Although analogies have been drawn between some types of meandering rivers and medium-to high-sinuosity, aggradational, leveed submarine channels, a number of different or additional processes operate in submarine channels. Analysis of several individual submarine channels suggests that they undergo much slower bend growth than alluvial rivers and may reach a planform equilibrium, in contrast to meandering rivers, in which bends progressively migrate downstream. Sinuous leveed submarine channels should therefore aggrade to produce isolated ribbons of thalweg deposits (of predictable 3D geometry), in contrast to the stacked channel belts characteristic of most alluvial meandering rivers. A simple model of the flow structure and flow evolution of turbidity currents traversing submarine channels is proposed, based on theoretical, experimental, and fieldderived concepts. It predicts that submarine channel flows are highly stratified, have significant supra-levee thicknesses, and form broad overbank bodies of low-concentration fluid moving along the entire channel length. The interaction between the broad body of overbank fluid and within-channel flow is controlled by the processes of towing and angular shear, whose possible effects on channel sedimentation and planform stability are explored.

Summary The literature on the structure and behaviour of gravity currents is reviewed, with emphasis on some recent studies, and with particular attention to turbidity currents, though reference is also made to comparable behaviour in pyroclastic flows. Questions of definition are discussed, in particular the distinction between dense currents, which may deposit en masse, and more dilute currents. High‐density dispersions may exist as a discrete, independently moving layer beneath a more dilute flow, as the basal part of a continuous density distribution or possibly as a transient depositional layer. Existing theory appears inadequate to explain the behaviour of some high‐density dispersions. Surge‐type currents are contrasted with quasi‐steady currents, which may be generated by a variety of mechanisms including direct feed by rivers in flood. Such fluvially generated currents provide one means of generating currents with reversing buoyancy. Geologically significant turbidity currents are impractical for direct study owing to their large scale and (often) destructive nature. Small‐scale laboratory currents offer a wealth of insights into turbidity current behaviour. This paper summarizes recent experimental studies that focus on the physical structure of gravity currents, with emphasis on the velocity and turbulence structure, the vertical density distribution and the stability of stratification. Preliminary quantification of the turbulence structure (including controls on turbulent entrainment, turbulent kinetic energy, Reynolds stresses and turbulence production) has been facilitated by recent technological developments that have allowed the measurement of instantaneous fluctuations in both velocity and concentration. Laboratory models, however, generally involve substantial simplification, and require compromises in some parameters to achieve adequate scaling of the parameters of most interest. Mathematical modelling also provides important insights into turbidity current behaviour. We discuss various approaches to modelling, ranging from simple hydraulic equations to systems of partial differential equations that explicitly treat conservation of momentum, fluid and sediment mass, and turbulent kinetic energy. The application for which the model is designed (i.e. to calculate mean head velocity or to create an instantaneous two‐dimensional contour plot of downstream velocity in a current) determines the complexity of the mathematical model required. The behaviour of suspension currents around topography is complex and depends upon the relative height of the topography, and upon the density and velocity structure of the current. Many interactions with topography are well described by the internal Froude number, Fri. Both reflection and deflection of currents may occur on the upstream side of topography, depending upon Fri. On the downstream side of topography, flow separation, lee waves or hydraulic jumps may occur.

The simple physical models which are popularly used to describe deposition from turbidity currents are based on the notion of unidirectional waning flow, resulting in the familiar Bouma sequence (Ta--e) or its high-density counterpart, the Lowe sequence (S1-3). Most geologists working on turbidite successions know only too well how wide is the range of facies which do not fit into the standard facies models. The application of simple equations of motion shows that deposition can occur beneath flows that are steady or even waxing. The various combinations of different spatial and temporal accelerations produce markedly different vertical and lateral variations in the resulting turbidite. On these grounds alone, we should expect not one but at least five basic types of sequence in turbidite beds, for which likely candidates can be found in many deep water elastic systems. This has important implications for deposit geometry, gradients in reservoir properties, and geological interpretation and correlation of well cores. Since many basins are confined, a prior/reasoning tells us to expect interactions between turbidity currents and topography, producing anomalous intrabed vertical sequences, multiple current directions and locally enhanced deposition. Flume experiments with radially spreading scaled sediment gravity flows demonstrate how the geometry of sandstone deposits may be controlled by topography. Interactions between the flow and topographic features are dependent upon the shape and orientation of the obstacle, and upon its size relative to the height of the flow. Some of the results are counter-intuitive, and may have significant impact on location of reservoir sands around basin-floor and marginal topography.It is invariably the case that there are insufficient geological, geophysical and petrophysical data to create an unique reservoir description or reservoir engineering model. Consequently, geological models of a variety of kinds and scales, whose effectiveness is dependent upon the validity of the assumptions incorporated into them -assumptions which are often themselves models -must be relied upon. Central amongst these assumptions concerning deep water depositional systems are models for transport and deposition of sediment by turbidity currents; these influence the way we think about (and make predictions of) a whole range of sedimentary phenomena, from the character of individual beds to the geometry of turbidite systems and their components. They also affect predictions concerning lateral changes in reservoir-significant rock properties such as grain size, and the way correlation of lithofacies and individual beds is approached. Clearly, the reliability of larger-scale models comes into ques-

The article surveys the current state of our understanding of turbidity currents, with an emphasis on their fluid mechanics. It highlights the significant role these currents play within the global sediment cycle, and their importance in environmental processes and in the formation of hydrocarbon reservoirs. Events and mechanisms governing the initiation of turbidity currents are reviewed, along with experimental observations and findings from field studies regarding their internal velocity and density structure. As turbidity currents propagate over the seafloor, they can trigger the evolution of a host of topographical features through the processes of deposition and erosion, such as channels, levees, and sediment waves. Potential linear instability mechanisms are discussed that may determine the spatial scales of these features. Finally, the hierarchy of available theoretical models for analyzing the dynamics of turbidity currents is outlined, ranging from dimensional analysis and integral models to both depth-averaged and depth-resolving simulation approaches. 135 Annu. Rev. Fluid Mech. 2010.42:135-156. Downloaded from arjournals.annualreviews.org by 68.6.124.193 on 12/23/09. For personal use only.

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