ANALYSIS:
PROJECT DESCRIPTION
The La Gorce neighborhood, West 59th Street, was targeted for this pilot project due historical flooding of low-lying homes along the street. The bioswale pilot project is located within the West 59th Street right-of-way, between Alton Road and Biscayne Bay. The runoff from the existing roads and sidewalks will be managed by a series of proposed bioswales.
For the purposes of this study, the design target is to manage the first 1.5-inches of the storm events within a typical year. Runoff from the impervious road and sidewalks will be directed to multiple depressed storage areas (bioswales), where the design volume runoff will be captured and not discharged to the stormwater collection system of underground pipes, unless the storm event is larger than the 1.5-inch design volume.
BIOSWALE OBJECTIVES
Bioswales can improve stormwater runoff water quality by allowing solids and other pollutants to settle out of the water naturally in the bioswale depressions and the engineered soils beneath the surface of the basin rather than passing into storm sewers and eventually the waterways and ecosystems.
The objective of this evaluation will focus on quantifying the benefits of using this approach and identifying feasible design alternatives to provide the water quality and volume attenuation benefits for the target demonstration site. A bioswale stormwater best management practice was selected by the City to be demonstrated at this site, since it will provide on-site retention of the runoff from lawns, driveways and roads within a residential neighborhood that is ripe for other infrastructure improvements.
BENEFITS
• Effective removal of suspended solids, nutrients (nitrogen and phosphorus) and other pollutants from stormwater before it enters the bay;
• Allows stormwater to infiltrate into the ground and replenishes the fresh water lens beneath Miami Beach – this supports robust green spaces;
• Captures the first flush (1.5-inches) of runoff from every rain event to remove contaminants; and
• Beautifies the area with attractive landscape and utilizes a variety of native plants to minimize maintenance requirements.
LIMITATIONS
• Bioswales are not intended to solve flooding in low lying areas;
• Periodic maintenance is required, for example the mulch will need to be replenished or replaced to maintain its functionality;
• Only the first 1.5 inches of large rain events will be captured – excess water will bypass the bioswale due to limited capacity; however, this adequately treats contaminated runoff per county and state regulations.
• Potential loss of parking when bioswales are located in swale areas currently used by property owners for parking.
• Costs of bioswales can be expensive. For this pilot project, the overall estimate cost is approximately $850,000.