Sediment deposition

1. Sediment deposition
Lecture 8
Jyoti Khatiwada

2. Settling velocity
• Deposition
The velocity at which a sediment particle drops to
the channel bed is called the settling velocity .
This depends upon the size and shape and
density of the sediment particle. Deposition may
be temporary on the channel bed and the
sediment may be moved again at a time of higher
flow. In other situations there is a net deposition
of sediment, and a deposition landform results,
e.g. floodplains and point bars on the inside of
meander bends.

3. • Settling velocity
• Streamlines around a sphere falling through a fluid. This illustration is
accurate for laminar flow, in which the particle Reynolds number is small.
This is typical for small particles falling through a viscous fluid; larger
particles would result in the creation of aturbulent wake.
• The settling velocity (also called the "fall velocity" or "terminal velocity") is a
function of the particle Reynolds number. Generally, for small particles
(laminar approximation), it can be calculated with Stokes' Law. For larger
particles (turbulent particle Reynolds numbers), fall velocity is calculated
with the turbulent drag law. Dietrich (1982) compiled a large amount of
published data to which he empirically fit settling velocity curves.Ferguson
and Church (2006) analytically combined the expressions for Stokes flow
and a turbulent drag law into a single equation that works for all sizes of
sediment, and successfully tested it against the data of Dietrich.Their
equation is
• In this equation ws is the sediment settling velocity, g is acceleration due to
gravity, and D is mean sediment diameter. V is the kinematic
viscosity of water, which is approximately 1.0 x 10−6 m2/s for water at 20 °C.

4. • The expression for fall velocity can be simplified
so that it can be solved only in terms of D. We
use the sieve diameters for
natural grains, g=9.8}, and
values given above for V and
R. From these parameters,
the fall velocity is given by
the expression:

5. Settling
velocity
of mud and
coarse Grain

9. • Terminal velocity is the highest velocity attainable by an object as it falls
through a fluid (air is the most common example, but the concept applies
equally to any fluid). It occurs when the sum of the drag force (Fd) and
thebuoyancy is equal to the downward force of gravity (FG) acting on the
object. Since the net force on the object is zero, the object has
zero acceleration.[1]
• In fluid dynamics, an object is moving at its terminal velocity if its speed is
constant due to the restraining force exerted by the fluid through which it
is moving.
• As the speed of an object increases, so does the drag force acting on it,
which also depends on the substance it is passing through (for example air
or water). At some speed, the drag or force of resistance will equal the
gravitational pull on the object (buoyancy is considered below). At this
point the object ceases to accelerate and continues falling at a constant
speed called the terminal velocity (also called settling velocity). An object
moving downward faster than the terminal velocity (for example because
it was thrown downwards, it fell from a thinner part of the atmosphere, or
it changed shape) will slow down until it reaches the terminal velocity.
Drag depends on the projected area, here the object's cross-section or
silhouette in a horizontal plane: an object with a large projected area
relative to its mass, such as a parachute, has a lower terminal velocity than
one with a small projected area relative to its mass, such as a bullet.

10. • The particle Reynolds number
is important in determining the
fall velocity of a particle. When
the particle Reynolds number
indicates laminar flow, Stokes'
law can be used to calculate its fall velocity. When the
particle Reynolds number indicates turbulent flow, a
turbulent drag law must be constructed to model the
appropriate settling velocity.
• Reynolds number is a dimensionless number used in fluid
mechanics to indicate whether fluid flow past a body or in
a duct is steady or turbulent. It is the ratio of inertial
forces to viscous forces
• Drag Coefficient = 24 / Reynold's Number

11. bb

12. Free settling
• The total amount of force exerted on a particle can be broken
down into four categories.
• Force due to Acceleration = Gravity Force - Buoyancy Force -
Drag Force
• For a particle, there are two stages when it falls. The
acceleration portion and then the portion of constant velocity,
also known as the terminal velocity or free settling velocity

13. Hindered Settling
• Hindered settling is called hindered settling for a
reason -- the added number of particles in an
enclosed area creates a slower-moving mixture
than would normally be expected.
• In this case, everything revolves around epsilon
(e), which is the volume fraction of the slurry
mixture occupied by the liquid

15. • The qualitative analysis of sediment transports in river
engineering problems, such as sedimentation in river
courses and morphological changes of river banks,
designing the settling basins of water conveyance
networks, and sedimentation of dam reservoirs, needs
to use a suitable relation to estimate the terminal fall
velocity, sometimes called settling velocity, of sediment
particles.
• The terminal fall velocity of a particle is the particle
downward velocity in a low dense fluid at equilibrium
in which the sum of the gravity force, buoyancy force
and fluid drag force being equal to zero. Fall velocity of
a particle, depends on the density and viscosity of the
fluid, and the density, size, shape, spherically, and the
surface texture of the particle.

16. Flocculation
Flocculation, in the field of chemistry, is a process
wherein colloids come out of suspension in the form of
floc or flake; either spontaneously or due to the
addition of a clarifying agent. The action differs from
precipitation in that, prior to flocculation, colloids are
merely suspended in a liquid and not actually dissolved
in a solution. In the flocculated system, there is no
formation of a cake, since all the flocs are in the
suspension.
Coagulation and flocculation are important processes
in water treatment with coagulation to destabilize
particles through chemical reaction between coagulant
and colloids, and flocculation to transport the
destabilized particles that will cause collisions with floc.

18. Subaqueous Gravity displacement sedimentation
• Bouma sequence
• The Bouma sequence specifically describes the ideal vertical
succession of structures deposited by low-density (i.e., low
sand concentration, fine-grained) turbidity currents.
The layers are as follows.
E: Massive, ungraded mudstone, sometimes with evidence of trace
fossils (i.e., bioturbation). The Bouma E layer is often missing, or
difficult to differentiate from the Bouma D layer below.
D: Parallel-laminated siltstone.
C: Ripple-laminated fine-grained sandstone. Often the ripple
laminations are deformed into convolute laminations and flame
structures.
B: Planar-laminated fine- to medium-grained sandstone. The base
of Bouma B often has features known assole markings, such as
flute casts, groove casts and parting lineation.
A: Massive to normally graded, fine- to coarse-grained sandstone,
often with pebbles and/or rip-up clasts of shale near the base. Dish
structures may be present. The base of the sandstone, below A, is
sometimes eroded into underlying strata.

19. subaqueous environments are characterized by a
spectrum of flow types with debris flows and mud
flows on one end of the spectrum, and high-
density and low-density turbidity currents on the
other end. It is also useful in subaqueous
environments to recognize transitional flows that
are in between turbidity currents and mud flows.

20. Deposits of transistional flow
• Grain flow deposits are characterized by a coarsening-upward distribution
of grain sizes (inverse grading) within the bed. This results from smaller
grains within the flow falling down in between larger grains during grain-
to-grain collisions, and thereby depositing preferentially at the base of
flow.[1] Although present as grain avalanches in terrestrial sand dunes,
grain flows are rare in other settings. However, inverse graded beds
resulting from grain flow processes do make up so-called "traction
carpets" in the lower intervals of some high-density turbidites.
• Liquefied flow deposits are characterized by de-watering features, such
as dish structures, that result from upward escaping fluid within the
flow. As with pure grain flows, pure liquefied flows seldom occur on their
own. However, liquefied flow processes are very important as grains
within turbidity currents begin to settle out and displace fluid upwards.
This dish structures and related features, such de-watering pipes, are
often found in turbidites.
• Debris flow deposits are characterized by a bimodal distribution of grain
sizes, in which larger grains and/or clasts float within a matrix of fine-
grained clay. Because the muddy matrix has cohesive strength, unusually
large clasts may be able to float on top of the muddy material making up
the flow matrix, and thereby end up preserved on the upper bed
boundary of the resulting deposit.[1]

21. • Low-density turbidity current deposits (turbidites) are characterized by a
succession of sedimentary structuresreferred to as the Bouma sequence,
which result from decreasing energy within the flow (i.e., waning flow), as
the turbidity current moves downslope.[4]
• High-density turbidity current deposits are characterized by much
coarser grain size than in low-density turbidites, with the basal portions of
the deposits often characterized by features that result from the close
proximity of the grains to each other. Thus, indications of grain-to-grain
interactions (i.e., grain flow processes), and interaction of grains with the
substratum (i.e., traction) are generally present in the lower portions of
these deposits. Complete Bouma sequences are rare, and generally only
the Bouma A and B layers are evident.[4]
• Hybrid event beds (HEB) transitional between mud flows and turbidity
currents are characterized by features indicative of both cohesionless
(turbulence-supported) and cohesive (mud-supported) flow with no
separating bed boundary between the two. In most cases, they are
represented by grain-supported textures that grade upward within the
bed into mud-supported textures. It is not uncommon for debris flows
and mud flows to evolve downslope into turbidity currents, and vice
versa. Also, flows internally may transition upward from one flow process
to another

22. Sedimentation from turbidity currents
• A turbidity current is most typically an underwater current of
usually rapidly moving, sediment-laden water moving down a
slope. Turbidity currents can also occur in other fluids besides
water. In the most typical case of oceanic turbidity currents,
sediment laden waters situated over sloping ground flow down-
hill because they have a higher density than the adjacent waters.
The driving force behind a turbidity current is gravity acting on
the high density of the sediments temporarily suspended within
a fluid. These semi-suspended solids make the average density of
the sediment bearing water greater than that of the surrounding,
undisturbed water. As such currents flow, they often have a
"snow-balling-effect", as they stir up the ground over which they
flow, and gather even more sedimentary particles in their
current. Their passage leave the ground over which they flow
scoured and eroded. Once an oceanic turbidity current reaches
the calmer waters of the flatter area of the abyssal plain (main
oceanic floor), the particles borne by the current settle out of the
water column. The sedimentary deposit of a turbidity current is
called a turbidite.

23. Alluvial fan
• An alluvial fan is a fan- or cone-shaped deposit of
sediment crossed and built up by streams. If
a fan is built up by debris flows it is properly called
a debris cone or colluvial fan.

24. Debris flow fan
• Debris flows are geological phenomena in which water-
laden masses of soil and fragmented rock rush down
mountainsides, funnel into stream channels, entrain
objects in their paths, and form thick, muddy deposits on
valley floors. They generally have bulk densities
comparable to those of rock avalanches and other types
of landslides (roughly 2000 kilograms per cubic meter),
but owing to widespread sediment liquefaction caused
by high pore-fluid pressures, they can flow almost as
fluidly as water.
• Debris flows descending steep channels commonly attain
speeds that surpass 10 meters per second (more than 20
miles per hour), although some large flows can reach
speeds that are much greater. Debris flows with volumes
ranging up to about 100,000 cubic meters occur
frequently in mountainous regions worldwide.

25. Debris flow fan

26. difference between debris flow fan
and stream flow fan
• An alluvial fan is a fan- or cone-shaped deposit of
sediment crossed and built up by streams. If a fan is
built up by debris flows it is properly called a debris
cone or colluvial fan. These flows come from a single
point source at the apex of the fan, and over time
move to occupy many positions on the fan surface.
Fans are typically found where a canyon draining from
mountainous terrain emerges out onto a flatter plain,
and especially along fault-bounded mountain fronts.
• A convergence of neighboring alluvial fans into a single
apron of deposits against a slope is called a bajada, or
compound alluvial fan

30. Braided river
• A braided river is one of a number of channel
types and has a channel that consists of a
network of small channels separated by small
and often temporary islands called braid bars
or, in British usage, aits or
eyots. Braided streams occur in rivers with
high slope and/or large sediment load.

36. • A meander, in general, is a bend in a sinuous watercourse or
river. A meander forms when moving water in a stream
erodes the outer banks and widens its valley, and the inner
part of the river has less energy and deposits silt. A streamof
any volume may assume a meandering course,
alternately eroding sediments from the outside of a bend and
depositing them on the inside. The result is a snaking pattern
as the stream meanders back and forth across its down-valley
axis. When a meander gets cut off from the main stream,
an oxbow lake forms.
• A floodplain or flood plain is an area of land adjacent to
a stream or river that stretches from the banks of its channel
to the base of the enclosing valley walls and
experiences flooding during periods of high discharge.
• An alluvial plain is a largely flat landform created by the
deposition of sediment over a long period of time by one or
more rivers coming from highland regions, from which alluvial
soil forms

38. Channel and bar deposit
• A bar in a river is an elevated region of sediment (such as sand
or gravel) that has been deposited by the flow. Types of bars
include mid-channel bars (also called braid bars, and common in
braided rivers), point bars (common in meandering rivers), and
mouth bars (common in river deltas). Bars are typically found in
the slowest moving, shallowest parts of rivers and streams,[1]
and are often parallel to the shore and occupy the area farthest
from the thalweg.
• The locations of bars are determined by the geometry of the
river and the flow through it. Point bars form on the inside of
meander bends in meandering rivers because the shallow flow
and low shear stresses there reduce the amount of material that
can be carried there. The excess material falls out of transport
and forms the bar.
• A stream channel is the path for water and sediment flowing
within the streambanks . A stream channel constantly adjusts to
changes in streamflow, sediment load, stream slope and
vegetation.

43. • ALL GLACIAL DEPOSITS are DRIFT. Glaciers are
powerful enough to carry tiny and huge rock
debris, and when they drop it, the ice drops it
indiscriminantly. Thus, material deposited by
ice is unsorted or mixed in size. This non-
sorted material is called TILL.

45. Periglacial deposits
• Periglaciation (adjective: "periglacial," also referring to
places at the edges of glacial areas) describes
geomorphic processes that result from seasonal
thawing of snow in areas of permafrost, the runoff
from which refreezes in ice wedges and other
structures. "Periglacial" suggests an environment
located on the margin of past glaciers. However, freeze
and thaw cycles influence landscapes outside areas of
past glaciation. Therefore, periglacial environments
are anywhere that freezing and thawing modify the
landscape in a significant manner.
• Tundra is a common ecological community in
periglacial areas.

46. • Periglaciation results in a variety of ground conditions but especially
those involving irregular, mixed deposits created by ice wedges,
solifluction, gelifluction, frost creep and rockfalls. Periglacial
environments trend towards stable geomorphologies.[4]
Coombe and head deposits – Coombe deposits are chalk deposits found
below chalk escarpments in Southern England. Head deposits are more
common below outcrops of granite on Dartmoor.
Patterned Ground – Patterned ground occurs where stones form circles,
polygons and stripes. Local topography affects which of these are
expressed. A process called frost heaving is responsible for these
features.
Periglacial lakes – Periglacial lakes are formed where the natural drainage
of the topography is obstructed by an ice sheet, ice cap or glacier.
Periglacial lakes are not typical of areas under the modern periglacial
definition, since most of them formed temporarily during the last
deglaciation and are not necessarily associated to landforms created by
the freezing of water (glaciers not accounted).
Solifluction lobes – Solifluction lobes are formed when waterlogged soil
slips down a slope due to gravity forming U shaped lobes.
Blockfields or Felsenmeer – Blockfields are areas covered by large
angular blocks, traditionally believed to have been created by freeze-
thaw action.

50. The end.
• Sincerely
• Jyoti Khatiwada
• Roll no 10/2072
• 9849828976