Considerable work has been done on determining the #ow retardance of plants used in vegetated waterways. This has, however, received little attention in Nigeria and other parts of Africa. This paper reports on a study conducted to determine the hydraulic roughness of three African grasses, as a means of controlling rill erosion. The plants are * atiponna (Desmodium tri-orum), star grass (Dactylontenium aegyptium) and stubborn grass (Sporobolus pyramidalis).
The experimental set up consisted of six * 14 m;0)6 m parabolic-shaped channels. The e!ect of plant height and #ow depth on roughness coe$cient n values was tested. Atiponna recorded values for n of 0)14}0)30, star grass, 0)09}0)20 and stubborn grass, 0)08}0)19 representing an increase of about twice the value of n within the period of study. Decreasing the #ow depth from 6)0 to 1)1 cm resulted in an increase in roughness coe$cient value from 0)13 to 0)23, 0)08 to 0)24 and 0)06 to 0)20 for atiponna, star grass and stubborn grass, respectively. Statistical analysis at 95% con"dence limit showed signi"cant di!erence in the values of n for the three grasses, while there was also signi"cant di!erence for the di!erent #ow depths.