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EARTH-SCIENCE

ELSEVIER Earth-Science Reviews 40 (1996) 209-227

Recognition, classification and mechanical description of debris flows

P. Coussot *, M. Meunier Cemagref, Division Protection contre les Erosions, B.P. 76 Domaine Uniuersitaire, 38402 St-Martin-d’H&es, France

Received 12 July 1995; accepted 8 December 1995

Abstract

Various types of flow or mass movement involving water and sediments occur on steep slopes in mountainous areas. Among them, debris flows are peculiar events during which a large volume of a highly concentrated viscous water-debris mixture flows through a stream channel. Throughout the world these phenomena cause considerable damage but remain poorly understood although a basic knowledge is already available concerning their recognition and propagation.

Firstly, a synthesis of the useful practical criteria of recognition is proposed. Debris flows must be seen as intermediate phenomena between hyperconcentrated flows (intense bed load transport) and landslides separated from them by sharp transitions of some characteristics (celerity, deposit nature and flow type). Two parameters, solid fraction and material type, thought to be appropriate for a sound and practical classification, are brought out, and the corresponding complete classification of flow and mass movements in mountain areas is presented. Two extreme debris flow types are thus distinguished: muddy debris flows and granular debris flows. A critical review of recent advances in debris flow dynamics is then proposed. It is pointed out that adequate work must be carried out in the field of non-Newtonian fluid mechanics. In particular, one fundamental rheological property of debris flow materials is the yield stress, which explains thick deposits on steep slopes and can be inferred from field measurements. Furthermore it can be used to estimate viscous dissipation within the bulk during flow. Relevant models predicting muddy debris flow dynamics are already available whereas further progress is needed concerning granular flows.

1. Introduction

Various types of flow or mass movement involving water and sediments occurring on steep slopes in mountainous areas are cited in literature: flood, solid transport, hyperconcentrated flows, mudflows, debris flows, Mars, granular flows, landslides, debris avalanches, etc. In some cases, it may appear rela-

* Corresponding author. Tel: + 33-76-762805; Fax: + 33-76- 513803, email: [email protected].

tively difficult to distinguish one phenomenon from another whereas each of them is easily recognized by a specialist of the corresponding field. Because of their peculiar characteristics and because of a lack of consensus on a specific classification, mudflows and debris flows remain little-known to the layperson whereas, throughout the world, they remain a perma- nent hazard in mountainous regions. The French expression ‘ ‘lave torrentielle” typically covers the range of peculiar subaerial events often called debris flows or mudflows during which a large volume of a viscous and highly concentrated water-debris mix-

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ture forms, develops and flows along a stream-bed (Demontzey, 1894; Thiby, 1914: Bernard, 1927: Mougin, 1931; Sauret and Colas, 1986). In the following, we shall use the single term debris flow to strictly cover this range.

The typical impressive characteristics of debris flows have been well conveyed by Johnson and Rodine (1984): “A wall of boulders, rocks of all sizes, and oozing mud suddenly appear around the bend in a canyon preceded by a thundrous roar. As the boulder-choked wall passes, the channel remains filled with a debris-laden torrent of mud and boulders clanking and grinding together. The debris flows across an alluvial fan, engulfing structures and cars in its path, covering roads, fields and pastures with a blanket of muck, and slowly coming to a stop as the debris spreads in a lobate form with steep terminal snout and margins.” This kind of phenomenon obviously occurs in mountainous regions, on different scales throughout the world. In Scotland, no hillslope flow deposits exceeding 500 m3 have been found (Innes, 1983), whereas the total solid volume transported within the Jiangjia gully by mudflows can reach 3-5 lo6 m3 every year (Li et al., 1983). Typically these flows cause considerable damage when they run out onto the alluvial fan and reach inhabited zones (Takahashi, 1978, 1981, 1991; Innes, 1983; Johnson and Rodine, 1984). This justifies major research efforts concerning their initiation, propagation and stoppage, in an interdisplinary approach involving various specialities: geography, geology, geomorphology, rheology, fluid mechanics, civil engineering, etc.

Debris flows or mudflows have been recognized and studied by a number of authors (Takahashi, 1981; Li et al., 1983; Costa, 1984; Costa and Williams, 1984; Johnson and Rodine, 1984; Davies, 1986; Pierson, 1986; Qian and Wan, 1986; O’Brien and Julien, 1988; van Steijn, 1988; Meunier. 1991; Whipple and Dunne, 1992) from different points of views: field observations, flow characteristics, material behavior, material components, etc. However, there is not yet general agreement on classification and flow modelling in literature. For example different terms are used by geologists to describe the same events (Innes, 19831, and very different approaches to debris flow behaviour have been developed (Iver- son and Denlinger, 198’7). Moreover, because these

flows involve a large solid fraction and partly behave like a solid (they can remain static on a steep slope). they are often classed as landslides by laypersons, and because they flow in streams and partly behave like liquid they are often classified as ordinary stream tlows. Nevertheless, none of these approaches make it possible to account for the whole range of debris flow characteristics.

The aim of this paper is to review present knowledge of debris flow recognition, classification and propagation in order to provide basic useful tools for practitioners, from geologists to engineers. In order to distinguish sound criteria for debris flow distinction and recognition we first compare the field characteristics of the main types of natural flows and mass movements on steep slopes. This leads us to propose a simple global classification of these movements as a function of only two parameters: solid fraction and material type. In the second section we propose a critical review of present knowledge of debris flow dynamics. Since we are dealing with laminar-free surface flows of non-Newtonian fluids, it is necessary, before flow modelling, to examine the rheological properties of the materials under consideration.

It is worth noticing that, since our aim is to present a simplified, logical, overall view of debris flow mechanisms in comparison with other phenomena, we do not review in detail all possible terms and processes, and we do not describe all possible debris flow characteristics. The reader who wishes to go deeper into these problems can refer to the references quoted below. In the last place, note that we are dealing with current events directly observable in the field. Then, with a view of the recognition and mechanical description of these phenomena, both their processes and products must be analysed and do not need to be clearly distinguished.

2. Classification of mass movements on mountain slopes and situation of debris flows

The distinction and classification of subaerial flows and mass movements occurring in stream basins has long been necessary and a number of classifications may be found in the literature (Bever- age and Culbertson, 1964; Varnes, 1978; Hansen,

P. Coussot, M. Meunier/ Earth-Science Reviews 40 (1996) 209-227 211

1984; Bradley and McCutcheon, 1985; Pierson and Costa, 1987; Sheko, 1988). The criteria used to distinguish phenomena may vary from one author to another: triggering mechanism, basin characteristics, sediment composition, solid fraction, relative bed roughness, velocity, duration, bed slope, material behavior, physical processes during flow, etc. As remarked by Bradley and McCutcheon (1985), these classifications either contradict one another when they are only based on quantitative criteria such as velocity, relative submersion, solid fraction, etc., or are incapable of covering all phenomena. When based on the analysis of the material microstructure or other parameters related to flow characteristics, a classification is difficult to use in practice because field observations carried out after the event do not generally provide the corresponding information. This is the case even for the very interesting classification of Pierson and Costa (19871, which requires knowledge of the deformation rate (expressed through the mean velocity). More generally it is doubtful that mechanical characteristics such as velocity, flow depth or width, are relevant parameters for a classification since they depend on boundary and initial conditions and thus may easily vary from one event to another or during the same event. Finally different terms may be used to describe the same phenomena depending on the scientific background of the authors. This may complicate scientific exchanges and slow down progress in this field.

2.1. Distinction between debris jlows and other mass movements

Here we review the specific properties of debris flows by comparing them to other flows and mass movements. We shall mainly use the terms of flows or mass movements as distinguished by Pierson and Costa (1987) except for earthflow. In the following, earthflow will be referred to as landslide, which is thought to give a clearer idea of the most probable physical processes involved.

2.1.1. Differences between debris jlows and normal or hyperconcentrated stream jlows

2.1.1.1. Transient nature. Debris flows take the form of strongly transient flows, often as almost periodic surges (with a period of a few minutes) of heavily

debris-laden slurry separated by periods of relatively low flow rate or zero flow (Johnson, 1970; Niyazov and Degovets, 1975; Li et al., 1983; Davies, 1986). A critical example is given by the Jiangjia Ravine mudflows which, during 5 h in June 1966, occurred in the form of 126 successive waves, with a volume ranging from 655 and 24,600 m3, a mean material density of 2220 kg/m3 (Li et al., 1983) and a velocity ranging from 3 to 13 m/s. Different explanations for this peculiar phenomenon have been proposed in the literature (Engelund and Wan, 1984; Davies, 1986; Trowbridge, 1987; Coussot, 1992; Wang et al., 1993): hydraulic instability (roll waves, Dressler and Pohle, 1953) typical of sufficiently rapid flows of any fluid on steep slopes, peculiar material behavior (minimum in flow curve, Coussot et al., 1993) which may give rise to unstable flows, and/or surges originated in pulsatile stream-bed erosion and transport processes (bank ruptures, breaking down of local dams, etc.). In comparison, few channelled water flows degenerate into roll waves under some specific conditions (large Froude number, small roughness and uniform channel) or sheetfloods over the alluvial fan (Blair and McPherson, 1994). More- over, the local flow intensity (discharge and flow depth) of normal (ordinary) stream flows or hyperconcentrated flows varies slowly in time and space, with a characteristic time of the same order as changes in hydrological conditions. From this point of view, there is a sharp difference between current stream flows and debris flows. Fundamentally, debris flows are sharp transient phenomena.

2.1.1.2. Number of phases from a jluid mechanics point of view. Within debris flow bulk the relative velocity of two close elements (water or solid) is small [see field observations of Pierson (1986), and video-tapes of Costa and Williams (1984) and Valla et al. (1981>], so that the whole mass apparently undergoes very large and approximately continuous deformations. Additionally the mechanical properties of this mass do not change significantly during shear. Thus debris flows involve a water-debris mixture which, as a first approximation, can be considered as a (one-phase) flow of a viscous fluid. On the contrary, within normal or hyperconcentrated stream flows the mean velocity of the coarsest solid particles which are pushed and rolled on the bed (bed

212 P. Coussot, M. Meunier / Eurth-Science Reuiews 40 (1996) 209-227

load) significantly differs from that of the water-solid suspension which flows around it (Smart and Jaeggi, 1983; Meunier, 1994; Coussot and Meunier, 1995). The velocity profile of the coarsest particles in a channel cross-section is, for example, quite different from that of the surrounding water. This situation may be observed for debris flows but the solid fraction transported in this way remains negligible. Indeed debris flow transport boulders whose diameter goes up to a few meters and which generally looks suspended in the mass [various explanations for this phenomenon have been proposed (yield stress, buoyancy, dispersive pressure, etc.) (see review of Davies, 198611. Nevertheless some excep- tionally large boulders are probably pushed or rolled by the flow. A critical example is given by a stone weighing about 3000 tons that was transported sev- eral kilometers by a debris flow in Japan (Takahashi. 198 11. In addition note that we use the term “ hyperconcentrated flow” for two-phase stream flows with intense bed load transport whereas it seems to refer to one-phase flows for Chinese researchers (Qian and Wan, 1986).

2.1.1.3. Deposit structure. Debris flow deposits are composed of the whole mass (including water just after stoppage) and differ only slightly in form from the stopped flow, even if they undergo slow settling and draining after flow stoppage. Since debris flow materials have a high density, are very viscous and strongly sheared and mixed during flow, no specific significant grain sieving appears within debris flow deposits (Costa, 1984; Coussot, 1992). (However, in many cases, the concentration of big boulders is higher close to the surge front.) Debris flow deposits contain a wide grain size distribution from clay to large boulders (up to a few meters wide) (Johnson, 1970; Takahashi, 1981; Pierson, 1986; Phillips and Davies, 1991; Coussot, 1992) generally with negligible grading or internal layer structures (Friedman et al., 1992). On the contrary, solid deposits on the same slopes (more than a few percent) that originated in other stream (normal and hyperconcentrated) flows, have generally been washed or sieved, so that particular grain size ranges are found. The coarsest solid fraction of these flow types generally deposits first whereas the fine suspension flows away as wash load before being deposited during flows on

smoother slopes or in local stagnation zones (Blair and McPherson, 1994). Thus a sharp difference of nature between debris flows and hyperconcentrated flows appears not only during flow but also after deposition: the former are essentially one-phase flows whereas the latter are two-phase flows.

2. I. 1.4. Solidfraction range. It is also noticeable that there is a jump in solid fraction between hyperconcentrated flows (from 1 to 25%, in general) and debris flows (from 50 to 90% in general). Note that this jump does not exist between the one-phase debris flow front and body and its two-phase hyperconcentrated tail: for example, the channelized debris flow on June 1983 in lower Rudd Canyon in Farm- ington, Utah, turned out to be a hyperconcentrated flow (loss of competence to suspend gravel and turbulence onset) for a solid volume fraction between 46 and 53% (Pierson, 1985a). In order to better understand the difference between these flows, it appears interesting to seek a physical explanation for this difference. For given solid material and flow characteristics, there certainly exists a kind of concentration (percolation) threshold beyond which sedimentation becomes negligible. This threshold, which could also take the form of a transition range, should correspond to the appearance of a continuous interacting network between solid particles (coarse and fine) (Cf. Coussot and Piau, 1995a), which give rise to material strength (or rigidity) (see Section 3.1.). When the solid concentration of the mixture has reached this critical value, we are essentially dealing with a single-phase flow. The grain size distribution and solid fraction of the flowing mixture will vary slowly due to bottom and bank erosion and lateral deposits (except if boundary conditions significantly change but they are assumed to remain constant). On the contrary, if the solid concentration is smaller than the critical value, coarsest particles will fall or stay close to the bottom. The mean solid fraction will decrease, which will soon reduce the ability to transport coarse solid particles in suspension (because of a decrease of the material strength). Then a new grain class will settle rapidly. This chain reaction will go on until reaching a limit of solid concentration at which the flow is able to erode and push or roll a critical amount of coarse particles of critical size over the stream-bed. This situation finally corre-


sponds to a stable hyperconcentrated flow. The above reasonings provide an explanation for the jump in concentration between hyperconcentrated flows and debris flows: under given material- and flow-conditions no stable flow with an intermediate concentration can exist. Thus the transition from one flow type to another should require strong or continuous changes in material or flow conditions.

2.1.2. Distinction between debris flows and landslides or debris avalanches

2.1.2.1. Velocity. In order to compare these phenomena from a dynamic point of view let us consider the mean velocity during the event when the movement can be observed. For debris flows we do not take into account the rest period after stoppage. For landslides we consider both the long active phase during which slow earth motions (soil creep) can be recorded and the possible sudden rapid motion due to large fractures or slippages during which landslides may reach a velocity comparable to that of debris flows or debris avalanches. Debris avalanches are granular mass movements that originate in rocky or granular mass ruptures. From a general point of view, considering the large friction angle of granular masses, they can move rapidly only on steep slopes, for otherwise they would abruptly stop. Once again, it is important to recall that we intend to represent general trends and, for example, do not take into account large debris avalanches which were observed to reach surprising distances, [the body of rock that was shaken loose from Shattered Peak, Alaska, by an earthquake in 1964, extended over kilometers (Sanders, 198111. On the basis of these assumptions, there is generally a clear difference between the velocity of debris flows (between 0.5 and 10 m/s) (Sharp and Nobles, 1953; Johnson, 1970; Morton and Campbell, 1974; Khang, 1980; Pierson, 1980; Takahashi, 1981; Ishikawa, 1982; Hong et al., 1985; Pierson, 1986; Khegai et al., 1992 also reports veloc- ities up to 20 m/s>, that of landslides (less than few centimeters per day) and that of debris (or rock) avalanches (greater than 10 m/s) (Kobayashi, 1992; Evans, 1993).

Within debris avalanches, according to most physical explanations of the flow process (Pierson and Costa, 19871, the role of water, if there is any,

remains minor because grains dilate and the ratio of water to air is small. On the contrary, water lubricates some relative motion of the granular material or is the vehicle of colloidal interactions between clay particles in debris flows. It would be difficult to find any other clear-out differences between these two phenomena.

2.1.2.2. Motion type. Landslides essentially originate in internal fractures (faults or slips) along specific surfaces whereas locally some parts can be continu- ously deformed. As a result, the bulk generally undergoes relatively small deformations so that the initial structure of the material may be partly observed in the final deposit. The latter takes the form of an agglomerate of more or less large pieces of undeformed soil. The relative motion of these pieces has been mainly caused by macroscopic fractures. A similar feature characterizes submarines slides: strata established back upslope may be observed within slump deposits (Dott, 1963). On the contrary, it clearly appears from debris flow deposits that the initial structure of the material has been completely broken and changed during flow without significant macroscopic fault surfaces. Because they flow over long distances, debris flows undergo extremely large deformations. As already remarked, some landslides may accelerate during the last motion phase and end up significantly destructured. This is also true for submarine slumps (Friedman et al., 1992). For example a huge slump off the northwest African continental margin is thought to have become a debris flow that covered an area of 30,000 km3 (Friedman et al., 1992). In this particular case, the difference between the material and deposits of debris flows and landslides can be slight. Nevertheless these two phenomena may be clearly distinguished by considering that debris flows take the form of rapid surges flowing over long distances in stream channels whereas landslides occur on any steep slope and generally move over relatively short distances. (However, in some cases, debris flows originate in a landslide: see Sec- tion 2.1.4.)

2.1.2.3. Deposit aspect. The morphologic features of debris flow deposits have been reviewed by Johnson and Rodine (1984): lobes, snout, lateral levees, me- dial deposits in the channel and deeply incised chan-

214 P. Coussot, M. Mrunirr / Earth-Scirnce Rwiews 40 (1996) 209-227

nels. In streams, lateral levees are due to a fluid flow depth decrease from the front to the tail of the surge: in a given cross-section the lateral parts of the flow are generally shallower (Johnson, 1970) than the central part. Because of the fluid yield stress (see definition in Section 3) the lateral parts will tend to remain static when the flow force acting on them decreases, i.e. when the local fluid discharge decreases, thus leaving lateral levees with a depth of approximately less than 1 m. This observation has been used by Johnson and Rodine (1984) to demon- strate that a quasi-static approach to debris flow (with a Coulomb model) is irrelevant and must be replaced by a fluid mechanics approach. This phenomenon is far less frequent with landslides but, if observed, the levees have a much larger depth. With- out going further into the details of the possible behavior of the material composing landslides this at least shows that their strengths are much greater than those of debris flows (yield stress, see Section 3). Other debris flow morphological characteristics (van Steijn et al., 1988) are also due to specific flow conditions associated with a high fluid yield stress but they may be similar, though in general at a different scale, to some landslide deposit characteristics.

2.1.3. Brief compurison with submarine debris jlows The structure of subaqueous deposits is generally

interpreted for distinguishing rapidly, gravity-dis- placed deposits from those formed by the normal bedded sequences of shale. A critical example is given by oceanic grounds in eastern North America, where “brecciolas” composed of rubble of carbonate rocks (angular fragments up to 0.6 m thick and 2.4 m long) interstratified with dark-colored marine shales formed during the Early Paleozoic times (Rickard and Fisher, 1973). These brecciolas or other peculiar deposits (Friedman et al., 1992; Hiscott and Aksu, 1994) are interpreted as products of turbidity currents, slides (or slumps) and debris flows. Con- cerning these submarine phenomena an extensive literature exists. Suggested classifications for example rely on motion characteristics (inferred from deposit aspect), sediment type and bed’s internal structure (Dott, 1963; Prior and Coleman, 1984; Ghibaudo, 1992). It is worth noticing that one of the main distinctive point between submarine debris

Rows and slides is similar to that between subaerial debris flows and landslides, that is, the initial structures of the second one are partly kept during flow (see Section 2.1.2.). In addition, turbidity currents are turbulent suspensions of sediment in water while submarine debris flows are assumed to be laminar (Friedman et al., 1992). Turbidity currents are quite different from subaerial hyperconcentrated flows in that, for the latter, turbulence can also play a significant role for particle support, but solid particles are essentially dragged by water.

2.1.4. Specificities of the debris jZow initiation process

In the following, we shall discuss the conditions and process of debris flow triggering but we shall not consider the conditions favourable to debris flow activity. The reader is referred to Innes (1983) or Sheko (1988) for some elements concerning the latter problem. From the literature it appears that two main types of debris flow initiation processes may be distinguished. The first one (Sheko, 1988) consists in the progressive transition of a landslide into a debris flow due to an energy increase following a slope increase or due to a water supply. [A number of submarine debris flows are thought to have started as slumps (Friedman et al., 1992)]. This class can be subdivided into two categories depending on the origin of the sliding mass: deposits of granular material accumulated in a zone after upstream erosion (Azimi and Desvarreux, 1974) or waterlogged upper layers of a slope or a bank. Criteria for motion initiation of such masses are derived from soil mechanics (Sheko, 1988; Takahashi, 1991). A theoretical analysis of the initiation of debris flows under roughly similar circumstances (debris accumulation becoming unstable) was provided by Takahashi (1981).

The second type of initiation process, which, from our experience, is the most frequent in the French Alps, essentially relies on a generalized erosion of the surface of the stream basin. Moreover, according to our initial definition (see Introduction), it should be considered as the basic origin of debris flows, since it is intimately linked to the stream hydrological conditions. Generally, in the stream basin, no specific sufficiently deep or large area of erosion can be found which may have provided a significant


fraction of the solid material of the debris flow. The basin and the stream-bed are eroded in an almost uniform way (Bossan, 1992). It is likely that debris flows are due to the conjunction of small-scale bank slides or collapses, bed erosion and solid transport (Davies, 1986). In some cases, all these phenomena can contribute to a chain reaction in the form of an irreversible increase in solid concentration, due to a progressive transition in transport and erosion capacity of the flowing fluid (see Section 2.1.1.) (Sheko, 1988). When debris flows are formed, the nature of stream erosion differs significantly from that of other flows: debris flows rapidly scrape the bed. Since this initiation process is closely associated to the way the stream works, we anticipate that this is the most common phenomena in many countries. This process generally requires a long channel stream to erode and mix a large solid volume with water. This is an intermediate process between solid transport and landslides but it is a critical development of solid transport in the stream-bed. It is thus extremely hard to predict its occurrence. Debris flow form under rare, unknown, specific conditions: stream-bed and bank erosion capacity, water discharge, slopes, etc. These statements are in complete agreement with our postulate of a threshold of solid fraction and flow energy beyond which debris flow can form. More generally, debris flows are probably non-linear phenomena which, in some cases, can develop or not under very slight changes in initial conditions.

Some other exceptional initiation processes have been observed which took their origin in gully dam breaks (Takahashi, 1981), moraine lake ruptures (Lliboutry et al., 19771, ice avalanches (Plafker et al., 197 1) or earthquakes (Solonenko, 1963). Further- more, in some cases, a previously deposited debris flow in the stream channel can be remobilized by additional upstream material and then reach the alluvial fan (Meunier, 1987). Finally, lahars, which can be defined as “volcanic mudflows or debris flows” and whose apparent flow properties are very similar to those of debris flows in streams, form in various specific ways (see Pierson, 1986): water coming either from the melting of snow or ice accumulated on the volcanic cone, or from storms following eruptions, or from heavy rainfalls, etc., mixes with volcanic ashes and other debris. The typical lahars that were triggered by 1980 Mount St. Helens erup-

tion, Washington, covered surprisingly long distances (80 km) (Major and Pierson, 1992) and deposited more than 14. lo6 m3 of material and were the object of extensive measurements of flow characteristics (Pierson, 1985b).

2.1 S. Commentaries Debris flows appear to be an intermediate phe-

nomenon between hyperconcentrated stream flows and landslides either from the point of view of their initiation process or from the point of view of their dynamic characteristics. Various differences have been pointed out between debris flows and other flows or mass movements. It is worth noticing that those phenomena are generally separated by sharp transitions in some characteristics: velocity (between landslides and debris flows), motion of the solid fraction, deposit aspects and flow nature. These remarks should help the field recognition of the different phenomena.

2.2. Simple classification of mass movements as a function of solid fraction and material type

To go further we wish to propose a simple classification of the main mass movements and flows occurring on natural steep slopes. A French version of this classification has already been presented in the literature by Meunier (1991). As noted above, it is rather difficult to find appropriate criteria. The present scheme does not provide a complete, precise solution to this problem but aims at giving a simple, synthetic, conceptual view of flow and mass movements in a single picture with approximately relevant and practical criteria. The different phenomena are separated on the basis of the composition of the flowing material.

The first criterion is the solid fraction type. Refer- ring to the markedly different behavior types of cohesive clay particles, on one hand, and non-cohesive pebbles in water on the other hand, we shall consider two main lines of investigation: fine, cohesive materials and coarse, cohesionless, granular materials. Because exact interaction processes within the bulk are complex and partly unknown, they cannot be separated exactly into clear different types. Thus a strictly physical definition of the materials corresponding to these two lines of investigation

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cannot be given yet. However we may note that a separation on the basis of such a criterion already exists for other flows and mass movements:

_ In river hydraulics two types of solid transport are usually distinguished depending on the way the material is transported: suspension, when the solid fraction is dispersed and flows with the water; bed load, when the solid fraction essentially moves close to the bed. The finest materials (clay and silt) are generally considered to flow in suspension whereas coarser particles move as bed load.

- Landslides generally contain a clay fraction which ensures the cohesion of the bulk and prevails in motion initiation. Debris avalanches, rapid granular flows or rockfalls contain a clay fraction which is either negligible or plays a minor role in flow, the main role being played by coarse particle interactions. However, there is currently no real evidence of these intuitive statements.

Our second criterion is the solid (volume or weight) fraction. Classifications of flows and/or mass movements on steep slopes based at least on this criterion have been proposed by different authors (Beverage and Culbertson, 1964; O’Brien,

1986; Pierson and Costa, 1987). It has been recognized that the sediment concentration globally increases when the flow varies from pure water flow to stream flow with solid transport, then to hyperconcentrated flow, debris flow and landslides or debris avalanches. A general strict distinction of these phenomena according to this single criterion is nevertheless not possible. However, for a given solid material and for given overall flow and bed characteristics, considering the reasonings of Section 2.1.1.) it should be possible to determine a solid fraction transition range limiting hyperconcentrated flows from debris flows and within which none of these flow types would be stable.

Our classification is presented in Fig. 1 in the form of an ellipse. The limits between the different mass movements are only conceptual and qualitative. In fact, they can vary slightly depending on the specific boundary and initial conditions and specific physical material properties. Coussot (1992) has proposed criteria for some of these limits: the transition from debris flows to landslides could correspond to the critical solid fraction and material type in which a fracture occurs after a sufficiently large deforma-

increasing water content

Increasing solid fraction

Fig. I. Classification of mass movements on steep slopes as a function of solid fraction and material type.

P. Coussot, M. Meunier / Earth-Science Reviews 40 (1996) 209-227 217

tion; the transition from hyperconcentrated flows to debris flows could correspond to a critical solid fraction and material type for which settling is negligible within the material during a given time. It is worth noticing that these criteria, though approximative, subjective and empirical, reflect common ideas concerning the main physical differences among hyperconcentrated flows, landslides and debris flows (see Section 2.1.).

3. Debris flow propagation

We do not wish to be exhaustive but only to propose a critical review of main advances in debris flow propagation modelling with a view to practical applications in the field ranging from geology to engineering. Beyond the different possible ways proposed in the literature we wish, at least, to show that, fundamentally, debris flows constitute a peculiar phenomenon whose propagation must be modelled with specific tools found in non-Newtonian fluid mechanics.

As a first step, a debris flow is a mass of a single viscous material undergoing large homogeneous deformations without significant changes to its mechanical properties. From direct field observations and estimations of the corresponding Reynolds numbers, it is apparent that we are generally dealing with laminar flows. Thus soil mechanics tools commonly used to describe the mechanical behaviour of landslides before and just after rupture or hydraulics tools commonly used to describe water flow or two-phase flows such as water + bed load are inade- quate for the description of debris flow dynamics. The appropriate tools must be found in fluid mechanics. Then an additional difficulty appears: a single parameter (viscosity) is insufficient to describe the constitutive equation of these complex suspensions in various flow conditions. Thus, in practice, for the description of the flows of such fluids, the adequate methodology consists in first determining the rheological behaviour (see for example Coleman et al. (1966) or Barnes et al. (1989)) of the flowing mixture. Afterwards the flow characteristics will be determined analytically or numerically by solving flow equations in which the constitutive equation has been introduced, and by taking into account the particular

initial and boundary conditions (cf. for example Batchelor, 1967; Middleton and Wilcock, 1994).

3.1. Rheology

3.1.1. General approach For most materials, the general expression of the

constitutive equation in the form of a relationship among stress tensor, strain rate tensor and time (Truesdell, 1974) remains unknown. Generally, only a few particular (possibly time-dependent) relationships between some components of these tensors are determined under particular conditions. As a first approximation, one can have a good idea of the characteristics of free surface flows by assuming that, within these flow types, the fluid essentially undergoes simple shear. In other words, to all intents and purposes, fluid layers, in the form of parallel planes, glide over one another (see for example Coleman et al. (1966) for a more general definition of simple shear). Under these conditions the extra- stress (deviatoric stress) tensor (obtained after sub- traction of the isotropic pressure term> can be written as a function of three variables: tangential stress (7) and first and second normal stress differences, all of which can depend on time and shear rate (7). The relationship among these variables is the constitutive equation in simple shear flow. From a practical point of view, it is nevertheless difficult to determine normal stress differences. When time effects are negligible, for the description of free surface flows, one will generally use only the relationship linking r and y in a steady state (flow curve), keeping in mind that finite, reasonable normal stress differences could slightly perturbate the flow (Coleman et al., 1966). For a flow over an inclined plane, y is equal to du/d y where u is the fluid velocity and y the current height above plane. In the following, we shall discuss only the form of the flow curve for debris flows. (A detailed review and analysis of possible behaviour types of mass movements and more specifically of landslides was proposed by Iverson, 1985.)

3.1.2. Rheological properties of debris flows The direct determination of the behaviour of de-

bris flow material with the help of rheometers is faced with the irretrievable problem that they gener-

218 P. Coussot, M. Meunivr /Earth-Science Rec,iews 40 (1996) 209-227

ally contain particles of various sizes including big boulders. Usual laboratory rotational rheometers can test about 1 cm3 of material. In the recent years. some large-scale rheometers were designed specifically for testing debris flow samples and they can contain a volume of material in the order of 1 m’ (Phillips and Davies, 1991; Major and Pierson, 1992; Coussot and Piau, 1995b). However if, for example, the coarsest particle diameter is 1 m, an appropriate coaxial cylinder rheometer would have (cf. Coussot and Piau, 1995b) a diameter of at least 120 m, a gap of 30 m and a depth of 300 m, and would contain at least 4106 m3 of material. Obviously such an experi- ment will not occur in the near future. In the mean- time it is necessary to derive, extrapolate or estimate the rheological behaviour of the whole mass by indirect methods, involving theory and experiments with fine materials or the fine fraction of debris flows. In addition, from a general point of view, rheometry with concentrated suspensions is a partic- ularly difficult task. In order to obtain relevant results concerning the rheological properties of the fluid, many precautions must be taken against dis- turbing effects (wall slip, fracture, edge effects. settling, etc.) and appropriate experimental procedures must be used (Mewis and Spaull, 1976: Mewis. 1979; Magnin and Piau, 1987; Magnin and Piau. 1990; Coussot et al., 1993; Mas and Magnin, 1994).

Debris flow material belongs to the class of suspensions. At present, in fundamental rheology, despite the works of Einstein (19561, Batchelor (19701, Batchelor and Green (1972) concerning dilute suspensions of non-interacting particles in a Newtonian fluid, relatively little is known about the relationship between the suspension macroscopic behaviour and their microstructure. For more concentrated or more complex suspensions (see reviews of Mewis and Spaull, 1976; Blanc, 1983; Kamal and Mutel, 1985; Metzner, 1985; Utracki, 19881, semi-empirical models or speculative theoretical models prevail either for concentrated suspensions of non-interacting particles (Frankel and Acrivos, 1967; Goddard, 1977; Adler et al., 1985; Marrucci and Denn, 1985) or for colloidal suspensions (Moore, 1959; Firth and Hunter, 1976; Quemada, 1977; Hunter, 1982; Wilde- muth and Williams, 1984; Dabak and Yucel, 1986; Tsenoglou, 1990; Doraiswamy et al., 199 1; Coussot et al., 1993). With debris flow material, the complex-

ity could scarcely be greater since the interactions at a mesoscopic scale are various and complex. Clay particles immersed in water give rise to colloidal interactions, larger grains interact with one another via solid frictions or collisions, clay particles and grains interact via unknown processes and the solid fraction is very high (Iverson and Denlinger, 19871. Furthermore the relative importance of these interactions is a priori unknown.

In the literature, work has taken two main courses. The first one was initiated by Takahashi (19781 who considered a debris flow material as a suspension of force-free particles in a viscous fluid, undergoing rapid flow. From this analysis Takahashi inferred the possibility of applying the theory of Bagnold (1954) concerning concentrated suspensions of uniform force-free spheres in rapid flow. Bagnold’s model assumes the existence of two regimes (macro-viscous and inertial) which are obtained respectively when energy dissipations are mainly due to interstitial fluid shear or to momentum transfer via particle collisions. Bagnold’s theory also showed that sheared layers of cohesionless particles can undergo a dispersive pressure resulting from collisional momentum transfer perpendicularly to flow direction, and thus can move away from the slope. The inertial regime has been further developed within a three-dimensional “kinetic” theory for dry rapid granular flows (Lun et al., 1984; Savage, 1984; see review of Campbell, 1990). Weaknesses of the final mathematical model of Takahashi (1981) have been reviewed by Iverson and Denlinger (1987). Moreover, though scientifi- cally satisfactory and widely used by geophysicists, in fundamental rheology, Bagnold’s work has never been seriously confirmed and some criticism can be given to his data interpretations (Campbell, 1990; Coussot and Piau, 1995a). Bagnold’s model may be relevant to describe the behavior of extremely rapid flows of dry masses of cohesionless particles, for which it is clear that collisional effects prevail, but should be considered as mainly speculative if used for water-debris mixtures such as debris flow materials. Finally, because it does not consider the possible appearance of a continuous network of interacting particles beyond a critical solid fraction at least at low shear rates, the initial Bagnold theory is unable to predict or give information about debris flow yield stress whereas this appears to be one of


the basic properties of natural events. Indeed, this yield stress, which corresponds to a minimum shear stress that needs to be overcome for flow to take place, is at the origin of deep debris flow deposits which are always observed sometimes even on steep slopes.

On the basis of various field observations (John- son, 1970; Coussot, 1992; Whipple and Dunne, 1992) leading to the conclusion that one of the main characteristics of debris flows is their yield stress, a second class of work considered them as basically viscoplastic fluids. Their yield stress, which may be estimated from field measurements, is then associated to the clay particle interaction network which forms through the material (M’Ewen and Pratt, 1957; Michaels and Bolger, 1962; Firth and Hunter, 1976; van Olphen, 1977; Coussot et al., 1993; Coussot and Piau, 1994a) and must be broken for flow to take place. Various flow curve models have been proposed that were derived from experimental data obtained with laboratory rheometers (Fei, 1982; Wan, 1982; Locat and Demers, 1988; O’Brien and Julien, 1988; Coussot and Piau, 1994a, 1995~; Wang et al., 1994) on the fine fraction of debris flows or with specific large-scale rheometers (Phillips and Davies, 199 1; Major and Pierson, 1992; Coussot and Piau, 1994b), from theoretical considerations (Chen, 1988; Julien and Lan, 1991) or from field observations (Fink et al., 1981; Johnson and Rodine, 1984; Pier- son, 1986; Whipple and Dunne, 1992). The simplest and most often used model is the Bingham model (Bingham and Green, 1919), initially proposed for debris flows by Johnson (1970) and Daido (1971). A large number of works approximated rheometrical data in simple shear using this model. Recent works (Nguyen and Boger, 1983; Coussot and Piau, 1994a, 1995b, c; Atapattu et al., 1995; Coussot, 1995) have proposed to use a more sophisticated one, the Her- schel-Bulkley model (Herschel and Bulkley, 1926) in order to take into account the shear-thinning behaviour of water-clay-grain mixtures or muds (Michaels and Bolger, 1962; Locat and Demers, 1988; Major and Pierson, 1992; Wang et al., 1994) also typical of most other concentrated suspension types when studied in a wide shear rate range. This model writes:

j=Oo7<7,;j#Oo7=7,+K~” (1)

where r is the shear stress magnitude, p the shear rate magnitude, and rc, K and IZ are positive parameters. The Bingham model corresponds to Eq. (1) with n = 1. For each model the only meaningful physical parameter is rc, i.e. the yield stress (Coussot and Piau, 1994a), which increases exponentially with solid fraction within a very wide range. Additionally there is a general rheological and structural similarity between the different suspensions obtained with a given solid material for different solid fractions (Coussot, 1995).

Two very distinct approaches to debris flow behaviour have been proposed in the literature, which may correspond to our separation of mass movements owing to the material components (granular or cohesive). Roughly similar separations were suggested by Fairchild (1985) and Scott (1988) from field observations of lahars. In the case of subaqueous flows, Middleton and Hampton (1976) also distinguished debris flow, for which the main sup- porting force of sediments is the interstitial fluid yield stress, from grain flow, for which grain-to-grain interaction plays this role. This separation type also holds good in the work of Coussot and Piau (1995a) who plotted the various water-debris mixtures in a diagram as a function of the solid fraction and the ratio of silt-and-clay to total solid fraction and, with allowance for settling and fracture limits (see Section 2.2.) proposed a rheological classification of water-debris mixtures as a function of these two parameters (see Fig. 2). These two criteria for a classification are finally analogous to those of our general classification (Fig. 1) and a very close corre- spondence can be established between the different areas of the two classifications. Two main debris flow types were then clearly distinguished: on the one hand, the “muddy debris flows” for which the fine fraction (containing clay) is large enough (say above 10%) for the fine particle-water mixture to form an interstitial fluid which lubricates grain motions and imposes its behavior type on the whole material; on the other hand, the “granular debris flows” for which the fine particle fraction is low enough for direct grain contacts to play a major role on the mass behaviour. It is worth noticing that this classification is a first approximation to reality and, in particular, does not take into account the various possible boundary conditions of the flows. Corre-

220 P. Coussot. M. Meunier/ Earth-Scirnw Reviews 40 (1996) 209-227

sponding muddy debris flows, which are thought to follow a Herschel-Bulkley model, mainly include materials modelled until now with the help of the Bingham model. For (concentrated) granular debris flow material, so far mainly described with the help of the model of Bagnold (1954), it appears that no clear flow curve type could be proposed. It was simply suggested, on the basis of experimental results on different concentrated granular material types (Phillips and Davies, 1991; Coussot, 1992; Kytomaa and Prasad, 1993; Coussot, 19941, that they could exhibit a minimum in the flow curve. In fact, because of the peculiar characteristics of granular materials in various flow conditions (arching, dilatancy, segregation, thixotropy, bistability, jamming, etc.) (cf. the review of Savage, 19841, it is unlikely that confined rheometrical tests can give a realistic idea of the material behaviour during free surface flows. This has, for example, motivated direct flow tests in a large-scale channel (Iverson et al., 1992). More works will be required to establish which process should be taken into account to describe and explain granular flow behaviour among collisions, Coulomb friction, dilatancy, interstitial fluid flow, pore pressure fluctuations, etc., at the different stages of the flow.

Future work will probably give a more exact view of the various behaviour types of water-debris mix-

100%

tures. However, from a practical point of view, it is worth noticing that the rheological models which involve a yield stress, which can be roughly estimated from field measurements or rheometrical tests, are fundamentally the best way of modelling debris flow behaviour. Indeed, during flow, since these fluids are suspensions with high yield stresses, viscous dissipations are generally close to the viscous dissipations computed by assuming that the shear stress within the fluid is equal to the yield stress. Furthermore any model which does not take into account this yield stress, such as Newtonian or power-law models, is incapable of predicting flow stoppage whereas this is often the crucial point either from a civil engineering or a geological point of view. Roughly similar results have been obtained for volcanic lava flows. Whereas initial works considered lavas as Newtonian fluids (Walker, 1967), recent works proved that these materials exhibit a yield stress which mainly decreases with temperature and increases with silica content (Shaw et al., 1968; Pinkerton and Sparks, 1978; McBimey and Murase, 1984). These results opened the way to relevant, though quite approximative, numerical or analytical models (see for example Danes, 1972; Dragoni. 1989). However, the effects of additional phenomena such as degassing, crystals growth, crust formation, thermal exchanges, etc., during lava flows, are still

Landslides (viscoplastic flow

then rupture)

Bed load transport (two-phase flows)

* Shear- thinning

yield stress fluids

0% Fine fraction (~0.04 mm) / Total solid 100%

Fig. 2. Conceptual rheological classification of mass movements as a function of fine content and solid fraction. The exact limits between the different parts of the diagram should be determined for each material and may slightly vary with flow characteristics. (from Coussot, 1992).

P. Coussot, M. Meunier/Ear?h-Science Reviews 40 (1996) 209-227 221

to be explained, and the above approaches are insufficient.

3.2. Flow modelling

Any model which aims at predicting flow characteristics for particular initial and boundary conditions solves flow equations (in an integrated or local form) along with an equation which refers more or less directly to material viscosity and which expresses the resistance of fluid against deformations: wall resistance law, simple shear flow curve, three-dimensional constitutive equation or internal viscous dissipation rate. Then, one or fewer parameters, which necessarily derive from fluid viscosity, are finally introduced in the general fluid mechanics equations. To get an idea of the importance of these ‘ ‘ viscosity parameters” for flow prediction, one simply needs to imagine the great differences in velocity and flow depth of steady water and honey flows through a given channel cross-section for the same discharge. Additionally, it is important to have a very clear picture of the fluid behaviour type within the shear rate range involved in the flow under study. If one wants to predict fluid stoppage along with rapid fluid flows, one has to determine the fluid behaviour within the widest possible shear rate range in order to obtain accurate predictions. These remarks should preclude attempts to predict flow characteristics without relevant presumptions concerning the fluid behaviour type and without a good idea of the values of the corresponding viscosity parameters (Bradley, 1988). In the case of debris flows, some works overcome this difficulty by fitting parameters to previous well-known events. However, this method is relevant only if a realistic behaviour model is used (Bradley and McCutcheon, 1985), which means that separate rheometrical tests should be carried out at least in order to determine the form of the behaviour.

The theoretical analysis of free surface flows for a fluid of known constitutive equation (at least in simple shear) relies on the set of equations that include flow curve, mass conservation and momentum balance. For instance, for a steady uniform flow over an infinitely wide inclined plane, this last equation writes:

r=pg(h-y)sini (2)

where h is the local fluid thickness, g the gravity, p the fluid density and i the plane slope. In this case the flow rate and velocity distribution through a cross-section can be found by integrating the set of Eqs. (1) and (2) ( see for example Liu and Mei, 1989). Also note that the asymptotic flow depth corresponding to static situation is reached when r = r,, which, from Eq. (21, makes it possible to roughly determine fluid yield stress from deposit thickness (Johnson, 1970; Hiscott and James, 1985) or form (Coussot et al., 1995).

Takahashi’s work has been extensively developed towards engineering applications (Takahashi, 199 1) via complex mathematical formulations. Though these developments are remarkable because they cover almost the whole range of problems encoun- tered in the debris flow field and because they probably provide some explanations for some peculiar effects observed with granular flows (Iverson and Denlinger, 1987), they are based on rheological hypotheses which cannot be considered as reliable (cf. Section 3.1.) and require the determination of a coefficient by preliminary fitting. Progress is needed concerning constitutive equations and flow properties of granular materials, either with or without a liquid interstitial phase.

In the case of fine muddy debris flows (usually called mudflows) we enter the complex field of viscoplastic fluid flows (Bird et al., 1982). Research has aimed essentially at providing theoretical relationships between free surface flow characteristics and rheological properties and/or rough experimental confirmation (Paslay and Slibar, 1958; Howard, 1963; Ward and O’Brien, 1980; Naik, 1983; Trow- bridge, 1987; Wang et al., 1993) or at describing the transition from a laminar to a turbulent regime @hang and Ren, 1982; Naik, 1983; Hanks, 1986). Particular attention should be paid to the careful work of Liu and Mei (1989) which provides an extensive (mainly theoretical) study of the spreading of a thin layer of a Bingham fluid on an inclined plane, and to that of Rickenmann ( 1990) who carried out an original study of the transport capacity of slurry flows. This type of study might be useful for possible future work to determine the mechanisms of transition from a two- phase to a one-phase flow in streams. Nevertheless, only recently systematic comparisons of rheological predictions with various free surface flow properties

222 P. Coussot, M. Meunier/Earth-Science Reviews 40 (19961209-227

were done (de Kee et al., 1990; Coussot, 1994; Chilton and Gregory-Smith, 1995). The latter works made it possible to be confident in the use of existing rheometrical measurements for free surface debris flow prediction. Until now this verification has concerned only fine suspensions. However it is reasonable to extrapolate this conclusion to all homogeneous materials whose behaviour type is dictated by the fine interstitial clay-water mixture.

Whether these materials (muddy debris f-lows) are considered to follow a Herschel-Bulkley model or a Bingham model, similarity laws to be used for a small-scale model are readily found from the complete set of flow equations. The usual Froude similarity still holds but the Reynolds number must be replaced by two numbers (for the above models) involving fluid rheological parameters and flow characteristics (Enos, 1977; Coussot, 1994). For example, for flow through a wide open channel, three relevant non-dimensional numbers governing this flow are for example:

(3) where V is the mean fluid velocity through a cross- section and h the fluid depth. For n = I, H, corresponds to the Bingham number. In order to ensure similarity between two fluids flows at two different scales, it appears rather easy to follow a “global” similarity, for example by using a model fluid with reduced rheological parameters (Coussot and Laigle, 1994).

Additionally it was proved (Coussot, 1994) that gradually varying mudflows exhibit the same characteristics (hydraulic jump, subcritical and supercritical regimes, instability, etc.) as water flows. Transient mudflow characteristics may also be predicted considering, as in usual hydraulics (Chow, 1959), that the local wall friction is equal to that of the uniform flow with the same local flow depth and discharge (Laigle and Coussot, 1994, 1995). Like any yield stress fluid, through a channel section reduction, the characteristics of mudflows are quite different from water flows: instead of eddy zones on both sides of the entrance they form dead regions within which the fluid remains static; at the exit, they leave lateral

regions which remain free of fluid (Coussot and Meunier, 1994).

For muddy debris flows small-scale models involving steady flows can at least be used to compare the effect of different structures (checkdams, weirs, etc.) built in streams (Coussot and Laigle, 1994; SOGREAH, 1994). As soon as the material behaviour has been estimated, numerical models can reasonably be developed (O’Brien et al., 1993; Laigle and Coussot, 1994, 1995). Here the transient nature of the flows is taken into account via the use of St-Venant derived equations. When the viscosity parameters are adequate these models must be seen as relatively good tools as long as one uses them for studying natural flows within small spatial and tem- poral scales. In particular the model of Laigle and Coussot (1995) was proved to be capable of predicting laboratory transient flows from independent rheometrical tests without any additional fitting, un- like the work of Takahashi (1991). Typically, we recommend these models for prediction of the extent of runout on the alluvial fan. Indeed the bouldery front, which develops at the front of a channelled debris flow, has a limited volume and thus should play a minor role when the front width has significantly increased. Moreover, erosion should be negligible over a relatively short distance.

For a complete and more exact prediction of channelled natural flows over long distances, either using a small-scale, analytical or numerical model, important unsolved problems remain: the behaviour and role of the bouldery front, the initial conditions (hydrogram) and the role of erosion, lateral deposits and large boulders dispersed in the bulk. This state- ment has for example motivated experimental studies involving model granular materials flowing on a conveyor belt (Davies, 1990) which were able to reproduce these phenomena but the question of the similarity of these laboratory flows with natural events remains open.

4. Conclusions

Debris flows are complex phenomena which are not yet well known. Their initiation is still not predictable. The material involved exhibit complex

P. Coussot, M. Meunier / Earth-Science Reviews 40 (1996) 209-227 223

properties and varies a priori from one flow to another. However, despite these difficulties, it is possible to distinguish some basic useful knowledge concerning their recognition, classification and mechanical properties.

Debris flows must be seen as intermediate phenomena between hyperconcentrated stream flows and landslides separated from them by sharp behaviour transitions, which make it possible to recognize them or their deposits in the field. As a consequence too, their dynamics cannot be treated by usual hydraulic or soils mechanics tools. Their propagation must be studied within the field of non-Newtonian fluid mechanics. Their fundamental rheological property is their yield stress which explains the form that deposits take in the field and can be inferred from field measurements. Additionally it can be used to provide an estimation of viscous dissipations during flow.

For the description of debris flow dynamics with common tools (analytical, numerical or small-scale models) very good knowledge of material behavior and initial and boundary conditions is required. Con- cerning muddy debris flows the present knowledge makes it possible to obtain a reasonable idea of debris flow runout zones. Concerning granular flows the existing knowledge is insufficient and should be developed intensively in the future, maybe on new bases.

Acknowledgements

The “Region RhGne-Alpes” and “P61e Grenoblois d’Etudes et de Recherches sur les Risques Naturels” support is gratefully acknowledged. G.V. Middleton, J.E. Sanders and an anonymous referee helped us improving an earlier version of this paper.

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Maurice Meunier is the head of a research unity working on stream erosion and hydraulics at Cemagref in Grenoble (France). He received high degrees in physics, mathematics from the Ecole Polytechnique (Paris) in 1965 and in agricultural and environmental science from the Ecole Nationale du Genie Ru- ral des Eaux et For&s in 1967. He has been working in various fields of hydraulics in France and Africa. For ten years he has focused on solid transport,

erosion and debris flows in mountain streams and has formed a team and developed various unique experimental equipments for studying these problems.

Philippe Coussot is a researcher in Cemagref (Grenoble) (a laboratory for engineering in environment and agricul- ture). He received high degrees in physics and mathematics from Ecole Polytechnique (Paris) in 1987. Then he worked on debris flows towards his Ph.D. both in Cemagref and in the Lab- oratory of Rheology. He received his Ph.D. in Mechanics from the Institut National Polytechnique de Grenoble in 1992. His works mainly concern both

the theoretical aspects of the rheology and flow characteristics of debris flow or concentrated suspensions and the protection against debris flow damages from a civil engineering point of view.

recognition, classification and mechanical description of ... ›...

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