Hydrodynamic Clarification of Rainfall-Runoff Particles

This study examined the event-based performance of an empty-sump screened hydrodynamic separation (HS) for particulate matter transported in rainfall-runoff from a source area urban watershed. Fully-captured runoff events were examined and the entire volume of runoff and particulate matter from each event was treated by the HS. The ability of the HS (in this case continuous deflective separation) to reduce particulate matter loads in rainfall-runoff was quite variable and was largely a function of the runoff granulometry and hydraulics. The HS was subject to highly variable hydraulic (flow rate) and hydrologic (duration, intensity) event characteristics, and the coupled variability of granulometry. The HS was cleaned after each event so that scour and discharge of previously captured particulate matter could be separated from event-based discharges. Given the variable clarification mechanisms of a HS, results demonstrated that a clean HS provided good separation capability for the coarse sediment fraction (> 75 μm), much less separation for the settleable fraction (25~75 μm) and low separation capacity for the suspended fraction (1~25 μm). Results consistently indicated that the mass fraction of sediment and settleable particles also increase compared to suspended particle fraction indicating a positive relationship between particle concentration and coarser particle size distribution (PSD). Such coupling and generation of coarser PSDs help explain the improved results at higher flow rates. Study results were representative of the flow and granulometry with the full cross-section of flow sampled and the entire gradation measured. Results indicate that the coupling of hydrology and particle transport influences particle size distributions (PSDs) in runoff flows. This study clearly demonstrates that performance by the HS was predominantly influenced by influent PSD far more strongly than hydraulics. The role of scour is not reported herein.