Hydraulic Roughness of some African Grasses

Considerable work has been done on determining the flow 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 triflorum), 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 effect of plant height and flow depth on roughness coefficient 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 flow depth from 6·0 to 1·1 cm resulted in an increase in roughness coefficient 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% confidence limit showed significant difference in the values of n for the three grasses, while there was also significant difference for the different flow depths.