The thick black line in Fig. 3 stands for the measured hydraulic gradient in the horizontal part in the case with water flowing
alone. The typical Reynolds number is 2 105 and the flow is fully turbulent. The curve is a fit of the form If ¼ k V2
2gD that gives a value for the friction coefficients kh = 0.156 and ks = 0.160. Corresponding rugosities can be deduced with the Colebrook formula. The estimated rugosity is 20 lm for the horizontal pipe.