Constraining the efficiency of turbidity current generation from submarine debris flows and slides using laboratory experiments

Results from a small set of laboratory experiments are presented here that help further constrain the processes governing the production of turbidity currents from impulsive failures of continental shelf and slope deposits. Three mechanisms by which sediment can be transferred from a parent debris flow to a less-dense turbidity current were observed and quantified. These mechanisms are grain-by-grain erosion of sediment from the leading edge of the parent flow, detachment of thin layers of shearing material from the head of the parent flow, and turbulent mixing at the head of the parent flow. Which transfer process dominates an experimental run depends on whether the large dynamic stresses focused on the head of the debris flow are sufficient to overcome a effective yield strength for the parent sediment+water mixture and on whether the dynamic stresses are sufficient to induce the turbulent flow of the parent mixture. Analysis of data from Marr et al. [Geol. Soc. Am. Bull. 113 (2001) 1377] and Mohrig et al. [Geol. Soc. Am. Bull. 110 (1998) 387] support the use of a shear strength to dynamic stress ratio in constraining necessary critical values for occurrence of the different production mechanisms. Direct sampling of turbidity currents using racks of vertically stacked siphons was used to measure both the quantity of sediment eroded from the heads of non-mixing parent flows and the distribution of particle sizes transported by the developing turbidity currents. Acoustic backscatter imaging was used to better resolve the internal boundary separating any turbulent mixing zone near the front of a flow from unmodified parent material.     

Dual-frequency ADCPs measuring turbidity

A pair of self-contained acoustic Doppler current profilers (SC-ADCPs) operating with different frequencies were moored on a muddy sea bottom at about 20 m depth in the Bay of Vilaine off the French Atlantic coast. With their acoustic beams oriented upwards, the SC-ADCPs ensonified most of the water column. The results of several months of in situ recorded echo intensity data spanning 2 years (2003 to 2004) from the dual-frequency ADCPs are presented in this paper. The aim was to estimate suspended particle mass concentration and mean size. A concentration index CI is proposed for the estimation of particle concentration. Based on theory the CI—unlike the volume backscatter strength—does not depend on particle size. Compared with in situ optical data, the CI shows reasonable precision but not increased with respect to that of the highest-frequency backscatter strength. Concerning the mean particle size, despite a lack of quantitative validation with optical particle-size measurements, the method yielded a qualitative discrimination of mineral (small) and organic (large) particles. This supports the potential of dual-frequency ADCPs to quantitatively determine particle size. A cross-calibration of the transducers of each ADCP shows that a specific component of the precision of the backscatter strength measured by ADCP depends on the acoustic frequency, the cell thickness and the ensemble integration time. Based on these results, the use of two ADCPs operating with distinctly different frequencies (two octaves apart) or a single dual-frequency ADCP is recommended.     

Field expressions of the transformation of debris flows into turbidity currents, with examples from the Polish Carpathians and the French Maritime Alps

Various transformation mechanisms can generate turbidity currents from subaqueous debris flows. Different transformation mechanisms have been described and interpreted in the past from laboratory experiments and from deposits, but the two approaches have not generally been linked. This has made the genetic interpretation and comparison of deposits difficult. In this paper a generic classification scheme of debrite–turbidite couplets is proposed based on transformation mechanisms inferred from laboratory experiments. Five different flow types (called A–E herein) and their resulting deposits are detailed, but they are all part of a continuous spectrum, and a mixture of types is likely to be found in the field. Type A flows are strong, dense debris flows that undergo little transformation. Their deposit will be a debrite overlain by a thin turbidite, which is separated from it by a clear grain size break. Type B flows are weaker and can develop waves at the debris flow-turbidity current interface. The deposit will be a debrite with a wavy top overlain by a turbidite that is thicker than for type A flows. For type C flows, the interfacial waves will grow so much that the debris flow disintegrates into separate parts. The deposit will consist of debrite lenses encased in a turbidite. Type D flows will undergo even more mixing than type C flows so that the debrite parts will be mixed. Their deposit will be a turbidite with laterally varying areas of debrite characteristics near the bed. Type E flows will be so transformed that the debris flow character has disappeared and the flow is a turbidity current with high sediment concentration. The deposit will be largely turbiditic. The flow types and deposits will be illustrated with some examples from two field areas: the Polish Carpathians and the French Maritime Alps.     

A turbidity survey of Narragansett Bay

A turbidity survey of Narragansett Bay, Rhode Island, was made during the summer months of 1971 and included measurements of the attenuation function for scalar irradiance for daylight and the volume attenuation function for white tungsten light at various depths. One hundred and three stations were made at 17 different locations. Variations in the optical parameters were large, one standard deviation at any given location ranging from 7 to 23 per cent of the mean value. This variation was only slightly dependent on the state of the tidal currents, depth of the location, or weather factors. The magnitude of turbidity variations was almost 4-fold over a north-south range of 31 km within the estuary, with clearest water at the southern mouths of the Bay. A good correlation exists between turbidity parameters and Autumn values of suspended-material concentration found by Morton (1967), with both data sets showing highest turbidity and suspended concentrations in the West Passage of the Bay. “Wedges” and “bulges” of clear water were detected throughout the Bay but were most evident at the southern (Atlantic Ocean) end.Although it was not possible to fully define the parameters producing these temporal and geographic variations in estuarine turbidity, it is suggested that knowledge of these parameters can assist those concerned with the physical and biological state of an estuary, as well as divers and photographers plying their trades within its boundaries.     

High-frequency turbidity currents in British Columbia fjords

The frequency of turbidity currents in Bute Inlet and Knight Inlet (British Columbia, Canada) was monitored. A prototype instrument (turbidity event detector) was deployed adjacent to prominent incised sea-floor channels. Approximately 25–30 turbidity currents occur annually. They appear closely correlated to periods of higher river discharge into the heads of the fjords. Two peaks in both discharge and turbidity current fequency occur, one in response to snow melt in late June–early July, the other to glacier melt in August. Virtually no turbidity currents were observed in winter. River mouth bars, channel deposits, and other deltaic sediments build up during lower discharge periods and are swept onto the steep delta front and into subaqueous channels, along with bedload, during floods.     

Scaling analysis of deposition from turbidity currents

Many oil-bearing sedimentary deposits are formed by the settling of particles from turbidity currents. Modeling sedimentary processes that form these turbidites enables the calculation of properties such as extent, depth, porosity and permeability of hydrocarbon-bearing reservoirs. This paper estimates the extent and thickness of turbidites from the initial conditions of the turbidity flow. This is achieved by the application of scaling analysis of the partial differential equations that govern the dynamics of and deposition from turbidity currents. We apply the results of scaling analysis to five modern submarine fans. The predicted and actual values of the dimensions of the fan deposits match well. We then compare the derived results against tabulated sizes of ancient turbidites. The comparisons are good as long as we correctly identify the flow regimes in which the deposition took place. The good agreements observed in the two cases show that the estimates obtained using scaling analysis can provide useful first-guess values for the dimensions of the deposits.     

Reconstruction of turbidity currents in Amazon Channel

Quantifying the characteristics of the turbidity currents that are responsible for the erosion, lateral migration and filling of submarine channels maybe useful for predicting the distribution of lithofacies in channel fill and levee reservoirs. This paper uses data from a well-studied submarine channel in Amazon Fan in an attempt to reconstruct the velocity, thickness, concentration, duration, recurrence rates and vertical structure of turbidity currents in this long sinuous channel. Estimates of flow conditions are derived from the morphology of the channels and the characteristics of the deposits within them. In particular, the availability of information on the sediment distribution with respect to the channel topography at the time of deposition allows for insights into the vertical structure of the flow, a key property that has been so far poorly understood. Integration of flow constraints from well and seismic data or from detailed analysis of outcrop with numerical flow models is a critical step toward a complete understanding of the flow and associated deposits. Turbidity currents in sinuous submarine channels, exemplified by Amazon Channel, are found to last for tens of hours and occur on a regular, quasi-annual basis. Model results suggest that these flows had, on average, velocities ranging from 2 to 4 m/s in the canyon/upper fan which decreased to 0.5–1 m/s in the lower fan, travelling in excess of 800 km. The model turbidity currents were subcritical over most of the channel length, indicating a low degree of water entrainment and low rate of deceleration down the channel. The formation of such long, sinuous channels is intrinsically associated with frequent, long-duration, subcritical turbidity currents carrying a silt-dominated sediment load.     

Experiments on non-channelized turbidity currents and their deposits

Large-scale experiments on non-channelized turbidity currents show that a wide flow opening angle forms and a rapid dilution of the current with distance takes place. The thickness of the deposit decreases radially away from the source, resulting in tongue-shaped isopachs. The mean grain diameter also decreases with distance while the sorting improves. With increasing distance, the following succession of bedforms was observed: (Non-deposition) → parallel lamination → ripples → parallel lamination. This corresponds to the B, C and D division of the Bouma sequence for turbidites. The experiments are in good agreement with the models presented by Bouma, Mutti and Ricci Lucchi, and Walker for classical turbidites in the depositional lobes of submarine fans.     

A coupled debris flow–turbidity current model

In the last decade, offshore pipeline engineering extended its action field to very deep waters and continental slopes. This implied the necessity to deal with continental slope instability and mass gravity flows. Mass gravity flows are rare and have random occurrence; therefore, considering also the technical difficulties, the direct measurement of the phenomena is practically impossible. This has encouraged the development of physical and numerical models for investigating the characteristics and intensity of the phenomena (Proc. OTC Conf., Houston, TX (2000); Proc. 19th OMAE Conference, New Orleans, LA (2000)). In order to provide design activities with reliable predictive tools, two numerical models, one for debris flows and the other for turbidity currents, have been developed. The two models are coupled by the bottom boundary conditions of the turbidity current model that depends on the instantaneous velocity of the debris flow model. The two models used together provide a tool for the evaluation of a mass gravity flow event starting as a debris flow and evolving into a turbidity current.     

Seasonal variation of bottom turbidity layer in Etauchi Bay

The beam attenuation coefficient, organic carbon (POC) and organic nitrogen (PON) contents of suspended materials in Etauchi Bay, which has little inflow of river water as well as very weak tidal current (maximum speed: 6.5cm·sec⁻¹), were measured as a function of depth for all seasons to understand a seasonal variation of bottom turbidity layer. In spring and summer, the beam attenuation coefficient in bottom layer and POC and PON contents of suspended materials in the surface water layer increased with time, which brought the occurrence of the bottom turbidity layer. From autumn to winter, however, their concentrations became low and constant over the whole depth almost independent of time. As a result, the bottom turbidity layer disappeared in winter and beam attenuation coefficient became constant over the whole depth. From these results, it may be considered that the bottom turbidity layer was produced by phytodetritus brought from surface water layer, rather than by resuspension of bottom sediment in Etauchi Bay.     

Double-diffusive density flows

In a hydrostatically stable binary mixture (e.g., stably stratified and horizontally density-homogeneous salt seawater), horizontal density gradients can arise due to the difference between the diffusivities of two substances. The horizontal gradients of hydrostatic pressure that are formed in this case lead to the generation of density flows which appear to have been previously ignored. Examples of such flows arising in a fluid with initially horizontally uniform density and pressure distributions are analyzed. Density flows can also arise due to different boundary conditions for two substances.     

Near bottom variations of turbidity in the St Lawrence estuary

Observations of the turbidity and velocity fields in the near-bottom waters of the St Lawrence estuary were obtained with a package which includes a self-recording attenuance meter and a currentmeter. The latter also measures salinity and temperature. Time series varying in length between 26 h and 26 days, and with repetition rates between one and 15 min are discussed for 3 typical open-channel and nearshore stations. A high-frequency sampling mode provides a means to observe the passage of a frontal disturbance over the bottom during the semi-diurnal cycle. With lower frequency records having lengths of one week to one month, contributions to the turbidity fluctuations due to the spring-neap oscillations, seasonal changes in run-off, and the sudden rise in solid discharge of local tributaries following storms, can be resolved. From turbidity polar diagrams, local onshore sources of particulate suspended matter can be identified. Among other advantages, it is possible from such records to time precisely the occurrence of turbidity peaks in relation to the ebb and flow velocities, to assess the importance of resuspension, and to specify exactly the time rate of change of the turbidity. On the whole, self-recording equipments provide a wealth of information unavailable from more traditional hydrocast sampling techniques.     

A model of turbidity maximum maintenance in the Irish Sea

A two-dimensional model has been developed in order to improve understanding of the processes which interact to maintain the sediment concentrations at an isolated turbidity maximum in the Irish Sea throughout the year. The model comprises two interchangeable populations of particles with different diameters, one of fine cohesive material, the other made up of flocs. Both populations are slow settling, and subject to horizontal diffusion, resuspension, settling, aggregation and disaggregation.The equations used to describe the processes of aggregation and the break-up of flocs in response to sediment concentration and turbulent shear have been developed by the tuning of the model to observations. Due to high turbulent shear at the turbidity maximum, the particles are predominantly fine, while the sediment in the surrounding water is made up of larger flocs. Diffusion of small particles out of the turbidity maximum balanced by the diffusion of aggregated material towards it provides a mechanism for its maintenance. The modelled sediment concentrations at the turbidity maximum can be reproduced year-on-year, with no loss due to diffusive processes. This is achievable with a limited, exhaustible source of material which is not replenished once resuspended.Through comparison with satellite imagery the correlation of the modelled sediment concentrations with observations is investigated, both spatially and temporally. The seasonal cycle is reproduced well by the model with winter and summer concentrations matching those observed. Spatially the model also performs well in both turbid regions and those where the surface sediment load is low.     

Laboratory testing of mathematical models for high-concentration fluid mud turbidity currents

The propagation characteristics of fluid mud turbidity currents were investigated experimentally and theoretically. Parameterizations for propagation phase transition times from slumping to self-similar and self-similar to viscous phases are proposed. Predictive capabilities of different mathematical models that fall into three different modeling approaches (force-balance, box, shallow water) were evaluated for fluid mud turbidity current propagation using our experimental observations. For the slumping and self-similar phases, the box and force-balance models showed superior predictive capabilities than the one-layer shallow water models with deep ambient condition. Fluid mud turbidity currents have a non-Newtonian rheology and their transition and propagation characteristics in the viscous phase differ vastly from the Newtonian currents. We derived and presented a viscous force-balance expression for the propagation of a non-Newtonian power-law fluid current. We then experimentally evaluated the predictive capability of this force-balance and the viscous shallow water model by Di Federico et al. (2006). Both models' predictions are observed to be in notably good agreement with the experimental data. The results of this study are expected to be useful for preliminary swift calculations of the fluid mud turbidity current propagation characteristics as well as in deciding whether more detailed calculations at the expense of complexity are required.     

Transverse infilling of the central Aleutian Trench by unconfined turbidity currents

Holocene sand layers cored from the central Aleutian Trench are dominated by volcaniclastic debris, and the only likely source is the central Aleutian volcanic arc. This creates something of an enigma because bathymetric obstructions seemingly prevent direct delivery of sediment via transverse canyons or channels. Turbidity currents are funneled through submarine canyons on the upper trench slope, but the flows become unconfined as they cross the midslope Aleutian Terrace. Evidently, the turbid flows maintain high enough velocities to climb over the trench-slope break; acceleration down the lower trench slope then allows forearc bypassing to occur without the aid of through-going channels.     

Welded slump-graded sand couplets: evidence for slide generated turbidity currents

Some massive channelized strata preserved in the rock record are characterized by a lower slump member which evolves upward to a turbidite. This merging is indicative of probable generation of sediment gravity flows from submarine sliding. Conditions essential for deposition of such sequences are short transport distance between point of failure and depositional site, and an environment likely to retain both facies. Fan valleys are a likely setting for welded couplets: flowing sand, initiated by the sliding event, comes to rest at nearly the same time and position as the slump mass deposited near the base of the valley wall and in the axis proper.     

Light regime and components of turbidity in a Mediterranean coastal lagoon

The underwater light regime of a Mediterranean coastal lagoon (Albufera des Grau, Balearic Islands) was studied during four years in order to characterise the spatial and temporal variations in the light attenuation coefficient (K) and to assess the relative contribution of the different water components to total light attenuation.     

海底峡谷内浊流流动与沉积特征数值模拟研究

数值模拟已成为研究海底浊流的重要方式,开展海底浊流的流动与沉积特征的数值模拟,对深水沉积体系特征研究、海底工程设施稳定性评价以及深海油气勘探均具有重要意义。本文基于不可压缩流体Navier-Stokes方程与湍流k-ε模型构建了浊流数值计算模型,设定不同粒径、速度与悬浮颗粒物浓度等初始条件,模拟并分析了单粒径悬浮颗粒驱动的持续入流的海底浊流沿海底连续坡折带的流动过程及沉积特征。模拟结果显示,浊流在斜坡段处于加速状态,水平段流速骤减并逐渐沉积,且浊流在小坡度的加速不改变浊流的沉积趋势。浊流由于环境水体的夹带效应逐渐增厚,且浊流头部形态与流动特征与实测资料吻合良好。本文另对多频次持续入流浊流进行了模拟,并将模拟结果与实测地层沉积特征对比,结果显示,多频次持续入流的海底浊流在纵向上可能会形成多个不连续鲍玛层序的叠积。     

Characteristics and generation mechanism of turbidity maximum in the Changjiang Estuary

INTRODUCTIONItiswellknownthattheregenerallyexistsahigherconcentrationsedimentzoneinrivermouththanthatinthe'upperandlowerones,whichiscalledturbiditymaximum(TM).ButnomoreintensiveresearchonthisphenomenonhadbeenconductedforalongtimesinceitwasfoundinFrenchGirondeEstuaryin1938.Itwasnotuntilthe1950swhenitattractedresearchersallovertheworldandwasregardedasacommonphenomenoninanestuary.Itplayedanimportantroleinestuarinesedimentationandenvironment.Sincethe1960s,lotsofstudiesonTMintheChangjiangE…