entrainment of debris in rock avalanches

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ABSTRACT

Many rock avalanches entrain and liquefy

saturated soil from their paths. Evidence for

this includes mud displaced from the mar-

gins of rock avalanche deposits, substrate

material smeared along the base of deposits,

extrusion of liquefied soil upward through

the deposits, and increases of volume. A

hypothesis first suggested in 1881 and since

reinforced by several authors suggests that

entrainment of substrate material increases

mobility. Although the process has been

discussed in the literature for more than

100 years, few detailed and quantitative

descriptions exist. The main purpose of this

paper is to describe two recent cases from

British Columbia, Canada, where rockslides

entrained substrate on a very large scale,influencing the character of the events.

Estimated volume balance curves, based

on detailed field mapping, are provided for

both cases. Dynamic analyses are carried

out using a numerical model and using the

same set of rheological parameters. The

mechanism of material entrainment and

displacement is discussed. The data suggest

that rapid rock failures entraining very large

quantities of saturated substrate material

represent a special type of landslide, tran-

sitional between rock avalanche and debris

avalanche. Many rock avalanches can thus

be seen as end members of a continuum of

phenomena involving rock failure followed

by interaction with saturated substrate.

Keywords: rock avalanche, debris avalanche,

dynamic analysis, runout, entrainment, Brit-

ish Columbia.

INTRODUCTION

The 1903 Frank Slide of southern Alberta,a rock avalanche of 36 106 m3, was the mostlethal landslide in the history of both Canada

and the United States, destroying a part ofthe town of Frank, with 73 fatalities. Most ofthe damage in Frank was not, however, due toimpact or burial by rock debris. Instead, homesand other buildings were impacted by a lateraloutflow of mud, the liquefied alluvium from thefloodplain of the Old Man River, expelled fromthe western margin of the landslide (McConnelland Brock, 1904).

Liquefied substrate can play a dominantrole in rock avalanche motion. Its entrainmentserves to increase the volume of the landslide,but may also lead to a change in the character

of material in the basal part of the moving mass,enhancing mobility. This is the oldest amongnumerous hypotheses that attempt to explain thehigh mobility of rock avalanches, having beenfirst proposed by Buss and Heim in 1881 (see below). Abele (1974, 1997), Sassa (1988) andVoight and Sousa (1994) are among its later pro-ponents. The underlying process of liquefactionby rapid undrained loading was documented byHutchinson and Bhandari (1971) for earth flowsand Sassa (1985) for debris flows. However, fewquantitative descriptions of relevant case his-tories exist to substantiate this theory for rockavalanches. In general, it is difficult to assess the

role of basal liquefaction in field studies, as thesurface of rock avalanche deposits typically dis-plays dry, coarse rock fragments. However, mudis often observed around the deposit margins(e.g., Buss and Heim, 1881; Cruden and Hungr,1986), and a lubricating layer of such materialmay well remain concealed beneath the coarsedebris. The situation is more transparent wherethe amount of debris entrained along the path isvery large, relative to the volume of rock in the

initial failure. In such cases, the flow of lique-fied soil unquestionably adds to the mobilityof the event as a whole. Such landslides have atransitional character and deserve special iden-tity both in name and in terms of descriptive

and analytical treatment. Their characteristicsalso shed light on the behavior of other rockavalanches, in which debris entrainment is lessprominent, but nevertheless present.

CONCEPTS AND TERMINOLOGY

When a rockslide mass disintegrates andfragments in the process of becoming a rockavalanche, an initial volume increase occurs.A few estimates of the volume increase exist inthe literature, ranging from 7% to 26% (Hungr,1981, p. 306). However, such field estimates are

not reliable, as they require accurate measure-ment of both source and deposit volumes. In allthe examples used in this paper, it is assumedthat fragmentation produces a volume increaseof ~25%. This is at the center of the typicalrange of measured porosities of loosely placedwell-graded crushed rock, which is 18%35%(Sherard et al., 1963, p. 657).

It is of interest to note that such a volumeincrease negates nearly all possibility of a fluid pore pressure existing in the fragmented rockitself, at least in the initial stages of motion.In-place water content of most rock masses isnegligible. The large pore space newly created

by fragmentation must thus be essentially dry,until sufficient time passes to allow water fromsome source to flow into it. On a typical rockavalanche path, however, significant water cancome only in association with saturated soilentrained from the path downslope of the toe ofthe rupture surface.

During or following fragmentation, furthervolume increase occurs by entrainment of sub-strate material, partly or completely liquefied by

Entrainment of debris in rock avalanches:

An analysis of a long run-out mechanism

Oldrich Hungr

Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, British Columbia V6T 1Z4,

Canada

S.G. EvansDepartment of Earth Sciences, University of Waterloo, 200 University Avenue West, Waterloo, Ontario N2L 3G1, Canada

E-mail: [email protected].

GSA Bulletin; September/October 2004; v. 116; no. 9/10; p. 12401252; doi: 10.1130/B25362.1; 12 figures; Data Repository item 2004126.

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ENTRAINMENT OF DEBRIS IN ROCK AVALANCHES

Geological Society of America Bulletin, September/October 2004 124

rapid undrained loading (Hutchinson and Bhan-dari, 1971). Rock avalanche substrate is gener-ally saturated in temperate climates, as provenby the often-observed presence of liquid mud inor near the debris (see below). Given the rapidmotion and large volume of rock avalanches,one can speculate that even incompletely satu-

rated and moderately coarse-grained soil canliquefy under the intense undrained loadingimparted by masses of fragmented rock (cf.Sassa, 1985; Dawson et al., 1998).

To quantify the entrainment process, anEntrainment Ratio (ER) can be defined as theratio between the volume of debris entrainedfrom the path and the expanded volume of rockfragments produced by the initial rock failure:

ERV V

V F= =

+( )Entrained

Fragmented

E

R FV 1, (1)

where VE

is the volume of the entrained material,

VRis the volume of the initial rockslide andFF isthe fractional amount of volume expansion dueto fragmentation (0.25). The total volume of thelandslide deposits equals V

R(1+F

F) + V

E. Scott

(1988) used a similar index, which he referred toas the Bulking Factor, for volcanic lahars.

Hungr et al. (2001) proposed that the termrock slidedebris avalanche be used todescribe landslides that begin by the failure ofa rock slope and proceed to entrain large quan-tities of debris (talus, colluvium, residual soil,glacial drift, alluvium, peat, or other materials)from their path. Since the presence of small

quantities of entrainment is often difficult todetect, it is suggested that the composite term beapplied only to those events where ER exceeds0.25. This definition contrasts with earlier usesof the term rock slidedebris avalanche (e.g.,Varnes, 1978). However, it is advantageous, asit clearly points out the important role of mate-rial entrainment. Landslides involving flow-likemotion of fragmented rock with only modestentrainment can be described by the well-established term rock avalanches, defined byHungr et al. (2001).

A numerical model for simulating thedynamic behavior of landslides entraining

mass was presented by Hungr (1995). Use ofthe model requires that the initial rock failurevolume and the rate of debris entrainment beknown. The latter can be expressed as a yieldrate (Y

i), the volume entrained from the path

and incorporated into the moving avalanche perunit length of the path, in units of m3/m (Hungret al., 1984). Obviously, the yield rate dependson many factors, especially density, gradationand degree of saturation of the path substrate,the slope angle, and the current mass of theavalanche. A specific yield rate value may be

observed in each segment of the path of a par-ticular description. There are at present no theo-retical or even empirical means of predictingyield rates a priori. Records of yield rates fromknown events are useful to establish precedentfor empirical predictions.

DEBRIS ENTRAINMENT IN ROCKAVALANCHES

Impact Loading of Colluvium

The 1939 landslide at Fidaz, Switzerland,began as a 1 105 m3 rock failure from the headscarp of the prehistoric Flims landslide, butgrew to a total volume of 4 105 m3 (ER = 2.2)by expanding during fragmentation and entrain-ing a part of the colluvial apron surroundingthe source cliff (Niederer, 1941). In 1953, a 1 104 m3 block of rock detached from a cliff atModalen, Norway, as described by Kolderup

(1955) and entrained talus, producing a flow of1.15 105 m3 (ER = 8.2). In Brazil, a rock slabwith a volume of 6000 m3 fell from a graniterock slope, bounced off a lower rock slope, andimpacted on talus. This mobilized a large debrisavalanche that destroyed a clinic, causing thedeath of ~30 people (Barros et al., 1988). (Thefinal volume was not reported in this case.)

Mobilization of Glacial and Residual Soils

Devastating rock avalanches occurred onthe west side of the north peak of Nevados

Huascarn, Peru, in 1962 and 1970. In January1962, ~2.53 106 m3 of glacier ice and grano-diorite broke from Huascarns ice capcoverednorthern summit. After falling 1000 m down analmost vertical slope, this mass struck Glacier511 and incorporated more glacier ice andlarge volumes of lateral moraine. In its passagethrough a series of steep-sided ravines, morematerial was added to the debris avalancheuntil it spread out on a fan where the town ofRanrahirca lay. The total deposited volume wasestimated to be 13 106 m3 (Morales, 1966),over four times the original detached volume(ER = 2.8).

In May 1970, another landslide was trig-gered by an earthquake from the same sourceon Huascarn. Considerable uncertainty sur-rounds the volume of the initial detachment.Plafker and Ericksen (1978) estimated that 50 106 m3 of material was involved in the initialfall (5 106 m3 of ice), but did not mention anyentrainment of morainal soil. Lliboutry (1975)estimated an initial volume of 9 106 m3. Ghig-lino Antnez (1970) estimated a total of 14 106 m3 (5 106 m3 rock and 9 106 m3 ice).According to his data, the total deposit volume

of the 1970 event increased to 53 106 m3 as aresult of the incorporation of ice, glacial till, andcolluvium from the path of the debris avalanchesuggesting an ER of 2.0. The extreme discrepancy between these various accounts illustratethe difficulty of making volume estimates in thefield.

A 1987 rockslide of 6 106

m3

from CerroRabicano in Regin Metropolitana, Chiledisintegrated and produced a debris avalancheof a total volume of 15 106 m3, by entrainingsnow, ice, and valley infill deposits (ER = 1.0)The rockslide-debris avalanche traveled 17 kmalong the Estero Parraguire River on an averageslope of 4.5. It formed a short-lived natural damat the confluence with Ro Colorado. Breach othe dam produced a debris flood along the largestream, which continued to a total travel distance of 57 km, descending a vertical distanceof ~3400 m (Hauser, 2002).

Another case for which data exist is the earth

quake-triggered 1984 Mount Ontake debriavalanche. The volume of the initial failure isknown to be 34 106 m3 (Oyagi, 1987). Oyagalso noted that the rock avalanche entrainedquantities of residual, colluvial, and alluviasoil from the valleys through which it passedDetailed mapping of the deposits (Endo et al.1989) showed that the volume of the deposits i56 106 m3, an ER of ~0.3 after a 25% increaseof the initial failure volume due to fragmentation is taken into account.

Interaction with Alluvium

Abele (1974; 1997) was one of the firsworkers to suggest a link between long travedistance of rock avalanches and interaction withvalley fills, on the basis of detailed field map ping of prehistoric rock avalanche deposits ithe Alps. He proposed a mechanism wherebya combined movement of a rockslide mass riding on water-saturated silt, sand, and gravel canincrease both run-out distance and the spreadingof the debris.

Field observers of historical rock avalanchehave often remarked on the presence of a splashzone around the margins of the debris. Heim

(1932) noted a spritzzone of fine, liquid soisurrounding the distal and lateral margins othe Elm Slide debris, contrasting with the steepwell-defined edge of the rock debris sheet from beneath which it had evidently been expelledSimilar splash margins were described indetail by Cruden and Hungr (1986) surroundingthe Frank Slide debris. The surface of the FrankSlide debris deposit at the foot of the proximaslope is several meters below the original bed othe Old Man River, showing deep erosion of thevalley deposits (McConnell and Brock, 1904)


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1242 Geological Society of America Bulletin, September/October 2004

Evans et al. (1994) found alluvial gravels andwood debris which had been entrained froma river floodplain and transported onto a rockshelf more than 600 m above the original valleyfloor by the runup of the prehistoric AvalancheLake rock avalanche in the Northwest Territo-ries, Canada. The 1964 Hope Slide in southern

British Columbia, the 1987 Val Pola Slide innorthern Italy, and the 1985 North NahanniSlide in Canada sent flow slides, composedlargely of liquefied valley fills, away from theirlateral margins (Mathews and McTaggart, 1978,Govi, 1989, Evans et al., 1987).

TWO RECENT CASE HISTORIES

Field Estimation of Volumes

An important aspect of the fieldworkdescribed here was the estimation of volumetricbalance of the landslides, which is required to

derive yield rates. Areal extent of the source,path, and deposit was well constrained, as theauthors are in possession of post-event mapsat 1:2000 (Eagle Pass) and 1:2500 (NomashRiver) scale. Thus, area estimates contain errorsof no more than a few percent. A major potentialsource of error, however, derives from the diffi-culty of determining depths of material removaland deposition. Successive photogrammetricmapping before and after the event wouldhave improved the situation, but only partially.Such mapping would not determine the depthof disturbance in areas where deposits replaced

previously eroded material. More important,both landslides were characterized by surfaceelevation changes of


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because a short rock tunnel extends preciselyacross the width of the slide path.

The approximate volume balance of the eventis reconstructed in Figure 4. Figure 4A is theslope profile, showing the rockslide source, the bench with the coarse deposits, and the mainslope segment of the path. Figure 4B is a plotof the estimated yield rate, Y

i, in units of m3/m.

The yield rate estimates have been derived fromfield estimates of the depth and area of thevarious entrainment and deposition segments,as described in the preceding paragraphs. Error bars indicate the probable range of depth esti-mation errors (25%).

Integrating the yield rate along the lengthof the path produces the mass balance curves

shown in Figure 4C. The maximum volume inmotion was 9.4 104 m3 of fragmented rock passing the toe of the source cliff. Depositionof rock fragments on the bench, partly offset byerosion, reduced the volume in motion to ~2 104 m3. Entrainment of glacial and colluviamaterial on the slope produced a second peak o

some 3.5 104

m3

, which was deposited in thelake at an estimated average deposit thickness o2.2 m. The volume estimate of material passingthe crest of the bench could be quite inaccurateas it is within the error margin of the initial failurevolume. Nevertheless, the qualitative descriptionof the sequence of events given above serves tosubstantiate the basic shape of the mass balancecurve. The fahrbschung (travel angle, beingthe vertical angle between the crown of the initiafailure and the toe of the deposits) of the land-slide is ~31 and the ER is 0.3.

Velocity of the landslide was roughly estimated at one location near the crest of the bench

at the left margin of the path, where slight superelevation of the debris occurred along a curving trajectory controlled by topography. Thevelocity of the slide was estimated applying theforced vortex equation for flow through bend(e.g., Chow, 1957):

u Rg= tan , (2)

Here, u is the estimated mean flow velocityR is the radius of the central streamline of theflow, derived from a map or from field observations, gis the gravity acceleration and is the

transverse angle between trim lines definingthe boundary of the flow path. The conditionsat a distance of 500 m from the crown of therockslide suggested a speed of ~8 m/s. Highevelocities undoubtedly occurred on the steepslope of the lower path, although no direct estimate is possible.

Nomash River Slide

The Nomash River is a small stream in thewestern part of the Insular Mountains, some20 km inland from the outer coast of VancouveIsland (Fig. 1). The geographic coordinates o

the site are 1264200W and 495900N. Thbedrock of the area consists of crystalline limestone with basaltic intrusions, belonging to theUpper Triassic Karmutsen Formation. A landslide occurred in the headwaters of the NomashRiver on April 25 or 26, 1999, during springsnow melt. The slide originated at the crest oa 430-m-high southwest wall of a U-shapedglacial valley. It moved across the valley floorchanged direction by 90 and moved northwesfor 1.2 km, following the channel of the riverand covering its floodplain.

Figure 1. Location plan.


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The source of the initiating rockslide is amountain slope angled at 50 to the northeastand composed of a sequence of white marbleinterbedded with basaltic sills (Fig. 5). The sillcontacts dip at shallow angles to the southeast,perpendicular to the slide motion. The concaverupture surface consisted of a series of intersect-

ing joints, including some stress-relief jointssub-parallel with the slope face and several ran-domly oriented joints and shears. The detachedmass was roughly tabular in shape, tapering tothe base and ~3 105 m3 in volume. Applyinga fragmentation volume increase factor of 25%,the slide produced 3.75 105 m3 of fragments,which collapsed into the valley, perpendicular tothe Nomash River.

The valley bottom is mantled by a groundmoraine consisting of silty sand. Glacial tillunderlies the narrow floodplain of the NomashRiver and extends up the sides of the valleyto a height of ~100 m above the valley floor.

Fine-grained colluvial aprons rise another 80 mhigher in certain parts of the valley slopes.The floodplain is typically 50100 m wide,composed of shallow sand and gravel depositsand organic matter. The depth of these alluvialdeposits is not known, but it is probably limitedto several meters.

The rockslide descended from an upper bed-rock slope, overriding a colluvial apron and till-covered slope, angled at 30 (Fig. 6). The col-luvium and glacial till at location A in Figure 6were scoured 100150 m wide to a maximumdepth of 8 m, leaving prominent side-scarps

along the sides of the slide path, visible in Fig-ure 7. A field survey of the area indicated that~360,000 m3 of material were eroded from theslope and incorporated into the moving mass.Based on two samples, the eroded material con-tained


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bulk of the bouldery rockslide debris was foundupstream of the second bend (Location C).Beyond this point, the debris consists primarilyof silty sand similar to the colluvium entrainedon the slope below the source, although consid-erable alluvial gravel and organic debris are alsoevident. Rock debris occurs in this distal part ofthe slide deposit mainly in the form of several

large boulders, up to 6 m in diameter.Debris yield rates were estimated as shown

in Figure 8B. Error bars show the estimatedmaximum error of erosion and/or depositiondepth estimates (25%). Figure 8C shows thevolume balance curve resulting from these esti-mates. The shape of the mass balance curve isvery different from that for the Eagle Pass Slide.Only one peak occurs, near a distance of 700 mfrom the crown of the rockslide, where the frag-mented rock was still in motion while the totalvolume of entrained debris approached its peak.

The total volume of moving material passingthrough this point is more than 7 105 m3. TheER is 0.9.

Using the forced vortex equation (Equation2) for flow in the second curve, at Location C,Figure 6, yielded a velocity estimate of 12 m/s.The superelevation of the flow was barelymeasurable in the final two bends upstream of

Location D, indicating that the velocity therehad decayed to


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once the entire volume of the moving mass haspassed. It is also possible to change the rheologyof the moving mass to include the change fromdry frictional to velocity-dependent, at the pointwhere saturated material is entrained.

Both case histories have been analyzed, withtopography based on detailed site maps. The

same set of rheological properties was used forboth cases. In each case, the initial rockslide wasrepresented by a frictional material with no porepressure and a constant dynamic friction angleof 30. At the point where entrainment of debrisbegan, the rheology of the moving mass waschanged to the Voellmy model (Hungr, 1995).

The two-parameter model of Voellmy (1955)combines a frictional coefficient (f) and a veloc-ity-dependent turbulence parameter (). Thefrictional coefficient (tangent of the bulk fric-tion angle as defined by Hungr [1995]) is theresult of dry friction of the material, modified bypore pressure assuming a constant ratio between

pore pressure and total normal stress on the baseof the flow. Thus, the friction coefficient may bea small fraction of the dry friction coefficient ofthe substrate soil (cf. Hungr, 1995). The resist-ing shear stress () at the base of the flow, is thenrepresented (in a simplified formulation) as:

= +hfu

cos2

, (6)

where h is the flow depth, the slope angle, the unit weight of the material, and u the meanvelocity. Voellmy (1955) developed this modelempirically for snow avalanches by combining

Coulomb frictional and Chzy formulas. Theturbulence term (which is similar, althoughnot exactly equivalent, to the Manning n)was intended to cover all velocity-dependentfactors in snow avalanche motion, includingturbulence of the air-snow dispersion andair drag on the top surface of the avalanche.Krner (1976) showed that, perhaps by coin-cidence, the model offers a good simulation ofvelocities for rock avalanches.

Successful application of the Voellmy modelto debris flows and debris avalanches maypossibly benefit from a similar coincidence

(e.g., Rickenmann and Koch, 1997; Ayotte and

Hungr, 2000). However, the model may havea more fundamental physical basis. Bagnold(1954) showed that both the shear stress and theeffective normal stress in a dense dispersion ofgrains in fluid, rapidly sheared at constant vol-ume, depend on the square of the shear strainrate. We may visualize a plug flow moving ona saturated basal layer of constant thickness h

m.

Initially, while the movement is slow, the resis-tance will depend on friction and pore pressurein the basal zone. As velocity increases, grainsin the shearing layer will begin interacting

dynamically with each other and with thefluid to produce dispersive effective normalstress. This would tend to expand the layer. If,however, pore pressure diffusion is too slow to permit expansion, a decrease in pore pressurewill occur. According to Bagnolds results, thepore pressure decrease should be proportional tothe square of velocity. The frictional resistancewill increase as represented by Equation (6). Arecent evaluation of Bagnolds (1954) experi-ments has placed the value of the exponent 2.0in Equation (6) in question (Hunt et al., 2002).

While this development may eventually result ina need to modify Equation (6), good empiricalresults as reported by Krner (1976), Rick-enmann and Koch (1997), Hungr and Evans(1996), this paper, and others justify the use ofthe simple Voellmy model at least until furtherexperiments and back analyses are completed.

As described by Hungr (1995), the DANmodel uses empirical calibration to constrain boththe rheological relationship and the specific resis-tance parameters used. Previous experience withrock avalanches shows that the Voellmy equation

provides good results in terms of velocity anddistribution of deposits. Details of the calibrationprocess have been discussed by Hungr and Evans(1996) and Ayotte and Hungr (2000). The presentback analyses differ from previous work in thattwo rheologies are used, dry frictional for the ini-tial rockslide and Voellmy for the flow lubricated by entrained liquefied soil. It must be stressedthat the only adjustable parameters in the analy-sis are the two Voellmy coefficients. The frictionangle of the initial sliding movement (30) is areasonable estimate for dry dynamic shearing of

a granular material or moderately rough joint sur-faces. The locations where the rheology changesfrom frictional to Voellmy are also not arbitrary,but correspond to the points where entrainment ofsaturated soil is known to occur.

The Voellmy parameters for both of the pres-ent cases have been selected by trial-and-errorasf= 0.05 and = 400 m/sec2. These parametersare in the range of properties used earlier in thesimulation of long-runout flow slides involv-ing coal mine waste (Hungr et al., 2002). Thelow magnitude of the fcoefficient implies the

Figure 5. Nomash River rockslidedebris avalanche: view of the source area. The rockslide

scar is ~150 m wide.


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presence of high pore pressure in the basal layerof the landslide.

The results of an analysis of the Eagle PassSlide are summarized in Figure 9. In Figure 9A,the profile of the slide path with a width profile

determined from the map are shown. Becauseof the length of the profile, all depths (measurednormal to the path) have been exaggerated 10in the diagram. The initial slide profile is shown by a thick dashed line. Thin lines show the

profiles of the moving mass at 10 s intervals upto 70 s. The thick line shows the final deposit.

The slide started as a flow of frictional material. A yield rate of 54 m3/m was introduced onthe bench, at a distance of 420 m. It was thenreduced to 36 m3/m at 480 m in accordance with

Figure 4B. The frictional rheology was retainedon the proximal part of the bench, due to thepresence of coarse talus as the surficial materiain this area. The transition to Voellmy materiawas made at a distance of 480 m, where thesurface material was finer-grained glacial andcolluvial soil. The separation of the landslidedeposit into an upper part (on the bench) andlower part (in the lake) is well represented bythe model.

The model showed a substantial decrease infront velocity at a distance of 500 m, near thecrest of the bench (Fig. 9B). The single fieldvelocity estimate at this location supports a sub

stantial slow-down, although this estimate wamade near the margin of the path and is therefore probably not representative of the speed othe center of the flow front.

A corresponding analysis of the NomashSlide is shown in Figure 10. The amount oerosion was specified according to Figure 8BThe yield rate was taken as 1000 m3/m at a horizontal distance of 360 m from the crown. Thematerial was changed from frictional to Voellmyat the same location to account for the presenceof saturated, fine-grained material susceptible

Figure 7. The middle part of the Nomash River slide path. The rockslide originated above

the upper left corner of the photo. Eroded side scarp in the colluvial apron beneath the

source area can be seen left of center, intersecting a twinned logging road. The lighter-col-

ored areas are covered by rockslide blocks.

Figure 6. Plan of the Nomash River landslide path and deposit, showing locations mentioned in the text. The square grid spacing is 200 m

in both directions. The contour interval is 10 m.


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to rapid undrained loading. The yield rate wasreduced to 180 m3/m at 630 m. The flow veloc-ity of the front is shown on Figure 10B, andcompared with the field velocity estimatesreported in the previous section. Depth profilesat 10 s intervals and the final distribution ofdebris are shown in Figure 10C.

In summary, the model is seen to simulatevery satisfactorily all main aspects of both caseswith the same set of input parameters. The keyto a successful simulation, however, is to changethe material rheology at that point of the pathwhere entrainment of saturated material occurs.To illustrate the effect of entrainment, theanalysis of the Nomash case was repeated withno entrainment and constant frictional rheol-ogy (Fig. 11). This model shows a small rockavalanche with limited displacement, buildinga steep apron at the toe of the slope. Thefahrb-schungis 33, slightly larger than the assumedfriction angle of 30 and much larger than the

14 calculated by the alternative model. Thus, incomparing Figures 10 and 11, the entrainmentof saturated material is seen to have a strongeffect on the runout in this example.

THE MECHANISM OF MATERIAL

ENTRAINMENT

The observations described in the previoussections suggest a sequence of events form-ing a rockslidedebris avalanche: In the earlystage, a rockslide of moderate volume occurs.Such a slide would ordinarily produce a steep

talus cone at the foot of the slope. However,the moving mass of rock fragments reaches adeposit of loose saturated material lying in thepath (colluvium, glacial till and alluvium in thepresent cases). It appears that the rockslide frontdoes not immediately displace the loose soil inits path, but overrides it partially, causing rapidloading and liquefaction of the saturated sub-strate as suggested by Sassa (1985). This notionwas tested by a small-scale model. A massof rock fragments was released from a steepslope, so as to reach a thin deposit of clayey siltsaturated to just below its liquid limit. The frag-ments displaced the silt layer only slightly, but

overrode it. In the final stage of the experiment,the rock mass was moving on top of a sheet ofmud formed from the silt substrate. The shearstrength and viscosity of the material was toogreat at this small scale to effect much total dis-placement; however, the mechanism of overrid-ing and rapid loading was demonstrated.

It is probable that much more displacementof the substrate material occurs at full scale,where the ratio between the applied stresses andthe liquefied shear strength is several orders ofmagnitude larger than in the laboratory model.

A hypothetical mechanism of a full-scaleflow is shown schematically in Figure 12. InFigure 12A, the potentially liquefiable layerof relatively loose, saturated soil on the valleyfloor and apron is being approached by a rapidlymoving mass of rock fragments from the upperslope. On collision, the soil partly or completelyliquefies under impact loading, and its strengthis reduced to a fraction of its dry frictionalvalue. Part of it is pushed forward by the leadingfront of the rock mass, while part is overridden(Fig. 12B). A wave of liquid debris is projectedforward, overriding and entraining new sub-

strate and carrying some of the rock fragmentswith it. The rock mass follows behind, slidingon a thin cushion of liquefied soil (Fig. 12C).Finally, the slide stops, the mud wave havingswept itself far forward into the deposition area,forming a forward-tapering tongue speckledwith transported boulders. All of these featurescorrespond closely to field observations made atthe Nomash River site.

Apparently, very little drainage or consolida-tion of the fine-grained mud occurs during theevent, as demonstrated by the fluidity of the

distal debris. This is not surprising, given thehigh velocity of the landslide motion and thelow permeability of the poorly sorted and rela-tively fine grained soils at the site. Consideringthe dominant material of the slide substrate tobe a poorly sorted silty sand, the coefficient ofconsolidation can be estimated as of the orderof 102 cm2/sec. Using consolidation theory(Terzaghi and Peck, 1967, p. 181), a pore pres-sure reduction of only 10% in a saturated layer50 cm thick would require ~40 min, muchlonger than the 2 min duration of the entirelandslide event, as estimated by the model.

Mixing during rapid flow would likely furtherhinder the consolidation process, making porepressure dissipation slower. Thus, no significantdissipation of the pore pressures generated byrapid loading should be expected.

Once liquefaction occurs, the flow front con-tinues riding on top of a sheet of liquid mud ina configuration justifying the assumptions of theVoellmy rheological model. This allows mostof the rock debris to be carried out consider-ably beyond the deposition point that would be predicted by a dry frictional model. In the

Figure 8. Nomash River rockslidedebris avalanche. (A) Path profile. (B) Estimated yield

rate distribution. (C) Mass balance curve.


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final stages, the flow front resembles a saturateddebris avalanche.

The above scenario is probably universal ina qualitative sense, but will have different char-acteristics depending on the relative quantitiesof rock and liquefiable soil and the shape of thepath profile. Thus, in the Nomash case, nearlyall of the rock debris reached the liquefactionregion and moved a considerable distance. The

peak discharge was formed by the joint flow ofrock and mud near the center of the path. Therewas sufficient excess of liquefied material toproject a long tongue of mud ahead of the rockdeposits. In the Eagle Pass case, the proximal part of the rock mass deposited on the bench,having been slowed down by non-liquefiabletalus deposits. Only the leading edge of therock debris reached saturated colluvium and setoff a debris avalanche on the steep, colluvium-mantled slopes below the bench. As a result ofthis, the Eagle Pass landslide displays a rather

low degree of mobility, as measured by its steepfahrbschung. The event had two discharge peaks, one corresponding to the rockslide andthe other to the debris avalanche phase.

It is easy to visualize cases similar to the Nomash Slide, but where the quantity of theliquefiable material relative to the rock debrisis less. Thus, the rock debris is still mobilizedby a cushion of mud, but the leading tongue of

debris may be small or missing altogether. Insome cases, the mud may be expelled sidewaysinstead of forward (e.g., the Frank Slide, men-tioned in the Introduction).

Part of the liquid material may be smearedalong the path while supporting coarser debris.This hypothesis was suggested by Johnson(1978), who found that the Blackhawk Slidedebris is underlain by a possibly continuous1-m-thick contorted layer of sandstone debris,derived from a rock unit originally located farupstream, below the toe of the rupture surface.

Clastic dykes issue from this basal layer upwardinto the overlying marble fragments. Widespread clastic dykes and other intrusions opockets of valley deposits have been describedin major rock avalanches in the Bavarian Alpby von Poschinger (2002). Upward extrusion oliquid organic sand through coarse debris was

observed in the distal reaches of long-runouflow slides from coal mine waste dumps (Hunget al., 2002).

DISCUSSION AND CONCLUSIONS:

THEORIES EXPLAINING THE

MOBILITY OF ROCK AVALANCHES

It has long been noted that many rock avalanches are excessively mobile, if consideredas shearing masses of dry broken rock (Heim1932). Furthermore, the degree of mobilityappears to increase approximately in proportion to the logarithm of the volume of the even

(e.g., Abele, 1974; Scheidegger, 1973; Hs1975). For many years, researchers have beenlooking for an explanation of this phenomenonThe main hypotheses advanced for this purposeinclude the following:

1. Mobilization by an air cushion, overriddenand trapped beneath the mass of the rock avalanche (Shreve, 1968).

2. Fluidization by similarly trapped air or bysteam generated by vaporization of groundwate(Goguel and Pachoud, 1972; and others).

3. Fluidization by dust dispersions (Hs, 1975)4. Rock melting or dissociation by the heat o

friction (Erismann, 1979).5. Mechanical fluidization, understood aa process of spontaneous reduction of frictionangle at high rates of shearing (e.g., Scheidegger, 1975; Campbell, 1989; and others).

6. Acoustic fluidizationreduction of thefriction angle resulting from acoustic-frequency vibrations at the base of the flowingmass (Melosh, 1979).

7. Increase in areal dispersion of debris as aresult of fragmentation (Davies and McSavenney1999).

8. Lubrication by liquefied saturated soientrained from the slide path (Buss and Heim

1881; Abele, 1974, 1997; Sassa, 1985).Many critical reviews and discussions o

these various mechanisms have appeared inthe literature (e.g., Hs, 1975; Hungr and Morgenstern, 1984; Hungr, 1990; Legros, 2002)Some of the critical arguments that have beenadvanced are reviewed as follows.

Hypotheses 1 and 2 postulate high gas pressures. If substantial average gas pressure existedbeneath a sheet of debris, those parts of the sheethat are thinner than average would becomefully fluidized and exhibit features characteristic

Figure 9. Eagle Pass, results of the dynamic analysis. (A) Flow profiles at 10 s intervals (all

depths exaggerated 10). (B) Velocity profiles, with a field velocity estimate indicated by

a cross. The model used frictional rheology with a bulk friction angle of 30 at a distance

of 0484 m and a Voellmy model with an f (frictional coefficient) of 0.05 and a (velocity-dependent turbulence parameter) of 400 m/s2 beyond 484 m. Entrainment yield rates wereapproximately as shown in Figure 4B.


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HUNGR and EVANS


of upward escape of gas such as normal grading,elutriation of finer particles, craters, and gradedfallout fans or cones. No description of suchphenomena can be found in the literature. Manyrock avalanche deposits exhibit reverse grading,which is opposite to what would result from gasfluidization (e.g., Cruden and Hungr, 1986).

Rock melting (Hypothesis 4) may occur in

some cases, where a rockslide is unusually thick.However, it almost certainly is not a widespreadphenomenon, as very few examples of meltedrock have been reported in the literature.

With regard to the mechanical fluidizationtheory (Hypotheses 3 and 5), it is yet to beshown experimentally that high rates of shearinglead to a reduction in the overall friction angle ofdry granular material. High rates of shearing cancause dispersion of the material and reduction inbulk density, but offer no mechanical advantageto the shearing movement. For example, Hungr

and Morgenstern (1984) observed flows of drysand in a laboratory flume at speeds of up to6 m/sec. These flows were


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