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42-704: The Northwest Atlantic Mid-Ocean Channel (NAMOC) is the main body of a turbidity current system of channels and canyons running on the sea bottom from the Hudson Strait , through the Labrador Sea , and ending at the Sohm Abyssal Plain in the Atlantic Ocean . Contrary to most other such systems which fan away from the main channel, numerous tributaries run into the NAMOC and end there. The density of those tributaries

84-418: 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

126-416: A decrease in height. The behaviour of turbidity currents with buoyant fluid (such as currents with warm, fresh or brackish interstitial water entering the sea) has been investigated to find that the front speed decreases more rapidly than that of currents with the same density as the ambient fluid. These turbidity currents ultimately come to a halt as sedimentation results in a reversal of buoyancy, and

168-468: A few to mention. Both Direct numerical simulation (DNS) and Turbulence modeling are used to model these currents. Sediment gravity flow A sediment gravity flow is one of several types of sediment transport mechanisms, of which most geologists recognize four principal processes. These flows are differentiated by their dominant sediment support mechanisms, which can be difficult to distinguish as flows can be in transition from one type to

210-484: A lesser extent debris flows and mud flows, are thought to be the primary processes responsible for depositing sand on the deep ocean floor. Because anoxic conditions at depth in the deep oceans are conducive to the preservation of organic matter , which with deep burial and subsequent maturation through the absorption of heat can generate oil and gas , the deposition of sand in deep ocean settings can ultimately juxtapose petroleum reservoirs and source rocks . In fact,

252-433: A secondary turbidity current on the ocean floor by the process of convective sedimentation. Sediment in the initially buoyant hypopycnal flow accumulates at the base of the surface flow, 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

294-614: 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. When large turbidity currents flow into canyons they may become self-sustaining, and may entrain sediment that has previously been introduced into

336-567: A well-defined advance-front, also known as the current's head, and are followed by the current's main body. In terms of the more often observed and more familiar above sea-level phenomenon, they somewhat resemble flash floods. Turbidity currents can sometimes result from submarine seismic instability, which is common with steep underwater slopes, and especially with submarine trench slopes of convergent plate margins, continental slopes and submarine canyons of passive margins. With an increasing continental shelf slope, current velocity increases, as

378-425: Is 35 to 45 kg/m , depending on the water properties within the coastal zone. Most rivers produce hyperpycnal flows only during exceptional events, such as storms , floods , glacier outbursts, dam breaks, and lahar flows. In fresh water environments, such as lakes , the suspended sediment concentration needed to produce a hyperpycnal plume is quite low (1 kg/m ). The transport and deposition of

420-430: 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 leaves

462-548: Is the highest near the Labrador Peninsula , but the longest tributary, called Imarssuak Mid-Ocean Channel (IMOC), originates in the Atlantic Ocean. Most topography data on the NAMOC originate from wide-range sonar scans. With a total length of about 3,800 km (2,361 mi), NAMOC is one of the longest underwater channels in the world. It is 100–200 m deep and 2–5 km wide at the channel floor. The rising levees of

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504-453: Is used to reproduce the physical processes which govern turbidity current behaviour and deposits. The so-called depth-averaged or shallow-water models are initially introduced for compositional gravity currents 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

546-566: The Gulf of Cadiz , where the ocean current leaving the Mediterranean Sea (also known as the Mediterranean outflow water) pushes turbidity currents westward. This has changed the shape of submarine valleys and canyons in the region to also curve in that direction. When the energy of a turbidity current lowers, its ability to keep suspended sediment decreases, thus sediment deposition occurs. When

588-443: The interstitial fluid is a liquid (generally water); a pyroclastic current is one in which the interstitial fluid is gas. When the concentration of suspended sediment at the mouth of a river is so large that the density of river water is greater than the density of sea water a particular kind of turbidity current can form called a hyperpycnal plume. The average concentration of suspended sediment for most river water that enters

630-453: The ocean is much lower than the sediment concentration needed for entry as a hyperpycnal plume. Although some rivers can often 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 . The sediment concentration needed to produce a hyperpycnal plume in marine water

672-458: The sediments in narrow alpine reservoirs is often caused by turbidity currents. They follow the thalweg of the lake to the deepest area near the dam , where the sediments can affect the operation of the bottom outlet and the intake structures. Controlling this sedimentation within the reservoir can be achieved by using solid and permeable obstacles with the right design. Turbidity currents are often triggered by tectonic disturbances of

714-451: The NAMOC (about 100 m above the sea bed) often hinder confluence of some tributaries, which instead run along NAMOC for hundreds of km. Its western (right-hand, max. height 250 m) levee rises some 100 m above the eastern one (max. height 150 m). This asymmetry is attributed to the Coriolis effect affecting the turbidity currents, which reach velocities of 6–8.5 m/s and deposit silt and clay over

756-473: The adjacent Venezuela , Guyana and Suriname continental margins. Simple numerical modelling has been enabled to determine turbidity current flow characteristics across the sediment waves to be estimated: internal Froude number = 0.7–1.1, flow thickness = 24–645 m, and flow velocity = 31–82 cm·s . Generally, on lower gradients beyond minor breaks of slope, flow thickness increases and flow velocity decreases, leading to an increase in wavelength and

798-405: The cabled observatory which provided direct observations, which is rarely achieved. 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

840-535: 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). 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. A buoyant sediment-laden river plume can induce

882-439: The channel. The levee is absent in some parts of the NAMOC, for example between 56°N and 57°N, due to the local side-flows of sand. The meandering of the NAMOC is relatively small compared to other underwater channels, such as Amazon Canyon . It is more developed in the northern part, with a period increasing from 25 km between 59°45'N and 56°N to 50 km between 56°N and 54°30'N. The channel becomes on average more straight towards

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924-401: 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

966-606: 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

1008-445: The current lifts off, the point of lift-off remaining constant for a constant discharge. The lofted fluid carries fine sediment with it, forming a plume that rises to a level of neutral buoyancy (if in a stratified environment) or to the water surface, and spreads out. Sediment falling from the plume produces a widespread fall-out deposit, termed hemiturbidite. Experimental turbidity currents and field observations suggest that

1050-522: 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

1092-517: 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 . Seafloor turbidity currents are often the result of sediment-laden river outflows, and can sometimes be initiated by earthquakes , slumping and other soil disturbances. They are characterized by

1134-553: 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

1176-415: The models rely upon simplifying assumptions in order to achieve a result. The accuracy of the individual models thus depends upon the validity and choice of the assumptions made. Experimental results provide a means of constraining some of these variables as well as providing a test for such models. Physical data from field observations, or more practical from experiments, are still required in order to test

1218-433: The next as they evolve downslope. Sediment gravity flows are represented by four different mechanisms of keeping grains within the flow in suspension. Although the deposits of all four types of sediment support mechanisms are found in nature, pure grain flows are largely restricted to aeolian settings, whereas subaqueous environments are characterized by a spectrum of flow types with debris flows and mud flows on one end of

1260-413: The ocean floor can help to decrease the amount of damage to telecommunication cables by avoiding these areas or reinforcing the cables in vulnerable areas. When turbidity currents interact with regular ocean currents, such as contour currents , they can change their direction. This ultimately shifts submarine canyons and sediment deposition locations. One example of this is located in the western part of

1302-430: The ocean, rivers cannot readily form plunging hyperpycnal flows. Hence convective sedimentation is an important possible initiation mechanism for turbidity currents. Large and fast-moving turbidity currents can carve gulleys and ravines into the ocean floor of continental margins and cause damage to artificial structures such as telecommunication cables on the seafloor . Understanding where turbidity currents flow on

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1344-626: The predicted history of turbidity currents in this area was determined, increasing the overall understanding of these currents. Some of the largest antidunes on Earth are formed by turbidity currents. One observed sediment-wave field is located on the lower continental slope off Guyana , South America. This sediment-wave field covers an area of at least 29 000 km at a water depth of 4400–4825 meters. These antidunes have wavelengths of 110–2600 m and wave heights of 1–15 m. Turbidity currents responsible for wave generation are interpreted as originating from slope failures on

1386-416: 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. In the most typical case of oceanic turbidity currents, sediment laden waters situated over sloping ground will flow down-hill because they have a higher density than the adjacent waters. The driving force behind a turbidity current

1428-410: The sea floor. The displacement of continental crust in the form of fluidization and physical shaking both contribute to their formation. Earthquakes have been linked to turbidity current deposition in many settings, particularly where physiography favors preservation of the deposits and limits the other sources of turbidity current deposition. Since the famous case of breakage of submarine cables by

1470-485: The shape of the lobe deposit formed by a lofting plume is narrower than for a similar non-lofting plume 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

1512-409: The simplifying assumptions necessary in mathematical models . Most of what is known about large natural turbidity currents (i.e. those significant in terms of sediment transfer to the deep sea) is inferred from indirect sources, such as submarine cable breaks and heights of deposits above submarine valley floors. Although during the 2003 Tokachi-oki earthquake a large turbidity current was observed by

1554-582: 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 continuum particle concentration field. 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

1596-531: The south, but it still contains abrupt turns due to local seamounts sea bed fractures. Turbidity current 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

1638-720: 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. The deposits of these transitional flows are referred to by a variety of names, some of the more popular being "hybrid-event beds (HEB)", linked debrites" and "slurry beds". Powder snow avalanches and glowing avalanches (gas-charged flows of super heated volcanic ash) are examples of turbidity currents in non-marine settings. Modern and ancient (outcrop) examples of deposits resulting from different types of sediment gravity flows. Sediment gravity flows, primarily turbidity currents, but to

1680-465: 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

1722-407: The velocity of the flow increases, turbulence increases, and the current draws up more sediment. The increase in sediment also adds to the density of the current, and thus increases its velocity even further. Turbidity currents are traditionally defined as those sediment gravity flows in which sediment is suspended by fluid turbulence. However, the term "turbidity current" was adopted to describe

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1764-410: 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. 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

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