Evaluation of Low-Cost Low Impact Development Practices in Southwest Florida for the Control of Urban Runoff

Stormwater management is required due to development and alteration of the natural environment. It is heavily regulated in Florida and at the national level. Over the last two decades, Low Impact Development (LID) has been promoted as a sustainable and environmentally friendly method of controlling urban runoff. Case studies, provided in Chapter 2, show that LIDs can restore watershed hydrology by balancing the water budget. The difference in runoff between pre-development and post-development appears to increase with soil perviousness. However, the potential for mitigating the impacts of urbanization through runoff reduction is also greater for pervious, sandy soils that dominate central and south Florida. A greater potential for urbanization mitigation in Florida’s highly pervious soils initiates more research in quantifying the benefits of LID. Southwest Florida is currently in its infancy when adopting LID on a broad-scale; however, several municipalities are in the process of incorporating LID into their stormwater management programs. Low Impact Development includes non-structural practices such as minimal site disturbance and maintenance of natural flow patterns as well as structural practices. There are numerous structural LID practices such as rain barrels, bioretention systems, infiltration trenches, green roofs, and pervious pavement. Structural LIDs can be divided into comparison categories such as low capital cost and high capital cost as well as rainwater harvesting and infiltration- based. Low capital cost options include rain gardens, which can range from $4.00 to $10.00 per cubic foot of runoff volume whereas high capital cost options include pervious pavements and green roofs, which can range from $120.00 and $225.00 - $360.00 per cubic foot of runoff volume, respectively. Given the order of magnitude difference in cost between the low capital cost and the high capital cost LIDs, the focus of this thesis will be on those practices which require a low initial capital investment. Additionally, the low-cost options are further divided into two categories, rainwater harvesting LIDs and infiltration-based LIDs. Rainwater harvesting (RWH) is a LID practice that attenuates peak flow during wet weather events and reduces potable water demand for uses that would not normally require water of potable quality. The two options for RWH are rain barrels and cisterns. The difference between the two is a matter of scale. Rain barrels are typically implemented in one or more barrels with a volume of approximately 55-gallons, where as cistern volumes start at the hundreds of gallons. Effective RWH design includes long-term supply and demand as well as physical site considerations. Southwest Florida’s climate pattern is not compatible with rain barrels for runoff reduction due to their small volume; however, they still offer modest potable water savings to homeowners. Given the type, duration, and frequency of storm events, cisterns can offer runoff reduction as well as reducing potable water demand. For example, in Tampa, Florida, to achieve approximately 70% catchment efficiency, an average sized home would need approximately fourteen 55-gallon rain barrels or a 750-gallon cistern. Conversely, for a single 50-gallon rain barrel that serves outdoor use only, the water-saving efficiency is about 10% for Tampa. When properly designed, infiltration-based LIDs mitigate groundwater disruptions that result from urbanization such as minimizing receiving water body hydromodifications, such as stream bank erosion, and reducing pollutant discharges to surface waters. Infiltration-based LIDs include systems such as bioretention, level spreaders, drywells, and “pocket” practices i.e. pocket wetlands. Infiltration-based LIDs may be wet or dry systems and rely on easily attainable construction materials such as gravel, sand, and native vegetation. This combination may have applicability in Florida due to flat slopes, sandy soils, and areas with occasionally high seasonal water table. National standards for LID design should be considered guidelines and adapted accordingly to regional conditions in Southwest Florida. It is possible to utilize any number of LIDs, though one of the key factors to success is proper knowledge of the seasonally high water table, especially along the coast line. Additional factors to ensure infiltration-based LID success include installing a pre-treatment filter strip, standardized infiltration rate testing, standardized materials specifications, proper sequence of construction, and diligent construction inspections during and following construction. The prospect of increased LID implementation within Southwest Florida appears promising. Municipalities are actively incorporating LID into their stormwater management recommendations. A behavioral study and interviews with staff from local governments regarding LID was conducted. The results indicate that Southwest Florida is facing many of the same barriers to implementation as other communities across the nation. These include lack of knowledge and education, lack of regionally specific design guidelines, and few “real world” pilot projects. Based on the behavioral study, it appears education could be the strongest key to LID acceptance. Over the course of three months in a graduate level urban hydrology course, opinions regarding LID in Southwest Florida went from not possible to positively inquiring how to increase implementation. Since the region faces most of the same barriers to implementation, it may be possible to use other cities’ methods to increase LID acceptance and implementation as a template while modifying them so they are regionally appropriate. A mnemonic device entitled “Let’s Make LIDs RADD” was created to assist engineers in implementing successful LIDs. Where “R” represents site reconnaissance, “A” stands for choosing the appropriate LID given the site conditions, the first “D” denotes conducting a drainage investigation, and the second “D” corresponds with finalizing the LID design based on the information gathered after conducting all previous acronym activities