In consequence there will not be a positive density
contrast, which is the prerequisite for the formation
of turbidity currents; instead the flow may become an
overflow plume. In lakes, on the other hand, river
water is often heavier, due both to suspended material
and to its being colder than the lake surface water. It
will then be able to follow the bed downslope, and
become a turbidite.
Submarine slides may also quickly evolve into turbidity currents. River sediment entering a marine basin
will mix with the seawater so that clays flocculate and
are deposited on the delta slopes. If the slope angle
becomes too steep, we get slides, which may result in
turbidity currents. When fine-grained sediments are
deposited on slopes from suspension they have a
very high water content. Compaction, sometimes
caused by earthquakes, will cause upward flow of
porewater and may result in liquefaction. This causes
the sediments to begin to flow even on gentle slopes
because friction is reduced, and they may then turn
into turbidity currents.
The forces driving a turbidity current are:
F 1 ¼ g Á Δρ Á V sin α
where g is the gravity constant, Δρ is the difference
between the density of the current and that of the
surrounding water, V is the volume of water along a
certain length of the channel, with the cross-section
(A) of a turbidity current with length L, and α the angle
of the slope. Acting against the movement are frictional forces (F 2 ) which, as long as the current is not
accelerating, must be equal to the gravitational forces.
These are shear forces against the bed, τ 1 , and against
the overlying water, τ 2 , plus internal friction and turbulence within the current which keep the sediments in
suspension. In order for the sediment grains to remain
in suspension, the turbulence must be sufficiently
strong to have an upward component which
corresponds at least to the settling velocity of the
coarsest grains. Turbulence is greatest near the bottom
of the current, where change in velocity as a function
of height above the bottom (velocity gradient) is
greatest. The largest grains in suspension will thus be
concentrated near the bottom of the current. Near the
bed, in addition to turbulence, we also have shear
stresses which will transport the grains in virtually
“pseudo-laminar” flow in a thin layer over the bottom.
If the concentration of large sand grains along the bed
becomes large, we also get dispersive energy because
of collisions between the grains (see Sect. 2.13).
We therefore find that both the concentration of
sediment in suspension, and maximum grain size in
suspension, decrease upwards from the bottom. If we
disregard internal friction, we obtain
g Á Δρ Á V sin α ¼ τ 1 þ τ 2
ð
ÞÁ A
where A is the area of contact with the bottom and the
overlying water. The ratio between the volume (V) and
the contact area A is approximately the thickness of the
flow H. The shear stresses are proportional to the
square of the velocity τ ¼ cv
2
ð
Þ .
The velocity (v) of a turbidity current is then:
v ¼ c Á g Á Δρ Á H: sin α
ð
Þ
1=2
Here the coefficient c includes the coefficient of
resistance for friction against the seafloor and against
the overlying water. This corresponds to Chezy’s
number for fluvial flow, so in many ways we can
regard a turbidity current as an underwater river.
We see from the above equation that thick turbidity
flows will have a higher velocity than thin ones and
that thick flows can flow on gentler slopes than thinner
ones. This is because the shear stress against the bottom and the overlying water is nearly independent of
the thickness of the flow. The flow velocity also
increases with increasing density of the sediment/
water mixture in the flow, but high density flows will
have higher internal friction and require higher
velocities to keep the material in suspension.
A turbidity current can be divided into head, neck,
body and tail. The sediment particles in the head area
move somewhat faster than the front of the current
itself. This leads to sediment being swept upwards and
then backwards towards the neck, where it mixes with
water from the overlying water mass. From there it is
carried backwards to the body and tail, where we find a
finer-grained, thinner suspension. When the turbidity
current loses velocity, the largest particles in the head
will settle out of suspension first because of reduced
turbulence. Gradually smaller and smaller grains will
settle and we get deposition of a bed which is fairly
massive, without internal structure, but which
becomes finer upwards. In most cases, apart from in
proximal turbidites, we also find deposition of some
2 Introduction to Sedimentology
45
contrast, which is the prerequisite for the formation
of turbidity currents; instead the flow may become an
overflow plume. In lakes, on the other hand, river
water is often heavier, due both to suspended material
and to its being colder than the lake surface water. It
will then be able to follow the bed downslope, and
become a turbidite.
Submarine slides may also quickly evolve into turbidity currents. River sediment entering a marine basin
will mix with the seawater so that clays flocculate and
are deposited on the delta slopes. If the slope angle
becomes too steep, we get slides, which may result in
turbidity currents. When fine-grained sediments are
deposited on slopes from suspension they have a
very high water content. Compaction, sometimes
caused by earthquakes, will cause upward flow of
porewater and may result in liquefaction. This causes
the sediments to begin to flow even on gentle slopes
because friction is reduced, and they may then turn
into turbidity currents.
The forces driving a turbidity current are:
F 1 ¼ g Á Δρ Á V sin α
where g is the gravity constant, Δρ is the difference
between the density of the current and that of the
surrounding water, V is the volume of water along a
certain length of the channel, with the cross-section
(A) of a turbidity current with length L, and α the angle
of the slope. Acting against the movement are frictional forces (F 2 ) which, as long as the current is not
accelerating, must be equal to the gravitational forces.
These are shear forces against the bed, τ 1 , and against
the overlying water, τ 2 , plus internal friction and turbulence within the current which keep the sediments in
suspension. In order for the sediment grains to remain
in suspension, the turbulence must be sufficiently
strong to have an upward component which
corresponds at least to the settling velocity of the
coarsest grains. Turbulence is greatest near the bottom
of the current, where change in velocity as a function
of height above the bottom (velocity gradient) is
greatest. The largest grains in suspension will thus be
concentrated near the bottom of the current. Near the
bed, in addition to turbulence, we also have shear
stresses which will transport the grains in virtually
“pseudo-laminar” flow in a thin layer over the bottom.
If the concentration of large sand grains along the bed
becomes large, we also get dispersive energy because
of collisions between the grains (see Sect. 2.13).
We therefore find that both the concentration of
sediment in suspension, and maximum grain size in
suspension, decrease upwards from the bottom. If we
disregard internal friction, we obtain
g Á Δρ Á V sin α ¼ τ 1 þ τ 2
ð
ÞÁ A
where A is the area of contact with the bottom and the
overlying water. The ratio between the volume (V) and
the contact area A is approximately the thickness of the
flow H. The shear stresses are proportional to the
square of the velocity τ ¼ cv
2
ð
Þ .
The velocity (v) of a turbidity current is then:
v ¼ c Á g Á Δρ Á H: sin α
ð
Þ
1=2
Here the coefficient c includes the coefficient of
resistance for friction against the seafloor and against
the overlying water. This corresponds to Chezy’s
number for fluvial flow, so in many ways we can
regard a turbidity current as an underwater river.
We see from the above equation that thick turbidity
flows will have a higher velocity than thin ones and
that thick flows can flow on gentler slopes than thinner
ones. This is because the shear stress against the bottom and the overlying water is nearly independent of
the thickness of the flow. The flow velocity also
increases with increasing density of the sediment/
water mixture in the flow, but high density flows will
have higher internal friction and require higher
velocities to keep the material in suspension.
A turbidity current can be divided into head, neck,
body and tail. The sediment particles in the head area
move somewhat faster than the front of the current
itself. This leads to sediment being swept upwards and
then backwards towards the neck, where it mixes with
water from the overlying water mass. From there it is
carried backwards to the body and tail, where we find a
finer-grained, thinner suspension. When the turbidity
current loses velocity, the largest particles in the head
will settle out of suspension first because of reduced
turbulence. Gradually smaller and smaller grains will
settle and we get deposition of a bed which is fairly
massive, without internal structure, but which
becomes finer upwards. In most cases, apart from in
proximal turbidites, we also find deposition of some
2 Introduction to Sedimentology
45