This paper investigates the relationship between trap efficiency and residence time in vegetative filter strips under unsteady flow conditions. A mathematical model, based on a kinematic‐wave formulation of overland flow and a mass balance equation for suspended sediments, is employed. An empirical equation is used to estimate sediment removal. The governing equations are written and solved numerically in terms of a set of dimensionless variables. The results indicate that the relationships between the trap efficiency and residence time depend on the shape of the inflow hydrograph. Further analysis of results identified another non‐dimensional parameter that can predict the trap efficiency if only the volume and the peak rate of inflow are known. While the mathematical model employed includes many elements of the sophisticated vegetative filter strip models previously reported, the resulting charts are as easy to use as the oversimplified design methods currently in use.