The magnitudes of the friction factors for the two beaches are very similar over the majority of the swash cycle. This may be somewhat surprising since one would expect higher friction factors for the rougher beach. (For reference, friction factors calculated assuming steady and uniform flow and evaluated from the measured depths and velocities, with equivalent roughness of 1.3 mm and 8.4 mm, are consistently higher for the rougher beach by approximately 50%.) The fact that the friction factors are not very different in the present experiments suggests that the high flow unsteadiness and non-uniformity of swash have significant influence on the bed shear stress for a given instantaneous velocity. The boundary layer development is likely to play a significant role and its effect can be observed in the results during the uprush. At x = 0.072 m the boundary layer has developed to only a limited extent, so the friction factors for the two beaches are very close. Further up the slope, beyond x = 2.377 m, the friction factors are higher for the coarser 8.4 mm beach. The increase in friction factor as flow reversal is approached is consistent with cf behaviour in uniform, steady flow in that cf is increasing with decreasing Reynolds number and higher relative roughness. In contrast, cf behaviour during the early stage of the backwash is different from cf behaviour in uniform, steady flow in that cf is initially low, despite the Reynolds number being low and the relative roughness being high. Somewhat later, cf values for the 8.4 mm beach catch up on the values for the 1.3 mm beach and then become higher than the 1.3 mm beach values, but only for locations higher up the beach. At the two lowest locations on the beach cf values are surprisingly similar during the backwash. This means that the boundary layer development is not sufficient to explain cf behaviour throughout the whole swash cycle.
beaches
two
two beaches
roughness
1.3 mm and 8.4 mm,
mm
rougher beach
friction factors
approximately 50%
%
x
0.072 m
m
x
2.377 m
m
beach
8.4 mm
mm
beach
8.4 mm
mm
beach
1.3 mm
mm
beach
1.3 mm
mm
lowest locations on the beach
two