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Deposition is the geological process in which
sediments Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently sediment transport, transported by the action of wind, water, or ice or by the force of gravity acting on the particles. ...
,
soil Soil is a mixture In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, comp ...

soil
and
rocks In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), rocks of which it is composed, and the proc ...

rocks
are added to a
landform A landform is a natural or artificial feature of the solid surface of the Earth or other planetary body A planet is an astronomical body Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws ...

landform
or
landmass A landmass, or land mass, is a large region In geography Geography (from Ancient Greek, Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, and phe ...
. Wind, ice, water, and
gravity Gravity (), or gravitation, is a by which all things with or —including s, s, , and even —are attracted to (or ''gravitate'' toward) one another. , gravity gives to s, and the causes the s of the oceans. The gravitational attracti ...

gravity
transport Transport (in British English British English (BrE) is the standard dialect A standard language (also standard variety, standard dialect, and standard) is a language variety that has undergone substantial codification of grammar and ...
previously
weathered ''Weathered'' is the third studio album by American Rock music, rock band Creed (band), Creed, released on November 20, 2001. It was the last Creed album to be released until ''Full Circle (Creed album), Full Circle'' came out in October 2009, wit ...
surface material, which, at the loss of enough
kinetic energy In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular ...
in the fluid, is deposited, building up layers of sediment. Deposition occurs when the forces responsible for sediment transportation are no longer sufficient to overcome the forces of gravity and
friction Friction is the force In physics, a force is an influence that can change the motion (physics), motion of an Physical object, object. A force can cause an object with mass to change its velocity (e.g. moving from a Newton's first law, st ...

friction
, creating a resistance to motion; this is known as the null-point hypothesis. Deposition can also refer to the buildup of sediment from organically derived matter or
chemical process In a scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in the real world. The usual test for ...
es. For example,
chalk Chalk is a soft, white, porous Porosity or void fraction is a measure of the (i.e. "empty") spaces in a , and is a of voids over the total volume, between 0 and 1, or as a between 0% and 100%. Strictly speaking, some tests measure the "acce ...

chalk
is made up partly of the microscopic
calcium carbonate Calcium carbonate is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held togethe ...

calcium carbonate
skeletons of marine
plankton Plankton are the diverse collection of organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology ...

plankton
, the deposition of which has induced chemical processes (
diagenesis upright=1.35, Permineralization in vertebra from ''Valgipes bucklandi'' Diagenesis () is the process that describes physical and chemical changes in sediments first caused by water-rock interactions, microbial activity, and compaction after the ...
) to deposit further calcium carbonate. Similarly, the formation of
coal Coal is a combustible , Germany ) , image_map = , map_caption = , map_width = 250px , capital = Berlin , coordinates = , largest_city = capital , languages_type = Official language , languages = German language, German , ...

coal
begins with the deposition of organic material, mainly from plants, in
anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: *Adhesive#Anaerobic, Anaerobic ad ...
conditions.


Null-point hypothesis

The null-point hypothesis explains how
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently sediment transport, transported by the action of wind, water, or ice or by the force of gravity acting on the particles. ...

sediment
is deposited throughout a shore profile according to its grain size. This is due to the influence of hydraulic energy, resulting in a seaward-fining of sediment particle size, or where fluid forcing equals gravity for each grain size. The concept can also be explained as "sediment of a particular size may move across the profile to a position where it is in equilibrium with the wave and flows acting on that sediment grain". This sorting mechanism combines the influence of the down-slope gravitational force of the profile and forces due to flow asymmetry; the position where there is zero net transport is known as the null point and was first proposed by Cornaglia in 1889. Figure 1 illustrates this relationship between sediment grain size and the depth of the marine environment. The first principle underlying the null point theory is due to the gravitational force; finer sediments remain in the water column for longer durations allowing transportation outside the surf zone to deposit under calmer conditions. The gravitational effect or settling velocity determines the location of deposition for finer sediments, whereas a grain's internal angle of friction determines the deposition of larger grains on a shore profile. The secondary principle to the creation of seaward sediment fining is known as the hypothesis of asymmetrical thresholds under waves; this describes the interaction between the oscillatory flow of waves and tides flowing over the wave ripple bedforms in an asymmetric pattern. "The relatively strong onshore stroke of the waveforms an eddy or vortex on the lee side of the ripple, provided the onshore flow persists, this eddy remains trapped in the lee of the ripple. When the flow reverses, the eddy is thrown upwards off the bottom and a small cloud of suspended sediment generated by the eddy is ejected into the water column above the ripple, the sediment cloud is then moved seaward by the offshore stroke of the wave." Where there is symmetry in ripple shape the vortex is neutralised, the eddy and its associated sediment cloud develops on both sides of the ripple. This creates a cloudy water column which travels under the tidal influence as the wave orbital motion is in equilibrium. The Null-point hypothesis has been quantitatively proven in
Akaroa Akaroa is a small town on Banks Peninsula in the Canterbury, New Zealand, Canterbury region of the South Island of New Zealand, situated within a Akaroa Harbour, harbour of the same name. The name Akaroa is Ngāi Tahu, Kāi Tahu Māori langua ...

Akaroa
Harbour, New Zealand,
The Wash The Wash is a rectangular bay A bay is a recessed, coastal body of water that directly connects to a larger main body of water, such as an ocean The ocean (also the sea or the world ocean) is the body of salt water which cove ...

The Wash
, U.K.,
Bohai Bay Bohai Bay () is one of the three major bays of the Bohai Sea The Bohai Sea or Bo Sea, also known as Bohai Gulf, Bo Gulf or Pohai Bay (), is a marginal sea approximately in area on the east coast of mainland China Mainland China, also ...
and West Huang Sera, Mainland China, and in numerous other studies; Ippen and Eagleson (1955), Eagleson and Dean (1959, 1961) and Miller and Zeigler (1958, 1964).


Deposition of non-cohesive sediments

Large-grain sediments transported by either bedload or suspended load will come to rest when there is insufficient bed shear stress and fluid turbulence to keep the sediment moving; with the suspended load this can be some distance as the particles need to fall through the water column. This is determined by the grain's downward acting weight force being matched by a combined buoyancy and fluid drag force and can be expressed by: : \frac \pi R^3 \rho_s g = \frac \pi R^3 \rho g + \frac \Complex_d \rho \pi R^2 w_s^2\, Downward acting weight force = Upward-acting buoyancy force + Upward-acting fluid drag force where: *''π'' is the ratio of a circle's circumference to its diameter. *''R'' is the radius of the spherical object (in m), *''ρ'' is the mass density of the fluid (kg/m3), *''g'' is the
gravitational acceleration In physics Physics is the that studies , its , its and behavior through , and the related entities of and . "Physical science is that department of knowledge which relates to the order of nature, or, in other words, to the regular s ...
(m/s2), *''Cd'' is the drag coefficient, and *''ws'' is the particle's settling velocity (in m/s). In order to calculate the drag coefficient, the grain's
Reynolds number The Reynolds number () helps predict flow patterns in different fluid flow situations. At low Reynolds numbers, flows tend to be dominated by laminar (sheet-like) flow, while at high Reynolds numbers flows tend to be turbulent In fluid dynam ...
needs to be discovered, which is based on the type of fluid through which the sediment particle is flowing, laminar flow, turbulent flow or a hybrid of both. When the fluid becomes more viscous due to smaller grain sizes or larger settling velocities, the prediction is less straightforward and it is applicable to incorporate
Stokes Law In 1851, George Gabriel Stokes Sir George Gabriel Stokes, 1st Baronet, (; 13 August 1819 – 1 February 1903) was an Anglo-Irish physicist A physicist is a scientist A scientist is a person who conducts Scientific method, scientific ...
(also known as the frictional force, or drag force) of settling.


Deposition of cohesive sediments

The cohesion of sediment occurs with the small grain sizes associated with silts and clays, or particles smaller than 4ϕ on the
phi Phi (; uppercase Φ, lowercase φ or ϕ; grc, ϕεῖ ''pheî'' ; Modern Greek: ''fi'' ) is the 21st letter of the Greek alphabet. In Archaic Greek, Archaic and Classical Greek (c. 9th century BC to 4th century BC), it represented an Aspir ...

phi
scale. If these fine particles remain dispersed in the water column,
Stokes law In 1851, George Gabriel Stokes Sir George Gabriel Stokes, 1st Baronet, (; 13 August 1819 – 1 February 1903) was an Anglo-Irish physicist A physicist is a scientist A scientist is a person who conducts Scientific method, scientific ...
applies to the settling velocity of the individual grains, although due to seawater being a strong
electrolyte An electrolyte is a medium containing ions that is electrically conducting Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that quantifies how strongly it resis ...

electrolyte
bonding agent,
flocculation Flocculation, in the field of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior and the ...

flocculation
occurs where individual particles create an electrical bond adhering each other together to form flocs. "The face of a clay platelet has a slight negative charge where the edge has a slight positive charge when two platelets come into close proximity with each other the face of one particle and the edge of the other are electrostatically attracted." Flocs then have a higher combined mass which leads to quicker deposition through a higher fall velocity, and deposition in a more shoreward direction than they would have as the individual fine grains of clay or silt.


The occurrence of null point theory

Akaroa Harbour Akaroa Harbour, is part of Banks Peninsula in the Canterbury region of New Zealand. The harbour enters from the southern coast of the peninsula, heading in a predominantly northerly direction. It is one of two major inlets in Banks Peninsula, ...
is located on
Banks Peninsula in the early 19th century Banks Peninsula is a peninsula of volcanic origin on the east coast of the South Island of New Zealand. It has an area of approximately and encompasses two large harbours and many smaller bays and coves. The South Is ...
,
Canterbury, New Zealand Canterbury ( mi, Waitaha) is a Regions of New Zealand, region of New Zealand, located in the central-eastern South Island. The region covers an area of , and is home to a population of The region in its current form was established in 1989 d ...
, . The formation of this harbour has occurred due to active erosional processes on an extinct shield volcano, whereby the sea has flooded the caldera, creating an inlet 16 km in length, with an average width of 2 km and a depth of −13 m relative to mean sea level at the 9 km point down the transect of the central axis. The predominant storm wave energy has unlimited fetch for the outer harbour from a southerly direction, with a calmer environment within the inner harbour, though localised harbour breezes create surface currents and chop influencing the marine sedimentation processes. Deposits of loess from subsequent glacial periods have in filled volcanic fissures over millennia, resulting in volcanic basalt and loess as the main sediment types available for deposition in Akaroa Harbour Hart et al. (2009) discovered through bathymetric survey,
sieve A sieve, fine mesh strainer, or sift, is a device for separating wanted elements from unwanted material or for characterizing the particle size distribution of a sample, using a screen such as a woven mesh or net or perforated sheet material. ...

sieve
and
pipette A pipette (sometimes spelled pipet) is a laboratory tool commonly used in chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composit ...

pipette
analysis of subtidal sediments, that sediment textures were related to three main factors: depth, distance from shoreline, and distance along the central axis of the harbour. This resulted in the fining of sediment textures with increasing depth and towards the central axis of the harbour, or if classified into grain class sizes, “the plotted transect for the central axis goes from silty sands in the intertidal zone to sandy silts in the inner nearshore, to silts in the outer reaches of the bays to mud at depths of 6 m or more”. See figure 2 for detail. Other studies have shown this process of the winnowing of sediment grain size from the effect of hydrodynamic forcing; Wang, Collins and Zhu (1988) qualitatively correlated increasing intensity of fluid forcing with increasing grain size. "This correlation was demonstrated at the low energy clayey tidal flats of
Bohai Bay Bohai Bay () is one of the three major bays of the Bohai Sea The Bohai Sea or Bo Sea, also known as Bohai Gulf, Bo Gulf or Pohai Bay (), is a marginal sea approximately in area on the east coast of mainland China Mainland China, also ...
(China), the moderate environment of the
Jiangsu Jiangsu (; ; Postal romanization, formerly romanized Kiangsu) is an eastern-central coastal Provinces of the People's Republic of China, province of the China, People's Republic of China. It is one of the leading provinces in finance, education, ...

Jiangsu
coast (China) where the bottom material is silty, and the sandy flats of the high energy coast of
The Wash The Wash is a rectangular bay A bay is a recessed, coastal body of water that directly connects to a larger main body of water, such as an ocean The ocean (also the sea or the world ocean) is the body of salt water which cove ...

The Wash
(U.K.)." This research shows conclusive evidence for the null point theory existing on tidal flats with differing hydrodynamic energy levels and also on flats that are both erosional and accretional. Kirby R. (2002) takes this concept further explaining that the fines are suspended and reworked aerially offshore leaving behind lag deposits of the main bivalve and gastropod shells separated out from the finer substrate beneath, waves and currents then heap these deposits to form chenier ridges throughout the tidal zone, which tend to be forced up the foreshore profile but also along the foreshore. Cheniers can be found at any level on the foreshore and predominantly characterise an erosion-dominated regime.


Applications for coastal planning and management

The null point theory has been controversial in its acceptance into mainstream coastal science as the theory operates in
dynamic equilibrium In chemistry, and in physics, a dynamic equilibrium exists once a reversible reaction occurs. Substances transition between the reactants and products at equal rates, meaning there is no net change. Reactants and products are formed at such a rate ...

dynamic equilibrium
or unstable equilibrium, and many fields and laboratory observations have failed to replicate the state of a null point at each grain size throughout the profile. The interaction of variables and processes over time within the environmental context causes issues; "a large number of variables, the complexity of the processes, and the difficulty in observation, all place serious obstacles in the way of systematisation, therefore in certain narrow fields the basic physical theory may be sound and reliable but the gaps are large" Geomorphologists, engineers, governments and planners should be aware of the processes and outcomes involved with the null point hypothesis when performing tasks such as
beach nourishment Beach nourishment (also referred to as beach renourishment, beach replenishment, or sand replenishment) describes a process by which sediment Sediment is a naturally occurring material that is broken down by processes of weathering Weat ...
, issuing building consents or building coastal defence structures. This is because sediment grain size analysis throughout a profile allows inference into the erosion or accretion rates possible if shore dynamics are modified. Planners and managers should also be aware that the coastal environment is dynamic and contextual science should be evaluated before the implementation of any shore profile modification. Thus theoretical studies, laboratory experiments, numerical and hydraulic modelling seek to answer questions pertaining to
littoral The littoral zone or nearshore is the part of a sea The sea, connected as the world ocean or simply the ocean The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% of the surface ...

littoral
drift and sediment deposition, the results should not be viewed in isolation and a substantial body of purely qualitative observational data should supplement any planning or management decision.


See also

* * * * * * * * * * * *


References

{{Rivers, streams and springs Sedimentology