Sediment deposition

Lecture 8Jyoti Khatiwada

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.

• 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.

• 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:

Settling velocity

of mud and coarse Grain

• 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.

• 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

bb

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

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

• 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.

FlocculationFlocculation, 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.

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.

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.

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]

• 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

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.

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.

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.

Debris flow fan

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

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.

• 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

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.

• 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.

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.

• 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.

The end.

• Sincerely• Jyoti Khatiwada• Roll no 10/2072• 9849828976