Can Surface Overflow Rate Predict Particulate Matter Load Capture for Common Urban Drainage Appurtenances?

Urban drainage appurtenances separate particulate matter (PM) and detritus unintentionally and by design. Such PM separation impacts conveyance, treatment, and maintenance practices. This study examines two common appurtenances: Gully pots (or catch basins) and screened hydrodynamic separators (HS). Under steady and controlled physical model testing, PM separation was measured for influent granulometry [particle size distributions (PSDs), PM specific gravity]. Catch basin separation ranged from 40 to 99% for a monodisperse (well-graded sand, SW) PSD and 60 to 83% for a hetero-disperse PSD. With similar testing, a clean HS (to avoid scour dominating PM separation), the HS was also loaded with a heterodisperse sandy silt (ML) and tested as a function of flow, with separation of 40 to 65%, as compared to 70 to 99% for the SW, similar to the catch basin. Physical model results were compared to the surface overflow rate (SOR) model, illustrating that the SOR overestimated PM separation by 3–13%. The SOR was extended to unsteady runoff events. For unsteady loading of an HS with complex hydrodynamics and short residence times, the SOR overpredicted measured PM separation by 3–22% on the basis of PM granulometry. For maintenance and coarse PM load inventories, the SOR can reasonably predict the fate of coarse PM, subject to Type I settling in an HS and catch basin units with similar PM separation behavior. If suspended PM mass dominates the particle size distribution (PSD), is the focus of treatment, or for units with long residence times, the continuous phase hydrodynamics must be coupled with a discrete phase model, requiring analytical or numerical models such as computational fluid dynamics (CFD). For conditions illustrated herein, the SOR is reasonably robust.