Turbidity current

thumb|right|upright=1.35|[[Turbidites are deposited in the deep ocean troughs below the continental shelf, or similar structures in deep lakes, by turbidity currents which slide down the slopes. ]]

thumb|right|upright=1.35|Longitudinal section through an underwater turbidity current

A turbidity current is most typically an underwater current of usually rapidly moving, sediment-laden water moving down a slope; although current research (2018) indicates that water-saturated sediment may be the primary actor in the process. Turbidity currents can also occur in other fluids besides water.

Researchers from the Monterey Bay Aquarium Research Institute found that a layer of water-saturated sediment moved rapidly over the seafloor and mobilized the upper few meters of the preexisting seafloor. Plumes of sediment-laden water were observed during turbidity current events but they believe that these were secondary to the pulse of the seafloor sediment moving during the events. The belief of the researchers is that the water flow is the tail-end of the process that starts at the seafloor.

However, the term "turbidity current" was adopted to describe a natural phenomenon whose exact nature is often unclear. The turbulence within a turbidity current is not always the support mechanism that keeps the sediment in suspension; however it is probable that turbulence is the primary or sole grain support mechanism in dilute currents (<3%). Definitions are further complicated by an incomplete understanding of the turbulence structure within turbidity currents, and the confusion between the terms turbulent (i.e. disturbed by eddies) and turbid (i.e. opaque with sediment). Kneller & Buckee, 2000 define a suspension current as 'flow induced by the action of gravity upon a turbid mixture of fluid and (suspended) sediment, by virtue of the density difference between the mixture and the ambient fluid'. A turbidity current is a suspension current in which the interstitial fluid is liquid (generally water); a pyroclastic current is one in which the interstitial fluid is gas. The average concentration of suspended sediment for most river water that enters the ocean is much lower than the sediment concentration needed to form a hyperpycnal plume. Although some rivers can have continuously high sediment load that can create a continuous hyperpycnal plume, such as the Haile River (China), which has an average suspended concentration of 40.5 kg/m<sup>3</sup>. Controlling this sedimentation within the reservoir can be achieved by using solid and permeable obstacles with the right design. Since the famous case of breakage of submarine cables by a turbidity current following the 1929 Grand Banks earthquake, earthquake triggered turbidites have been investigated and verified along the Cascadia subduction Zone, the Northern San Andreas Fault, a number of European, Chilean and North American lakes, Japanese lacustrine and offshore regions and a variety of other settings.

Canyon-flushing

When large turbidity currents flow into canyons they may become self-sustaining, and may entrain sediment that has previously been introduced into the canyon by littoral drift, storms or smaller turbidity currents. Canyon-flushing associated with surge-type currents initiated by slope failures may produce currents whose final volume may be several times that of the portion of the slope that has failed (e.g. Grand Banks).

Slumping

Sediment that has piled up at the top of the continental slope, particularly at the heads of submarine canyons can create turbidity current due to overloading, thus consequent slumping and sliding.

Convective sedimentation beneath river plumes

thumb|Laboratory images of how convective sedimentation beneath a buoyant sediment-laden surface can initiate a secondary turbidity current.

A buoyant sediment-laden river plume can induce a secondary turbidity current on the ocean floor by the process of convective sedimentation. so that the dense lower boundary become unstable. The resulting convective sedimentation leads to a rapid vertical transfer of material to the sloping lake or ocean bed, potentially forming a secondary turbidity current. The vertical speed of the convective plumes can be much greater than the Stokes settling velocity of an individual particle of sediment. Most examples of this process have been made in the laboratory, but possible observational evidence of a secondary turbidity current was made in Howe Sound, British Columbia, where a turbidity current was periodically observed on the delta of the Squamish River. As the vast majority of sediment laden rivers are less dense than the ocean,

Deposits

right|thumb|Turbidite [[interbedded with finegrained dusky-yellow sandstone and gray clay shale that occur in graded beds, Point Loma Formation, California.]]

When the energy of a turbidity current lowers, its ability to keep suspended sediment decreases, thus sediment deposition occurs. When the material comes to rest, it is the sand and other coarse material which settles first followed by mud and eventually the very fine particulate matter. It is this sequence of deposition that creates the so called Bouma sequences that characterize turbidite deposits.

Because turbidity currents occur underwater and happen suddenly, they are rarely seen as they happen in nature, thus turbidites can be used to determine turbidity current characteristics. Some examples: grain size can give indication of current velocity, grain lithology and the use of foraminifera for determining origins, grain distribution shows flow dynamics over time and sediment thickness indicates sediment load and longevity.

Turbidites are commonly used in the understanding of past turbidity currents, for example, the Peru-Chile Trench off Southern Central Chile (36°S–39°S) contains numerous turbidite layers that were cored and analysed. From these turbidites the predicted history of turbidity currents in this area was determined, increasing the overall understanding of these currents. This sediment-wave field covers an area of at least 29 000 km<sup>2</sup> at a water depth of 4400–4825 meters. These turbidity currents ultimately come to a halt as sedimentation results in a reversal of buoyancy, and the current lifts off, the point of lift-off remaining constant for a constant discharge. Experimental turbidity currents and field observations suggest that the shape of the lobe deposit formed by a lofting plume is narrower than for a similar non-lofting plume

Prediction

Prediction of erosion by turbidity currents, and of the distribution of turbidite deposits, such as their extent, thickness and grain size distribution, requires an understanding of the mechanisms of sediment transport and deposition, which in turn depends on the fluid dynamics of the currents.

The extreme complexity of most turbidite systems and beds has promoted the development of quantitative models of turbidity current behaviour inferred solely from their deposits. Small-scale laboratory experiments therefore offer one of the best means of studying their dynamics. Mathematical models can also provide significant insights into current dynamics. In the long term, numerical techniques are most likely the best hope of understanding and predicting three-dimensional turbidity current processes and deposits. In most cases, there are more variables than governing equations, and the models rely upon simplifying assumptions in order to achieve a result.

Oil exploration

Oil and gas companies are also interested in turbidity currents because the currents deposit organic matter that over geologic time gets buried, compressed and transformed into hydrocarbons. The use of numerical modelling and flumes are commonly used to help understand these questions. Much of the modelling is used to reproduce the physical processes which govern turbidity current behaviour and deposits. and then later extended to turbidity currents. The typical assumptions used along with the shallow-water models are: hydrostatic pressure field, clear fluid is not entrained (or detrained), and particle concentration does not depend on the vertical location. Considering the ease of implementation, these models can typically predict flow characteristic such as front location or front speed in simplified geometries, e.g. rectangular channels, fairly accurately.

Depth-resolved models

With the increase in computational power, depth-resolved models have become a powerful tool to study gravity and turbidity currents. These models, in general, are mainly focused on the solution of the Navier-Stokes equations for the fluid phase.

With dilute suspension of particles, a Eulerian approach proved to be accurate to describe the evolution of particles in terms of a set of continuum particle concentration fields each representing one unique particle kind of density and/or diameter. Under these models, no such assumptions as shallow-water models are needed and, therefore, accurate calculations and measurements are performed to study these currents. Measurements such as, pressure field, energy budgets, vertical particle concentration and accurate deposit heights are a few to mention. Both Direct numerical simulation (DNS) and Turbulence modeling are used to model these currents.

Notable examples of turbidity currents

Within minutes after the 1929 Grand Banks earthquake occurred off the coast of Newfoundland, transatlantic telephone cables began breaking sequentially, farther and farther downslope, away from the epicenter. Twelve cables were snapped in a total of 28 places. Exact times and locations were recorded for each break. Investigators suggested that an estimated 60 mile per hour (100 km/h) submarine landslide or turbidity current of water saturated sediments swept 400 miles (600 km) down the continental slope from the earthquake's epicenter, snapping the cables as it passed. Subsequent research of this event have shown that continental slope sediment failures mostly occurred below 650 meter water depth. The slumping that occurred in shallow waters (5–25 meters) passed down slope into turbidity currents that evolved ignitively. that is well-correlated to other evidence of earthquakes recorded in coastal bays and lakes during the Holocene. Forty–one Holocene turbidity currents have been correlated along all or part of the approximately 1000 km long plate boundary stretching from northern California to mid-Vancouver island. The correlations are based on radiocarbon ages and subsurface stratigraphic methods. The inferred recurrence interval of Cascadia great earthquakes is approximately 500 years along the northern margin, and approximately 240 years along the southern margin. During the 2006 Pingtung earthquake off SW Taiwan, eleven submarine cables across the Kaoping canyon and Manila Trench were broken in sequence from 1500 to 4000 m deep, as a consequence of the associated turbidity currents. at Port Valais. These papers were possibly the earliest identification of a turbidity current and he discussed how the submarine channel formed from the delta. In this freshwater lake, it is primarily the cold water that leads to plunging of the inflow. The sediment load by itself is generally not high enough to overcome the summer thermal stratification in Lake Geneva.
The longest turbidity current ever recorded occurred in January 2020 and flowed for through the Congo Canyon over the course of two days, damaging two submarine communications cables. The current was a result of sediment deposited by the 2019–2020 Congo River floods.

References

External links

Turbidity current in motion
Start of a turbidity current .
Depth-resolved simulation of turbidity currents.

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