Modeling Particulate Matter Resuspension and Washout from Urban Drainage Hydrodynamic Separators

Abstract

Given the spatial complexity of urban drainage systems, the numerous system appurtenances and unit operations are a maintenance challenge. Maintenance can be the Achilles heel for best management practices (BMPs), and washout is of crucial importance for hydrodynamic separators (HS) that commonly function as preliminary unit operations to separate coarse particulate matter (PM) in urban systems. This study combines physical and computational fluid dynamics (CFD) models to quantify washout from two common HS types; screened (SHS) and baffled (BHS). Washout of particle size distributions (PSDs) for a range of flows is examined. Trajectory analysis of PM illustrates entrainment and washout as a function of PSDs and PM deposit depth. Velocity distributions identify washout-critical PM sizes and areas. Bed profiles are modeled by integrating velocity distributions across washout areas. A Shields washout criterion contradicts physical and CFD results, illustrating the portability challenges of open-channel approaches to nonuniform complex flows in BMPs. The driving parameter for washout is flow intensity at the PM deposit interface. The physically validated CFD model reproduces washout as a function of PSD and flow rate. A baffled HS provides volumetric isolation of deposited PM and generates low washout as tested with a fine hetero-disperse PSD. In comparison to the low washout of the baffled HS for a finer and hydraulically isolated PM deposit, a screened HS generates higher washout for a coarse monodisperse PM deposit, and over 50% of the volumetric domain has an upward velocity vector.