Final Report: South Dade Stormwater Treatment Area Evaluation

Brown, Mark; Cohen, Matt; Dudas, Joel

Final report to the South Florida Water Management District. PROJECT SUMMARY: This research explored how to best ecologically engineer watersheds to minimize the effects of land use change caused by rapid population growth. Specifically, various urbanization condition and spatial wetland network designs were tested for their relationships to stormwater hydrology and water quality. The area studied was the C-102 and C-103 sub-basins in the rapidly growing region to the south of Miami, Florida. Since the project watershed was in a heterogeneous landscape with unique hydrologic characteristics, and because spatial evaluations of wetlands are not well supported by existing models, a new type of spatial model was developed. A cell-based hydrologic model, using local environmental and management variables, was developed using Geographic Information Systems software. The spatial model then employed an iterative technique to simulate the temporal dynamics characteristic of the hydrology found in both low-relief landscapes and in wetlands. Model development involved calibration, validation using alternate urbanization and rainfall conditions, and sensitivity analyses of four important model processes. Calibration was primarily challenged by instabilities related to the spatial lateral runoff algorithms. The calibrated model did not simulate as accurately during extremes of high or low rainfall conditions. Simulation accuracy was not found to be considerably different between the simulation results of two discrete sub-basins. The development steps revealed that the influences of input data, mathematical equations, and parametric settings on simulation results were somewhat scale-dependent. Model results suggested that urbanization has increased the annual watershed discharge of constituents into Biscayne Bay by approximately one order of magnitude. In addition, simulation results indicated that the conversion of wetlands into farms, residences, and canals has fundamentally changed the primary deposition pathway of the phosphorus cycle from biological to chemical. Simulation results of land use scenarios where 10% of the urbanized landscape was configured into nine different spatial arrangements of stormwater treatment wetlands suggested that although all wetland networks treated stormwater pollutants, the treatment achieved was variable. Reductions in annual discharge of constituents to the ocean achieved by the treatment wetland networks was between 14.8% and 41.3% for Total Phosphorus, and between 34.2% and 63.9% for Total Suspended Solids. Comparisons of wetland network simulation results suggested that location of wetlands in areas where the landscape naturally converges water and constituents reduced constituent discharges into the ocean more than did wetlands located either randomly or only in close proximity to pollutant sources.

Final Report: South Dade Stormwater Treatment Area Evaluation

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