Particle dynamics in stormwater treatment areas

Abstract

In south Florida, USA, Stormwater Treatment Areas (STAs) are used to reduce Phosphorus (P) loading into the northern portion of the Everglades. Phosphorus is well retained in the STAs (e.g. P biological uptake and P-sorption on adequate substrata) but a small, yet significant portion of it, leaves the STAs as particulate P (PP) in the discharge water. Thus, this study was conducted to improve the understanding of mechanisms and factors that affect PP removal efficiency of the STAs. Specifically, this study employed a suite of field and analytical methods to determine the effects of hydraulic conditions on the erosion, entrainment, transport, and settling of particles in the STAs. Prediction of the flux of sediment by numerical models requires estimates of the density, settling velocity, size, and fractal dimension of the particles and estimates of the critical bed shear velocity. We found that the STA particles are porous aggregates as in other similar shallow water bodies, but have a relatively high settling velocity, likely due to the high amount of CaCO3 and relatively low amount of organic matter in the sediment. Correlations between diurnal variations in windspeed and suspended sediment concentrations suggest that wind generated turbulence was responsible for resuspended matter in the water column. However, the wind-generated shear stresses applied to the bed were much smaller that the critical shear stress measured at the bed. These results suggest that much of the suspended material came from entrainment of materials from the surfaces of the surrounding submerged aquatic vegetation rather than erosion and entrainment of sedimented materials from the bed. Using calibrated acoustic backscatter from an acoustic doppler velocimeter, an analogy of Type 2 erosion was applied to estimate the critical shear stress of particles from the stems and leaves of submerged aquatic vegetation. The estimated critical stress was close to zero, supporting the idea that this unconsolidated and easily resuspended material comprises the bulk of the suspended sediment during normal operating conditions.