Spatial comparisons in wetland loss, mitigation, and flood hazards among watersheds in the lower St. Johns River basin, northeastern Florida, USA

Wetlands provide invaluable ecosystem services to moderate the effects of flooding. In the United States, Sect. 404 of the Clean Waters Act requires that mitigation of impacted wetlands from issued Environmental Resource Permits (ERPs) occur in the same watershed. This study investigated spatial patterns of impacted wetlands and compulsory mitigation (e.g., preservation and/or purchase of mitigation bank credits) in three watersheds of the lower St. Johns River basin, Florida, from ERPs submitted in 2006–2019. Each watershed was stratified by the Federal Emergency Management Administration’s (FEMA) designated 100-year floodplain, 500-year floodplain, and outside the floodplain. Flood hazards (e.g., projected 2030 Sea Level Rise, seasonal nuisance high tides, and storm surge) were compared among the three floodplain categories with ERP data, age and value of residential structures, FEMA field assessment following Hurricane Irma in 2017, and a composite flood hazard score. Results indicated that wetland loss occurred primarily in the 100- and 500-year floodplains, and wetlands were at greatest risk from projected 2030 SLR and nuisance high tides. These ERPs planned to preserve upland and wetlands by purchasing credits from banks located in areas with low risk of flooding. Older houses were typically in areas with greater flood hazard exposure, with higher home values. This study demonstrated that a watershed evaluation of flood exposure should be considered for future development, regardless of floodplain category. Given the significant social and economic costs from flood damage, community planners should prioritize protection of wetlands and open spaces in areas most vulnerable to flood hazards.