“First Flush,” Power Law and Particle Separation Diagrams for Urban Storm-Water Suspended Particulates

Commensurate with development of in situ storm-water control and treatment is the need to quantify the delivery and granulometry of the suspended particulate fraction in storm water. Consistent with this need, this study examined the so-called “first flush” phenomenon for suspended particles with a measured range from 2 to 75 μm (typically <50 μm), the appropriateness of a single- versus multiple-power-law model of particle-number density (PND), and the application of process selection diagrams for particle separation. In comparison to a defined concentration “first flush” during the early portion of the examined rainfall-runoff events, results indicate that a disproportionately high and in some cases a continuous suspended particle delivery phenomenon that followed the hydrology of the event occurred. Such results suggest that the entire event may require treatment, not solely the commonly designated “first flush” based on indices such as suspended solids. While a single-power law reasonably represented granulometric characteristics of suspended storm-water particulates, and in theory a continuously size-based power law is the most accurate representation; within the given suspended particle-size range a multiple-power law provided reasonable simplicity and accuracy for total PND, surface area, and particulate volume. Despite a wide range of hydrologic conditions for a series of nine rainfall-runoff events examined, process selection diagrams based on the number-volume mean size $\left(l_{\mathrm{nv}}\right)$ and total PND led to a similar conclusion. Based on the combination of $l_{\mathrm{nv}}$ and PND in the urban catchment sedimentation in a typical transportation land use drainage facility is capable of removing 90% of particulate matter by mass within a detention time of 120 min.