distinct morphodynamic scaling relationship exists between high- and low-
energy environments. Results from sediment transport studies in low-energy
systems may therefore be applicable to large-scale systems. The existence of
such a scaling factor also implies that the physical connection between waves,
currents, and morphology, both within and between beach systems, are related to
variations in wave energy.
Brander (1999b) applied the Shields parameter S as a criterion for predicting
sediment entrainment.
τ
S=
(C13)
ρ ( s - 1) gD
where
τ=
bed shear stress
(ρs-ρ)/ρ
s=
ρ=
density of seawater
acceleration due to gravity
g=
median sediment grain size
D=
Under combined waves and currents the bed shear stress is:
(
)
τ = ρ uc2 + uw
2
(C14)
where uc and uw = the time-averaged shear velocity computed following a method
outlined by Nielsen (1992) for currents and waves, respectively.
Brander (1999b) observed that sediment entrainment in the feeder current
and rip channels of a circulation cell is different. Sediment entrainment in the
feeder channel is minimized at low tide because increased wave breaking and
dissipation across nearshore bars inhibits wave motion and reduces the value of
S. Therefore, despite higher flow velocities in the feeder current at low tide, the
potential for transport is inhibited by the reduction in sediment entrained.
Sediment entrainment in the rip channel, however, is maximized at low tide
and minimized at high tide. Because flow velocities are also maximized at low
tide (as discussed in the previous section), the offshore transport is greatest then.
At high tide, both sediment entrainment and flow velocities are minimized, but
still significant. Thus, although much smaller at high tide, transport in the rip
channel occurs throughout the tidal cycle (Brander 1999b).
Wave and current contributions should not be considered separately, but
superimposed (Grant and Madsen 1979). Nonlinear interactions exist between
waves and currents, and the sediment transport under their combined action is
likely to differ from the sum of the individual contributions (Beach and Sternberg
1992).
C9
Appendix C
Literature Review of Cross-Shore Sediment Transport by Rip Currents
Previous Page
