Hydraulic Resistance in Grass Swales Designed for Small Flow Conveyance

Grass swales, originally used for erosion control in agricultural settings, are now widely employed in urban environments as an effective best management practice for controlling pollutants in stormwater runoff. In particular, vegetated swales are quite successful in removing heavy metal concentrations when the depth of flow is small relative to grass height. However, guidance materials currently available for design of vegetated channels focus on larger depths of flow (large flow conveyance/erosion control), and for such conditions the hydraulic resistance exerted by the vegetation can be significantly different than that observed when the depth of flow is small (remediation). Utilizing a series of laboratory channels, small-flow retardance curves have been developed in the present work for Bluegrass, Centipede, and Zoysia grass species. These “small-flow” curves extend the well-known Stillwater n versus VR diagram by approximately 1 order of magnitude, to smaller values of VR. Experimental results should provide valuable design guidance to those faced with the need to hydraulically design a swale intended for shallow depths of flow.