Flow of different material mixtures in a rotating drum

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1 Flow of different material mixtures in a rotating drum R. Kaitna Institute of Mountain Risk Engineering, University of Natural Resources and Applied Life Sciences (BOKU), Vienna, Austria D. Rickenmann Institute of Mountain Risk Engineering, University of Natural Resources and Applied Life Sciences (BOKU), Vienna, Austria, also at Swiss Federal Research Institute WSL, Birmensdorf, Switzerland Keywords: rotating drum, flow regimes, rheology, dimensionless numbers ABSTRACT: Debris flows can be considered as mixtures of coarse particles dispersed in a viscous matrix, composed of fine sediment and water. Experiments in a rotating drum with a diameter of 2.5m have been carried out to study the flow behaviour of different material mixtures, ranging from granular avalanches to viscous fluids and grain fluid mixtures of varying sediment concentration and matrix viscosity. The bulk shear stress at the channel boundary was measured for the different mixtures. The macroscopic flow behaviour is described on the basis of the evolution of flow resistance with increasing flow velocity, and is used as a criterion to differentiate between the frictional, viscous, and collisional flow regime. Our results are only in partial agreement with a classification of flow regimes based on threshold values of dimensionless Bagnold and Savage number found in literature. 1 INTRODUCTION Flow mechanics of a particle fluid system is a compound of complex interactions within and between the solid and fluid phase (e.g. Coussot & Ancey 1999; Iverson & Denlinger 2001). In debris flow research the flowing mixture is often divided into the liquid matrix, composed of water and fine sediment in suspension, and the solid phase, consisting of coarse particles dispersed in the fluid. Considering the main components involved, the following forces may play a significant role for flow resistance: viscous forces within the matrix particle particle forces particle fluid forces Depending of the relative concentration of fine and coarse sediment, the adjectives viscous or granular are often used for classification. In debris flow mixtures, consisting of sediments of particle sizes of different proportions and water content, it is necessary to identify the relevant processes of energy dissipation. In order to describe the flow behaviour of debris flows, often rheologic models are chosen. Rheologic models relate shear stress and shear rate, mostly in a form of the general Herschel Bulkley model n τ = τ y + Kγ (1)

2 where τ = shear stress, τ y = yield stress, γ = shear rate, and K, n = coefficients. The exponent n assigns the rate of increase of τ with shear rate. If n < 1, the fluid exhibits a shear thinning flow behaviour and is termed visco-plastic fluid. When n > 1, a progressive decrease of the importance of viscosity with respect to shear rate determines flow behaviour ( shear thickening or dilatant ). In case of n = 1, the Herschel Bulkley model reduces to the Bingham model, which was often used in debris flow modelling. Parameters for the different rheologic approaches have been back-calculated from observations and deposits of debris flow events (e.g. Tecca et al. 2003) or measured from partial samples up to limited maximum grain sizes (e.g. Cui et al. 2005; Coussot & Piau 1995). It was found to be problematic to relate unique values to catchments of similar geomorphologic or geologic characteristics or even to different debris flow events within the same watershed (e.g. Rickenmann et al. 2005; Naef et al. 2006). Other approaches to model debris flows (e.g. Takahashi 1991; O Brien & Julien 1993) are based on the work of Bagnold (1954), who conducted shear experiments with dispersions of neutrallybuoyant wax particles in a glycerine-water-alcohol mixture. Bagnold observed a linear relationship between shear rate and shear stress for low shear velocities and called this the macro-viscous flow regime. Measurements of normal stress showed the same proportionality to the shear rate and Bagnold determined the effective dynamic friction coefficient as the ratio between normal stress and shear stress to be For higher shear velocities the shear stress and normal stress were found to be proportional to the square of the shear rate γ and were independent of the fluid viscosity. The effective dynamic friction coefficient was determined to be Bagnold assumed that the momentum transfer is mainly produced by elastic collisions between the particles of parallel layers in relative motion. The momentum transmitted through each collision is proportional to the relative velocity between two colliding particles, which is proportional to γ in a linear velocity field and laminar shearing. For the same reason the collision frequency is also proportional to γ. The shear stress (and normal stress) in a collision dominated flow regime is therefore proportional to the square of the shear rate (Coussot 1997). Hunt et al. (2002) carried out a thorough review of the work of Bagnold (1954) and found substantial shortcomings and inconsistencies in Bagnold s experiments and data analysis. In the last decades more attention was given to the role of solid constituents and geotechnical models have been employed to describe the motion of debris flows (e.g. Savage & Hutter 1989; Iverson 1997; Pitman & Le 2005). The basis of these models is the well known Coulomb equation, given with τ = σ ' tanφ + c (2) where σ = effective normal stress; φ = angle of internal friction; c = cohesion or intrinsic yield strength. The effective stress is calculated as the difference of total normal stress σ and pore fluid pressure u: σ ' = σ u (3) The essential difference to common rheologic formulas is the dependence of shear resistance on normal stress and pore pressure. There is no fixed relationship between shear resistance and shear rate and the pressure of the pore fluid is the critical state variable (Iverson 2003). Conducting a dimensional analysis using δ, the characteristic grain diameter as characteristic length, ρ s δ 3 as characteristic mass and 1/γ as characteristic time, several dimensionless parameters can be derived. Their chief significance is the ratio that they form (Iverson 1997): The Savage number N SAV is the ratio of inertial shear stress associated with grain collision to quasi-static shear stress as a result of Coulomb friction and can be written as N SAV 2 2 ρ γ δ = (4) s ( ρ ρ ) gh tanφ s f

3 where ρ s and ρ f = density of the solids and the fluid respectively, g = acceleration due to gravity, H = flow depth. Savage & Hutter (1989) inferred that if N SAV > 0.1, grain collision stresses may affect flow dynamics significantly. Based on diverse data of field debris flows, Iverson & Denlinger (2001) argue, that many geophysical flows fall within the friction dominated rather than collision dominated regime. However, scrutiny of Eq. 4 reveals, that the most uncertain parameters to be assessed for natural flows (γ and δ), scale to the power of 2 and thus calculation of the N SAV is very sensitive to the definition of a characteristic diameter and shear rate (which might only be estimated within one order of magnitude for natural flows). A number that assesses the role of viscous fluid stresses in granular mixtures was first formulated by Bagnold (1954). The Bagnold number N BAG represents the ratio of grain collision stresses to viscous stresses and is defined as υ 3 2 s ρsγδ N BAG = (5) υ * υ 3 µ s where µ = dynamic viscosity of pore fluid with suspended sediment, υ s = volumetric solid concentration, and υ = maximum possible value of υ s in a dense-packed configuration. Bagnold (1954) reports that if N BAG < 40, the flow resistance is mainly due to viscous forces ( macroviscous regime). Values of N BAG > 450 indicate a collision dominated flow, in which bulk normal and shear stress are proportional to γ 2. Iverson (1997) and Parson et al. (2002) substituted the term [υ 1/3 s /(υ* 1/3 -υ 1/3 s )] 1/2 with υ s /(1-υ s ), altering the threshold values to 15 and 180, respectively. The Bagnold number is then written as 2 υsρsγδ N BAG = (6) (1 υ ) µ s As already mentioned Bagnold s work on granular shear flow has been subject to a critical review by Hunt et al. (2002), which poses the question of the reliability of the given threshold numbers. In this paper we discuss experiments with a vertically rotating drum which has been constructed to study the flow of artificial and natural mixtures of particles and fluids. The advantage of such setup is that observation of a stationary surge is possible over an extended period of time. Sensors have been installed at different locations in the drum channel and at the axis and relevant flow parameters measured. In order to investigate the role of particles in a viscous fluid, experiments have been carried out with different concentrations of PVC grains dispersed in fluids of various viscosities. The tests cover mixtures ranging from dry granules to viscous fluids. 2 EXPERIMENTS 2.1 Experimental setup In this section an overview of the experimental facility, data acquisition techniques and tested material is given. A more detailed description can be found in Kaitna (2006) and Kaitna et al. (2007). The experiments have been carried out in a rotating drum with a diameter of 2.46 m (Fig.1). The inner surface of the rotating drum is polished to avoid possible flow instabilities due to irregular bottom curvature. The rectangular cross-section has a width of 0.45 and is confined on one side by stainless steel, on the other side by acrylic glass to allow observations from the side. In order to avoid slip at the flume bottom the inner cylinder is roughened by a glass-fibre 5 x 5 mm mesh of approximately 1 mm height. Some centimetres behind the tail of the surge a brush is installed to clean the drum inner surface from stuck material.

4 Figure 1. Left: Side view of the rotating drum; right: cross section of the experimental setup The total flow resistance of the mixture, i.e. bottom shear stress and lateral shear stress at the channel walls, is measured with a torque flange, which is directly installed at the rotation axis of the drum between the bearings and the engine (Fig. 1). The net-torque is derived by subtracting the average torque of the empty drum from the recorded torque during an experiment. The average shear stress of the mixture exerted on the channel bottom may then be estimated by assuming a uniform distribution of bottom shear stress and a triangular shear stress distribution on the side walls. Surface velocity is determined from digital video analysis. The flow of each test has been recorded by a digital video camera mounted directly above the surge. Using tracer particles the mean particle velocity (= velocity relative to the laboratory floor) is determined from nine subsequent frames. In order to obtain an average value, the surface velocity for a larger region in the body of the wave was derived, excluding the very snout, where the particles start to descend under the surge, and the tail, where more unsteady (and pulsing) flow was produced by the brush. For steady state the mean flow velocity of the tested material relative to the drum bottom equals the rotation speed of the drum at the channel bed and is registered by a sliced ring fixed at the rotation axis and a static photo-electric sensor that registers an impulse at every degree of rotation, thus the exact angular position of each sensor installed within the drum can be determined. The time derivative yields the rotation velocity in revolutions per minute [rpm] which can be converted into mean flow velocity in [m/s] with respect to the drum circumference. Normal stress and shear stress measurement are carried out using force plates of 60 mm diameter, which are attached to load cells. The force plates are arranged pair wise, one measuring in radial direction (normal force), and the other measuring in tangential direction (shear force). This arrangement is installed twice within the rotating drum, displaced by 180, giving two independent normal force and shear force measurements within the middle third of the channel section (Fig. 2). One laser sensor and one ultra-sonic sensor are installed in the middle of the channel section directly above each unit measuring normal force and shear force. Both sensors are attached to the rotating parts of the facility; hence two independent longitudinal flow depth profiles at each revolution are obtained. Pore fluid pressure fluctuations are registered by a pressure transducer that is attached to a reservoir filled with water. The side of the reservoir adjacent to the tested material is closed by a synthetic foil and a steel mesh of 2 mm diameter. Pressure fluctuations in the pore fluid include suspended material up to grain sizes of the steel mesh.

5 Figure 2. Left: Section of the rotating drum with normal and shear force plate (arrows indicate the direction of measurement), above the force plates is the ultra-sonic sensor; right: sketch of the pore fluid pressure device. 2.2 Materials Tests have been carried out with fluids of different viscosity, unimodal mixtures of particles and fluid, and dry granular material. The viscous fluid was either water (mimicking particle fluid mixtures with low matrix viscosity) or the synthetic polymer Carbopol Ultrez10 produced by the company Noveon Ltd. The polymer is purchased as a powder and can be dispersed in deionised water, modifying the rheologic behaviour depending on concentration. The general flow behaviour of the fluid shows a viscoplastic rheology (Noveon 1995) and can be described by a shear thinning Herschel Bulkley model. The advantage for application in laboratory open channel flows is that the mixture is homogeneous and fully transparent. Moreover rheologic parameters that have been derived from drum tests can be independently determined in a conventional coaxial cylinder rheometer (Bohlin Visco 88BV). The viscosity of the liquid polymer is sensitive to changes of ph value, thus care was taken to avoid contamination with mud, dust or other substances that could alter ph - value. For the tests with dry granular material and in order to assess the effect of particles in a non- Newtonian fluid, mono-dispersed PVC particles have been used. The particles have a cylindrical shape of 10 mm diameter and approximately mm height. The specific density of the particles is For comparative reasons also results from tests with debris flow material up to grain sizes of 5 mm in diameter are discussed in this study. The material was taken from a fresh deposit in the Scalära torrent near Trimmis/Chur in Eastern Switzerland and analysed by Schatzmann (2005). Coussot (1994) postulated a minimum value of 10 % of particles smaller than 0.04 mm for mixtures, where viscous forces dominate the flow behaviour (i.e. shear stress is a function of shear rate); the tested deposit contained 9 % of material smaller than 0.04 mm, thus rheological investigations where considered useful (Schatzmann 2005). A detailed analysis of these experiments can be found in Kaitna et al. (2007). 2.3 Experimental procedure The synthetic polymer was produced some days in advance of the tests and stored in a closed container. The debris flow mixtures were produced one day in advance of an experiment. Before each experiment the material was mixed using a drill for at least ten minutes and then filled into the drum. The measurement system was then started and the speed of the drum increased. At each preselected velocity, measurements were performed during about 10 to 20 rotations. Velocity was increased stepwise up to a speed where the flow visibly starts to become unstable and/or turbulent. Then the speed was incrementally decreased again. During each state of constant speed tracer particles of 5 mm diameter have been dropped twice onto the surface of the flow for derivation of sur-

6 face velocity by particle tracking. Samples of the material in the drum were taken before, during, and after each experimental run. In case of experiments with Carbopol, the samples were analysed subsequently in a coaxial cylinder rheometer (Bohlin Visco 88 BV) to detect possible changes of viscosity due to contamination with dust or mud during the tests and to compare rheologic parameters estimated from the free surface flows in the drum with rheologic measurements. For the natural debris flow material, the water content was measured and for selected samples grain size analyses were carried out. 3 RESULTS 3.1 Qualitative description The most obvious difference between dry granular material and a fluid was that it was difficult to establish stationary flow conditions for dry granular flows (PVC grains without interstitial fluid). At different levels of agitation some interesting features can be observed. Table 1 exemplarily gives a qualitative description of the macroscopic flow behaviour of the PVC particles. Table 1. Qualitative description of flow behaviour of dry granular mixtures tested in the rotating drum. Rotation velocity [rpm] Mean velocity [m/s] Flow description Relatively stable flow, irregularly collapsing, snout of the surge slightly moving forwards and backwards Relatively stable flow, collapses lead to moderate migration of the snout Like V = 4.9 rpm Onset of pulsing flow, collapses lead to strong movement forwards, after some seconds the original flow developed Rhythmical pulsing flow, interval about 2 seconds, migration distance about 1/5 of flow length; collapses move mass further forward Rhythmical pulsing flow, like at V = 8.9; every second interval collapses Rather un-rhythmical pulsing flow, less pronounced collapses Less pronounced pulsing behaviour, less rhythmical Rather stable for some time, at intervals of about 4 seconds pulsing (or pushing from the back?) causes the front to propagate forward The presence of an interstitial fluid significantly dampens this unsteady flow, already for the grain fluid mixtures with low fluid viscosity (i.e. water and Carbopol C CA = 0.05%), the tip of the surge varied only for some centimetres at constant rotation velocity. Also no collapsing flow has been observed for these mixtures. Since only uni-sized particle fluid mixtures have been tested no development of a coarse grained debris flow snout was possible. However, due to geometry of the experimental setup, at low fluid viscosities and low rotation velocities a fluid wave preceded the body of the granular mixture. The reason for this is that the coarse particles settled quicker than they could flow to the front and subsequently have been dragged to the tail of the surge at lower depths of the flow. Considering a relatively large particle diameter of 1 cm, the permeability is high, thus fluid migration through the pores is easily possible. At medium velocities the mixture could often be regarded as

7 homogeneously mixed, whereas at high velocities a dry front region was observed and identified by comparing measurement of pore fluid pressure, flow depth and normal stress. For dry granular mixtures dilatancy effects have been observed. The bulk density of the static mass of PVC grains of 10 cm diameter was determined with kg/m³, corresponding to a static volume concentration of ~0.60. With increasing level of agitation the mean volume concentration of the surge decreased linearly to values smaller than 0.48 (corresponding to a bulk density of 700 kg/m³). 3.2 Total shear stress at different rotation velocities Experiments with fluid particle mixtures have been carried out on the basis of Carbopol mixtures of different concentrations (0.05%, 0.08%, 0.12%, and 0.15%). Fig. 3 shows exemplarily plots of the development of total shear stress of the mixtures with increasing shear rate of the tests with pure fluid, fluid particle mixtures and natural debris flow mixtures. It is noted, that the estimation of an average shear rate for the surge is strongly simplified and amongst others based on the assumption of a linearly sheared layer underneath an unsheared region, commonly termed rigid plug. The assumption of an unsheared region is supported by the video analysis of the tracer particles on the surface. A detailed description of the analysis for homogeneous fluids can be found in Kaitna and Rickenmann (in press). However, a general trend of the development of shear stress with increasing velocity can be detected. The flow curve of the clear Carbopol (square symbols in Fig.3a) shows a typical shear thinning behaviour, which can be well described by a Herschel Bulkley model, in a limited shear rate range also with a Bingham model. Reference measurement in a conventional rheometer supports the results from the drum experiments. The solid and dashed lines in Fig.3a show the minimum and maximum flow curves of the samples taken before and after the test. It is noted that this is an example of a significant loss of viscosity during a test, which was not observed during the other experiments. Addition of PVC particles to Carbopol 9 (Fig.3a) makes the shear stress increase by more than a factor of two, whereas the general flow behaviour is still dominated by viscous resistance of the fluid (i.e. shear stress depending on shear rate). The Carbopol used in this experiment had a relatively high viscosity (Carbopol powder concentration C CA = 0.12 % per weight). (a) (b) Figure 3. (a) Increase of shear stress with estimated shear rate for mixtures Carbopol 9 (0.12%) and Carbopol + PVC mixtures with C v = 0.130, C v = 0.242, and C v = 0.318, and (b) for debris flow material with a sediment concentration of C v = 0.49, C v = 0.57, C v = 0.60, and C v = 0.61.

8 The flow behaviour of the debris flow mixture presented in Fig.3b corresponds to that of a homogeneous fluid rather than to granular material. There is a clear increase of shear resistance with increasing shear rate. Similarly to the artificial mixture higher Cv values result in an increased flow resistance but do not change the general flow behaviour. The macroscopic flow resistance of this mixture could be well described by a rheologic model. The shear stress of the dry granular mixtures does not exhibit dependence on velocity; hence a rheologic interpretation was not possible. A bulk friction angle is computed as the ratio of mean shear stress and mean normal stress of the total surge. Mean normal stress is calculated from data of the normal force plate. Mean bed shear stress can be derived by different ways: from torque flange measurement, directly from shear stress measurement of the shear force plates installed at the circumference of the drum, and from back-calculation from surge geometry. The latter can be carried out either with flow depth data of the laser sensor or from data of the normal force plate. Fig.4 shows the bulk friction angle of different mixtures derived from flange data. (a) (b) Figure 4. (a) Bulk friction angle in relation to mean flow velocity for different volumes (30 40 litres) of flowing PVC particles and (b) development of bulk friction angle with mean velocity for artificial particle fluid mixtures Water+PVC (C v = 0.54), W+PVC (C v = 0.44), W+PVC (C v = 0.50), W+PVC (C v = 0.60), and Carbopol (0.05%)+PVC (C v = 0.55). It is noted that slip at the channel bottom may occur since bed roughness is smaller than mean particle size, thus the friction angle calculated represents an average of bed friction angle and internal dynamic friction angle of the material. It can be seen that the values are more or less constant and range between 20 and 25 (Fig. 4a), indicating a frictional flow regime for all experimental conditions. The static friction angle for the 10 mm PVC cylinders derived from tilting board tests is determined with 30. Armanini et al. (2005) carried out flow experiments with PVC grains of around 3 mm diameter and derived a static friction angle of 35 and a dynamic friction angle of 31. The presence of the pore fluid decreases the frictional resistance of the PVC grains significantly (Fig.4b). For the experiments carried out with water and a low concentrated Carbopol mixture (0.05%), lubricated contacts (cf. Ancey 2001a) and particle friction act in combination with viscous fluid forces. It can be seen that the bulk friction angle ranges between 5 and 8 for mixtures of water and PVC grains and Carbopol and PVC grains ( Carbopol (0.05%)+PVC (C v = 0.55) ), which is significantly lower than for the dry granular mixtures (~20-25 ). The flow resistance of the Carbopol PVC mixture is of the same magnitude as for the water PVC mixtures, although viscosity of the interstitial fluid is increased by one order of magnitude compared to water. The friction angle

9 of mixture Water + PVC (C v = 0.60) is increased, since it is only partially water saturated. One can conclude that flow resistance is not exclusively controlled by the boundary layer which was water saturated as in the other mixtures but also by the upper parts of the surge. In the diagram two regions can be identified: At lower velocities (< ~ 1.3 m/s) the bulk friction angle is more or less constant, indicating flow resistance due to (Coulomb) friction. It is noted that for the mixtures of low interstitial fluid viscosity segregation effects (a fluid front followed by the main surge as mentioned earlier) were observed. Due to this segregation, the determined flow resistance may be dominated by frictional contacts of the higher concentrated part of the flow. At rotation velocities > ~ 1.0 m/s the mixtures are visibly more homogeneous. There is a trend of flow resistance to increase for all mixtures with mean flow velocity. This increase of flow resistance can be attributed to either the increasing influence of the fluid present within the mixture (i.e. viscous forces) or to the increasing influence of collisional momentum exchange between the particles. The latter hypothesis seems quite unrealistic since the presence of an interstitial fluid is expected to damp momentum exchange due to particle collision. 3.3 Dimensionless numbers and flow regimes Since the dimensionless numbers outlined in section 1 (Introduction) represent the ratios of the main sources of flow resistance (frictional, collisional and viscous forces), an attempt was made to calculate N SAV and N BAG and to classify our experimental flows according to the macroscopic flow behaviour. The distinction between solid and fluid phase for PVC particle Carbopol and water mixtures is obvious, whereas for the debris flow material mixtures it is difficult to define a threshold particle diameter, separating particles belonging to the interstitial muddy suspension and to the solid phase. In literature different values can be found, between d < 0.06 mm and d < 20 mm. In this study a threshold value of 0.1 mm was used, based on the discussion by Schatzmann (2005) for viscous debris flows. Determination of the shear rate of the granular mixtures for which no rheologic interpretation was carried out (i.e. estimation of a representative shear rate) is associated with some uncertainties. Here shear rate was defined as the ratio of surface velocity and mean flow depth. For all mixtures N BAG was calculated using Eq. 6. Fig. 5 shows the plot of Bagnold number in relation to Savage number for all experiments carried out in the rotating drum. Additionally threshold values found in literature are drawn (dashed lines). Flow regimes were classified on the basis of the development of bulk shear resistance as measured during the drum experiments. In our experiments, for N BAG < 100 all mixtures showed a velocity dependent flow resistance. Tests carried out with PVC Carbopol mixtures of high matrix viscosity and natural debris flow material fall in this class. Bagnold (1954) reports a threshold value smaller than 15 for the macroviscous regime and larger than 200 for the inertial regime (cf. section 1). Considering N SAV and the threshold value of 0.1 (Savage and Hutter 1987, Iverson 1997), separating a frictional flow regime (constant ratio of shear stress and normal stress) and a collisional flow regime, nearly all PVC + Carbopol mixtures with N BAG < 100 would fall into the collisional regime (N SAV is between 0.01 and 20), which is contrary to our observations. In our experiments incrementally adding PVC particles increased the apparent viscosity of the mixture, but the general flow behaviour compared to a viscous fluid without particles was not altered.

10 Figure 5. Savage number against Bagnold number for all (particle) experiments carried out in the rotating drum. The classification of the grouped data delineated by ellipsoids is based on the evolution of flow resistance with mean flow velocity. For the experiments where viscous forces are expected to be less dominant (N BAG > 100) a clear distinction between frictional regime (constant ratio of shear stress and normal stress) and collisional regime could not be made. The experiments with PVC water mixtures, especially at low velocities, are affected by segregation effects, where a shear rate independent flow resistance was detected (in this case a bulk analysis using mean values is no longer valid). However, for higher velocities, the measurements showed an increasing flow resistance. For these tests N BAG is between 1000 and and N SAV larger than 0.1, which would correspond to a collisional dominated regime. After Bagnold (1954) flow resistance in the inertial regime (dominance of collisional momentum exchange) is associated with a significant drop of the bulk friction angle contrary to the observations for the drum experiments. It is noted that Bagnold eliminated the effect of gravity by using neutrally buoyant grains, whereas in this study frictional contacts due to gravity forces are relevant. For the dry granular mixtures shear rate independent resistance was observed up to Savage numbers in the range of Savage and Hutter (1989) gave a threshold value of 0.1 between the frictional and collisional regime, which is on average smaller by one order of magnitude. 4 DISCUSSION It has been shown that simple rheologic models are successful in describing the flow of fine grained slurries lacking a considerable amount of coarse particles (e.g. O Brien and Julien 1988; Coussot 1994; Parson et al. 2001). As observed in this as well as other studies (e.g. Ancey and Jorrot 2002, Schatzmann 2005, Ancey 2001a) rheologic properties of a viscous fluid are altered by addition of coarse particles. As the fraction of coarse particles increases, resistance forces due friction and collision between grains become significant. From experiments with PVC particles dispersed in water, Armanini et al. (2005) report that frictional behaviour is characterised by slow deformations (shear rates < 10 s -1 ) and rate independent shear and normal stresses which are proportional to each other. These observations correspond to the flow behaviour identified for drum experiments in the frictional regime. Collisional behaviour observed by Armanini et al. was characterised by rapid deformations (shear rates > 20 s -1 ) and a quadratic dependence of the shear and normal stresses on shear rate. However, contrary to Bagnold (1954), who carried out experiments using neutrally buoyant particles, Armanini et al. found the

11 τ/σ ratio to increase at higher shear rates. In this study a similar behaviour was observed for the PVC water mixtures. It may be pointed out that Armanini et al. measured shear rate and solid concentration along the vertical flow depth and identified distinct sub-layers dominated by either frictional or collisional behaviour. It is possible that different flow regimes over the flow depth might also have been present in our rotating drum experiments. In this study frictional behaviour is observed up to N BAG = 10 6 for dry granular mixtures and viscous behaviour at N SAV larger than 0.1 for PVC Carbopol mixtures. This illustrates (1) the need to carry out experiments with different solid and fluid materials (e.g. varying size, shape, friction angle, and fluid viscosity) and (2) the importance to use more than one dimensionless parameter to identify different flow regimes. In case of more complex situations, e.g. development of excess pore pressure, additional parameters are needed. In the context of debris flows there is a need to better understand the role of fine material, which may be regarded as part of the fluid phase changing fluid properties. In the literature threshold values were proposed for particle diameters separating the fluid and solid phases for example as 0.06 mm (O Brien and Julien 1988) and 20 mm (Malet et al. 2003). 5 CONCLUSION A vertically rotating drum has been built to investigate the flow behaviour of debris flow material. The experimental setup and the measurement devices were tested with different kind of material, from low viscous homogeneous fluids to dry granular material. It was shown that the rotating drum can be used to observe the macroscopic flow behaviour and measure the relevant flow parameters of such fluids over an extended time period. With regard to debris flow mechanics it is important to understand the role of different energy dissipation processes at different energy levels within the bulk mixture. In this study the bulk shear stress at the channel boundary was measured for the different mixtures. We used the evolution of flow resistance with mean flow velocity as a criterion to differentiate between the frictional, viscous, and collisional flow regime. Our results are only in partial agreement with a classification of flow regimes based on threshold values of dimensionless Bagnold and Savage number found in literature. More experiments are needed to verify the threshold values of dimensionless numbers separating different flow regimes estimated in this study. REFERENCES Ancey, C. 2001a. Role of lubricated contacts in concentrated polydisperse suspensions. Journal of Rheology 45: Ancey, C. 2001b. Dry granular flow down an inclined channel: Experimental investigations on the frictionalcollisional regime. Physical Review E: Ancey, C. & Evesque, P Frictional-collisional regime for granular suspension flows down an inclined channel. Physical Review 62(6): Ancey, C. & Jorrot, H Yield stress for particle suspensions within a clay dispersion. J. Rheol. 45: Armanini, A., Capart, H., Fraccarollo, L. & Larcher, M Rheological stratification in experimental freesurface flows of granular-liquid mixtures. J. Fluid Mech. 532: Bagnold, R.A. (1954): Experiments on a gravity-free dispersion of large solid spheres in a Newtonian fluid under shear. Proceeding of the Royal Society London 225: Coussot, P Steady, laminar flow of concentrated mud suspensions in open channel. Journal of Hydraulic Research 32(4): Coussot, P Mudflow Rheology and Dynamics. IAHR Monograph Series. Rotterdam: Balkema. Coussot, P. & Piau, J.M A large scale field coaxial cylinders rheometer for the study of the rheology of natural coarse suspensions. J. Rheol. 39:

12 Coussot, P. & Ancey, Ch Rheophysical classification of concentrated suspensions and granular pastes. Phys. Rev., E 59: Cui, P., Chen, X., Waqng, Y., Hu, K. & Li, Y Jiangia Ravine debris flows in south-western China. In Jakob, M.; Hungr, O. (eds.): Debris-flow Hazards and Related Phenomena. Berlin, Heidelberg: Springer: Hungr, O Analyses of debris flow surges using the theory of uniformly progressive flow. Earth Surface Processes and Landforms 25: Hunt, M.L., Zenit, R., Campell, C.S. & Brennen, C.E Revisiting the 1954 suspension experiments of R.A. Bagnold. Journal of Fluid Mechanics 452. Cambridge University Press: UK: Iverson, R.M The physics of debris flows. Reviews of Geophysics 35(3): Iverson, R.M The debris-flow rheology myth. In D. Rickenman & C.L. Chen (eds), Debris-Flow Hazards Mitigation: Mechanics, Prediction, and Assessment; Proceedings of the 3rd International DFHM Conference, Davos, Switzerland, September 10-12, 2003: Rotterdam: Millpress. Iverson, M.R. & Denlinger, R.P Flow of variably fluidized granular masses across three-dimensional terrain: 1. Coulomb mixture theory. Journal of Geophysical Research 106(B1): Kaitna, R Debris flow experiments in a rotating drum. Thesis, University for Natural Resources and Applied Life Sciences (BOKU) Vienna, Austria. Kaitna R. & Rickenmann D. (in press) A new experimental facility for laboratory debris flow investigation. Journal of Hydraulic Research. Kaitna, R., Rickenmann, D. & Schatzmann (2007): Experimental study on the rheologic behaviour of debris flow material. Acta Geotechnica. DOI /s z. Malet, J.-P., Remaitre, A., Maquaire, O., Ancey, C. & Locat, J Flow susceptibility of heterogeneous marly formations: implications for torrent hazard control in the Barcelonette Basin (Alpes-de-Haute- Provence, France). In D. Rickenman & C.L. Chen (eds), Debris-Flow Hazards Mitigation: Mechanics, Prediction, and Assessment; Proceedings of the 3rd International DFHM Conference, Davos, Switzerland, September 10-12, 2003: Rotterdam: Millpress. Naef, D., Rickenmann, D., Rutschmann, P. & McArdell, B.W Comparison of flow resistance relations for debris flows using a one-dimensional finite element simulation model. Nat. Hazards Earth Syst. Sci. 6: Noveon Carbopol Ultrez10 polymer for personal care applications. Datasheet. Available at: O Brien, J.S. & Julien, P.Y Laboratory analysis of mudflow properties. Journal of Hydraulic Engineering 114: O Brien, J.S., Julien, P.Y. & Fullerton, W.T Two-dimensional water flood and mudflow simulation. Journal of Hydraulic Engineering 119(2): Parson, J.D., Whipple, K.X. & Simoni, A Experimental study of the grain-flow, fluid-mud transition in debris flows. Journal of Geology 109: Pitman, E.B. & Le L A two-fluid model for avalanche and debris flows. Phil. Trans. R. Soc. A. 363(1832): Rickenmann, D., Laigle, D., McArdell, B.W. & Huebl, J Comparison of 2D debris-flow models with field events. Computational Geosciences. Online. Savage, S.B The mechanics of rapid granular flows. Adv. Appl. Mech. 24: Savage, S.B. & Hutter, K The motion of a finite mass of granular material down a rough incline. J. Fluid Mech. 199: Schatzmann, M Rheometry for large particle fluids and debris flows. Versuchsanstalt für Wasserbau, Hydrologie und Glaziologie der ETH Zürich, Mitteilungen 187. Zürich: Eigenverlag. Takahashi, T. (1991): Debris flow. IAHR Monograph Series. Rotterdam: Balkema. Tecca, P.R., Galgaro, A., Genevois, R. & Deganutti, A.M Developement of a remotely controlled debris flow monitoring system in the Dolomites (Aquabona, Italy). Hydrological Processes 17: