor” elevation has been set at (more or less) existing grade due to the City’s no-fill policies. Moving forward, this “low floor” elevation will be higher to ensure that stormwater from private property can drain to the road, now set (or planned to be set) at a higher elevation.
Further, the City may propose requiring raised seawall heights while under a construction permit as well.
Why not just use backflow preventers to prevent tidal flooding?
Backflow preventers are devices that can be placed at the end of stormwater outfall pipes. When thoroughly maintained, they can be effective in preventing what’s known as “sunny day flooding” where saltwater comes back through the outfall pipe, and in some cases, into the street. These valves function by closing when the water elevation on the downstream face (at the outfall in the Bay) is higher than the water elevation on the upstream face (i.e. within the roadway). The valve remains closed and only opens when the water elevation on the upstream face becomes higher than the water elevation on the downstream face.
Many roadways within the City are at an elevation of about 2 feet. Tide gauge data published by NOAA shows that, on average, the City may experience a few tides each year that slightly exceed this elevation. If a backflow preventer valve was present during these times, the tidal saltwater would be kept out of the road. The issue occurs when it rains. The City is responsible for getting the rainfall off of the street for safety and to prevent rainfall flooding. With a backflow preventer, the collected rainfall must now stack up higher than the tide elevation in order for the valve to open up and let that rainwater out. In today’s scenario, this may mean that only a couple of inches of rainfall would need to sit on the road for the valve to open and allow the stormwater to drain.
While the City now sees infrequent tides above 2 feet, sea level rise could result in everyday high tides exceeding 4 feet by the year 2100. If the roads remain at an elevation of 2 feet and the tide rose above 4 feet, the backflow preventer will stop the tide from backing into the street, but at this point, it becomes a much more significant concern during rain events. Remember, backflow prevention valves only open when the upstream elevation is higher than the tide. This means that rainfall would need to “stack” in the road to an elevation of about 4 feet just for the valve to open and release the rainwater. For this example, this would mean that more than two feet of rainfall flooding within the roadway would need to occur for the valve to open. If we keep our roads at the same elevation and the tide elevations increase as predicted, using backflow prevention valves as our sole strategy will result in significant adverse impacts to daily life.
Why plan to the year 2100?
The proposed plan will require significant funding and time to elevate City roadways incrementally and responsibly. Private property will be elevated through the natural progression of development meaning that any roadway elevation project will need to match new and existing structures. Elevating roadways in a non-segmental manner could result in the roadway being higher than the finished floor elevations of many structures. This would result in stormwater being collected within a yard or driveway rather than within the road. The City places a high priority on keeping the “feel” of the City the same. If roadway elevating is not performed in the appropriate incremental manner, it is likely that the improvements will have a negative impact to those who live, work and play in our community.
The projected rates of sea level rise have also been considered. The annual change is not predicted to be linear. Therefore, the increase in sea level rise between the years 2050 and 2051 will be greater than the increases projected from 2030 to 2031. Selecting a shorter planning period could result in a false feeling of accomplishment when the true challenge still lies ahead.
The goal is to increase the likelihood that Treasure Island remains a thriving community in the year 2100. Selecting a longer planning period ensures that we begin to prepare today for the exponential rise of seas predicted in the future. If the process doesn’t begin in the near term, the necessary improvements will become too expensive by the time that the tide is impacting daily life on the island. A methodical, purposeful program to account for tomorrow’s challenges spreads out the financial liability, minimizes the hardships on the citizenry and ultimately increases our chances of success.
When will the elevation projects start?
The City is in the process of revising the Comprehensive Plan with supporting code changes proposed shortly thereafter. Grant funding is being pursued to assist in the development of a technical manual that will establish the core concepts of the program and provide technical guidance to the construction and development community. Elevation projects cannot commence until these steps are taken. It is believed that this process will be complete by the end of 2023.
Will elevation to the 5.1 feet goal keep my property safe from storm surge?
Unfortunately, storm surge is a threat many residents of Pinellas County and other coastal communities face. FEMA’s Base Flood Elevations, which are somewhat predictive of surge events, range from 11 to 17 feet throughout the City. This elevation is well above the roadway elevation goals set forth within this program. However, adherence to minimum finished floor elevations does provide a level of protection against structural flooding during these severe events.