Modeling Sediment Reduction in Grass Swales and Vegetated Filter Strips Using Particle Settling Theory

Two of the most common and simple stormwater control measures are swales and filter strips. However, an overly simplistic one-size-fits-all design approach typically is used for these practices. To provide more flexibility in design, a coupled hydraulics and particle-settling model was created to predict swale and filter strip total suspended solids (TSS) reduction as a function of catchment area, longitudinal slope, side slope, cross section type (triangular swale, trapezoidal swale, or filter strip), and length. The hydraulics and hydrology models were based on Manning's equation and the rational method, respectively, with the underlying requirement that the water quality design storm does not exceed the height of the grass. The particle-settling model was underpinned by the Aberdeen equation. The model predicts that triangular swales produce the least and filter strips the most TSS removal because of increased hydraulic retention time; trapezoidal swales had on average 10% greater TSS removal than triangular swales with similar design characteristics. Grass filter stormwater controls performed better with decreasing slope, increasing length, smaller catchment area, and shallower side slopes. Filter strips registered the majority of their TSS reduction within the first meter of their width and were insensitive to increases in longitudinal slope. This water quality design storm-abating modeling approach could be coupled with modeling for infrequent-return-interval storms to obtain dual benefits of sediment reduction and conveyance. This water quality model may provide regulatory agencies a tool to provide variable TSS credit to swales and filter strips as a function of design. (C) 2016 American Society of Civil Engineers.