Littoral and marine sediments

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Definition of Marine sediment:

Natural unconsolidated granular material with sediment density greater than water.

This is the common definition for Marine sediment, other definitions can be discussed in the commodity

Contents

  • 1 Origin of coastal and marine sediments
    • one.one Clastic sediments [one]
    • 1.2 Biogenic sediments [4]
    • 1.three Chemical sediments
  • 2 Mineral composition of clastic sediments
  • three Physical and chemical properties of sediment
    • 3.one Autumn velocity
    • 3.two Flocculation
    • 3.3 Sediment transport
    • 3.4 Sediment deposits in coastal waters
    • 3.five Graded sediment
    • 3.6 Bedforms
    • 3.seven Sediment sorting
    • iii.8 Seabed erosion
    • iii.nine Sediment contamination and bioavailability
  • 4 Spatial distribution of sediments
    • 4.1 Oceans
    • iv.ii Beach and foreshore
    • 4.3 Estuaries
  • 5 Related manufactures
  • half-dozen References

Origin of coastal and marine sediments

Continental shelves are for the most office formed of sediment deposits that may achieve thicknesses in excess of 1 km. These sediments have several origins:

Clastic sediments [1]

Clastic sediments (clast = fragment) are ultimate weathering products derived from rock. Most clastic sediments are terrigenous materials eroded from land. Erosion of rocky shores and submarine rock also produces clastic sediments. About 85% of all coastal marine sediments are clastic sediments[1].

The products of terrestrial rock weathering are transported mainly by rivers (present-time fluvial sediment supply is indicated in Fig. ane), but likewise past current of air, ice and waves. The transported particles and fragments abrade each other and erode the surfaces over which they laissez passer, thus contributing to the further breakdown of rock fragments. Fourth dimension and altitude are important factors: the longer the journeying to the body of water, the more take chances there is for mineral grains to be rounded and reduced in size past abrasion. During this journey, grains are sorted according to density, size and shape, and chemically less stable minerals are dissolved. The nearly mutual solid products of weathering are rock fragments, quartz and clay minerals. Quartz is the only common mineral of igneous and metamorphic rocks that is both hard (resistant to abrasion) and chemically stable at the Earth's surface. This is the reason why quartz is the predominant mineral in present solar day embankment and river sands and is as well common in virtually ancient sandstones. The rate at which weathering occurs depends on the local climate, with rapid breakdown in tropical areas, favoured by high rainfall and temperatures, and with slow weathering occurring in deserts, where water is absent. In loftier-breadth zones, alternate frost and thaw and glacier dynamics are of import weathering agents.

Figure one: Annual discharge of sediments from world's catchment basins. The width of the arrows indicates the relative sediment discharge. Numbers are boilerplate discharges in millions of tonnes. Redrawn after Milliman and Meade (1983)[two].


A minor fraction of terrigenous material is deposited from the atmosphere. Information technology consists of dust carried by the wind and ash emitted by volcanoes and contains mainly very fine sand (quartz), simply also silt and clay particles with pregnant amounts of aluminum (Al), atomic number 26 (Iron), magnesium (Mg) and calcium (Ca ). Supply of atmospheric dust may be a modest factor in most areas today, except from major deserts, e.thousand., the Sahara, but in the past it has been substantial and has led to the accumulation of extensive loess deposits, e.1000., in Communist china. The exceptionally high sediment load of the Xanthous river is due to erosion of these loess deposits, which are finally discharged into the Bohai Bounding main.

In chill and subarctic areas and where mountains are loftier enough and then equally to generate significant quantities of snow, glaciers produce substantial amounts of sediment with the meltwater flow through subglacial tunnels.

Clastic sediments tin be classified according to grainsize, see Table 1. Grainsizes are indicative of the advancement of the weathering process; small grains derive from weathering of larger grains. Sediment samples generally contain grains of different sizes. Sedimentary deposits are named afterwards the largest present grains (clasts). Common rock types (indurated sedimentary deposits) are [3]:

  • Breccia, Gritstone, Conglomerate: sedimentary rocks consisting of large clasts (boulders, cobbles, pebbles, gravel), cemented together past finer sediments. Breccia is composed of big angular elements and Gritstone of minor angular elements; Conglomerate is composed of rounded elements.
  • Diamict: (indurated) unsorted (glacial) sediment.
  • Sandstone: sedimentary rock equanimous of sand-sized particles (mainly quartz) with finer grained material (feldspar, micas, lime, clay) that infills space in betwixt, and cemented after deposition by minerals precipitated in the pores.
  • Arkose: sandstone with a high content of feldspar.
  • Greywacke: sandstone equanimous of poorly sorted athwart grains (quartz, feldspar, small rock fragments).
  • Flagstone: sandstone mainly composed of quartz and feldspar that is easily split in layers.
  • Siltstone: sedimentary rock composed mainly of quartz, shell fragments and assorted heavy minerals.
  • Mudstone: sedimentary stone composed mainly of clay minerals and sometimes lime and organic thing.
  • Marl: lime-rich mudstone.
Table 1. Sedimentary particles, grainsize and the corresponding sedimentary stone type. The grainsize range is expressed in d[mm] and in [math]\phi=-log_2 d[mm][/math]. Adapted from (Hugget, 2007)[ane].
particle type particle name grainsize d[mm] grainsize [math]\phi[/math] rock blazon
Gravel Boulders > 256 < (-8) Conglomerate, breccia,

gritstone, diamict

Cobbles 64-256 (-eight)-(-6)
Pebbles 4-64 (-half-dozen)-(-ii)
Granules two-4 (-2)-(-1)
Sand very coarse i-2 (-1)-0 Sandstone,arkose,

greywacke, flags

fibroid 0.v-1 0-ane
medium 0.25-0.five 1-2
fine 0.125-0.25 2-iii
very fine 0.0625-0.125 3-4
Silt 0.002-0.0625 4-ix Siltstone, mudstone, marl
Clay < 0.002 > 9

Biogenic sediments [4]

Most biogenic sediments are erosion products of unconsolidated or consolidated marine detrital deposits or erosion products of framework-edifice organisms (e.g. coral reefs). These sediments, called "bioclastic sediments", are composed of fragments of organic skeletal materials . They mainly consist of calcium carbonate (CaCO3) in the form of calcite crystals or aragonite crystals. Other biogenic sediments, such as dead plant remains (peat), have a loftier organic component and are termed organic-rich biological sediments. Biogenic sediments may constitute a substantial fraction of the bed material in coastal environments where the supply of terrigenous textile is pocket-sized (no nearby river deltas). Many beaches in the tropics and subtropics consist almost entirely of carbonate sands which are derived from adjacent reefs composed of corals, skeletal material, shell fragments or precipitated calcium carbonate. Fine bioclastic sediments may also accrue in sheltered coastal environments in temperate climate zones. An instance is the inner part of the mega-tidal Baie du Mont Saint Michel (almost no fluvial sediment input), where tidal flats are composed of so-called tangue – low-cohesive fine sediment with a high biogenic content [five].

Chemic sediments

Chemical sediments grade by atmospheric precipitation of minerals out of solution equally the h2o becomes saturated, which is often due to evaporation. The about mutual chemical sediments formed in this way are calcite (CaCOiii), gypsum (CaSOfour.2H2O) and halite (NaCl). They are a mutual blazon of sediment of sabkhas, back-barrier coastal plains which are ubiquitous in barren climates.

Mineral composition of clastic sediments

Figure 2 shows the average composition by weight of chemical elements in the lithosphere. These elements combine to class minerals. The chief minerals in the lithosphere (world crust and uppermost mantle) are [6]:

Figure two: Composition by weight of chemic elements in the lithosphere [one].

Table two. Specific density of common minerals from which sediments are composed.
mineral specific density
Quartz 2.65
Aragonite two.95
Calcite 2.71
Orthoclase 2.55-2.63
Albite two.6-2.65
Anorthite two.72-2.75
Mica 2.8-three.1
Illite 2.half-dozen-2.nine
Montmorillonite 1.7-2
Vermiculite 2.four-2.7
  • Feldspars: Aluminium silicates. The most common forms are: orthoclase KAlSi3O8, albite NaAlSiiiiO8 and anorthite CaAliiSi2O8. These minerals are fairly difficult, often with pinkish, white, or gray in color. Most 40% of the lithosphere consists of feldspars.
  • Quartz: silicon dioxide crystal (SiOii);
  • Mica: complex silicate minerals comprising elements Thou or Na and metallic cations such every bit Al, Mg and Fe, that form crystals with a canvass-like system of atoms;
  • Dirt: hydrous silicates that contain metal cations, generally aluminium. Most mutual clay minerals are kaolinite (Al2SitwoO5(OH)4), illite, montmorillonite, vermiculite and bentonite. Their bones building blocks are sheets of silica (Si) tetrahedra and oxygen (O) and hydroxyl (OH) octahedra. Micas and dirt minerals result from chemical weathering of igneous rocks and feldspars.

The specific densities (density relative to h2o [math]s=\rho_{mineral}/\rho_{water}[/math]) of these minerals is given in Table two.

Physical and chemical properties of sediment

In this section several physical and chemical properties of coastal and marine sediments are shortly introduced. More detailed treatments can exist constitute in other Coastal Wiki articles indicated in the text.

Autumn velocity

Figure 3: Fall velocity of quartz spheres (specific density [math]s=\rho/\rho_{h2o}=2.65[/math]) in still water at temperature [math]\theta=20^{o}C[/math]. The fall velocity at other temperatures [math]\theta[/math] can be deduced by multiplying grainsizes [math]d[mm][/math] past [math]1-0.016*(\theta-twenty)[/math].

The seabed of oceans and coastal waters are mainly formed past settling of sediment particles out of interruption. Which particles settle where is mainly determined past the particle fall velocity, availability and turbulence. Nether energetic weather just coarse grades may settle. The fall velocity of sediment particles depends on size and density. Considering the density of different types of sediments is similar (Table ii), the grain size is determinant. The larger the grain size, the higher the autumn velocity. The fall velocity as a function of the grainsize is displayed in Fig. 3. Indicated is the fall velocity of round quartz grains in even so water at low concentration. In reality, sediment particles do not accept a round shape, which slightly affects the fall velocity. The bodily fall velocity in flowing water is very different, due to up and down motions caused by turbulent fluid motions ('turbulent eddies'). The concentration of sediment particles also plays a role. At very high concentration, the fall velocity decreases, considering particles increasingly interfere with each other, a miracle that is chosen hindered settling[7].

Flocculation

Figure 4: The ratio of floc fall velocity and fall velocity of the constituent particles [8].

If the fall velocity for very small particles, such as clay minerals, is derived from settling quartz spheres (Fig. 3), it seems so depression that they almost never reach the lesser from break. The ubiquitous mud beds in coastal waters betoken to a different settling mechanism. This mechanism consists of flocculation. Clay minerals accept a big area relative to their weight, and electrical surface charges. This ensures that they hands bind to one another and to other substances in the h2o. Clays are therefore classified as cohesive sediments. A substance that serves as a powerful binder is and so-called EPS, extracellular polymeric substances [ix]. These large organic molecules (polysaccharides, proteins, nucleic acids and lipids) are exuded by living organisms and therefore omnipresent in coastal waters. Flocs abound much faster in natural seawater than in salinized distilled h2o[x]. In add-on to EPS, bacterial colonization also plays a role in flocculation [eleven]. Flocculation is further influenced by factors such equally salinity and pH of the water. When flocs abound, they non only capture smaller flocs that settle more slowly, simply also other suspended sediments, such as dirt, silt and fine sand. Frequent encounters between sediment particles are important for floc growth. Flocculation is thus enhanced with a loftier concentration of suspended material and with a certain (low) caste of turbulence (frequently expressed as the turbulent shear rate [math]G[/math], see beneath), dependent on the suspended sediment concentration[12]. Nevertheless, when turbulence is strong (the general instance in estuaries), large flocs are broken downward. Flocs settle much faster than the individual constituent particles - a gene of a thousand or more than, run across Fig. iv. The largest flocs are agglomerates of microflocs. These then-called macroflocs (typical size > 160 [math]\mu[/math]grand) have the highest autumn velocity, only they are less stable than the smaller microflocs (typical size < 160 [math]\mu[/math]k). The various factors that decide the autumn velocity are captured in the following empirical formulas, for macroflocs [math] w_{M}[/math] and microflocs [math] w_{\mu}[/math], respectively [13] [14]:

[math] w_{M}=\Large\frac{g B_M}{Thousand}\normalsize \left(\Large \frac{c}{ \rho}\normalsize \right)^g \left(\Large \frac{G d_\mu^2}{\nu}\normalsize \right)^{0.33} \exp \left[-\left( \Large \frac{u_{*One thousand}}{\sqrt{\tau / \rho}}\normalsize \right)^{0.463} \right] \; ,\qquad w_{ \mu}= \Large \frac{g B_\mu}{Chiliad}\normalsize \left(\Large \frac{Gd^2}{\nu}\normalsize \right)^{0.78} \exp \left[-\left(\Big \frac{u_{\mu}}{\sqrt{\tau / \rho}}\normalsize \right)^{0.66} \right] , [/math]

where the index [math]Yard[/math] designates the macroflocs and the index [math]\mu[/math] the microflocs. Other symbols stand for: [math]d[/math] the grainsize of the constituent particles, [math]d_{\mu}[/math] the grainsize of the constituent microflocs, [math]\tau[/math] is the near-bed shear stress and [math]c [/math] the suspension concentration in [math] kg/l [/math]. The turbulent shear rate [math]G[/math] is given by [math] G = \sqrt {\epsilon / \nu }=\sqrt{\Large\frac{\tau}{\rho \nu}\frac{dU}{dz}\normalsize} , [/math] where [math]\epsilon[/math] is the free energy dissipation rate per unit mass, [math]\nu [/math] the kinematic viscosity and [math]U(z)[/math] the current velocity equally a function of depth [math]z[/math]. For the Tamar and Gironde estuaries the following parameter values were established: [math] B_M = 0.13, \; B_{\mu} = 0.6, \; thou = 0.22, \; u_{*M} = 0.067 m/s, \; u_{*\mu} = 0.025 m/southward, \; d_\mu = 10^{-four} 1000, \; d = x^{-5} m [/math]. The settling velocities observed in the Tamar and Gironde are 0.5-1 mm/s for microflocs and nigh v times larger for macroflocs.

Further details on flocculation can exist institute in the article Flocculation cohesive sediments.

Sediment transport

The processes that underlie sediment transport are strongly related to turbulent flow structures in a thin purlieus layer near the bed and above. Because these processes are very complicated and non fifty-fifty fully understood, empirical formulas are used in practice for the description of sediment transport. These formulas are described in the articles Sand transport and Sediment transport formulas for the littoral environs. These articles mainly deal with non-cohesive sediments. Transport of cohesive sediments is dealt with in the articles Dynamics of mud transport and Sediment deposition and erosion processes. For measurement of sediment transport see: Measuring instruments for sediment ship, Laboratory and in situ assay of samples.

Sediment deposits in littoral waters

A more detailed introduction to sediment deposition and erosion is given in the article Sediment deposition and erosion processes.

Degradation of sediment on the seabed takes identify when conditions are suitable. These atmospheric condition are dissimilar for each type of sediment.

When a sediment-laden water mass reaches an expanse of lower menstruum strength and wave activity, function of the carried material is deposited. Sediment particles with the greatest fall velocity settle showtime and particles entrained as bedload (rolling and jumping along the bottom) come to rest. When the electric current strength and moving ridge activity further decrease, the fine suspended material also settles. Most sedimentation takes place in the period around slack tide (flow reversal). Part of the deposited material will afterward exist resuspended by the recovering tidal flow, but another function will remain. This can atomic number 82 to temporary or permanent deposition. Deposits are temporary if they are comparatively consolidated and re-eroded during conditions of strong currents (eastward.grand., spring tide) or strong wave action (storm). Permanent deposits have a layered grapheme; each layer represents a deposition period. Successive layers may contain different types of sediment if they have been deposited nether different conditions. The layered sediment deposits in estuarine channels sometimes exhibit thin intermediate mud drapes deposited in short periods around high-water or low-water slack tide)[15].

Graded sediment

Figure 5: Classification of different types of graded sediment beds. Redrawn from Blair and McPherson (1999)[sixteen].

Sediment deposits in littoral areas are non always layered; mixed deposits occur ofttimes. These deposits are referred to as graded sediment. Layered sediment deposits can be mixed when the seabed is strongly perturbated. Mixing also takes place through bioturbation: soil animals (mainly worms and molluscs) bring material from deeper layers to the surface through ingestion and excretion [17] [18]. Deposition of mixed sediment also takes identify by excretion of filter feeders in the form of fecal pellets. Terminology for mixed deposits is indicated in Fig. 5 for unlike mixtures of gravel, sand and mud. Mud is itself a mixture of particles with a grain size of less than 0.063 mm, consisting of very fine sand, silt, clay and organic matter.

Run across also: Biogeomorphology of coastal systems, Sandy shore habitat, Meiofauna of Sandy Beaches.

Spontaneous segregation of graded sediment can occur under certain conditions. This segregation generates contiguous patches of different types of sediment. The underlying feedback mechanism links sedimentation of fine textile on a smooth muddy seabed to a locally reduced degree of turbulence [19]. As a result, fine cloth volition mainly deposit in places where fine material already dominates on the seabed, and then that these muddy patches increment in size and in mud content. This continues until no fine cloth is available whatsoever more from neighboring patches of coarser deposits from which the fines have been winnowed.

Bedforms

Figure 6: Cross-stratification of a migrating dune. Pinnacle: Migrating dune in unidirectional menstruation. The layer inclination follows the leeside gradient; this is called form-concordant. Bottom: Migrating dune in alternating menses (moving ridge orbital menses, tidal flow) with dominant menstruum direction. The stratification at the stoss side of the dune follows the leeside dune gradient of the previous opposite-flow period; this is called form-discordant. Laminae represent to episodic avalanching events along the slip face up. The skid face angle [math]\alpha[/math] is close to the subaqueous bending of quiet of the sediment. The discontinuity plane corresponds to the partially eroded surface of a previous bedform. The laminae and discontinuity planes are preserved in the stratigraphic tape. Adapted from [20]

Sediment layers are generally horizontal, just tin can also have a wavy grapheme. These undulations are acquired by the fact that the interaction between flow and sediment bed does not conduct linearly, pregnant that minor disturbances of the apartment sediment bed can grow exponentially. This leads to the emergence of a large range of bedforms, as explained in the commodity Stability models. The smallest bedforms, ripples, arise in places where bed sediments are sandy and where currents and wave activeness are non very strong but sufficient to set up sediment in motion. This is explained in the articles Moving ridge ripples and Wave ripple formation, for situations where sediment movement is mainly determined by waves. When moving ridge activity increases, while the background current remains pocket-sized, ripples tin can transform into patchy seabed structures called hummocks [21]. Bed ripples exert friction on tidal currents and have a great influence on the catamenia velocity. More details tin can exist institute in the articles Bedforms and roughness and Bed roughness and friction factors in estuaries. In addition to ripples, much larger bedforms can also arise, for example: megaripples (moving ridge length of several meters), transverse and longshore confined (wave length of the order of ane hundred meters, meet Rhythmic shoreline features), dunes (besides called sandwaves, wavelength of tens to hundreds of meters), cheniers (sand ridges on dingy shorelines, wavelength of the order of 1 hundred meters[22]) and shoreface-connected ridges and tidal ridges (wavelength of several kilometers, see Sand ridges in shelf seas). Unlike bedforms scales mentioned may be found superimposed on each other. In the underlying sedimentary layers, bed ripples are not e'er preserved, but the larger bedforms can generally exist observed quite well.

The typical layer structure of the seabed that occurs under the stoss slope of migrating bedforms, such every bit (mega)ripples or dunes, is called cantankerous-stratification. The layer structure consists of parallel laminae formed by sediment avalanching at the migrating bedform crest (more precisely, at the brinkpoint, sligtly downstream of the crest). The laminae are strongly dipping in the direction of bedform migration, co-ordinate to the bending repose, meet Fig. 6.

The bedforms described above do not occur with deposits of fine cohesive material (mud). Dewatering of freshly deposited mud is a slow process and mud layers therefore remain fluid for a long time equally and so-called fluid mud, run into for example the commodity Dynamics of mud transport. In one case consolidated, the erosion resistance is high, then that no bed ripples can grade. Mud layers therefore have a smoothen surface and exert little friction on the flow.

Sediment sorting

Sediment sorting is the spatial segregation of different types of sediment deposits. As indicated earlier, sediment deposition requires suitable hydrodynamic weather condition, which depend on the type of sediment. Fine sediments cannot settle in loftier-energy environments (strong waves, strong currents). Temporary settling is possible when currents are weak (neap tide, slack tide) and in the absence of strong moving ridge action. However, in situations where depression-free energy and high-energy weather alternating, these temporary deposits will disappear. Although currents generally carry a mixture of unlike types of sediment, remaining deposits volition just comprise grainsizes that cannot be resuspended nether the most energetic weather among the alternating atmospheric condition that occur at a specific location. Grain size analysis of sediment deposits therefore can provide an indication of the maximum hydrodynamic shear stresses that occur at a particular location[23] [24]. A qualitative overview of the types of sediment deposits that form the seabed top layer under different hydrodynamic conditions is indicated in Table 3 for sandy coastal environments, together with the associated sediment transport modes and bedforms. These bedforms are themselves crusade of sediment sorting. In low-energetic environments (weak currents, waves) the coarsest sediments accumulate in the troughs (swales) by preferential downwards move along the skid face of the bedforms[21]. In the instance of stronger waves and currents the coarsest sediments are plant at the bedform crest where fine sediments are most easily brought in suspension [25].

Table 3. Sediment sorting in sandy coastal environments: correspondence between hydrodynamic conditions (highest energy weather amidst the usual alternating tidal, climatic and seasonal weather) and seabed deposit; [math]U_c[/math] is the maximum depth-averaged tidal current velocity and [math]U_w[/math] is the wave-orbital velocity at the seabed. The associated bedforms and sediment transport modes are also indicated. The table is adapted from Terwindt (1981) [26] and Martinius and van den Berg (2011)[15].
Hydrodynamic weather Sediment Bedform Sediment transport mode
Stiff currents [math]\small U_c \gt 1 k/s[/math] Medium-fibroid sand 3D dunes Suspended load
Medium currents [math]\small 0.5\lt U_c\lt ane m/s[/math] Fine-medium sand, mud drapes Ripples, 2D dunes Bedload, suspended load
Weak currents [math]\small U_c\lt 0.5 m/s[/math] Mud, fine sand Flat bed (mud), ripples (fine sand) Suspended load (mud), bedload (fine sand)
Strong waves [math]\small-scale U_w\gt 1 one thousand/s[/math] Fine-fibroid sand Apartment bed Suspended load, canvass menstruation
Medium waves [math]\small 0.three\lt U_w\lt one m/s[/math] Fine-medium sand, mud drapes Wave ripples Bedload, suspended load
Weak waves, currents [math]\modest U_w\lt 0.three m/due south, U_c\lt 0.5 m/s[/math] Fine sand, mud layers Moving ridge ripples (fine sand), flat bed (mud) Bedload, suspended load
Strong currents, stiff waves [math]\small U_c, U_w\gt i m/due south[/math] medium-coarse sand Flat bed Suspended load, sail flow

Seabed erosion

When exposed to turbulent shear stresses due to currents or waves, sediment particles are dislodged from the seabed and lifted i by 1 or in lumps. For this, the shear stress must exceed a certain critical value, [math]\tau_{cr}[/math], which depends on many factors, in particular the grainsize (see Fig. 7). The white gap in Fig. 7 shows that seabed erosion is influenced by other factors than shear stress and grain size. These factors are[27]:

Fig. 7. Seabed erosion or no erosion depending on shear stress equally a function of the median sediment grainsize [math]d_{l}[/math]. The white zone represents the influence on seabed erosion of various factors other than shear stress and grain size. [Pa]=[Nm-2]=[kg thou-1s-2].

  • Specific density. The sediment specific density is not considered in this effigy, because it is rather similar for most mineral sediments. This does not apply to sediments of biotic origin, in which case besides density, biochemical bonds also strongly affect erodibility.
  • Angularity of sediment particles. Rounded grains are less susceptible to uptake than athwart grains[28].
  • Sediment sorting. When sediment deposits are a mixture of coarse and fine sediment, the surface layer will mainly consist of coarse particles that protect the underlying fine sediment from erosion (and then-called bed armoring issue). However, the fibroid grains will be more exposed and are therefore more easily expelled from the sediment bed.
  • Superlative turbulent intensity. The shear stress represents the boilerplate turbulent intensity. However, seabed erosion is sensitive to incidental peaks of high turbulent intensity (associated with turbulent bursting events). Some seabed erosion therefore can occur already at quite low values of the shear stress[29].
  • Consolidation. Fine sediment can be easily resuspended when it has just been deposited. Freshly deposited fine sediment has a large pore volume filled with seawater that can exist ready in move under the influence of horizontal pressure gradients of the surface h2o. Over time, the seawater is forced out of the pores by the weight of the consolidating deposit. The bulk density of the eolith increases while the permeability decreases. This causes a abrupt increase of the disquisitional shear stress for erosion[30].
  • Clay content of the eolith. The molecular structure of dirt particles enables the formation of electrochemical bonds that significantly increase the resistance of the deposit against erosion. This issue already occurs at a dirt content of just ten%. A deposit with a sufficiently high clay content retains the water for a long time and consolidates slowly. Once consolidated, the eolith forms a hard layer that can withstand very high shear stresses. Meet Sediment degradation and erosion processes for further details.
  • Biota. Living organisms tin can strongly change the erosion strength of seabed deposits. Diverse large organic molecules, called EPS (extracellular polymeric substances), secreted by micro-organisms, demark sediment particles together and form biofilms at the sediment surface, especially in intertidal areas. Erosion resistance of sediment deposits is likewise favored by the formation of microbial mats, often called algal mats. Erosion resistance is diminished by other organisms, bivalves in detail, that disturb the sediment bed by and then-called bioturbation and graze on microalgae. More than details can be found in Biogeomorphology of littoral systems.

The amount of sediment [math]E[/math] [kg s-11000-2] that is brought in suspension when the shear stress [math]\tau[/math] exceeds the critical stress [math]\tau_{cr}[/math] for initiation of erosion, primarily depends on the excess shear stress [math]\tau - \tau_{cr}[/math]. However, it also depends on many other factors. For fine non-cohesive sand, the relation tin exist represented past the empirical formula[31]

[math]East = M \, (\Big\frac{\tau}{\tau_{cr}}\normalsize -1)^n ,[/math]

with [math]northward \sim [/math] 1 and [math]M \sim [/math] 0.05-0.1 kg m-2s-ane. For fine cohesive sediment, [math]M[/math] and [math]n[/math] strongly depend on the degree of consolidation and must be determined experimentally. The values constitute for [math]due north[/math] are generally much larger than 1 and the values of [math]Grand[/math] much smaller than 0.one kg 1000-twosouthward-1, see Sediment deposition and erosion processes for further details. In exercise, the coefficients [math]1000[/math] and [math]n[/math] change with time because the consolidation caste of the seabed changes with depth.

Sediment contamination and bioavailability

As indicated before, fine sediments - dirt particles in detail - can easily bind with other substances in the water. This binding is chosen sorption: adsorption (=surface binding) or absorption (=uptake). This holding allows fine sediments to filter out dissolved contaminants from the h2o. The h2o becomes less polluted, but the contamination of bed sediments, on the other paw, increases. The segmentation of pollutants between the dissolved phase and the sediment-spring phase is represented by the parameter [math] K_d [/math]. This partition, i.e. the value of [math] K_d[/math], depends on the type of pollutant and the type of sediment.

Laboratory tests evidence that for inorganic contaminants, for example heavy metals such equally atomic number 82 (cation Atomic number 822 +), cadmium (Cd2 +) and copper (Cu2 +), the partition depends on the sediment grainsize [32]. The smaller the grainsize (i.e. greater surface/volume), the stronger is the sorption (large [math] K_d [/math]), almost independently of the concentration of dissolved heavy metals. Biogeochemical processes determine the degree to which trace metals (especially cadmium and copper) are jump to sediment, depending on several factors, such equally salinity and oxygen content. In an estuary, the ratio between the bound and labile (bioavailable) fraction of trace metals is variable in space and fourth dimension. For example, measurements in the Scheldt estuary show that the labile fraction is greatest in the salt-fresh transition zone[33].

For organic pollutants such equally PCB due south (polychlorinated biphenyls) and PAH s (polyciclic aromatic hydrocarbons), sorption to sediment is primarily determined past the organic carbon content in the sediment, rather than by the grain size[34]. This is a well-known feature in water sanitation technology, where active carbon is used to filter contaminants from waste water.

Contaminants are much less toxic to marine organisms when they are spring to sediment than when they are dissolved in water [35]. The bioavailability decreases farther as the sediment bed gets older. Perturbation of the sediment bed by dredging or by bioturbation, however, plays a office. When sediment is worked up from a deeper anoxic layer to a higher oxic layer, attached metals are released by desorption. Burial of sediments to deeper soil layers, on the other hand, reduces bioavailability. The bioavailability of contaminants in soils can be reduced by calculation agile carbon[35].

Spatial distribution of sediments

The blazon of sediment occurring on the seafloor varies greatly from place to place. The presence of nearby sediment sources plays an important part, but hydrodynamic atmospheric condition strongly influence which sediments are deposited where. Because certain types of sediment are but deposited under specific weather, sorting occurs of the different types of sediment that are supplied. In the post-obit, a curt clarification is given of resulting sediment deposits in different marine environments.

Oceans

The deep oceans take four primary sediment sources [4]:

  1. Clastic fluvial sediments. Large quantities of fine sediments (silt, dirt and fine sand) are deposited by the globe's major rivers at the edges of the continental shelfs (meet Fig. ane). These deposits destabilize the continental shelf slope, causing the sediment to slide down to the body of water flooring by slumping or through gravity currents. These deposits are confined forth the shelf break and are non further spread over the ocean flooring.
  2. Atmospheric grit. A big part of the fine sediments entrained past air current from barren land areas and ash from volcanic eruption plumes is deposited in the oceans (eastward.g., red clay deposits, manganese nodules). This contribution to ocean sedimentation is relatively small present.
  3. Carbonaceous ooze. The remains of calcareous algae, mainly coccoliths and foraminifera, are a major component of the seafloor sediment in large parts of the ocean. Below depths of iii to 4 km, however, they do non occur, because the scales dissolve here as a result of too high a pressure level and too high a concentration of carbon dioxide.
  4. Siliceous ooze. The remains of silicate plankton, mainly diatoms and radiolaria, boss in other parts of the body of water - especially in the central function of the Pacific and in the Antarctic ocean.

Embankment and foreshore

Figure eight: Relation between fall velocity (sediment blazon) and beach slope as a function of the Dean parameter [math]\Omega[/math]. Redrawn subsequently (Flemming and Fricke, 1983)[36].

Sediments on the foreshore and subaerial embankment are derived from nearby rivers or from ancient offshore river deposits. The foreshore is a highly energetic environs due to the activity of waves, which preclude settling of fine sediments. Sand and gravel are the dominant seabed sediments, depending on the supply of these sediments. The coarsest sediments are found in the breaker zone, specially in the higher office where the waves collapse on the beach. The finer fractions are found in the offshore zone and also on the backshore and the dunes, which are fed from the dry beach by aeolian transport. The type of sediment on the shoreface largely determines the equilibrium shoreface slope for a given wave climate. The human relationship betwixt sediment grain size, wave climate and shoreface gradient tin can be characterized by the Dean parameter

[math]\Omega = H / (w T) , [/math]

where [math]H[/math] is the significant offshore wave summit (earlier breaking), [math]w[/math] the sediment fall velocity and [math]T[/math] the peak wave menstruum. This relationship is schematically shown in Fig. eight.

Figure 9: Correlation betwixt sediment grainsize and related beach slope from 78 field studies, adjusted from Bujan et al. (2019)[37]. The violet surface area indicates the scatter of the information points.

Figure 8 suggests a positive correlation between sediment grainsize and embankment slope. This is substantiated by a study of Bujan et al. (2019)[37], who compiled information on grainsize and associated embankment slope ([math]\beta[/math]) from a big number of field sites. The event is shown in Fig. 9, where the violet area represents more than 95% of the information points. The dark bluish line is a fit to the observed values represented by the empirical relation [math]\tan \beta = -0.154(D_{50}-0.125)^{-0.145} + 0.268[/math]. The big bandwidth around this line reflects the different characteristics of the field sites (moving ridge climate, tides, beach profile, sediment sorting, ...) and the unlike data collection and assay methods used in the underlying studies. Grainsize and embankment gradient are positively correlated. For big grainsizes, wave up- and downrush is reduced past infiltration into the the sediment bed. This is a plausible explanation for the weak dependence of beach slope on grainsize for coarse sediment, compared to the potent dependence for fine-medium sediment[37].

For a more than detailed discussion of the shoreface equilibrium contour the reader is referred to the article Shoreface profile, which deals with sandy coasts. Coasts with abundant fine sediment sources are dealt with in the article Coastal mud belt.

Estuaries

The sediment distribution in estuaries is particularly circuitous. Sediment deposits depend on the supply of river sediment, the supply of sediment from the ocean and on local flow and wave weather, in relation with a by and large intricate topography. In addition, deposits are influenced by strong fluctuations in river belch, tides and wave weather at various timescales. Even so, the sediment distribution in estuaries more often than not displays a characteristic pattern that is shown in Fig. 10 and discussed below.

Figure ten: Typical example of the spatial distribution of bed sediments in a coastal obviously estuary, schematically represented (not at scale). Adapted from Dalrymple et al. (1992)[38].


Gravel transported by rivers is mostly deposited upstream and does not reach coastal plain estuaries. This is the instance when catchment areas are situated far from the sea; long rivers discharge mainly sandy and muddy sediments into broad coastal plain estuaries. In the example of mountainous coasts, estuaries are generally small-scale drowned river valleys (rias); they may receive substantial amounts of gravely sediments, which are further discharged to the coastal zone by strong river floods. About coastal plain estuaries also receive sand and mud imported from the sea past tidal flood currents. The estuarine morphology is in morphodynamic equilibrium if, averaged over a long period, the inflow of sand and mud is balanced past consign to the ocean past outflowing ebb currents[39], see Morphology of estuaries. This is non a static equilibrium, because sand and mud are moving around the estuary all the time, including temporal deposition and erosion.

Morphodynamic equilibrium does not imply that the concentration of suspended sediment in an estuary is uniform. Concentration maxima occur in convergence zones where temporal settling of sediments takes place (or permanent settling in the instance of not-equilibrium). These convergence zones depend not just on flow strength and flow pattern, just also on send, settling and erosion characteristics of sediment particles. The areas of highest suspended concentration and deposition thus differ for sand and mud [twoscore].

Convergence of sand send takes place on the outer delta, where the ebb flow expands and decelerates. Within the estuary, convergence zones of sand transport are marked by sand banks and sand bars which take grown at the junctions of ebb and flood flow channels and at the inner bends of flow meanders.

Convergence of mud transport (silt and dirt) occurs in the transition zone between flood ascendant near-bed flow (due to tidal asymmetry and estuarine density-driven circulation) and ebb dominant flow (river menstruation enhanced past ebb tidal currents). This transition zone has the highest mud concentrations, both in the h2o cavalcade (turbidity maximum) and in the channel bed (for more details, run across the articles Estuarine turbidity maximum, Dynamics of mud transport, Sediment deposition and erosion processes). Besides, mud concentrations are high in sheltered areas where fine sediments can settle without immediately beingness resuspended, such as salt marshes and mangroves. In these areas, waves and currents are strongly reduced by vegetation, which enhances sediment degradation. Run across also: Dynamics, threats and management of salt marshes and Mangroves.

Related articles

Sediment deposition and erosion processes
Dynamics of mud transport
Shoreface profile
Estuarine turbidity maximum
Littoral mud belt

References

  1. ↑ i.0 i.1 one.ii 1.3 Huggett, R.J. 2007. Fundamentals of geomorphology. Routledge, Taylor & Francis
  2. ↑ Milliman, J. D. and Meade, R. H. 1983. Earth-broad delivery of sediment to the oceans. Journal of Geology 91, ane–21
  3. ↑ Selley, R.C. 2005. Sedimentary rocks: Mineralogy and Classification. In: Encyclopedia of Geology (Editors: Richard C. Selley, L. Robin M. Cocks and Ian R. Plimer). Elsevier.
  4. ↑ four.0 4.1 Taylor, K.Chiliad. 2008. Sediments and sedimentation. In: An Introduction to Concrete Geography and the Surround (J. Holden, editor), Pearson Education Express
  5. ↑ Desguée, R., Robin, N., Gluard, L., Monfort, O., Anthony, E.J. and Levoy, F., 2011. Contribution of hydrodynamic weather condition during shallow water stages to the sediment residual on a tidal apartment: Mont-Saint-Michel bay, Normandy, French republic. Estuarine, Coastal and Shelf Science, 94, 343-354
  6. ↑ Anderson, R.South. and Anderson, Southward.P. 2010. Geomorphology: The Mechanics and Chemical science of Landscapes. Cambridge University Press. p. 187.
  7. ↑ Winterwerp, J.C. 2002. On the flocculation and settling velocity of estuarine mud. Continental Shelf Research 22: 1339–1360
  8. ↑ Migniot, C. 1968. A study of the concrete properties of various forms of very fine sediments and their behaviour under hydrodynamic action. La Houille Blanche 7, 591–620
  9. ↑ Grabowski, R.C., Droppo, I.G. and Wharton, G. 2011. Erodibility of cohesive sediment: the importance of sediment properties. World Science Reviews 105 (3-4): 101-12
  10. ↑ Skinnebach, K.H., Fruergaard, Grand. and Andersen, T.J. 2019. Biological effects on flocculation of fine-grained suspended sediment in natural seawater. Estuarine, Coastal and Shelf Science 228, 106395
  11. ↑ Linley, Due east.A.S. and Field, J.Chiliad. 1982. The nature and significance of bacterial aggregation in a nearshore upwelling ecosystem. Estuarine, Littoral and Shelf Scientific discipline 14: 1-11
  12. ↑ Mietta, F., Chassagne, C., Manning, A.J. and Winterwerp, J.C. 2009. Influence of shear rate, organic matter content, pH and salinity on mud flocculation. Bounding main Dynamics 59: 751–763
  13. ↑ Dyer, 1000.R. and Manning, A. J. 1999. Observation of the size, settling velocity and constructive density of flocs and their fractal dimension. J.Ocean Res. 41: 87-95
  14. ↑ Soulsby, R.L., Manning, A.J., Spearman J. and Whitehouse, R.J.S. 2013. Settling velocity and mass settling flux of flocculated estuarine sediments. Marine Geology 339 (2013) ane–12
  15. ↑ 15.0 fifteen.1 Martinius, A.W. and van den Berg, J.H. 2011. Atlas of sedimentary structures in estuarine and tidally influenced river deposits of the Rhine-Meuse-Scheldt system. EAGE Publ. ISBN 978-90-73834-11-8
  16. ↑ Blair, T.C. and McPherson, J.G. 1999. Grain-size and textural classification of coarse sedimentary particles. Journal of Sedimentary Research 69: vi–xix
  17. ↑ Baumfalk, Y.A. 1979. Heterogeneous grain size distribution in tidal apartment sediment caused by bioturbation activity of Arenicola marina (polychaeta). Netherlands Journal of Sea Research 13: 428-440
  18. ↑ Gallagher, E.D. 2008. Bioturbation. Biol. Ocean. Processes, EEOS 630
  19. ↑ Murray, A.B. and Thieler, E.R. 2004. A new hypothesis and exploratory model for the formation of large-calibration inner-shelf sediment sorting and 'rippled scour depressions'. Continental Shelf Res. 24: 295-315
  20. ↑ Reineck, H.-E. and Singh, I.B. 1973. Depositional sedimentary environments. Springer, Berlin, 439 pp.
  21. ↑ 21.0 21.i Van den Berg, J.H. and Nio, S.D. 2010. Sedimentary structures and their relation to bedforms and catamenia weather. EAGE Publications
  22. ↑ Augustinus, P.G.E.F. 1989. Cheniers and chenier plains: a general introduction. Marine Geology 90: 219-229
  23. ↑ Ward, S.L., Neill, S.P., Van Landeghem, K.J.J. and Scourse, J.D. 2015. Classifying seabed sediment type using simulated tidal-induced bed shear stress. Marine Geology 367: 94–104
  24. ↑ Escobara, C.A., Mayerle, R. and Restrepo, D. 2019. Estimation of sediment grain sizes in a mesotidal expanse, Dithmarschen Bight, High german North Sea. Marine Geology 417, 106006
  25. ↑ Van Oyen, T., Blondeaux, P. and Van den Eynde, D. 2013. Sediment sorting along tidal sand waves: A comparison between field observations and theoretical predictions. Continental Shelf Inquiry 63: 23–33
  26. ↑ Terwindt, J.H.J. 1981. Origin and sequences of sedimentary structures in inshore mesotidal deposits of the North Sea. Spec. Publs. Int. Ass. Sediment. 5: four-26
  27. ↑ Yang, Y., Gao, Due south., Wang, Y.P., Jia, J., Xiong, J. and Zhou, L. 2019. Revisiting the problem of sediment motion threshold. Continental Shelf Research 187, 103960
  28. ↑ Paphitis, D. 2001. Sediment movement under unidirectional flows: an assessment of empirical threshold curves. Coastal Applied science. 43: 227-245
  29. ↑ Mohtar, W.H.K.West., Lee, J.W., Azha, N.I.M. and Cheng, Northward-S. 2020. Incipient sediment motion based on turbulent fluctuations. International Journal of Sediment Enquiry 35: 125-133
  30. ↑ Mohr, H., Draper, S., White, D.J. and Cheng, Fifty. 2021. The effect of permeability on the erosion threshold of fine-grained sediments. Coastal Engineering 163, 103813
  31. ↑ Mehta, A. J. and Partheniades, E. 1982. Resuspension of deposited cohesive sediment beds. Procs. 18th International Conference on Coastal Applied science, Cape Town, South Africa
  32. ↑ Schorer, M. 1997. Pollutant and organic matter content in particle size fractions. Freshwater contamination, IAHS publ. 243.
  33. ↑ Gaulier, C., Zhou, C., Gao, Y., Guo, W., Reichstädter, M., Ma, T. Baeyens, West. and Billon, G. 2021. Investigation on trace metal speciation and distribution in the Scheldt estuary. Science of the Total Environment 757, 143827
  34. ↑ Karickhoff, S.W., Brown, D.Southward. and Scott, T.A. 1979. Sorption of hydrophobic pollutants on natural sediments. Water Research xiii: 241-248
  35. ↑ 35.0 35.1 National Enquiry Council. 2003. Bioavailability of Contaminants in Soils and Sediments: Processes, Tools, and Applications. Washington, DC: The National Academies Press. doi: 10.17226/10523
  36. ↑ Flemming, B.Due west. and Fricke, A.H. 1983. Beach and nearshore habitats every bit a function of internal geometry, primary sedimentary structures and grain size. In: McLachlan, A., Ersamus, T. (Eds.), Sandy Beaches equally Ecosystems. Dr. West. Junk Publishers, The Hague, pp. 115–132
  37. ↑ 37.0 37.1 37.2 Bujan, N., Cox, R. and Masselink, Thou. 2019. From fine sand to boulders: Examining the relationship between beach-face slope and sediment size. Marine Geology 417, 106012
  38. ↑ Dalrymple, R.Westward., Zaitlin, B.A., Boyd, R., 1992. Estuarine facies models: conceptual basis and stratigraphic implications. Periodical of Sedimentary Petrology 62, 1130–1146
  39. ↑ Dronkers, J. 2017. Convergence of estuarine channels. Cont. Shelf Res. 144: 120-133
  40. ↑ Dronkers, J. 2017. Dynamics of coastal systems. World Scientific Publ. Co., 753 pp.
The main authors of this article are Job Dronkers and Janrik van den Berg
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