Coastal Watershed Model and A Resilient Watershed Management Plan For The Highly Urbanized City of St. Petersburg, FL

The City of St. Petersburg is developing a city-wide watershed master plan (WMP) to improve and manage the stormwater within the City by considering the sea level rise (SLR) and climate change impacts. As such, a watershed model is developed for the City covering 26 highly urbanized basins in the most densely populated County in Florida, Pinellas County. The city is a peninsular and low-lying area bordering the Gulf of Mexico and Tampa Bay, and shares many boundaries with County-maintained watersheds. Jacobs has modeled the watershed and completed the floodplain analysis phase. This model will be used to analyze level of service analysis and identify sustainable BMP projects to mitigate watershed flooding and improve water quality. One of the bigger challenges for developing this WMP was the vast network of stormwater features from both aged systems and new development being incorporated into the 62 sq. Mile area, making it one of the largest and most detailed WMP ever done within the State of Florida. Due to its size and scale of model, the WMP was broken into 7 reasonable groupings based on the hydraulic and hydrologic characteristics within the City limits and allowing for localized focus for BMP and LOS analysis. It is anticipated that the plan will help develop a capital improvement projects with a total budget of over 600 million USD to be implemented over the next 20 years to reduce flooding, water quality impacts, and harden against climate change and SLR. To develop the hydrologic and hydraulic (H&H) model data features, Jacobs' team has evaluated and completed data collection and collation for various sources including the City's Stormwater inventory which consisted of 30,000 structures, 4,000 as-built plans records, and 600 City Atlas Sheets. This resulted in further data gap analysis, field reconnaissance using ArcGIS Collector app, and survey for more than 1,000 sites and 2,200 stormwater structures, resulting in the model capture of almost 500 miles of stormwater pipe. Furthermore, Jacobs incorporated Flood Modeller FAST (FAST) results, Jacobs proprietary modeling software which does a 'rapid inundation' flood analysis on the terrain model where only the terrain volumetric characteristics are utilized for flood analysis. FAST results helped with sub-basin refinements and identifying possible locations where stormwater inventory could be missing.
Utilizing ArcGIS, and the Geographical Watershed Information System, GWIS, database structure for WMP evaluation, we were able to QC data discrepancies and better delineate a localized level of watershed basin development with efficient importation into the modeling software ICPR4. Based on the data assembled, collected and compiled, a very detailed watershed model was developed to capture the stormwater management infrastructure within the City. The high level of detail will help with better level of service (LOS) analysis for the localized systems and regional systems, and help with evaluating a more robust BMPs to mitigate the flooding at both regional and local scales. Currently, the model is finishing up the Floodplain Analysis phase where Jacobs has developed the 100-year/ 24-hour storm floodplains and has calibrated and verified using the 13 surface water data loggers and 19 rain gauges installed throughout the city 2 years in advance, along with historical water levels, public input from mail-outs, and online public forums. The calibration and verification was conducted using the unnamed multi-day event from August 2019 and tropical storm Eta from November 2020. For the100-year/ 24-hour storm simulation, Jacobs has updated the St Petersburg tidal gauge information for Mean High High Water (MHHW) to account for the increase in tide since the last epoch (1983-2001). This provided a more reasonable tidal boundary condition to use in the model. Furthermore, Jacobs again utilized the Flood Modeller results to cross check the floodplain delineation and also identify possible stormwater infrastructure maintenance needs.
To develop a more resilient WMP, models are also being developed to incorporate future years 2050 and 2100 conditions with respect to SLR and future rainfall projections for conditions. SLR estimates were gathered from latest publications from both National Oceanographic and Atmospheric Administration (NOAA) and Tampa Bay Climate Science Advisory Panel (CSAP). Jacobs has conducted historic rainfall analysis and projected rainfall totals for various design storm events for the future years. The incorporation of year 2050 aligns with the other City initiatives, such as Vision 2050 and consistent with the City's policies. The incorporation of 2100 condition helps the City improve their Community Rating System (CRS) rating from 5 to 4 providing a 30% discount on flood insurance for its residents.
The next phase of the WMP is to conduct the LOS analysis which focuses on both flood extent and duration impacts to both roadways and building structures. After the LOS criteria is established for each of the 12,000+ basins within the City's model, a prioritization is given for BMP analysis to those areas most impacted regarding most potential for reasonable improvement in terms of flood reduction extent and duration and water quality improvements. For BMP analysis, apart from the traditional stormwater improvement, BMP solutions will also focus on low impact development (LID), green infrastructure, where feasible. Some of the other potential BMPs to be evaluated are injection wells to not only reduce flooding problems, but also recharge the groundwater table which may also help with saltwater intrusion from all sides of the City. The City is also interested in potential development of new potable water sources from excess runoff to capture and store fresh water inland instead of discharging to nutrient limited estuaries. Cost benefit analysis for the evaluated BMPs will help the City prioritize the projects that provide the most benefit and in the most economical way. It will allow them to evaluate BMPs on both a localized level, potentially helping isolated neighborhoods with flood issues, but also on a regional level where entire corridors of the City could be better protected from flood impacts. The City's progressive approach in completing this resilient watershed management plan will help protect it's assets and will be able to better plan for the future and prioritize it's funding efficiently.