Experiments on Vegetative Flow Resistance for Erosion Control using Grass Species from the Caribbean Region

Abstract

<b>Abstract.</b> Well-designed and maintained vegetative channels are a cost-effective way of conveying water for agricultural applications and preventing soil erosion. Two considerations for good channel design are flow resistance and stability. Flow resistance is related to friction and form stresses generated between water and vegetative cover and, stability refers to the prevention of channel degradation due to erosion. Fulfillment of both conditions for vegetative lining is a challenging problem because vegetation resistance depends not only on soil and flow conditions but also on the species, density, biomechanical properties, and plant morphology. An experimental setup was built to study the vegetative resistance of grass species using actual vegetation. <fig><graphic xlink:href=23052_files/23052-00.jpg id=AEEF9213-4B17-4B3C-9A21-A94D35ACAF4D></graphic></fig> The flume was constructed such that plant species could be inserted into the flume using modular trays accommodated side by side to form a testing reach. The rest of the channel bottom was raised to the elevation of the planting trays forming a continuous surface. A transparent acrylic window on each side of the measurement distance allows for visual observation of the plant‘s conditions during the experiments. Four grass species were studied: Bermuda, Bahía, Pangola, and Zoysia. The methodology includes particle velocities obtained with an ADV to form a velocity profile over the vegetation, fit the data to a modified logarithmic profile which provided the shear stress over the deflected vegetation, a gradually varied flow profile to obtain the energy grade line, and solution of momentum and Manning‘s equation to solve for the Manning‘s coefficients. 14 experiments were conducted. Results show that the velocity profile above the deflected vegetation is described by a modified von Karman logarithmic law (Christensen, 1985). Manning‘s coefficients obtained were between 0.115 and 0.282 (Table 1), which are within expected results for small hydraulic radii and shallow water flows (Sturm, 2010). The results also were within the limit of Manning‘s n values for channel and overland flows, thus can be used for overland flow and shallow channel flows. Relations developed by USDA-NRCS (Sturm, 2010) were used to determine the degree of retardance in terms of Manning‘s coefficient. Results showed that retardance degrees in the Puerto Rico Erosion and Sedimentation Control Handbook for Developing Areas (USDA, 2005) apply to the species tested in this project. A new retardance degree for Pangola grass was recommended when the plant height is near 0.15 m tall.