While computational fluid dynamics (CFD) is utilized to simulate particulate matter (PM) separation and particle size distributions (PSDs) from unit operations, the role of computational parameters and runoff hydrograph unsteadiness to simulate intra-event elution of PM has not been examined. An Euler-Lagrangian CFD model is utilized to simulate discrete (Type I) separation of PM by a common hydrodynamic unit operation subject to unsteady runoff events and a hetero-disperse PM gradation. Utilizing a baffled hydrodynamic separator (HS) this study illustrates the potential of a CFD model to predict PM elution as a function of runoff hydrograph unsteadiness. The study hypothesizes that accurate simulation of unit behavior as a function of unsteadiness is dependent on mesh size (MS), time step (TS) and PSD discretization number (DN). Full-scale physical model and CFD results are compared. Intraevent results demonstrate that MS, TS and DN parameters significantly influence prediction of transient PM elution, PSDs and computational effort. Results demonstrate that each parameter generates error for transient PM elution as influenced by the unsteadiness level. In contrast, TS, MS and DN selection each have a statistically significantly smaller influence on event-based PM mass balances and PSDs. (C) 2011 Elsevier B.V. All rights reserved.