4,512 Zones of Transport. Littoral transport occurs in two modes: bedload transportthe motion of grains rolled over the bottom by the shear
of water moving above the sédiment bed; and suspended-toad transport, the
transport of grains by currents after the grains hâve been lifted from
the bed by turbulence.
Both modes of transport are usually présent at the same time, but it
is hard to distinguish where bedload transport ends and suspended-load
transport begins. It is more useful to identify two zones of transport
based on the type of fluid motion initiating sédiment motion: the offshore zone where transport is initiated by wave-induced motion over rip, and the surf zone where transport is initiated by the passing breakIn either zone, net sédiment transport is the product of two processes: the periodic wave-induced fluid motion that initiâtes sédiment
motion, and the superimposed currents (usually weak) which transport the
pies
er.
sédiment set in motion.
a. Offshore Zone
reach a depth where the
sédiment on the bottom.
Waves traveling toward shallow water eventually
water motion near the bottom begins to affect the
At first, only low-density material (such as seaweed and other organic matter) moves. This material oscillâtes back and
forth with the waves, often in ripple-like ridges parailel to the wave
crests. For a given wave condition, as the depth decreases, water motion
immediately above the sédiment bed increases until it exerts enough shear
to move sand particles. The sand then forms ripples with crests parai e
to the wave crests. These ripples are typically uniform and periodic,
and sand moves from one side of the crest to the other with the passage
of each wave.
the wave height, the veloc, fîmes the wave neignt,
As depth decreases to a value seve
ximateiy sinusoïdal to a is
ity distribution with time changes
t asSociated with the brie
tribution that has a high shorewar
P
veiocities associated wi
passage of the wave crest, and lower s
Q of the trough. As the
the longer time interval occupied y
. e passing crest decreases
shoreward water velocity associate
a cioud of sand erupts upwa
begins to reverse direction over a ripp » crest. This cloud o san
from the lee (landward) side of te
?
t|ie trough. At these s
drifts seaward with the seaward
, o£ suspended sédiment i
depths, the distance traveled V^the std concentration at a
or more ripple wavelengths, so that the
maximums during th^hgjSby
above the ripples usually exhibits a
tfre suSpension clou s
age of the wave trough. These maxim
oach of the next wave c
®
the two nearest upstream ripples.
maîning suspended in the c ou
reverses the direction of the sJoreward as bedload.
landward flow also drags material
érosion or
For the nearshore profile to be ^q-1 s1c^r™dWauay,ft°"1.»
on
accretion, the average rate at «hach sand
sand as added.
’he bottom must be balanced by te
net residual curren
currents,
Any net change will be determined y
motion by the waves,
bottom which transport sédiment se
4-53
of water moving above the sédiment bed; and suspended-toad transport, the
transport of grains by currents after the grains hâve been lifted from
the bed by turbulence.
Both modes of transport are usually présent at the same time, but it
is hard to distinguish where bedload transport ends and suspended-load
transport begins. It is more useful to identify two zones of transport
based on the type of fluid motion initiating sédiment motion: the offshore zone where transport is initiated by wave-induced motion over rip, and the surf zone where transport is initiated by the passing breakIn either zone, net sédiment transport is the product of two processes: the periodic wave-induced fluid motion that initiâtes sédiment
motion, and the superimposed currents (usually weak) which transport the
pies
er.
sédiment set in motion.
a. Offshore Zone
reach a depth where the
sédiment on the bottom.
Waves traveling toward shallow water eventually
water motion near the bottom begins to affect the
At first, only low-density material (such as seaweed and other organic matter) moves. This material oscillâtes back and
forth with the waves, often in ripple-like ridges parailel to the wave
crests. For a given wave condition, as the depth decreases, water motion
immediately above the sédiment bed increases until it exerts enough shear
to move sand particles. The sand then forms ripples with crests parai e
to the wave crests. These ripples are typically uniform and periodic,
and sand moves from one side of the crest to the other with the passage
of each wave.
the wave height, the veloc, fîmes the wave neignt,
As depth decreases to a value seve
ximateiy sinusoïdal to a is
ity distribution with time changes
t asSociated with the brie
tribution that has a high shorewar
P
veiocities associated wi
passage of the wave crest, and lower s
Q of the trough. As the
the longer time interval occupied y
. e passing crest decreases
shoreward water velocity associate
a cioud of sand erupts upwa
begins to reverse direction over a ripp » crest. This cloud o san
from the lee (landward) side of te
?
t|ie trough. At these s
drifts seaward with the seaward
, o£ suspended sédiment i
depths, the distance traveled V^the std concentration at a
or more ripple wavelengths, so that the
maximums during th^hgjSby
above the ripples usually exhibits a
tfre suSpension clou s
age of the wave trough. These maxim
oach of the next wave c
®
the two nearest upstream ripples.
maîning suspended in the c ou
reverses the direction of the sJoreward as bedload.
landward flow also drags material
érosion or
For the nearshore profile to be ^q-1 s1c^r™dWauay,ft°"1.»
on
accretion, the average rate at «hach sand
sand as added.
’he bottom must be balanced by te
net residual curren
currents,
Any net change will be determined y
motion by the waves,
bottom which transport sédiment se
4-53