IN,.. Sediment Transport
87
The total sediment transport rate (qt) can be computed from the vertical
distribution of fluid velocities and sediment concentrations, as follows "
qt~
d+n
I UCdz
0
(39)
in which •
U
C
d
= local instantaneous fluid velocity at height z above bed
= local instantaneous sediment concentration at height z above bed
= water depth to mean surface level
= water surface elevation
Defining " U = u + u and C = c + c
(40)
in which •
U
C
U
C
= time and space-averaged fluid velocity at height z
= time and space-averaged concentration at height z
= oscillating fluid component (including turbulent component)
= oscillating concentration component (including turbulent component)
By introducing eq. (40) in eq. (39) and averaging over time and space, we get •
__
d
d~
qt = f ucdz + i ucdz
(41)
0
0
in which "
__
d
q e = j ucdz
0
= current-related sediment transport rate
d~
qw = j ucdz
0
= wave-related sediment transport rate
Current-related sediment transport is defined as the transport of sediment
particles by the time-averaged (mean) current velocities (longshore currents, rip
currents, undertow currents). Both the current velocities and the sediment
concentrations are affected by the wave motion. It is known that the wave motion
reduces the current velocities near the bed, but strongly increase the near-bed
concentrations due to its stirring action. Wave-related sediment transport is
defined as the transport of sediment particles by the oscillating fluid components
(cross-shore orbital motion). The oscillating components (u and c ) may also be
affected by the current velocities.