Vegetative Flow Resistance for Erosion Control Using Grass Species from the Caribbean Region

Vegetative channels convey runoff effectively, prevent flooding, protect soil against erosion, avoid the formation of gullies, maintain adequate water quality, and reduce the generation of sediments. The degree of retardance as a measure of the drag force on the vegetation cover is a crucial parameter for determining resistance to water flow in these channels. This research developed a new procedure to assess Manning's coefficient and retardance index for vegetation using four Caribbean Area grass species: Bahia Grass (Paspalum notatum), Zoysia Grass (Zoysia sp.), Pangola Grass (Digitaria eriantha), and Bermuda Grass (Cynodon dactylon). The USDA-NRCS Caribbean Area identified 16 species as vegetative lining for waterways and proposed a classification according to the degree of vegetative retardance. The category for these species was developed for use in the Western Gulf Region and required validation for the Caribbean region. A modified universal log law fitted the velocity distributions above the vegetative lining in the experiments. The energy equation for a gradually varied flow, the momentum equation, and Manning's equation provided a solution for Manning's coefficient. The procedure allowed us to assign a degree of retardance to the species obtained by comparing Manning's coefficients with those from USDA-NRCS. Results show that retardance degrees published in the Puerto Rico Erosion and Sedimentation Control Handbook for Developing Areas apply to the species tested in this project. This research identified a new retardance degree for Pangola grass when the plant height is less than 0.15 m. The experimental values for Manning's coefficient are also recommended for use in overland flow conditions.