A comparison of the frictional pressure
drop obtained from the surface roughness of 1.189 lm,
1.378 lm and 3.312 lm to the smooth surface (0.936 lm) found
that the average pressure drop increased by 3.90%, 8.25% and
19.24%, respectively. As described in Section 4.1, it can be
explained that the main flow of water along the channel is
disturbed by the recirculation flow in the troughs as the water
velocity and surface roughness increase. This leads to the enhancement
of turbulence intensity and an increase in pressure drop.