STORMWATER SEDIMENT TRANSPORT IN GRASS SWALES DURING SMALL RAINS

Grass swales are vegetated open channels that collect and transport stormwater runoff. They are often used as an alternative to concrete gutters to transport runoff along streets due to their low cost. However, they also offer several advantages in stormwater quality management, especially in their ability to infiltrate runoff. This paper describes another benefit of grass swales: their ability to trap particulates during low flows. A series of detailed laboratory tests were conducted to describe sediment transport processes for stormwater in grass swales. Field verifications of these processes are also described in this paper. As expected, runoff hydraulics, especially depth of flow, along with swale length, affect the transport of particulates of different sizes. Shallow flows (less than the grass height) provided consistently high removal rates, while deeper flows (and especially along with relatively low sediment concentrations) had poorer sediment trapping abilities. Obviously, long swales and large particle sizes are an effective combination, but the smallest particles are likely to be effectively transported along most swales. There appeared to be equilibrium concentrations for different particle sizes that were not further reduced, irrespective of swale length, likely associated with combinations of scour of material from the underlying soil or of previously trapped sediment, and the carrying capacity of the water.