**1. Introduction**

262 Hydrodynamics – Natural Water Bodies

Straub, L.G. & Anderson, A.G. (1958). Experiments on self-aerated flow in open channels. *Journal of Hydraulic Division*, ASCE Proc., v.87, n.HY7, pp. 1890-1-1890-35. Tozzi, M.J. (1992). Caracterização/comportamento de escoamentos em vertedouros com

Wilhelms, S.C. & Gulliver, J.S. (2005). Bubbles and waves description of self-aerated spillway flow. *Journal of Hydraulic Research*, Vol. 43, No.5, pp. 522-531. 2005 Wood, I.R.; Ackers, P. & Loveless, J. (1983). General method for critical point on spillways.

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Thesis – Universidade de São Paulo, São Paulo.

paramento em degraus [Characterization of flow behavior in stepped spillways]. Dr Thesis. University of São Paulo, São Paulo, Brazil, [in Portuguese]. 302 pp. Dr

> Gravity (or density currents) currents are a general class of flows (also known as stratified flows) in which flow takes place because of relatively small differences in density between two flows (Middleton, 1993). Gravity currents that are driven by gravity acting on dispersed sediment in the flow were called *sediment gravity flows* (Middleton & Hampton, 1973). Sediment gravity flows may occur in both subaerial (e.g. avalanches, pyroclastic flows and so on) and subaqueous ambients (e.g. bottom currents, turbidity currents, debris flow – see Simpson, 1997) and may flow above, below or inside the ambient fluid. The distinction regarding sediment gravity flows and open-channel flows is due to the order of magnitude of the density difference between the fluids. Sediment gravity flow are generally of the same order of magnitude, whilst open-channel flow the difference in density between the flow (e.g. rivers) and the ambient air is much higher than that.

> The interest in these types of flows are mainly due to four factors: (i) phenomenon comprehension highlighting the origin, transport and deposition processes; (ii) their great magnitude and unpredictability (potential environmental hazards); (iii) the lack of monitoring these events in nature and; (iv) because of their economic significance, since some deposits generated by such currents are prospective reserves of hydrocarbon.

> Despite the great progress addressing theoretical and analytical evaluation of these phenomena, particularly on the origin, transport and deposition of this class of flow, even today, they are not completely comprehended. Generally, the complexity of the phenomenon can be expressed by: (i) interaction between the flow and the bed morphology; (ii) the quantity and the composition of sediment transported and (iii) the complex mixing processes. As a consequence, the origin and the hydrodynamics properties of these flows are less understood than open-channel flows (Baas et al., 2004). Simple definitions, such as volumetric concentrations of sediments, its composition and size distribution of solid particles in the mixture as well as the sediment-support mechanisms are difficult to measure in nature which is also an indicative of such complexity.

> Kneller & Buckee (2000) commented that difficulties in understanding the dynamics of suspended sediment are extremely complex by virtue of turbulence. In that case, the phenomenon is: non-linear; non-uniform (variation in space) and unsteady (variation in time). If the flow contains large loads of sediments and/or cohesive sediments in suspension this complexity increases even more. Besides the variation of density with time and space (open boundary conditions), the mechanical properties (rheology) of the suspensions

Sediment Gravity Flows: Study Based on Experimental Simulations 265

and composition) is transported ahead by the flow (transfer zone). Concomitantly, dynamical and depositional processes occur along time and space, causing flow transformations, such as: sediment transport, erosion and/or deposition, mixing,

The sediment gravity flows which maintain buoyancy flux throughout movement are called *conservative* (i.e. do not interact with their boundary). Otherwise, flows are called *nonconservative sediment gravity flows* (i.e. open boundary interaction such as erosion and

Generally, gravity currents are divided geometrically into three distinct parts: head, body

Fig. 1. Schematic of a sediment gravity flow (description of all terms is provided in the list of

The *head* or front of the current is roughly shaped as a semielipse. In most cases, the head is thicker than the body and tail, because of the resistance imposed by the ambient fluid (fluid resistance) to its advance. The head plays an important role on flow dynamics because is characterized by strong three-dimensionality effects and intense mixing (Simpson, 1997). The most advanced point of the front is called *nose* and it is located slightly above the bottom surface, as a result of the no-slip condition at the bottom as well as the resistance (shear) at upper surface (Britter & Simpson, 1978). In the head, two types of instabilities are the main responsible for mixing with the ambient fluid (*entrainment*). The first type of instability is a complex pattern of *lobes and clefts* caused by second order gravitational instabilities at front surface (Kneller et al., 1999; Simpson, 1972). The second type of instability is a series of billows associate to Kelvin-Helmholtz instabilities (Britter & Simpson, 1978), which takes place just behind the head and produced by viscous shear at the head and body (upper surface). This zone behind head creates a large-scale turbulence mixing and also divides the head from the body (symbolically called: *neck* of the flow). Generally, the velocity of the *body* is greater than the head velocity by 30% or 40% (Baas et al., 2004; Kneller & Buckee, 2000). One reason for this is the presence of a large billow behind the head which cause a locally diluted zone (entrainment of ambient fluid). Thus, in order to the flow maintain its constant rate of advance, the current increases the velocity of the body to compensate the deficit of density created (Middleton, 1993). The body is divided into two zones: near the bottom zone, where the density is higher; and above this, a suspended/mixing zone, where the mixing with the fluid ambient occurs. The interface

entrainment (Elisson & Turner, 1959) and so on (Fig. 1).

deposition).

nomenclature).

and, tail.

involved (thixotropy, viscosity and gravitational forces) must be taken into account as well as the sediment-support mechanism and the influence of shear stress on the upper layer (Kuenen, 1950). Because of such uncertainty and complexity, many terms, concepts, models and particular descriptions (over than 30) have being introduced and applied to interpret these classes of flows and deposits along the years (e.g. Gani, 2004; Lowe, 1982; Middleton & Hampton, 1973).

Sediment gravity flows can be divided into five broad categories according to Parsons et al., (2010). Each flow type has a range of concentrations, Reynolds numbers, duration, grain size and rheology behaviour, enclosing a general overview of the flows transformation along time and space (Fischer, 1983). Two types of flows have been regularly studied along the last 60 years: turbidity currents and debris flows. Both represent the contrast of the sediment gravity flows categories (not considering mass flows, like slides and slumps - see also Middleton & Hampton, 1973). Succinctly, the main properties attributed and well accepted in the literature to turbidity currents are: diluted (low-density), Newtonian behaviour, turbulent regime, and Bouma sequence type deposit (Bouma, 1962) usually called turbidites. On the other side, debris flows are characterized by great influence of noncohesive material, non-Newtonian behaviour, matrix strength, bipartite and chaotic (ungraded) deposits.

The interest of many fields of academy and industry do not only concern the comprehension of those two particular types of flows. In fact, all classes of sedimentary gravity currents are motivating researchers to face the problem from different approaches and methods, for instance: studies based on outcrops analogy (generally by sedimentologists and correlated areas); numerical and analytical modelling (which is improving through time) and, finally experimental simulation which has been a powerful tool of visualization and measurement of flow dynamics properties as well as of generated deposit.

The scope of this chapter is to outline the experimental study on sediment gravity flows in order to characterize and comprehend this phenomenon regarding their rheological behaviour, hydrodynamics and depositional properties. The simulations covered a wide range of concentration and/or different amount of cohesive sediments in the mixture. The properties of the flow and deposit were evaluated, classified and compared to literature background. The chapter is structured in five sections; first, a general description of sediment gravity flows will be presented followed by the experimental approach applied. Then, the rheology tests it will be reported and finally, the careful evaluation of the experimental results in terms of time-space and vertical profiles will be described in order to extrapolate the results to natural sediment gravity flows.
