2. Characteristics and Motion of Sédiments
31
of water over seabed influences the direct shear stress on the bed, or when
the seabed is susceptible to such stresses due to fine and loosely movable
sédiments. In the event of océan waves transmitting substantial amounts
of sédiments, it can hâve the propensity to control the wear of seabed
(abrasion). Meanwhile, the abraded sédiments are crushed down, thereby,
reducing in size and more smoothed (attrition). Although sédiment settles
down in a slow moving or still water in ponds or lakes and océan, river
channel deposits and beach sands are some of examples of fluvial transport
and déposition.
Sédiments in océan is transported either as bed load which usually is
coarser or suspended load which usually is finer. The velocity of the flow
reaches a critical value for initiating the motion of the sédiments which
dépend on it grain size. This is referred to as the entrainment velocity. Nevertheless, even if the velocity falls below the entrainment velocity the grains
will continue to move due to the reduced (or removed) friction between the
grains and the river bed. Ultimately, when the flow velocity reduces, in
particular falls below its threshold or if the grain size is large the sédiments
finds its way to the river or seabed.
Sands from medium to coarse and clays which are non-sensitive will
typically move as there or thereabouts firm material for which the velocities
are fairly low. Causes of this volume will tend to give up relatively rapidly
when the side angle is eut down under some critical point.
Edgers and Karlsrud [1982] proposed three potential mechanisms for
these huge run-out. The mechanisms include (1) turbidity currents, (2) viscous flow models, and (3) progressive liquéfaction.
2.13.2 Incipient sédiment motion
Laboratory experiments conducted extensively indicates two criteria for
movement initiation of level bed sédiment with D^q grain size distribution
between 0.1 mm and 0.2 mm [Hallermeier, 1980]. The appropriate threshold
flow velocity for sand motion as applied in the field is
^max(-d)
■z
\
T 0.5
8 ( — - 1 ) gD50
. \7
/
J
(2.22)
where
wmax(-d) is peak fluid velocity at the sédiment bed.
«max(-d) can be determined using Airy’s wave theory given as
tvH coshfc(d + z) .
.
u - —--------. , , :— sin(«x - cri)
T
smh kd
(2.23)
31
of water over seabed influences the direct shear stress on the bed, or when
the seabed is susceptible to such stresses due to fine and loosely movable
sédiments. In the event of océan waves transmitting substantial amounts
of sédiments, it can hâve the propensity to control the wear of seabed
(abrasion). Meanwhile, the abraded sédiments are crushed down, thereby,
reducing in size and more smoothed (attrition). Although sédiment settles
down in a slow moving or still water in ponds or lakes and océan, river
channel deposits and beach sands are some of examples of fluvial transport
and déposition.
Sédiments in océan is transported either as bed load which usually is
coarser or suspended load which usually is finer. The velocity of the flow
reaches a critical value for initiating the motion of the sédiments which
dépend on it grain size. This is referred to as the entrainment velocity. Nevertheless, even if the velocity falls below the entrainment velocity the grains
will continue to move due to the reduced (or removed) friction between the
grains and the river bed. Ultimately, when the flow velocity reduces, in
particular falls below its threshold or if the grain size is large the sédiments
finds its way to the river or seabed.
Sands from medium to coarse and clays which are non-sensitive will
typically move as there or thereabouts firm material for which the velocities
are fairly low. Causes of this volume will tend to give up relatively rapidly
when the side angle is eut down under some critical point.
Edgers and Karlsrud [1982] proposed three potential mechanisms for
these huge run-out. The mechanisms include (1) turbidity currents, (2) viscous flow models, and (3) progressive liquéfaction.
2.13.2 Incipient sédiment motion
Laboratory experiments conducted extensively indicates two criteria for
movement initiation of level bed sédiment with D^q grain size distribution
between 0.1 mm and 0.2 mm [Hallermeier, 1980]. The appropriate threshold
flow velocity for sand motion as applied in the field is
^max(-d)
■z
\
T 0.5
8 ( — - 1 ) gD50
. \7
/
J
(2.22)
where
wmax(-d) is peak fluid velocity at the sédiment bed.
«max(-d) can be determined using Airy’s wave theory given as
tvH coshfc(d + z) .
.
u - —--------. , , :— sin(«x - cri)
T
smh kd
(2.23)