Retrofitting coastal neighborhoods: low impact (re)development in Indian Beach and Sapphire Shores
Low Impact Design Practices
Common stormwater management systems collect water from impervious surfaces, route it for treatment to a single detention or retention location such as a pond and then discharge it to a surface water body. Low Impact Design (or Development) incorporates one or more depressions that hold stormwater for designed periods of time allowing nutrients and other potential contaminants to be assimilated by the vegetation, for sediment to settle out and for the water to percolate through the soils. All of these mechanisms reduce the volumes and increase the quality of stormwater that is ultimately discharged to surface water offsite.
While LID can be incorporated into the design of new residential or commercial stormwater management systems, its real potential lies in using it to retrofit existing developed areas that have no current form of stormwater treatment. Part of the flow can rerouted through bioswales, which are depressions containing sod or, preferably, native vegetation underlain with soils that facilitate percolation. Bioswales can be as simple as small depressions in native soils or they can be more complex and contain underdrains or very specific materials designed to control the rate of percolation or aide in the removal of pollutants. Whether bioswales are simple or complex, they will retain and treat stormwater runoff that would otherwise go untreated and contribute to surface water contamination.
Sarasota County has embraced LID as a viable addition to traditional stormwater treatment methods and has developed a technical LID manual that is currently undergoing peer review. Sarasota County also commissioned a study to determine the efficiency of grassed swales in reducing pollutant loads in stormwater (PBS&J, 2010) and found that they removed particulate loading significantly. They were found to be less effective in reducing concentrations of dissolved constituents; however, since the grassed swales did reduce overall loading there would be a concomitant reduction in dissolved constituents in the final discharge.
Indian Beach and Sapphire Shores Project Areas
The Indian Beach/Sapphire Shores neighborhoods are located just south of the New College campus and west of Tamiami Trail. The neighborhoods were platted and developed over 75 years ago, long before local and state environmental authorities adopted stormwater treatment requirements. The primary engineering concern at the time was to protect the new residents and structures from flooding when natural habitats were converted to impervious surfaces and higher volumes of stormwater runoff were generated. The neighborhoods were located on coastal ridges immediately adjacent to Sarasota Bay which served as a convenient repository for the newly created runoff containing sediment and contaminants primarily in the form of nitrogen and phosphorus.
Today, we know that stormwater runoff is the primary cause of water pollution worldwide. Amendments to the Federal Clean Water Act in the early 1970s started a wave of adoptions of Federal, State and local environmental regulations specifically aimed at improving water quality in the Nation’s surface waters. While there should be no doubt that these regulations significantly improved the quality of stormwater (and other) surface water discharges for new developments, they did little to reduce the volume and improve the quality of existing discharges. Most communities developed prior to the 1970s continue to discharge untreated stormwater to surface waters. Because coastal areas were highly prized by early settlers, it is these areas that remain the primary sources of contamination and nutrient enrichment in adjacent bays and estuaries. Indian Beach and Sapphire Shores are two such neighborhoods. And while there is no legal requirement for residents to retrofit their stormwater management system, they and Sarasota Bay would benefit greatly from a program designed to improve the quality of water originating in their neighborhoods and discharging to Sarasota Bay.