**1.1 Sediment gravity flow anatomy**

In nature, subaqueous sediment gravity flow behaves like a river system, i.e., originating (source zone), flowing (transfer zone) and decelerating up to the point where all suspended sediment settled down (depositional zone). In general, the initiation of sediment gravity flows is strongly related to two processes of sediment remobilization in the natural field: Firstly, by the occurrence of catastrophic events such as earthquakes, sedimentary failures, storms and volcanic eruptions which cause high instabilities and remobilize large amount of sediments instantaneously (Normark & Piper, 1991); Secondly, by continuous river supply in which the river discharge is connected into water body (usually reservoirs, lakes and oceans) generating plumes and/or hyperpycnal flows due the density difference (positive or negative). After the process starts, the mixture of suspended sediment (concentration, size

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 &

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

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

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

In nature, subaqueous sediment gravity flow behaves like a river system, i.e., originating (source zone), flowing (transfer zone) and decelerating up to the point where all suspended sediment settled down (depositional zone). In general, the initiation of sediment gravity flows is strongly related to two processes of sediment remobilization in the natural field: Firstly, by the occurrence of catastrophic events such as earthquakes, sedimentary failures, storms and volcanic eruptions which cause high instabilities and remobilize large amount of sediments instantaneously (Normark & Piper, 1991); Secondly, by continuous river supply in which the river discharge is connected into water body (usually reservoirs, lakes and oceans) generating plumes and/or hyperpycnal flows due the density difference (positive or negative). After the process starts, the mixture of suspended sediment (concentration, size

of flow dynamics properties as well as of generated deposit.

extrapolate the results to natural sediment gravity flows.

**1.1 Sediment gravity flow anatomy** 

Hampton, 1973).

(ungraded) deposits.

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

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 deposition).

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

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

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

Sediment Gravity Flows: Study Based on Experimental Simulations 267

sediment-support mechanisms may occur simultaneously, such as: *hindered settling*, in which grains deposition is inhibited because the number of particles increases in an certain zone, creating a slower-moving mixture than would normally be expected (effect of population of grains); *dispersive pressure:* in which the grains are held in suspension by their interaction forces (collision) and; *matrix strength*: a mixture of interstitial fluid and fine sediment (cohesive), which has a finite yield strength that supports coarse grains (Lowe,

Fig. 3. The difference between the internal dynamics of the turbidity current (a) and debris

The effect of high concentration on the dynamics of sediment gravity flows is expressed by changes in the mixture and flow properties such as: density of the fluid; increase of the potential energy and momentum of the flow and; viscosity of the mixture (rheological behaviour). Also, the settling velocity of particles is strongly influenced by the increase in fluid concentration mainly because: the fall of the particles induces an upward movement of water; the buoyancy of the particle increases due to high-density fluid, and by the interaction between particles (effect of population - *hindered settling*). The transport capacity of the flow tends to increase with high sediment concentration; however, these changes also

In contrast, the presence of cohesive sediment implies a different scenario in which the flocs of cohesive particles will settle down during the flow, creating a clay/mud near-bed layer with high content of water inside. Despite the fact the turbulence can be produced in this clay/mud layer (due to shear flow), there is also a significant increase in viscous forces (non-Newtonian behaviour), which could reduced the flow ability to transport great amounts of sediment

In order to understand the hydrodynamic of natural sediment gravity, an experimental study was performed with different types of sediments, such as: non-cohesive particles

depend on the composition of sediment present in suspension.

**2. Apparatus and experimental simulations** 

1979; Middleton & Hampton, 1973).

flow (b).

downstream.

between these layers (bipartite flow) point out a discontinuity in the body (water-column stratification) that is reflected by an abrupt gradient of velocity, concentration and viscosity (Postma et al., 1988).

The third part of sediment gravity flow is characterized by a deceleration zone and final dilution stage of the current, normally called *tail*.

In terms of dynamics properties of the flow, sediment gravity flows differ significantly from open-channel flows (e.g. rivers) regarding their velocity profile. In that case of sediment gravity flow, the main difference is due to the fact that is not possible to ignore the shear effects in the upper surface of the current (*see* Fig. 2 a, b). Then, the sediment gravity flows velocity profile has null values at the upper and bottom surfaces and values grow towards to the middle (balance of drag forces acting on those surfaces), creating a front point (maximum value) usually at 0.2 to 0.3 times the height of the current. Depending on the concentration and composition of sediments in suspension, both velocity and concentration profiles may present completely different shape (Fig. 2 c) as the inner dynamic of the flow became more complex (e.g. matrix strength, cohesive forces).

Fig. 2. Vertical profiles of velocity, concentration and shear stress for: a) open-channel flows; b) turbidity current and; c) debris Flow.

The two most known classes of sedimentary gravity flows (described earlier) have differences regarding their internal dynamics. The dynamics of turbidity currents is complex due to the processes of erosion and deposition. Because of this, the three-dimensional representation of this phenomenon through analytical equations is not simple, which leads to simplification (e.g. shallow water flows – Parson et al., 2010; Parker et al., 1986). In the same way, the debris flows are extremely complex too, as the existence of yield strength caused by the high density and the presence of clay implies in shear-like flow and plug-like flows as illustrated in Fig. 3.

Generally, the hydrodynamic of a sediment gravity flow is closely associated to sedimenttransport capacity (total amount of sediment transported by the flow) and competence (ability of the flow to carry particular grain size) as well as to the sediment-support mechanism, whose the main role is to keep the sediments in suspension for a long period of time (and distance). For each class of flow may occur different mechanisms of sediment-support, as it depends on grain-size and composition, concentration of sediments and the rheological properties of the mixture.

For turbidity currents, the main sediment-support mechanisms are vertical component of turbulence and buoyancy. However, for flows of high concentration (high-density) several

between these layers (bipartite flow) point out a discontinuity in the body (water-column stratification) that is reflected by an abrupt gradient of velocity, concentration and

The third part of sediment gravity flow is characterized by a deceleration zone and final

In terms of dynamics properties of the flow, sediment gravity flows differ significantly from open-channel flows (e.g. rivers) regarding their velocity profile. In that case of sediment gravity flow, the main difference is due to the fact that is not possible to ignore the shear effects in the upper surface of the current (*see* Fig. 2 a, b). Then, the sediment gravity flows velocity profile has null values at the upper and bottom surfaces and values grow towards to the middle (balance of drag forces acting on those surfaces), creating a front point (maximum value) usually at 0.2 to 0.3 times the height of the current. Depending on the concentration and composition of sediments in suspension, both velocity and concentration profiles may present completely different shape (Fig. 2 c) as the inner dynamic of the flow

Fig. 2. Vertical profiles of velocity, concentration and shear stress for: a) open-channel flows;

The two most known classes of sedimentary gravity flows (described earlier) have differences regarding their internal dynamics. The dynamics of turbidity currents is complex due to the processes of erosion and deposition. Because of this, the three-dimensional representation of this phenomenon through analytical equations is not simple, which leads to simplification (e.g. shallow water flows – Parson et al., 2010; Parker et al., 1986). In the same way, the debris flows are extremely complex too, as the existence of yield strength caused by the high density and the presence of clay implies in shear-like flow and plug-like

Generally, the hydrodynamic of a sediment gravity flow is closely associated to sedimenttransport capacity (total amount of sediment transported by the flow) and competence (ability of the flow to carry particular grain size) as well as to the sediment-support mechanism, whose the main role is to keep the sediments in suspension for a long period of time (and distance). For each class of flow may occur different mechanisms of sediment-support, as it depends on grain-size and composition, concentration of sediments and the rheological

For turbidity currents, the main sediment-support mechanisms are vertical component of turbulence and buoyancy. However, for flows of high concentration (high-density) several

viscosity (Postma et al., 1988).

dilution stage of the current, normally called *tail*.

b) turbidity current and; c) debris Flow.

flows as illustrated in Fig. 3.

properties of the mixture.

became more complex (e.g. matrix strength, cohesive forces).

sediment-support mechanisms may occur simultaneously, such as: *hindered settling*, in which grains deposition is inhibited because the number of particles increases in an certain zone, creating a slower-moving mixture than would normally be expected (effect of population of grains); *dispersive pressure:* in which the grains are held in suspension by their interaction forces (collision) and; *matrix strength*: a mixture of interstitial fluid and fine sediment (cohesive), which has a finite yield strength that supports coarse grains (Lowe, 1979; Middleton & Hampton, 1973).

Fig. 3. The difference between the internal dynamics of the turbidity current (a) and debris flow (b).

The effect of high concentration on the dynamics of sediment gravity flows is expressed by changes in the mixture and flow properties such as: density of the fluid; increase of the potential energy and momentum of the flow and; viscosity of the mixture (rheological behaviour). Also, the settling velocity of particles is strongly influenced by the increase in fluid concentration mainly because: the fall of the particles induces an upward movement of water; the buoyancy of the particle increases due to high-density fluid, and by the interaction between particles (effect of population - *hindered settling*). The transport capacity of the flow tends to increase with high sediment concentration; however, these changes also depend on the composition of sediment present in suspension.

In contrast, the presence of cohesive sediment implies a different scenario in which the flocs of cohesive particles will settle down during the flow, creating a clay/mud near-bed layer with high content of water inside. Despite the fact the turbulence can be produced in this clay/mud layer (due to shear flow), there is also a significant increase in viscous forces (non-Newtonian behaviour), which could reduced the flow ability to transport great amounts of sediment downstream.

#### **2. Apparatus and experimental simulations**

In order to understand the hydrodynamic of natural sediment gravity, an experimental study was performed with different types of sediments, such as: non-cohesive particles

Sediment Gravity Flows: Study Based on Experimental Simulations 269

Additionally, all flows were recorded with a digital video-camera placed on the side of the tank in order to evaluate the time series of geometric features of the current (see Fig. 1), such as: *the current height (ht*); *thickness of the body (hb)* defined as the height of the body not considering the mixing zone at the upper surface and; *thickness of the internal layer (hi)*, which considers the interface layer created by the presence of a more concentrated zone near the bottom. The depositional properties (e.g. deposition rate) were also evaluated through the

After the experiment, the ambient fluid was slowly drained and the final deposit properties

The rheology is the study of deformation and flow of matter and is a property of the fluid that expresses its behaviour under an applied shear stress. Through the rheological characterization of mixtures (water and sediment), it is possible to establish the relationship between shear stress and strain rate (shear rate), and consequently the coefficient of dynamic viscosity (and/or apparent) as well as the constitutive equations in terms of

In natural flows, the non-conservative condition of the sediment gravity flows, i.e. erosion and deposition during the movement, modifies the mechanisms of transport and deposition of particles within the flow (e.g. local concentration, size and composition of grains in

Based on this, a rheological characterization of mixtures was carried out aiming to establish such property of the mixtures and verify its behaviour for different initial conditions. To do that, it was used a Rheometer device with two types of spindle (cone plate and parallel plate). For the tests, the mixtures were prepared following the same proportions of sediment used in the experimental work and also considering the same temperature (~ 19°C). The rheogram - output data of the Rheometer consisting in the ratio of shear stress and strain rate - was compared to typical rheological models found in literature. The simplest

(due to the definition of Newton's law of viscosity) and it can be expressed for two-

The equation (1) shows a linear relationship between the imposed shear stress and strain rate (gradient of deformation). As a consequence, the viscosity of the fluid or

deviation from linearity between the stress-strain curve converts the rheological property to non-Newtonian behaviours, which can be generally divided into four more groups: *plastics* in which there is no deformation of the flow until the critical initial stress (yield strength -

is overcome; *dilatant and pseudoplastic*, in which the deformation (strain rate) is expressed by a power law type (if coefficient of power law n > 1 then the fluid is *dilatant* otherwise (n < 1) is *pseudoplastic*) and; *Herschel-Bulkley* in which the fluids has a plastic behaviour (yield

*<sup>0</sup>*) followed by a power law behaviour. The *Herschel-Bulkley* model can be

*u z*

*x*

 

*x)* related to strain rate *(u/z)* is the *Newtonian* model

(1)

) is constant for all values of shear rate. Any

*0*)

(e.g. thickness, grain-size and mass balance) were measured (and/or sampled).

video images.

**3. Rheology of mixtures** 

volumetric concentration and presence of clay.

rheological model of imposed stress *(*

dimensional flow in the x – z plane as:

mixture (*coefficient of dynamic viscosity -* 

expressed for two-dimensional flow in the x – z plane as:

strength -

suspension), which impact also their rheological behaviour.

represented by very fine sand and silt sized glass beads, and cohesive particles represented by kaolin clay. Both sediments have density approximately of 2600 kg/m³. In total, 21 experiments (Fig. 4) were carried out with eight values of bulk volumetric concentration (2.5%, 5%, 10%, 15%, 20%, 25%, 30% and 35%). In addition, for each value of concentration were used three different proportions of clay in the mixture from 0% (pure non-cohesive flows) passing to 50% (mixed) and finally, 100% (pure cohesive flows).

Fig. 4. Initial properties of the mixtures simulated and the particles properties.

The experiments were performed in a 2D Perspex tank (4.50 m long x 0.20 m wide x 0.50 m height). A 120 litres mixture was prepared in a mixing box (full capacity of 165 litres) connected at the upstream part of the tank through a removable lock-gate (0.21 m wide and 0.70 m high). An electric-mechanical mixer was installed within that box to assure the full mixing of sediment mixture. The tank also had a dispersion zone (approximately 1.00 m length) in which the water (and flow) were drained after the experiment.

In all sets of experiments were used lock-exchange methodology characterized by the instantaneously release of the mixture (lock-gate opening) reproducing a catastrophic event on nature. As soon as the mixture entered into the channel, the dense flow was generated.

In order to measure the flow properties during the experiments, a group of equipments was installed within the tank. Four UHCM's *(Ultrasonic High-Concentration Meter)* were set along the vertical profile (at 1.0; 3.2; 6.4 and 10 cm from the bottom) to acquire time-series concentration data, whilst ten UVP's (*Ultrasonic Doppler Velocity Profiler*) of 2 MHz transducers were set along vertical profile (15 cm) to register time-series of velocity data. Both equipments were located at 340 cm from the gate. With both velocity and concentration data, the hydrodynamic properties were established for all flows such as: time series of velocity and concentration, mean vertical profiles, non-dimensional parameters for the head, body and tail zones.

Additionally, all flows were recorded with a digital video-camera placed on the side of the tank in order to evaluate the time series of geometric features of the current (see Fig. 1), such as: *the current height (ht*); *thickness of the body (hb)* defined as the height of the body not considering the mixing zone at the upper surface and; *thickness of the internal layer (hi)*, which considers the interface layer created by the presence of a more concentrated zone near the bottom. The depositional properties (e.g. deposition rate) were also evaluated through the video images.

After the experiment, the ambient fluid was slowly drained and the final deposit properties (e.g. thickness, grain-size and mass balance) were measured (and/or sampled).
