Influence of Sediment Concentration and Size Distribution on Hydrodynamic Separator Performance Using SHSAM: MS4 Sites in Nebraska

Abstract Understanding the particle size distribution (PSD) of roadway runoff is essential for selecting stormwater treatment practices and evaluating whether hydrodynamic separators can meet regulatory sediment removal targets. This study examines PSDs and total suspended solids (TSS) from Nebraska roadway catchments and compares them with published values to place the local measurements within the broader range of roadway and urban sediment conditions. The work also uses Sizing Hydrodynamic Separators and Manholes (SHSAM) modeling to identify when hydrodynamic separators may function effectively as a stand-alone treatment option and when additional measures may be needed. PSDs of stormwater samples measured at three Municipal Separate Storm Sewer System (MS4) roadway sites that discharge directly to storm drains in Nebraska are compared with values reported in the literature, and hydrodynamic separator performance is evaluated using SHSAM modeling for multiple catchment scenarios. Results showed that the Nebraska PSDs exhibit substantial variability but remain within the range of values commonly reported for roadway dominated catchments. Modeling indicated that hydrodynamic separators can generally achieve high removal of sand-sized particles for small catchments representative of Nebraska conditions, with more than 50% removal of the sediment load associated with particles larger than 63 μm in catchments smaller than about 2 ha, but removal of finer particles remains limited across all modeled catchment sizes. However, performance declines for PSDs with higher proportions of fine particles, indicating that additional treatment may be required. The study identifies that hydrodynamic separators serve as an effective treatment option when roadway sediment contains a substantial coarse fraction and the treatment area is small. The findings also show that hydrodynamic separators offer meaningful treatment credit at sites where a significant portion of sediment originates from outside the right-of-way, when allowed by the permitting process.