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Journal of Hydrology 391 (2010) 175–187

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Journal of Hydrology

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Segregation process of water-granular mixtures released down a steep chute

Barbara Zanuttigh a,*, Paolo Ghilardi b

a University of Bologna, DICAM, Viale Risorgimento 2, 40136 Bologna, Italyb University of Pavia, Dipartimento di Ingegneria Idraulica e Ambientale, Via Ferrata 1, 27100 Pavia, Italy

a r t i c l e i n f o s u m m a r y

Article history:Received 2 February 2010Received in revised form 8 May 2010Accepted 15 July 2010

This manuscript was handled by GeoffSyme, Editor-in-Chief, with the assistance ofJohn W. Nicklow, Associate Editor

Keywords:SegregationMixturesExperimentsDebris flow

0022-1694/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.jhydrol.2010.07.016

* Corresponding author. Tel.: +39 0512093754; faxE-mail addresses: [email protected] (B.

(P. Ghilardi).

The objectives of this contribution are to analyse and quantify the segregation process of water saturatedmixtures flowing down steep chutes.

Experiments were carried out using two granular mixtures, a steep chute of variable length and a col-lecting box divided by sectors, in which the flowing material freely falls down at the chute outlet. Theflow characteristics, the filling in time of the box itself, the grain-size composition of the box content withvarying the distance from the chute outlet are examined by means of laser level transducers, image anal-ysis and sieving.

Segregation is effective and rapid along the flow direction, showing a modest dependence on mixturegrain-size composition. The grain-size composition profile in time is reconstructed, showing coarser par-ticles concentrated at the flow front, on the flow surface and at the sides, whereas fine fractions prevail atthe tail and at the bottom.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction by a factor of six or so if boulders are present at the flow front

In several classifications of slope movement types, as the one byVarnes (1978) or the one by Takahashi (1991), debris flows andmud flows are distinguished based on grain size distribution, onpredominant movement rate and/or on the initiation mechanism.However, according to field observations, for instance on the Mt.Yakedake (Suwa and Okuda, 1985) and at Wrightwood (Johnsonand Rodine, 1984), it is clear that several of these fluid/solid fea-tures are simultaneously present in the same debris flow (Iversonand Vallance, 2001). In fact, during flow propagation a significantsegregation process may occur whose consequence is the accumu-lation of the coarser materials at the front and of the finer materialsat the tail.

The segregation mechanism is strictly related with the debriserosive power, which was observed, for instance, by Jian et al.(1983), Okuda et al. (1980) and Davies (1990) was experimentallyinvestigated by Rickenmann et al. (2003). The erosive power maybe relevant during the mobilisation and the development phase,when the front becomes deeper and picks up boulders, and it isvery reduced in the following propagation phase, when the flowis completely segregated and solid interaction forces dominate atthe front and fluid resistance forces prevail at the tail (Iverson,1997). Moreover, the impact force of a mudflow can be increased

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: +39 0512093263.Zanuttigh), [email protected]

(Watanabe and Ikeya, 1981). For these reasons the quantificationof the segregation mechanism has a remarkable importance intechnical applications.

According to some experimental observations, Takahashi(1991) gives the following interpretation about the mechanism ofparticle segregation. As soon as a debris flow is originated, largerparticles move upward while the smaller ones move downward;therefore, if a debris flow suddenly stops an inverse grading inthe deposited layer may be evident. If the flow continues, becauseof the higher velocity of the upper layers, larger particles moveahead accumulating at the front. In fact, when a particle reachesthe front, it tumbles down to the bottom and is buried in the flow,but, if it is larger than the surrounding particles, it appears soonagain on the top of the flow and moves ahead. Takahashi thus ex-plains the accumulation of boulders at the debris front as a conse-quence of the repetition of such mechanism along the traveldistance. Suwa (1988) claims that the big boulders observed atthe front have often a size nearly equal to the flow depth, so theycannot be lifted only by dispersive forces and the mechanism oftheir accumulation towards the front is rather due to their highervelocity with respect to the smaller boulders and the surroundingfluid. An empiric statistical approach was proposed by Savage andLun (1988), who experimentally analysed a binary mixture of PVCspheres flowing down an inclined chute. They defined a probabilis-tic distribution function of void spaces and a characteristic diame-ter, whose value represents the possibility of the single particlebeing captured by the lower layers, and statistically solved the

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176 B. Zanuttigh, P. Ghilardi / Journal of Hydrology 391 (2010) 175–187

problem by means of the maximum entropy approach. More re-cently Gray and Ancey (2009) have investigated how coarse richfronts are connected to the inversely graded flow behind and pre-sented a solution for the recirculation of larger grains at the flowfront.

Only Takahashi (1991) carried out experiments specificallyaimed at quantifying the segregation process both along and trans-versal to the flow of a complex muddy mixture flowing down aninclined chute with erodible bed and falling downstream into acollecting box divided by sectors. Some results on segregationspeed and boulder concentration at the front can be qualitativelyappreciated. However, the explanations of the experimental set-up and of the results are insufficient to allow the repetition of theseexperiments and/or a more complete analysis of the data.

A qualitative contribution on the segregation process of a binarymuddy mixture was given by Davies (1990) in his experiments

Fig. 1. Side view of the experimental set-up, showing the gate, the wheel system, thtransducers, high and normal speed cameras).

aimed at analysing in details the debris wave formation and com-position. The increase of granular concentration in a water flowover a non-erodible bottom, moving at a constant speed, inducea gradual change from a dispersed to an heterogeneous granulardistribution inside the fluid matrix. Local accumulations of grainsprogressively develop into stationary granular waves formed by adry front, a central uniformly deep body and a fluid tail.

Vallance and Savage (2000) performed bi-disperse segregationexperiments with interstitial fluids of various densities and foundthat the segregation rate is reduced by the density contrast be-tween the fluid and the grains. Thornton et al. (2006) derived a the-oretical framework for segregation that yields this result.

Zanuttigh and Di Paolo (2006) analysed the segregation processof complex dry granular avalanches, by adopting an experimentalset-up similar to the one proposed by Savage and Lun (1988) andTakahashi (1991).

e chute, the collecting box and the positions of the instrumentation (laser level

0.4 m

0.1 m

(a)

(b)

B. Zanuttigh, P. Ghilardi / Journal of Hydrology 391 (2010) 175–187 177

The experiments here described are carried out using sedi-ment–water mixtures that are characterised by complex poly-dis-perse sediments. The mixtures are suddenly released down a steepchute of variable length. Downstream of the chute, the free-fallingmaterial is collected in a box divided into longitudinal and trans-versal sectors, following an approach similar to Zanuttigh and DiPaolo (2006).

Aim of this paper is to analyse in depth rapidity and efficiency ofthe segregation process, accounting for the presence of interstitialfluid and for a complex poly-disperse sediment matrix.

The paper is composed of four main parts. Section 2 describesthe experimental set-up and the measurements. Section 3 analysesthe grain-size composition of the material inside the collecting boxdepending on the space left to the material flowing down thechute, i.e. on the chute length. In Section 4 grain-size profiles alongthe flow are reconstructed based on the image analysis of the boxfilling in time. Possible extension of these results to debris flowevents are finally discussed in Section 5 by comparing the resultswith other available experimental works and with fieldobservations.

Fig. 2. (a) Top view of the collecting box, dimensions in cm; labels name central andside cells as in the text and (b) side view of the collecting box with lateral PVC wallsand indication of the distance from the chute outlet.

(b)

(a)

d>4 mm 2<d<4 mm 1<d<2 mm d<1mm

(c)

d>4 mm 2<d<4 mm 1<d<2 mm d<1mm

Fig. 3. (a) Grain-size curves for the two mixtures adopted in the experiments; (b)coloured fractions in mixture 1; (c) coloured fractions in mixture 2. (For interpre-tation of the references to colour in this figure legend, the reader is referred to theweb version of this article.)

2. The experimental set-up

This section describes in detail the experimental equipment, thegranular mixtures used in the tests, the test conditions and themain characteristics of the flow obtained in the chute.

2.1. Equipment

The equipment consists of an inclined chute, a sluice-gate and acollecting box fixed downstream the chute (Fig. 1).

The chute is 0.10 m wide and 6 m long and is kept at a constantslope of 20� with respect to the horizontal. The chute bottom isroughened with 0.3 mm rough sandpaper that avoids both thematerial sliding on the boundary as a solid body and the entrap-ment of finer particles inside the bottom roughness. Chute sidesare made of glass walls to allow optical measurements and videoobservations.

A gate is fixed inside the chute at different distances from thechute outlet ranging from 4 to 1 m, in order to vary the chutelength and thus to test the rapidity of the segregation process. Awheel system guarantees a quick opening of the gate.

The collecting box is 0.37 m long, 0.10 m internally wide (as thechute) and 0.72 m deep with PVC transparent walls 0.01 m thick toallow image analysis (Fig. 2a). The box is subdivided into sectors bymeans of transverse and longitudinal internal walls 0.003 m thick,so that the falling material is divided into three transverse and sixlongitudinal parts. The width of central sectors is twice that of lat-eral cells; cell length is about 0.05–0.06 m each, to obtain an ade-quate representation of the flow stratigraphy with respect to flowdepth without producing grain blockage. A frontal septum and sidecheeks were installed to avoid water dispersion (Fig. 2).

The device affects, due to cell number and size, the resolutionwith which one can get results on flow stratigraphy.

2.2. Granular mixtures

Experiments were carried out using two marble mixtures hav-ing the same lithological compositions but different grain-sizeassortment. The mixtures were prepared to represent two typicalconditions that frequently occur in the field (Berti et al., 1999). Inparticular, the first mixture (mixture 1) is characterised by widelyranging grain size and its distribution curve was derived followingthe method by Füller for the perfect distribution curve of the frac-tions used in the preparation of concrete (Burlamacchi, 1994); in

Table 1Main flow characteristics: vl and vi, velocity derived from laser transducers and fromimage analysis respectively; yp, peak flow depth from the laser level transducerclosest to the chute outlet.

d Mixture 1 Mixture 2

vl, m/s vi, m/s yp, m vl, m/s vi, m/s yp, m

4.0 1.79 1.70 0.035 1.49 1.50 0.0452.0 1.76 1.55 0.027 1.45 1.40 0.0431.5 1.58 1.65 0.036 1.41 1.47 0.0551.0 0.84 0.70 0.042 0.83 0.94 0.046


several works in fact debris are recognised to qualitatively flow ‘‘asfresh concrete”, among the others, Davies (1990) and Iverson(1997). The second mixture (mixture 2) was chosen to be less dis-persed and biased towards the coarser fraction. Curves of grain-size composition are shown in Fig. 3a.

In order to get a qualitative idea of the segregation process, themixture grain-size was divided into four classes based on d50 thatis the median or 50th percentile of sediment size: coarse fraction,d50 > 4 mm; middle-coarse fraction, d50 = 2–4 mm; middle-finefraction, d50 = 1–2 mm; fine fraction, d50 < 1 mm. Each class wasrepresented by the same marble grains but different natural col-ours, see Fig. 3b and c.

The properties of the two mixtures were measured without anycompaction or vibration and produced similar results: the bulkdensity in air qb is 2610 kg m�3 for both mixtures; internal frictionangle / is 29� for mixture 1 and 34� for mixture 2; porosity isaround 0.32 for mixture 1 and 0.37 for mixture 2.

2.3. Tests

Eight tests were globally carried out, by using the two mixturesand placing the gate at four distances from the chute outlet: 4 m,2 m, 1.5 m, 1 m. Each test was performed twice to check therepeatability of configuration and results.

Volumetric concentrations of water cw and particles cs togetherwith the optimal shape of the sediment volume stored upstreamthe gate were experimentally derived after some preliminary tests.The optimal value of cw for generating a mature debris event afterthe gate sudden release was found to be equal to 60% for both mix-tures. This value of cw is in agreement with field observations (Iver-son, 1997) where it usually ranges between 40% and 60%. For bothmixtures, the sediment mass used in each experiment was 7 kgand average mixture density qm was 1644 kg m�3.

For each experiment, sediment saturation was obtained intotwo subsequent phases. First the whole 7 kg sediment mass waspoured into a 5 l box and then the water was added until no morecould be retained. After about 2 h, the remaining water (up to4.05 l globally) and then the content of the box were poured up-stream the gate. The volume of sediment was finally shaped asthe trapezoid drawn in Fig. 4, where specific attention was paidto the slope angle – equal to / and to the absence of water ontop of the material in contact with the gate.

2.4. Measurements

Measurements (see Fig. 1) were carried out by means of:

� two laser level transducers placed at a mutual distance of 0.2 m,with the downstream one placed at 0.1 m from the chute outlet.

Fig. 4. Storage of water and mixture upstream the gate before its sudden release

Laser level transducers acquired overall the test the local flowdepth and allowed to determine the front velocity;� a high-speed camera with 640 � 480 px resolution and an aver-

age acquisition rate of 50 frames/s. The high-speed camera wasadopted to register during the whole test the falling process ofthe mixture jet into the collecting box;� a standard camera with an average acquisition rate of 25

frames/s to record the advance of the flowing material and itsfront velocity;� a sieve with the following filters: 8–4.76–4–2–1–0.5–0.25 mm,

to estimate the fractions of the material filling in the collectingbox, more details below;� a high precision balance with resolution ±0.01 g (for weights

lower than 5 kg) and a mechanical balance with resolution±20 g (for weights greater than 5 kg).

When the flow stopped, side images of the filled box were takento qualitatively register the vertical stratigraphy and the disposi-tion of the granular fractions and of the water in the side sectors.Therefore the water and the granular content of the side sectorswas separated and then carefully weighed, joining the materialfallen in transversely corresponding sectors. The granular materialwas also sieved, leading to one sieving of the side sectors for eachtest.

After emptying the side sectors, side images of the segregatorbox were taken again to visually evaluate the stratigraphy andthe disposition of the granular fractions in the central sectors.The material in each central sector was carefully weighed andsieved, for a total of six sievings for each test.

2.5. Flow characteristics

Main characteristics of the flows generated in the chute afterthe gate sudden release are summarised in Table 1. Fig. 5 showsfor each test the flow depth in time acquired at the downstream la-ser level transducer. The surge forming is evident: the records

. From left to right, (a) scheme and (b) side view of one test with mixture 1.


present a steep front, a central body of quite constant depth and atail in which flow depth slowly decreases. The flow front appears tohave a sharper shape in case of mixture 1 rather than for mixture 2.

Peak flow depth is on average 0.03 m for mixture 1 and 0.04 mfor mixture 2, being also slightly different the front velocities:around 1.4 m (up to 1.5 m if one considers the image analysis only)for mixture 1 and around 1.3 m for mixture 2. In all cases the Fro-ude number derived from front velocity and peak flow depth is

(a)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Flow

dep

th (c

m)

Time (s)

L=4 m

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Flow

dep

ths

(cm

)

Time (s)

L=1.5 m

(b)

0.00.51.01.52.02.53.03.54.04.5

Flow

dep

th (c

m)

Time (s)

L=4 m

0.00.51.01.52.02.53.03.54.04.55.05.5

Flow

dep

th (c

m)

Time (s)

L=1.5 m

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

Fig. 5. Flow depth in time measured by the laser lev

greater than 1, and specifically it ranges from 1.3 to 3.4 for mixture1 and from 1.2 to 2.2 for mixture 2.

Mixture 1 due to its better grain inter-locking shows globallya more fluid behaviour than mixture 2, with greater velocitiesand lower peak depth. The deeper front of mixture 2 can becertainly related to its coarser matrix and –as we will see inthe next section – to segregation of the coarser fraction tothe front.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Time (s)

L=2 m

0.00.51.01.52.02.53.03.54.04.5

Time (s)

L= 1m

0.00.51.01.52.02.53.03.54.04.5

Time (s)

L=2 m

0.00.51.01.52.02.53.03.54.04.55.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0Time (s)

L=1 m

el transducer, (a) mixture 1 and (b) mixture 2.


3. Analysis of the segregation process

This section presents the most significant results obtained fromthe tests described above, in detail: the segregation process per-pendicular and parallel to the flow direction; the process speedby varying the space left to the mixtures for flowing down thechute; effects on the segregation process due to the differentgrain-size composition of the mixtures.

3.1. Segregation parallel to flow direction and segregation speed

� For each test, the segregation process parallel to the flow direc-tion was analysed first by weighing the water and the solid frac-tion filling each one of the central cells of the collecting box.� Then by separately sieving the granular material filling each

cell.

Table 2 summarises the results of the whole test for all chutelengths and for both mixtures. Figs. 6–9 detail the results for bothmixtures and for two specific chute lengths, 4 m and 1.5 m, whichare shown to be particularly relevant for this analysis.

For mixture 1 and 4 m chute length, Fig. 6a shows the percent-age of water and granular fractions distributed in the cells andFig. 6b details the results obtained by sieving the solid fractionsin each cell.

The solid material is concentrated at the flow front, being C5and C6 the most filled in cells (Fig. 6a). The water content tendsto decrease with decreasing the distance from the chute outlet:this trend may be explained if one considers that the flow frontis faster and therefore more dilated, so more water can penetrate.Indeed in the cells closer to the chute outlet the water content,even if lower than in the other cells, prevails on the granular frac-tion and proves the formation of the typical fluid tail. The coarseand medium-coarse fractions monotonically increase whereasthe medium-fine fraction decreases with increasing the distance

Table 2Results from sieving the dried material. Percentage of the granular material contained in th

Mixture 1

d < 1 mm 1 mm < d < 2 mm 2 mm < d < 4 mm d > 4 mm

L = 4 mC1 60.9 21.5 6.3 11.4C2 51.3 28.0 13.8 6.9C3 41.6 27.5 22.1 8.8C4 28.7 23.3 27.0 21.0C5 16.9 17.3 29.0 36.8C6 9.9 11.6 29.6 48.8

L = 2 mC1 52.8 29.8 12.5 4.9C2 38.9 28.0 24.5 8.6C3 31.1 24.8 28.3 15.8C4 24.0 21.3 28.5 26.2C5 15.7 14.6 28.0 41.7C6 8.5 8.5 24.2 58.8

L = 1.5 mC1 41.8 20.0 17.8 20.4C2 41.4 26.2 18.3 14.1C3 34.2 24.7 24.5 16.6C4 20.2 20.4 24.5 34.9C5 13.1 13.8 24.1 49.0C6 9.6 9.8 26.4 54.2

L = 1 mC1 36.0 22.4 22.9 18.6C2 21.5 16.2 23.1 39.1C3 29.5 21.5 22.8 26.2C4 19.8 18.0 23.4 38.7C5 12.4 13.7 31.2 42.8C6 17.5 17.0 23.5 42.1

from the chute outlet. In cells C5 and C6 around the 80% of thematerial is composed by the coarse and medium-coarse fractions,whereas in cells C1 and C2 it can be found a mirror-like behaviour,i.e. medium-fine and fine fractions are up to the 80% of the cell con-tent (Fig. 6b).

Fig. 7a and b shows respectively the percentage of water/solidand the grain-size composition in each cell for mixture 2 and4 m chute length.

Cell C6 is poorly filled in only by the solitary boulders thattend to precede the flowing front. A part from this cell C6, thewater content (Fig. 7a) decreases with decreasing the distancefrom the chute outlet as for mixture 1 (Fig. 6a). Regarding waterdistribution, the same comments given above (Fig. 6a) stillhold and can be applied also to the figures presented in thefollowing.

The flowing front falls in cell C5, which results almost only filledin by medium-coarse and coarse fractions. The percentage of thecoarse and medium-coarse fractions (Fig. 7b) increases from 25%in C1 to 90% in C6.

The composition of the material filling the central cells is de-scribed in Fig. 8a and b for mixture 1 and 1.5 m chute length.As for mixture 2 when the chute length is equal to 4 m(Fig. 6a), the cell C6 is filling by the boulders preceding the frontand by water due to turbulence (Fig. 8a). As for the tests withchute length equal to 4 m (Figs. 6a and 7a), with the exceptionof cell C6, the water content decreases with decreasing distancefrom the chute outlet.

The quantity of the coarser material (Fig. 8b) decreases withdecreasing the distance from the chute outlet as for all the casespreviously analysed (Figs. 6b and 7b). It can be appreciated thatthere is still evidence of the coarser material in C1 (20% of the total,double than in Fig. 6b) and of the finer material in C6 (around 20%of the total, as in Fig. 6). The segregation trend is thus clear but thecoarse and the fine fractions are not separated as for results at 4 mchute length (Fig. 6a and b).

e central cells divided into the four main classes. Mixtures 1 and 2, all chute lengths.

Mixture 2

d < 1 mm 1 mm < d < 2 mm 2 mm < d < 4 mm d > 4 mm

L = 4 mC1 42.5 31.8 15.2 10.4C2 32.0 29.5 22.2 16.3C3 20.9 23.4 28.4 27.3C4 10.1 15.4 28.1 46.3C5 3.8 8.1 29.7 58.4C6 2.5 4.8 23.1 69.6

L = 2 mC1 39.9 30.9 11.9 17.4C2 26.6 32.7 25.0 15.7C3 17.6 22.9 27.6 31.9C4 9.0 14.2 25.1 51.8C5 4.4 9.0 27.6 59.1C6 4.8 8.8 25.3 61.1

L = 1.5 mC1 27.7 28.9 20.1 23.4C2 18.2 20.8 25.4 35.5C3 13.9 18.0 25.5 42.6C4 7.1 14.0 19.5 59.5C5 4.9 9.6 27.1 58.4C6 4.6 9.0 24.3 62.2

L = 1 mC1 20.5 23.0 24.5 31.9C2 15.2 19.9 28.3 36.6C3 10.6 14.0 23.8 51.5C4 5.2 8.0 24.9 61.8C5 10.4 14.3 32.9 42.3C6 1.8 2.4 14.9 80.9


Fig. 9a and b represents the distribution of water and grain frac-tions for mixture 2 and 1.5 m chute length. The results are similarto the ones already shown in Fig. 8a and b for the correspondingcase with mixture 1.

Fig. 9a shows that the material is concentrated in cells C4 andC5. The material filling C6 is essentially composed by bouldersanticipating the flow front and by water due to turbulence. BesidesC6, the concentration of the coarser material decreases withdecreasing the distance from the chute outlet, i.e. from C5 to C1.

Fig. 9b shows that the finer material decreases and the coarserincreases from C1 to C6. This trend denotes that the segregationprocess is started. Indeed there is a non-negligible percentage ofcoarser material in C1 (23% of the total, more than double thanin Fig. 7b) and of fine material in C6 (around 5% of the total, morethan double than in Fig. 7).

Fig. 10 summarises the experimental observations by graphi-cally representing the results reported in Table 2. It shows the per-centage of medium-fine and medium-coarse fractions filling eachcentral cell of the box at different chute lengths. For mixture 1,Fig. 10a, it can be appreciated that the difference in grain-size com-position of each cell is really modest when comparing the resultsobtained at L = 1.5 m and L = 2 m. For mixture 2, Fig. 10b, the per-centage of the medium-fine and medium-coarse fractions fillingeach cell is almost the same when considering L = 2 m and L = 4 m.

If one examines the flow characteristics synthesized in Table 1and the flow records reproduced in Fig. 5, it can be observed thatflow depth, flow velocity and the shape of the surge are similarwithin the test conditions starting from L = 1.5 m for mixture 1and from L = 2 m for mixture 2. With the term ‘‘complete segrega-

(a)

0

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C1 C2 C3 C4 C5 C6

Hei

ghts

(cm

)

Cells

d>4 mm2 mm <d<4 mm1 mm <d<2 mmd<1 mmWater

(b)

11.4 6.9 8.821.0

36.848.8

6.3 13.822.1

27.0

29.0

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21.528.0

27.5

23.3

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60.951.3

41.628.7

16.9 9.9

0%10%20%30%40%50%60%70%80%90%

100%

C1 C2 C3 C4 C5 C6

Perc

enta

ge (%

)

Cells

d>4 mm 2 mm<d<4 mm 1 mm<d<2 mm d<1 mm

Fig. 6. Percentage of material filling the central cells of the collecting box: (a)including water and (b) considering only the granular material divided into the fourclasses and re-normalising to 100%. From left to right, chute length of 4 m. Mixture 1.

tion” we will thus refer in the following to the configuration whenthe percentage of the medium-fine and medium-coarse fractionsfilling each cell remains almost constant with changing chutelength and similar conditions of the flowing material. .

In conclusion, from the joint analysis of Figs. 6–9 and of the re-sults reported in Table 2 and in Fig. 10 one can get the followingresults:

� both mixtures 1 and 2 are perfectly segregated after travellingfor 4 m down the chute inclined at 20�, with the coarser mate-rial concentrated at the front and decreasing from the front tothe tail, leading to an inverse behaviour of the finer material;� both mixtures 1 and 2 are not segregated after travelling for 1 m

down the chute inclined at 20�, being the granular fractions dis-tributed in a complex way that essentially depends on the mix-ture assortment before the gate sudden release;� after travelling for 1.5 m, the segregation process is evident in

both mixtures, but there is still high percentage of coarser frac-tions at the flow tail;� after travelling for 2 m, based on the comparison with the

results achieved at 4 m, the segregation process appears com-pleted for mixture 1, and still under way for mixture 2.

It can be concluded that complete segregation occurs for a tra-vel distance between 1.5 and 2 m for mixture 1, and between 2 and4 m for mixture 2. If one considers an average peak flow depth of0.03 m for mixture 1 and of 0.04 m for mixture 2, the complete seg-regation is reached respectively at 66 and 100 times the averagepeak flow depth.

(a)

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80

C1 C2 C3 C4 C5 C6

Hei

ghts

(cm

)

Cells


(b)

10.4 16.327.3

46.358.4

69.6

15.222.2

28.4

28.1

29.723.1

31.8

29.5

23.4

15.4

8.1 4.8

42.532.0

20.910.1

3.8 2.5

0%10%20%30%40%50%60%70%80%90%

100%

C1 C2 C3 C4 C5 C6

Perc

enta

ge (%

)

Cells


Fig. 7. Percentage of material filling the central cells of the collecting box: (a)including water and (b) considering only the granular material divided into the fourclasses and re-normalising to 100%. From left to right, chute length of 4 m. Mixture 2.


3.2. Segregation perpendicular to flow direction

In all tests, the careful blending of the mixture and the instanta-neous opening of the gate allowed an almost symmetric filling ofthe segregator. In the free falling from the chute outlet to the col-lecting bin, the flowing material did not tend to dilate and thereforethe filling of the side cells was very modest with respect to the cen-tral ones (never exceeding the 10% of the whole flowing mass).

For these reason, and also to allow the image analysis of the fall-ing into the central cells, the side cells placed on the camera sidewere left open at the bottom. Moreover, the modest quantity ofmaterial entrapped in the remaining side cells on the opposite sidewas weighted and sieved all together.

Grain-size composition of side cells is given in Table 3 and is com-paredwiththeaveragecompositionobtainedinthesametestsforthecentral cells. From image analysis, it was observed a greater filling ofthe side cells in tests with mixture 2 rather than in tests for mixture 1.

It can be appreciated that the content of the side cells, for bothmixtures and in all the tests, show a greater percentage of the coar-ser fraction than the central cells. There is thus also a segregationof coarser fractions from the central body of the flowing materialtowards the chute walls. This result does not surprise if one consid-ers the tendency to the deposition of large boulders at levees dur-ing debris events, see for instance Johnson and Rodine (1984),Iverson (1997), Zanuttigh and Lamberti (2007).

3.3. Grain-size composition effect

Based on the results regarding segregation speed – already re-ported in Section 3.1 – the segregation process appears to slightly

(a)

0

10

20

30

40

50

60

70

80

C1 C2 C3 C4 C5 C6

Hei

ghts

(cm

)

Cells


(b)

20.4 14.1 16.634.9

49.0 54.217.818.3

24.5

24.5

24.126.4

20.0 26.224.7

20.4

13.89.8

41.8 41.4 34.220.2

13.1 9.6

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

C1 C2 C3 C4 C5 C6

Perc

enta

ge (%

)

Cells


Fig. 8. Percentage of material filling the central cells of the collecting box: (a)including water and (b) considering only the granular material divided into the fourclasses and re-normalising to 100%. From left to right, chute length of 1.5 m.Mixture 1.

depend on grain-size composition. In fact the mixture character-ised by a coarser matrix (mixture 2) needs to travel down the chutefor a greater distance before reaching a totally segregatedconfiguration.

This delay can be justified with the mixture porosity greater formixture 2 than for mixture 1. Indeed the greater water quantity in-side the pores can reduce the collision among particles and thusalso particle mixing and consequent segregation.

When segregation process is completed, it appears moremarked for mixture 2 than for mixture 1, as one can observe fromthe comparisons between Figs. 7b–9b and Figs. 6b–8b respectively.

4. Reconstruction of flow grain-size composition profiles

The main purpose of this experimental analysis is to find outhow grain-size composition changes in time and space inside theflow. A method was developed to reconstruct such profiles basedon the measurements and the results already presented in the pre-vious sections.

The material flowing at the chute end in a certain time step Dthas to be equal to the amount of material falling inside the box inthe same Dt. The continuity equation can be written as

DVf ¼X18

1

DVif ¼X18

1

AicDyic ¼ ubyDt ð1Þ

where DVf is the total volume variation of the material insidethe bin within a given Dt, DVif is the volume variation of the

(a)

0

10

20

30

40

50

60

70

80

Hei

ghts

(cm

)

Cells


(b)

23.435.5 42.6

59.5 58.4 62.220.1

25.425.5

19.5 27.1 24.3

28.9

20.818.0

14.09.6 9.0

27.718.2 13.9 7.1 4.9 4.6

0%10%20%30%40%50%60%70%80%90%

100%

C1 C2 C3 C4 C5 C6

C1 C2 C3 C4 C5 C6

Perc

enta

ge (%

)

Cells


Fig. 9. Percentage of material filling the central cells of the collecting box: (a)including water and (b) considering only the granular material divided into the fourclasses and re-normalising to 100%. From left to right, chute length of 1.5 m.Mixture 2.

(b)

(a)

Fig. 10. Percentage of medium-fine and medium-coarse fractions filling each cell of the box for different chute lengths: (a) mixture 1 and (b) mixture 2. These plots representand compare the results reported in Tab. 1.

Table 3Results from sieving the dried material. Comparison between the percentage of the granular material contained globally in the central cells and in the side cells divided into thefour main classes. Mixtures 1 and 2, all chute lengths.

Mixture 1 Mixture 2

d < 1 mm 1 mm < d < 2 mm 2 mm < d < 4 mm d > 4 mm d < 1 mm 1 mm < d < 2 mm 2 mm < d < 4 mm d > 4 mm

L = 4 m L = 4 mc 26.5 18.0 17.7 37.7 c 28.9 19.2 19.3 32.6s 19.8 16.8 28.0 35.4 s 8.9 11.5 25.2 54.4

L = 2 m L = 2 mc 41.8 18.0 16.0 24.1 c 26.6 24.5 16.7 32.2s 17.3 15.8 28.4 38.6 s 5.8 9.4 23.7 61.1

L = 1.5 m L = 1.5 mc 39.3 25.8 14.7 20.2 c 36.6 27.9 16.0 19.6s 17.2 18.6 30.7 33.5 s 9.8 11.9 22.5 55.8

L = 1 m L = 1 mc 39.1 25.3 21.0 14.6 c 28.5 20.8 17.3 33.4s 17.9 17.8 29.1 35.3 s 7.8 11.6 25.5 55.1


material filling a single cell within a given Dt, Aic is the area ofa single cell, Dyic is the variation of material depth inside each

bin cell during a given Dt, b is the chute width, y and u are flowdepth and velocity.


Depths were accurately measured at a frequency of 50 Hz withthe transducer placed about 0.10 m upstream of the chute outlet.The time step Dt in Eq. (1) is variable and corresponds to the timesteps at which an appreciable variation of volumes inside the boxoccurred (see, for instance, Fig. 11a and b); both Dt and the compo-nents of DVif for each cell were derived from the videos.

The procedure based on Eq. (1) is composed of the followingsteps, to be repeated for each test.

Based on the previous quantitative results of the sieving, anaverage particle diameter is derived for each cell C1–C6 from aweighted average of the diameters of the present fractions andtheir corresponding quantities.

Then the videos of the free falling of the mixtures at the chuteoutlet inside the collecting box are analysed (example inFig. 11b) and the frames where the filling of each cell is appreciableare selected.

The jet has a variable thickness comprised between the trajec-tories of the fastest (on flow top) and slowest particles (at chuteand flow bottom). Based on the selected frames, the associatedtime intervals, the cell or the cells that are contemporarily filledin and the height of the material filling each cell at a given instantare registered.

Finally the change in deposition height (and thus in volume) ineach cell can be transformed by means of Eq. (1) into a given depthof the flowing material down the chute. In fact, the remaining un-known in Eq. (1) is only the associated depth: first member of Eq.(1) is now known, the chute width is constant, the time interval isfixed from the image analysis, the velocity is derived from the dis-

(a)

0

10

20

30

40

50

60

70

C1 C2 C3

Hei

ghts

(cm

)

0.12 s 0.24 s1.40 s 1.72 s

(b)

Fig. 11. Filling process of the segregator box derived from the frame by frame analysis oimages captured at 0.24 s, 1.40 s and 2.80 s respectively from left to right. Chute length

tance from the chute outlet covered by the trajectories of the jet inthe time interval.

The flow depth profile recorded by the transducer can thus firstbe divided along the time axis, depending on the selected timeintervals. Then for each time interval the flow depth profile canbe also divided along the depth axis, depending on the height ofthe material present in the corresponding cell/cells. Finally eachportion of the profile delimited in this way can be associated tothe average cell diameter derived for the corresponding cell.

The grain-size composition profiles obtained for mixtures 1 and2 for two chute lengths, 4 m and 1 m, are reported in Figs. 12 and13 respectively. The tests for L = 1 m and L = 4 m are selected toshow the profiles for the two ‘‘extreme” chute lengths. Moreover,during these tests the pictures of the material filling the box wereparticularly clear and allowed a more accurate analysis. It can beobserved that in both Figs. 12 and 13 the length of the time axisand the time intervals along it are not exactly the same. As ex-plained above, the time interval is determined by the possibilityto well describe the process based on the available pictures. As re-gards the length of the time axis, this is related to different flowconditions. In the case of L = 4 m, the flow front is deeper and nar-rower, so that there is almost no flowing material after 3 s and it isanyway the same material analysed in the last time interval. AtL = 1 m instead the flow front is lower and wider so that it is nec-essary to analyse the filling-flowing process till 4–5 s. Profiles inFig. 12 shows for both mixtures the concentration of coarser parti-cles at the front and at the top of the flowing body. The flow tail iscomposed only by the finer fractions. When the chute length is

C4 C5 C6Cells

0.44 s 0.80 s 1.00 s2.00 s 2.80 s

f the video recordings: (a) percentage filling versus the central cells and (b) sampleequal to 4 m, mixture 2.

(a)

(b)

Time [s]

Time [s]

Flow

dep

th [c

m]

Flow

dep

th [c

m]

Fig. 12. Profile showing the composition of the flowing material in time. Chute length equal to 4 m, (a) mixture 1 and (b) mixture 2.


equal to 1 m, see Fig. 13, it can be appreciated in both mixtures theformation of two waves. The preceding wave is composed by med-ium and fine fractions, whereas most of the coarser particles ap-pear to be lifted on top of the front of the second wave. Lookingat the differences between the reconstructed profiles for each mix-ture it can be appreciated the change of grain-size compositionwhen segregation is not evident as in Fig. 13 with respect to Fig. 12.

5. Discussion

The flow of a water saturated granular mixture – due essentiallyto permeability effects – is at best an analogy of a field debris flow,not a theoretically justified model and it is therefore not wise toextrapolate quantitative results from the laboratory to the field.Nevertheless, the experimental results show some at least qualita-tive matches with debris flows that are now discussed.

The profiles obtained from these experiments are in agreementwith debris flow profiles documented by several field observationsand reconstructed schemes. The evidence of boulders on the debrissurface and of bouldery surges followed by muddy tails is proven –among others- by images captured at Mt. Yakedake, Japan (Suwaand Okuda, 1985, event of September 5, 1983); at Moscardo (Aratt-ano and Trebbo, 2000, events of June 22 and July 8, 1996), ItalianAlps; at Fully (Rickenmann, 2000, event of October 15, 2000), SwissAlps. The coarser composition of the flow front of the debris, whosebottom and tail consist of medium-fine particles, is observed andschematised by Suwa and Okuda (1985) at Mt. Yakedake, is re-corded by Marchi et al. (2002) at Moscardo, is shown by videos

at Fully (Arattano and Trebbo, 2000), is reconstructed throughin situ measurements and video recordings at Acquabona by Gene-vois et al. (2001), is visible from close views of the debris front inthe Curah Lengking river, Semeru, Indonesia (Lavigne et al., 2003).

Nevertheless, the profiles here reconstructed are only one of thepossible longitudinal profile a real debris flow can show in thefield. For instance, debris flows may develop as surges formed bya plug flow of boulders pushed downstream by the medium-finermaterial, as in Nojiri Rive, Kagoshima, Japan (Iverson, 1997) or inTumulat creek, Portland, Oregon (Johnson and Rodine, 1984).When the debris matrix is muddy, debris flow often appear as aturbulent mud wave or a train of mud waves in which bouldersare well-mixed and sporadically show-up; similar waves are ob-served at Jiang-Jia Ravine, China, (Davies et al., 1991; Jian et al.,1983); Illgraben (events of June 28, 2000 and June 28, 2001) andDorfbach (event of May 20, 1995), Swiss Alps, (Rickenmann et al.,2003); Kamikamihori, Japan (Okuda et al., 1980); Mt Thomas,New Zealand (Pierson, 1980); Belvedere glacier (event of August27, 1991), Bormio (event of July 22, 1992), Marderello (event of Au-gust 13, 1993), Ardenno (event of June 26–27, 1998), all in the Ital-ian Alps (Arattano and Trebbo, 2000).

Regarding segregation speed, the results presented here can bediscussed by comparison with other available works.

Savage and Lun (1988) used a binary mixture of spherical par-ticles, a rough bottom chute inclined of 26� with respect to the hor-izontal, an initial flow depth of 1.5 cm. By adopting a percentage ofsmall particles equal to 10% and 15%, the complete material segre-gation occurred at about 55 cm and 65 cm from the chute outlet

(a)

(b)

Time [s]

Time [s]

Flow

dep

th [c

m]

Flow

dep

th [c

m]

Fig. 13. Profile showing the composition of the flowing material in time. Chute length equal to 1 m, (a) mixture 1 and (b) mixture 2.


respectively. By increasing the chute inclination to 28�, the dis-tance at which segregation was completed increased. Accordingto Savage, this unexpected result can be explained as follows. Asthe chute slope increases, the flow bulk material flows at a higherrate and this leads to a decrease of the bulk solid fraction (or cor-respondingly, an increase of the voids fraction) and to an increaseof the single particle velocity fluctuations. Under such conditions,diffusive mixing of the particles becomes more important and seg-regation effects are thus inhibited. The effects of diffusive remixingat higher flow rates and slope inclinations has also been observedby Gray and Chugunov (2006).

The experiments carried out by Takahashi (1991) give someindications on the water effect on segregation speed. A constantwater discharge (0.002 m3/s) and a known concentration of vari-ously assorted granular material was made flow down a rectangu-lar rough chute inclined of 26� with respect to the horizontal. It canbe qualitatively observed that with increasing the distance theflow covers, the forefront becomes dryer and coarse componentsbecomes prevalent with respect to the fine ones. By assuming,based on the set-up geometry, the normal flow depth to be around2.5 cm, the complete vertical segregation occurs at about 80 times

the normal flow depth (chute 2 m long) but indeed the concentra-tion of coarser particles at the front significantly increase after thistravel distance and longitudinal segregation can be consideredcompleted, since only results for the front are given, at about 160times the normal flow depth (chute 4 m long).

The experiments described here considered variously assortedwater saturated mixtures that are suddenly released down a rect-angular rough chute inclined of 20� with respect to the horizontal.In substantial agreement with the results by Takahashi (1991), thesegregation process is evident at about 50 times the average peakflow depth and is perfectly completed at about 67 and 100 timesthe average peak flow depth for the widely distributed (mixture1) and for the coarser matrix (mixture 2) respectively. The earliercompletion of the segregation process for mixture 1 rather thanfor mixture 2 – keeping constant the other parameters – may leadto the conclusion that widely range mixtures segregate faster thanmixtures with narrower band of sizes.

In order to extend the results for the segregation speed to realcases, the dynamic flow characterization (laboratory to prototypescale effects) cannot be neglected, but at this stage it is not possibleto quantify their effect. Experiments carried out by Iverson (1997)


at the USGS debris flume are very meaningful: measurements ofpore pressures in time show an appreciable rise just after the pas-sage of the deepest flow front, enhancing the presence of a drycoarse front and a fluid fine tail. Debris flows appear to completelysegregate in very short reaches, of the order of few tens of metersfor a 10 m3 debris volume (Major and Iverson, 1999).

6. Conclusions

The experiments carried out by sudden realising water satu-rated mixtures in a steep chute demonstrated that these water-granular flows segregate rapidly and efficiently along the flowdirection. Transverse segregation of the flowing material is alsoappreciable, with concentration of coarser particles at the sides.

In substantial agreement with the results by Takahashi (1991),the segregation process is evident at about 50 times the averagepeak flow depth and is perfectly completed at about 67 and 100times the average peak flow depth for the widely distributed (mix-ture 1) and for the coarser matrix (mixture 2) respectively. The seg-regation speed appears to decrease with decreasing the band ofsediment sizes. Indeed the greater water quantity inside the porescan reduce the collision among particles and thus also particlemixing and consequent segregation.

Grain-size composition profiles show the coarser particleslocalised at the front and on the flow surface, the medium fractionprevailing inside the flow body and at the flow bottom, the finefraction composing the flow tail. Such profile type is in agreementwith debris flow profiles in several documented events, among theothers, in the Alps, at Acquabona and Moscardo, Italy, and at Illgra-ben and Fully, Switzerland; in Asia, at Curah Lengking, Indonesia,and at Mt. Yakedake, Japan; in the USA at Wrightwood, California.

Acknowledgements

The experiments described in this paper were carried out with-in the MsC activities of four students from the University of Paviaand from the University of Bologna: Lorenzo Cesarotti, Daria LuisaCorte, Francesca Guerzoni, Andrea Serradimigni.

We wish to gratefully acknowledge two Anonymous Reviewerswho significantly contributed to paper improvement with theircomments and suggestions.

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http://dx.doi.org/10.1029/2005RG000175

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