Behavior Of Turbidity Currents Research Articles

From flood to turbidity current: combined models to simulate continent to ocean sediment transport in the Var system, France

Turbidity currents are a form of gravity-driven flow that occurs in subaqueous environments. These currents contain a low volumetric percentage of particles but are very important in transporting them from coastal to deep-marine environments. In the Var sediment routing system, southeastern France, there is a sedimentary record of turbidity currents formed by submarine landslides and flooding of the Var River. In 2006, there was continuous monitoring of the marine section that recorded four turbidity currents. To explore the connectivity between the terrestrial source and marine basin we linked two reduced complexity models of sediment transport and landscape change. The aim of the paper is to combine two models, one dedicated to the prediction of river water and sediment run-off, and the second dedicated to the transport, erosion, and sedimentation by a turbidity current. The landscape evolution model CAESAR-Lisflood was used to model discharge and sediment yield. The behaviour of turbidity currents was modelled with the cellular automata model CATS. From calibration of the two models against observations of discharge and suspended sediment in the Var River system, we find that in 2006, two rainfall events would have led to hyperpycnal turbidity currents. These two events match well with two of the four-recorded events. Focusing on the largest event we find that the source pulse of sediment from the terrestrial environment is short-lived, less than half a day, but contains a significant quantity of fine particles. The event transferred on the order of 105 m3 of suspended sediment from source to sink. This study demonstrates that hyperpycnal turbidity currents can be generated with concentrations as low as 2-6 kg/m3 at the Var River mouth far below the theoretical 40 kg/m3 threshold, suggesting active convective sedimentation in the surface plume and sea-water dilution at the flooding river mouth. A substantial amount of sediment (35% of the input volume) is directly transferred towards the deepest part of the system during a short-living hyperpycnal turbidity current. However, a considerable (65%) part remains in the surface as a hypopycnal plume feeding the hemipelagic sedimentation.

Detailed Seafloor Imagery of Turbidity Current Bedforms Reveals New Insight Into Fine‐Scale Near‐Bed Processes

AbstractHigh‐resolution imagery of the morphological character of modern active turbidite systems are critical for understanding the complexity of turbidity current processes occurring on the seafloor. Here, we describe a 30 cm‐resolution autonomous underwater vehicle repeat bathymetric dataset that allows to significantly increase the geomorphological detail of crescentic bedforms in turbidite systems. This repeat imagery shows the erosion produced by dense basal layers at the base of turbidity currents, the inception of plunge pools, and the controls that hydraulic jump troughs have on subsequent flow path of weaker turbidity currents. Transverse small‐scale scours located in cyclic step troughs suggest that weak flows follow local relief, therefore explaining the wide variability of turbidity current flow indicators observed in outcrops. This imagery demonstrates that turbidite systems are formed by the superimposition of erosion surfaces and depositional patterns recording contrasted flow behavior and provides new views on turbidity current behavior in natural environments.

Geomorphology of a bended submarine canyon in Wanhu Seamount region, northern South China Sea: Insights from manned submersible observation and measurement

Submarine canyons can transport large volumes of terrigenous material from the continent to the deep-sea environment and are the primary conduits of source-to-sink systems. The large-scale morphologic features of canyons have been well studied using seismic reflection and the bathymetry data. However, detailed analyses of the small-scale elements of canyon morphology are relatively few because of the limited resolution of traditional field investigations. In this paper, the multi-scaled geomorphology of a bended submarine canyon (canyon C4) located in Wanhu Seamount region (northern South China Sea) is described and discussed based on the in-situ high-definition video data, high-precision near-bottom acoustic data, and push-cores obtained using the manned submersible vehicle. The NNW-trending submarine canyon diverted to 122.5° at the foot of Wanhu Seamount, and the fresh erosional surfaces were recorded using the video data at the canyon axis bend. A total of eight sediment waves were generated at the overbank of the bending region. These waves contain wavelengths and wave heights decreasing in an upslope direction from the canyon axis towards the Wanhu Seamount. The C-M pattern (coarsest one percentile grain size versus median grain size values), grain cumulative probability curves, and grain size gradations derived from the sediment cores suggested that these sediment waves were induced by a turbidity current. The bending of the canyon resulted from the deflection and reflection of the turbidity currents, which were constrained by Wanhu Seamount. The abnormal geomorphologies of some sediment waves were related to the back-squeezed underflow of the turbidity currents. The turbidity current behavior was compared and found to be in agreement with the experimental and modeling simulation results mentioned in the literature. However, the morphologic setting in the natural case was more complex than that observed from the experiments, with the flow behaviors being more complicated. Future studies are required for more detailed comparison between the natural case and simulations to better understand the turbidity current behavior in a bended submarine canyon.

Effect of an obstacle on the depositional behaviour of turbidity currents

ABSTRACTTurbidity currents are responsible for much of the sedimentation in reservoirs. In order to control these flows, various methods such as placing an obstacle in their path have been proposed. In this study, the effect of inlet sediment concentration and obstacle height on the behaviour of turbidity currents is investigated experimentally. For this purpose, some experiments were carried out with different inlet concentrations and various obstacle heights. Velocity and concentration profiles were measured using an acoustic Doppler velocimeter. To examine the depositional behaviour of turbidity current, suspended sediment flux was calculated using velocity and concentration profiles of the current. The results indicate that increasing the obstacle height reduces the sediment flux upstream of the obstacle, and therefore more sedimentation occurs in the region. At low inlet concentrations, it was shown that the sediment flux is approximately constant along the channel and is independent of the obstacle height.

Disconnected submarine lobes as a record of stepped slope evolution over multiple sea-level cycles

The effects of abrupt changes in slope angle and orientation on turbidity current behavior have been investigated in numerous physical and numerical experiments and examined in outcrop, subsurface, and modern systems. However, the long-term impact of subtle and evolving seabed topography on the stratigraphic architecture of deep-water systems requires fine-scale observations and extensive 3-D constraints. This study focuses on the Permian Laingsburg and Fort Brown formations, where multiple large sand-rich systems (Units A–F) have been mapped from entrenched slope valleys, through channel-levee systems, to basin-floor lobe complexes over a 2500 km2 area. Here, we investigate three thinner (typically <5 m in thickness) and less extensive sand-rich packages, Units A/B, B/C, and D/E, between the large-scale systems. Typically, these sand-rich units are sharp-based and topped, and contain scours and mudstone clast conglomerates that indicate deposition from high-energy turbidity currents. The mapped thickness and facies distribution suggest a lobate form. These distinctive units were deposited in similar spatial positions within the basin-fill and suggest similar accommodation patterns on the slope and basin floor prior to the larger systems (B, C, and E). Stratigraphically, these thin units represent the first sand deposition following ­major periods of shut-down in sediment supply, and are interpreted as marking a partial re-establishment of sand delivery pathways creating “disconnected lobes” that are fed mainly by flows sourced from failures on the shelf and upper slope rather than major feeder channel-levee systems. Thickness and facies patterns throughout the deep-water stratigraphy suggest seabed topography was present early in the basin formation and maintained persistently in a similar area to ultimately form a stepped slope profile. The stepped slope profile evolved through three key stages of development: Phase 1, where sediment supply exceeds deformation rate (likely caused by differential subsidence); Phase 2, where sediment supply is on average equal to deformation rate; and Phase 3, where deformation rate outpaces sediment supply. This study demonstrates that smaller systems are a sensitive record of evolving seabed topography and they can consequently be used to recreate more accurate paleotopographic profiles.

A new model for turbidity current behavior based on integration of flow monitoring and precision coring in a submarine canyon

Abstract Submarine turbidity currents create some of the largest sediment accumulations on Earth, yet there are few direct measurements of these flows. Instead, most of our understanding of turbidity currents results from analyzing their deposits in the sedimentary record. However, the lack of direct flow measurements means that there is considerable debate regarding how to interpret flow properties from ancient deposits. This novel study combines detailed flow monitoring with unusually precisely located cores at different heights, and multiple locations, within the Monterey submarine canyon, offshore California, USA. Dating demonstrates that the cores include the time interval that flows were monitored in the canyon, albeit individual layers cannot be tied to specific flows. There is good correlation between grain sizes collected by traps within the flow and grain sizes measured in cores from similar heights on the canyon walls. Synthesis of flow and deposit data suggests that turbidity currents sourced from the upper reaches of Monterey Canyon comprise three flow phases. Initially, a thin (38–50 m) powerful flow in the upper canyon can transport, tilt, and break the most proximal moorings and deposit chaotic sands and gravel on the canyon floor. The initially thin flow front then thickens and deposits interbedded sands and silty muds on the canyon walls as much as 62 m above the canyon floor. Finally, the flow thickens along its length, thus lofting silty mud and depositing it at greater altitudes than the previous deposits and in excess of 70 m altitude.

Long-range sediment transport in the world's oceans by stably stratified turbidity currents

Submarine fans, supplied primarily by turbidity currents, constitute the largest sediment accumulations on Earth. Generally accepted models of turbidity current behavior imply they should dissipate rapidly on the very small gradients of submarine fans, thus their persistence over long distances is enigmatic. We present numerical evidence, constrained by published field data, suggesting that turbidity currents traveling on low slopes and carrying fine particles have a stably stratified shear layer along their upper interface, which dramatically reduces dissipation and entrainment of ambient fluid, allowing the current to propagate over long distances. We propose gradient Richardson number as a useful criterion to discriminate between the different behaviors exhibited by turbidity currents on high and low slopes. This article is protected by copyright. All rights reserved.

Experimental Investigation of the Effect of Inlet Concentration on the Behavior of Turbidity Currents in the Presence of Two Consecutive Obstacles

Turbidity currents are often the main processes of sediment transport in deep waters and reservoirs. To prevent sedimentation in critical locations, various methods, such as placing solid obstacles in the path of these flows, have been proposed. In the present study, the effect of inlet concentration on the behavior of turbidity currents in the presence of two consecutive triangular obstacles was investigated experimentally. For this purpose, a series of laboratory experiments were conducted with various inlet concentrations. In each experiment, velocity and concentration profiles were measured using an acoustic Doppler velocimeter. The velocity of the current head and local Froude number were also determined along the test channel. The results show that a region with negligible velocity and a notable concentration develops upstream of each obstacle, and as the inlet concentration increases, this region becomes larger. In addition, the results also indicate that the sediment deposition rate is higher upstream of the first obstacle compared to that of the second obstacle. Furthermore, observations show that three recirculation regions are developed between the obstacles.

The Influence of A Lateral Slope On Turbidite Lobe Development On A Modern Deep-Sea Slope Fan (Villafranca Deep-Sea Fan, Tyrrhenian Sea)

Abstract In deep-sea fans, fan lobes form from the stacking of the deposits of turbidity currents and other sediment gravity flows beyond the channel mouth. Slopes lateral or oblique to the main direction of turbidity currents exiting channel confinement can profoundly affect flow behavior and the distribution of facies and lithology in fan lobes. In this paper, we report on the analysis of multibeam and CHIRP subbottom data aimed at the study of the effects of a lateral slope on turbidity-current behavior and resultant geomorphology and stratigraphy of a modern fan lobe. The Villafranca fan lobe, the subject of this study, develops on the 0.5° dipping seafloor of the Gioia intraslope basin in the southeastern Tyrrhenian Sea. The Villafranca is a transient fan lobe that has its downslope limit coinciding with a 200-m-high bathymetric step formed by the erosional flank of the Stromboli slope valley, perpendicular to the fan-lobe trend. A “lobe complex” hierarchical level is assigned to the Villafranca fan lobe. The lobe complex develops beyond the mouth of the Villafranca leveed channel, which trends N and is oblique to the regional NW-dipping slope. The depositional topography created by an adjacent channel levee wedge to the west is the cause of the observed obliquity. The western and the eastern “lobes” constitute the Villafranca lobe complex. Both lobes have mainly NW-trending channels, and therefore they do not conform to the classical divergent pattern of channels observed in deep-sea fan lobes. The channel trend is a result of the inability of flows to freely spread sideways, due to the presence of the lateral slope and the dip of the seafloor at the channel mouth. The western lobe has an up-dip apex with respect to the eastern one and is characterized by channels that nucleate on the downslope side of a channel-mouth bar and that die out distally before reaching the flank of the Stromboli slope valley. The eastern lobe, in contrast, has channels that are depositional upslope but on approaching the flank of Stromboli slope valley become mainly erosional or bypass features. The two lobes develop concomitantly, therefore an evolution due to compensational stacking with the two lobes being in different stages of development is here disregarded. It is therefore feasible that turbidity currents, which due to different initiating mechanisms have distinct magnitude and efficiency, deposit their load either in the eastern or in the western lobe. Another possibility is that single turbidity currents exiting the Villafranca channel mouth form two separate flow portions with different properties, which independently but simultaneously feed the eastern and the western lobe. In both cases, the effect of the lateral slope and the obliquity between the dip of the slope and the trend of the channel mouth are here considered as the primary factors controlling the differences in depositional style of the two lobes.

Three-dimensional paleomorphologic reconstruction and turbidite distribution prediction revealing a Pleistocene confined basin system in the northeast Nankai Trough area

Integrated three-dimensional (3-D) paleomorphologic and sedimentary modeling was used to predict the basin architecture and depositional pattern of Pleistocene forearc basin turbidites in a gas hydrate field along the northeast Nankai Trough, off central Japan. Structural unfolding and stratigraphic decompaction of the targeted stratigraphic unit resulted in successful modeling of the paleobathymetry at the time of deposition. This paleobathymetry was characterized by a simple U-shaped paleominibasin. Subsequent turbidity current modeling on the reconstructed paleobathymetric surface demonstrated morphologically controlled turbidity current behavior and selective turbidite sand distribution within the minibasin, which strongly suggests the development of a confined turbidite system. Among three candidate inflow patterns, a northeasterly inflow pattern was determined as most likely. In this scenario, flow reflection and deflection caused ponding and a concentration of sandy turbidite accumulation in the basin center, which facilitated filling of the minibasin. Such a sedimentary character is undetected by seismic data in the studied gas hydrate reservoir formation because of hydrate-cementation–induced seismic anomalies. Our model suggests that 3-D horizon surfaces mapped from 3-D seismic data along with well-log data can be used to predict paleobasin characteristics and depositional processes in deep-water turbidite systems even if seismic profiles cannot be determined because of the presence of gas hydrates.

Experimental investigation of the effect of obstacles on the behavior of turbidity currents

Turbidity current is produced when a particle-laden fluid flows under lighter ambient fresh fluid. The streaming of particle-laden fluid is called a turbidity current and this kind of current is an important mechanism for sediment transportation in lakes and oceans. In the present research, the main concentration is on the effect of obstacle with an isosceles right triangular cross section on the behavior of turbidity current. A series of laboratory experiments were carried out with various obstacle heights and different inlet densimetric Froude numbers. In each experiment, velocity profiles upstream and downstream of the obstacle were measured, using an acoustic Doppler velocimeter. Kaolin was used as the suspended material. Experiments showed that the density current with lower inlet Froude number, reacts to the presence of the obstacle more rapidly compared to the currents with higher values of inlet Froude number. At the upstream section of the obstacle, an increase in the height of the obstacle resulted in a reduction in the current inertia and the maximum velocity. Also the local Froude numbers had an abrupt increase when the dense layer passed the obstacle.

Effect of selected parameters on the depositional behaviour of turbidity currents

Turbidity currents containing kaolin particles were studied experimentally in a channel and the velocity and concentration profiles were measured using an acoustic Doppler velocimeter. These experiments were performed to investigate the depositional behaviour of turbidity currents. The suspended sediment flux was evaluated by experimental and analytical methods and the results of these two methods were in a good agreement. To evaluate the suspended sediment flux, it was necessary to recognize the suspended sediment zone from the upper shear layer region and near the bed depositional area as well. The method of determination of these areas is discussed. The effects of important parameters including the inlet Froude number and bed slope were also studied. Based on these results, it can be concluded that the non-dimensional sediment flux decreases at the vicinity of the hydraulic jump with the inlet Froude number, whereas an increase in the bed slope reduces the non-dimensional sediment flux.

Turbidity current modelling on geological time scales

A numerical model has been developed to simulate sedimentary architecture resulting from turbidity currents over geological time. The model is based on the cellular automata paradigm. The automata exchange matter and energy and are built in order to reproduce the physical processes which govern turbidity current behavior. The simulated architecture is the result of a given set of geological events. For each of these events a steady state is computed that is assumed to be representative of the average effect of a turbidity current on the construction of the sedimentary architecture. Using model capacities, we reproduce the first sequence of the Lower Pab channel complex formation and a unit of lateral stacked channels of the Angolan deep offshore Girassol field. The model results show that the simulated sedimentary facies distribution is realistic and this may allow a better prediction of the deposits organization in turbiditic systems. Moreover, this model improves stratigraphic modelling resolution and can take over diffusive methods at reservoir scale. The advantages of this numerical approach are a better identification and understanding of deposits heterogeneities and may provide a better representation of the spatial organization, geometry and variability of the geological units that form reservoir architecture in deep-sea clastic systems.

The role of physicochemical processes in controlling sediment transport and deposition in turbidity currents

Experimental results are presented that illustrate the role of chemical and electrostatic particle–particle interactions in controlling sediment transport in turbidity currents. These experiments were conducted in a small laboratory channel to allow good control of the background water chemistry. Both kaolinite clay and silica particles were used to generate the turbidity currents, and both sodium and calcium chloride were used to prepare background solutions with a wide range of ionic strengths (salinities). The zeta potential of kaolinite was found to vary considerably under the range of experimental conditions used, and this produced a significant variation in the size distribution of these particles. As a result, kaolinite deposition from the turbidity current increased greatly with background salt concentration. Silica remained highly negatively charged under all experimental conditions, and the behavior of silica turbidity currents did not change with salinity. These results demonstrate that the behavior of turbidity currents is dependent on the composition of the associated sediments and the background water chemistry.

The role of dense brines in the formation of vent-distal sedimentary-exhalative (SEDEX) lead–zinc deposits: field and laboratory evidence

A majority of the world's sediment-hosted exhalative (SEDEX) lead–zinc deposits are vent-distal. They are not underlain by a discordant alteration zone or stockwork vent complex that would indicate the path by which ore fluids reached the seafloor. The absence of a vent complex, together with sulfide mineral replacement of host rock mineral assemblages has led several investigators to suggest that, in spite of the well-layered nature of these deposits, mineralization was formed by sub-seafloor lateral migration of ore fluids along permeable strata. Field observations, supported by simple laboratory experiments, however, suggest an alternative process for characterizing the genesis of vent-distal SEDEX deposits. Cool, saline brines (e.g., ~120 °C and >15 wt% NaCl equiv.) are denser than seawater and, upon discharging into the sea, would flow away from the discharge vent as bottom-hugging fluids, similar to the behavior of turbidity currents. Their high densities and velocities prevent them from mixing with overlying seawater, thereby precluding significant cooling and dilution of the ore fluid. Upon coming to rest in a seafloor depression, the addition of H2S and/or dilution of the ore fluids to lower salinities result in the eventual precipitation of a vent-distal SEDEX deposit. Furthermore, the dense ore-forming fluid can sink into permeable sediments beneath the brine pool by displacing less dense pore water. The ore fluids are thus capable of effectively overprinting and/or replacing pre-existing minerals in the consolidating sediment pile.

INFLO1: A model predicting the behaviour of turbidity currents generated at river mouths

INFLO1 is an ANSI standard FORTRAN-77 predictive sedimentation model that simulates the behaviour of a hyperpycnal inflow. From data on discharge and sediment load at the river mouth as well as the basin geometry, INFLO1 predicts the flow parameters of the resulting turbidity current—flow thickness, velocity, volume and sediment discharge, and interstitial fluid density—and the deposition and erosion of sediment. Sensitivity analyses show that the simulated deposit morphology is most affected by changes in the drag coefficient and that the internal deposit architecture, parameterized by vertical variations in mean grain size, is most sensitive to the form of the flood hydrograph. Model predictions compare favourably with data derived from the 1663 Saguenay event. Given that hyperpycnal inflows are relatively frequent both on recent and geological timescales, INFLO1 allows for the reconstruction of flow parameters for past events yielding valuable paleoclimatic information and the assessment of the potential affects of future hyperpycnal inflows on sedimentation patterns and subaqueous structures.

Hydraulic Jumps in Sediment‐Driven Bottom Currents

Laboratory experiments were conducted to observe the behavior of turbidity currents in the vicinity of a slope transition. Both sediment‐laden and saline hydraulic jumps were produced. The vertical structure of the currents was found to depend on flow regime. The saline and turbid hydraulic jumps showed similar characteristics. The amount of water entrained by the flows through a jump was small. The change in flow regime caused a marked reduction of the bed shear stress downstream of the jump. In nature, a turbidity current experiencing a hydraulic jump will drop most of its bedload immediately downstream from the jump, while the suspended load will respond more gradually to the change in flow regime and will deposit sediment over a distance far exceeding 1,000 times the jump height.

Sediment-laden gravity currents with reversing buoyancy

There are many natural occurrences of sediment-laden gravity currents in which the density of the interstitial fluid is less than that of the ambient fluid, although the bulk density of the current is greater. Such currents are driven by the excess density of suspended particles. However, after sufficient particles have sedimented, the current will become buoyant, cease its lateral motion and ascend to form a plume. Examples of such currents include brackish underflows in deltas, turbidity currents and pyroclastic flows. Experimental studies are described which show that, due to sedimentation, sediment-laden gravity currents decelerate more rapidly than saline currents of the same density. There is little difference in the experiments between a sediment-laden current with neutrally buoyant interstitial fluid and one with buoyant interstial fluid until sufficient sediment has been lost to cause the latter kind of current to lift-off. A marked deceleration is then observed and a plume is generated, with lift-off occurring along the length of the current. The resulting buoyant plume then generates a gravity current below the upper surface of the fluid in the tank. The deposit from a current with buoyant fluid shows a fairly abrupt decrease in thickness beyond the lift-off distance and has a flatter profile than that from a simple sediment current. A theoretical model is presented, which is based on the two-layer shallow-water equations and incorporates a model of the sedimentation in which particles are assumed to be uniformly suspended by the turbulence of the current. The model shows good agreement with the observed lengths of the experimental currents as a function of time and predicts the lift-off distance reasonably well. These processes have implications for the behaviour of turbidity currents, the interpretation of turbidites, mixing processes in the oceans and the lift-off of pyroclastic flows.

Turbidity current simulation in a diverging channel

Spatially varied turbidity currents with erosion and deposition of sediment occurring in slightly diverging‐sloping channels with a rectangular cross section are simulated. The model consists of cross section integrated equations for conservation of volume and mass, x momentum equation, and empirical relationships for water entrainment and sediment entrainment. The continuity equation for the sediment bed is coupled with the equations for the flow in order to simulate the change of bed elevation due to erosion and deposition of turbidity currents. The gradually varied flow equations are solved. The simulations show (1) underflows in diverging channels do not reach a normal condition, (2) three different types of turbidity currents can be simulated, (3) turbidity current behavior is sensitive to particle size, water viscosity, channel slope, as well as channel diverging angle when channel aspect ratio is small.

Experiments on turbidity currents over an erodible bed

Experiments were conducted to determine the behaviour of turbidity currents laden with non-cohesive silt (silica flour) moving down a slope the bed of which is covered with similar silt. Each current was sustained with constant inlet conditions for six to eight minutes. The motion of the head was not studied; measurements were concentrated on the continuous part of the current that was essentially constant in time but developing in space. Only supercritical currents were studied. The currents were free to erode sediment from, and deposit sediment on, the bed. Measurements of vertical profiles of downstream velocity and sediment concentration allowed for the development of approximate similarity relations. These relations can be used to estimate most of the shape factors that occur in the vertically-integrated equations of motion. The values do not deviate grossly from those found by using a top-hat function, i.e. a step function, such that velocity or concentration equals its average value in the layer ...