DETERMINATION OF ROUGHNESS COEFFICIENT FOR WATERCOURSE DESIGN

Manning's roughness coefficient was evaluated for three water courses two lined (trapezoidal and rectangular concrete) and one in unlined condition. The derive values tended to be somewhat higher than those listed in available literature, because of small size of water courses and the tropical vegetation involved. A large variation of the coefficient depending on the channel conditions was evident. The results pointed out that moderate vegetation growth in earthen channels can increase the roughness coefficient causing increase in the depth of flow and thus increase the losses by 2 to 3 times. The study concluded that by cleaning water courses, roughness can be reduced and as a result the conveyance losses can be reduced significantly. Design roughness coefficient for concrete lined and earthen water courses are recommended as 0.014 and 0.035 respectively.