Catastrophic Debris Streams (Sturzstroms) Generated by Rockfalls

KENNETH J. HSU Geological Institute, Swiss Federal Institute of Technology, Zurich, Switzerland

ABSTRACT sturzstrom. Key words: sturzstrom, debris I was fascinated by Heim's accounts and streams, rockfalls, . chose the topic "Sturzstrom" as a theme of Large rockfalls commonly generate my inaugural speech in 1967 after my ap- fast-moving streams of debris that have INTRODUCTION pointment to Zurich. A theoretical analysis been called "sturzstroms." The geometry of and some preliminary experiments pro- sturzstrom deposits is similar to that of Large masses of rocks crashing down a duced evidence in support of Heim's claim , flows, and . steep slope commonly generate a stream of that the broken debris of large masses of Sturzstroms can move along a flat course broken debris, which often moves at fantas- rock flowed, but exchanges of written for unexpectedly large distances and may tic speeds over very gentle slopes for unex- communications failed to dissuade my surge upward by the power of their pectedly long distances. The famous Elm friend Ron Shreve from his conviction. momentum. A currently popular hypothesis rockfall of Switzerland, 1881, produced Furthermore, the necessary experimenta- to account for their excessive distance of such a debris stream; it buried a village and tion to deliver a decisive argument was transport suggests that sturzstroms slide on killed 115 persons. Albert Heim (in Buss never carried out, as my interests drifted air cushions. Contrary to that hypothesis, and Heim, 1881, p. 143) stated shortly elsewhere. Eventually the old manuscript evidence is herein presented to support after the catastrophe that he had never was taken out and dusted off when I Heim's contention that sturzstroms indeed known of such an example. However, he learned of the discoveries of sturzstrom de- flow. later realized (1932, p. 84) that cata- posits on the (Howard, 1973a, The flow of a sturzstrom can be com- strophic debris streams had been seen be- 1973b; Guest, 1971). Now there is pared to flow of a mass of concentrated fore, notably by Ebel in 1749 after the sufficient proof that the air-lubrication cohesionless grains in a fluid medium. Fric- rockfall of the Diablerets and by Meyer in theory may not be dependable. Perhaps tional resistance to such grain flow is, ac- 1807 at Goldau. there is some merit in offering an alterna- cording to Bagnold, less than that for slid- Heim referred to the rockfall debris tive physical model to describe the ing of rigid bodies because of the buoyancy stream as "Sturzstrom" or "Triimmer- mechanics of sturzstrom generated by large of an interstitial fluid which serves to re- strom." The word "Strom," which can be rockfalls. duce the effective normal pressure of the en- translated as stream or current, was em- trained grains. The presence of sturzstrom phasized because Heim believed that debris THE PHENOMENON OF deposits on the Moon indicates that the in- streams flowed like a liquid. He repeatedly STURZSTROMS terstitial fluid is not necessarily a com- referred to their geometrical similarity to pressed gas or a wet mud. The dispersion of lava flows and to glaciers, but he realized "" has been defined as "a gen- fine debris and pulverized rock dust among the great differences in the mechanics of the eral term covering a wide variety of mass the colliding blocks may have provided an various flowing mechanisms. The apparent movement . . . involving . . . downslope uplifting stress during the motion of some mobility of large rockfalls and the tendency transport, by means of gravitational body, terrestrial and lunar sturzstroms. of their debris to spread out also impressed stresses, of and rock material en masse" Scale models to provide kinematic simu- Kent (1966), who postulated fluidization of (Gary and others, 1972). Terzaghi (1950) lation of sturzstroms may have practical catastrophic rockfalls. specified that "landslide refers to rapid dis- application. Preliminary results suggest that Heim's and Kent's suggestions were dis- placement"; a similar movement proceed- a bentonite suspension of a certain consist- puted by Shreve, who advocated the idea ing at an imperceptible rate is called creep. ency is a suitable material for scale models that rockfalls slide and do not flow. In a The German expression for landslide is and that the flow of thixotropic liquids is series of papers, Shreve (1966, 1968a, "Bergsturz" (mountain fall). Heim (1932) kinematically similar to sturzstroms. The 1968b) proposed the air-lubrication distinguished some 20 different kinds of parameter "excessive travel distance" is in- hypothesis to account for the unusually landslides; those capable of generating a troduced to replace the expression "equiva- long distances of transport of large stream of rock debris were called lent coefficient of " as a measure of rockfalls. He drew his conclusions mainly "Felsstiirze," or "rockfalls" in English. mobility of sturzstroms. There is, on the on the basis of his studies of the Blackhawk Neither "landslide" nor "rockfall" fully de- whole, a positive semilog correlation of the slide in California and the Sherman slide in scribes the motion of very large fallen rock excessive travel distance to the size of the Alaska, although he took into considera- masses, because they not only slide and fall, fallen mass. Exceptions to the rule include tion rockfalls elsewhere, including the fam- but as Heim (1932) demonstrated, they also on the one extreme the unusual mobile ous Elm event studied by Heim. Shreve's flow. Huascaran rockfall which gave rise to a hypothesis received wide recognition (for Heim used several synonyms for the sturzstrom with a dense interstitial mud examples, see Howard, 1972); few recent flowage of broken fragments from a large and, on the other extreme, the least mobile papers on landslides have failed to mention rockfall: "Triimmerstrom," "Sturzstrom," Vaiont rockslide which remained a sliding air lubrication as a possible mechanism for "Schusstrom," "Fallstrom," "Wurfstrom," block and failed altogether to generate a this phenomenon. "Steinstrom," and "Blockstrom." The term

Geological Society of America Bulletin, v. 86, p. 129-140, 8 figs., January 1975, Doc. no. 50117.

129

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/1/129/3428976/i0016-7606-86-1-129.pdf by guest on 01 October 2021 130 K. J. HSt)

"Triimmerstrom" was the one most fre- 1972). The German equivalent of the term the German expression be kept but be writ- quently used by Heim and could be trans- "Lawine" has not been applied to describe ten in lower case and not italicized, so that lated as "debris flow." Unfortunately, this rockfalls and associated motions. In fact, sturzstrom could be part of the geological English expression has been used by Heim (1932) made a point of distinguishing vocabulary that includes many foreign geologists to describe slower movements rockfalls and their sturzstroms from stone words (for example, karst, cwm, flysch). A such as mudflows (Gilluly and others, avalanches (Steinlawinen): the broken de- sturzstrom can be defined as a stream of 1968; Johnson, 1970; Hampton, 1972). bris from a sturzstrom is generated through very rapidly moving debris derived from the The well-known of Wrightwood, the distintegration of a fallen rock mass, disintegration of a fallen rock mass of very California, for example, had a maximum whereas the debris from a stone avalanche large size; the speed of a sturzstrom often speed of less than 20 km/hr (Sharp and No- is composed of loose rocks and stones orig- exceeds 100 km/hr, and its volume is com- ble, 1953), an order of magnitude smaller inally perched on or above a steep slope. monly greater than 1 X 106 m3. than the estimated maximum speed of the Apparently catastrophic debris flows have The sturzstrom generated by the Elm sturzstrom at Elm. The term "avalanche" occurred very infrequently in English- rockfall is probably the example that has has been suggested to describe a cata- speaking countries, and 1 could find no been the object of the most study. The Elm strophic debris flow (Howard, 1973a). suitable English word available to describe event has been characterized by Heim However, this term commonly refers to the the phenomenon. I suggest, therefore, that (1882) as a drama of three acts: the fall downslope motion of "a large mass of snow the term introduced by Heim be adopted. (Bergsturz), the jump (Luftsprung), and the or ice" (A.G.I., 1957); only the new Glos- Because a literal translation of the word surge (Flachstrom und Brandung) (see sary of Geology stretched the definition to into "fallstream" is not informative and Fig. 1). include a fallen rock mass (Gary and others, somewhat misleading. I recommend that A vivid account of the fall phase was

Figure 1. The rockfall and the « turzstrom at Elm (reproduced from Heim, 1882). The upper figure is a sketch map of the sturzstrom and the lower map a profile. Heim described the Elm event as a drama of three acts, the fall, the jump, and the surge. The geometry of the sturzstrom deposits is very similar to a . No scale was given for these sketches; the horizontal distance from the top of the breakaway rim to the tip of the sturzstrom is a little more than 2 km (see Fig. 2).

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/1/129/3428976/i0016-7606-86-1-129.pdf by guest on 01 October 2021 CATASTROPHIC DEBRIS STREAMS GENERATED BY ROCKFALLS 131

given by an eyewitness of the Elm rockfall motion was colorfully described by a sur- "Oh, yes, Sepp," I shouted, "1 am here." I was (Heim, 1882, p. 87)1: "When the falling vivor, who was only one jump ahead of the pleased that somebody else was also alive. Then rock began to slide, the forest moved like a speeding debris (Heim, 1882, p. 94-95): my son started to dig me out. herd of galloping sheep, and the pine swirled in confusion; then the whole mass The debris mass did not jump, did not skip, and The stones would have ridden over, or did not fly in the air, but was pushed rapidly suddenly sank." pushed away, the house and the old man along the bottom like a torrential flood. The flow This description suggests that the rockfall was a little higher at the front than in the rear, with it, if the stones had moved as a sliding ceased to behave as a rigid body in the very having a round and bulgy head, and the mass mass. Instead the debris flowed around initial stage of movement. The fallen block moved in a wave motion. All the debris within him, and he did not suffer any injury other started to disintegrate, yet the broken parts the stream rolled confusedly as if it were boiling, than burial and filling and the accompany- moved in unison like a herd of galloping and the whole mass reminded me of a boiling ing mental shock. This phenomenon near sheep. The statement that the whole mass cornstew. The smoke and rumble was terrifying. the outer edge of the sturzstrom is remark- sank is not exactly accurate, for one-eighth I turned and ran, and a single jump saved me. edly similar to a curious observation by of the fallen mass broke off from the main When the sturzstrom drove past me like a speed- Sharp and Noble (1953, p. 557) near the block and never did make it to the bottom ing train, its outer edge was only a meter away. outer limit of a mudflow: "A cabin and During my last jump, small stones were whirling of the slope. This uppermost part of the shed . . . were buried to the eaves by debris. around my legs, being stirred up by the wind. original block moved a short distance from Otherwise I was not hurt by any fallen stones, . . . Curiously neither building appears to the rim of the slide and then got stuck in the nor did I feel any particularly strong air pressure. have been moved or to have suffered dam- seat of the breakaway scar. Those were the age other than burial and filling, probably black sheep who deserted the galloping Except for its tremendous speed, this de- because of low impact velocity and herd and remain up there even today, de- scription of the Elm sturzstrom, with homogeneous flow." spite the canons of the Swiss Army, which phrases such as "round and bulgy head," Despite a local vertical-velocity gradient, have been fired in vain in a futile campaign "wave motion," and so on, reminds me of the detritus from the very rear never quite to dislodge that hanging mass (Zweifel, the conventional characterization of caught up with the front runners. Heim 1883; Hsu, 1969b). mudflow movement (Sharp and Noble, (1932) observed that the stratigraphic In the second stage at Elm, the fallen 1953; Johnson, 1970; Hampton, 1972). I sequence of the detached block was pre- mass hit the flat floor of a slate quarry and doubt if anyone would be tempted to com- served in the sturzstrom: the rearmost for- became completely disintegrated. The de- pare this "torrential flood" or "boiling mation of the detached block constituted bris was deflected and shot horizontally cornstew" to a hovercraft sliding on an air the rear of the sturzstrom deposit, whereas forward, as a witness described (Heim, cushion. the foremost formation was found as debris 1882, p. 89): Heim emphasized that the surge of the in the very front. He explained the Elm sturzstrom did not fly, but hugged the phenomenon by saying that the debris in Then I saw the rockmass jump away from the ground very closely while it was running the rear was left behind because much of its ledge. The lower part of the block was squeezed along the flat valley bottom. A pipe, kinetic energy was sacrificed by collision, by the pressure of the rapidly falling upper part, originally buried at about 1-m depth, was and thereby the debris in front was pro- became disintegrated and burst forth into the air. pelled forward. Carrying his logic a step . . . The debris mass shot with unbelievable speed dug out by the sturzstrom and was later northward toward the hamlet of Untenal and found more than 1 km downstream among further, Heim (1932, p. 96) thought that over and above the creek, for I could see the alder the debris of a lateral ridge. The surge also the entire sturzstrom mass did not stop its forest by the creek under the stream of shooting carved parallel furrows, genetically akin to motion at a single instant. The rear may debris. the groove casts found at the bottom of have already come to a halt some 10 to 30 turbidite beds. Houses at the outskirts of sec before the whole sturzstrom came to Heim compared this jumping phenome- Elm were removed at their foundation and rest. Yet the debris at the very front of a non to the spraying of a waterfall striking a pushed away bodily. Yet on a local scale, sturzstrom was probably not the last to rock ledge. The undersurface of the rockfall Heim (1932, p. 95) inferred a vertical veloc- stop, and its sudden halt at the distal end was sharply defined, as is that of a water- ity gradient: "The bottommost debris caused the oncoming debris behind to pile fall, and the witness could see houses, trees, reached a standstill first. The overlying de- up as transverse ridges (see Shreve, 1968a, and fleeing people and cattle under the bris rolled over at a great speed." His inter- p. 40). We might compare a sturzstrom to a moving debris. The upper surface, on the pretation was probably influenced in part relay race. The runners for the first leg of other hand, was a cloud of stones and dust, by the strange account of a septagenarian, the relay teams started to slow down and like the black smoke issuing from a stream the oldest inhabitant of the village who was rest after they passed their batons to those locomotive. While the main mass flowed inside one of the last houses hit by the of the second leg. The baton, as a symbol of northward, a layer of broken debris was left sturzstrom, but survived to tell the tale kinetic energy, was transferred from one behind; it buried Untertal. The flowage of (Buss and Heim, 1881, p. 40): racer to the next. Finally, the anchor men the debris can be said to begin in this sec- took the batons. The final man of the win- ond phase, for a waterfall is not a slide, but ning team is the pace setter, but he is not the I stood at the kitchen door, which was also the a very rapid flow. last to stop his motion. If he should run out house door, and heard and saw with fear how the of steam and collapse suddenly near the Once the sturzstrom reached the bottom mountain came down. I thought that my wife of the slope, a branch was sent directly was at our son's next door, and wanted to go finish line, he might be piled up from be- north and surged up the side of the valley to there to fetch her. But then, the house crashed hind by his pursuers. a height of 100 m. The main surge, how- down, and I was caught by wind and skidded ever, went down the Sernf Valley and had back into the kitchen. I suddenly realized that I GEOMETRY OF STURZSTROM was rooted to the spot where I stood — I don't to turn 60° toward the northwest. The tip of DEPOSITS the sturzstrom moved another 1.5 km along know how it came about, and I was buried stand- a nearly horizontal valley floor (Fig. 1). Its ing and up to my neck among the broken lumber and stones. I could not move my arms and legs The geometry of the sturzstrom deposits and was tortured by the extreme anxiety for my is similar to that of a lava flow or a glacier, 1 This and other passages quoted from texts originally wife. After a long and horrible wait I finally not only in general outline but also in inter- written in German have been freely translated. heard the voice of my son. "Is anybody here?" nal structure. The debris tongue of Elm, for

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/1/129/3428976/i0016-7606-86-1-129.pdf by guest on 01 October 2021 132 K. J. HSt)

Figure 2. A sketch profile of the geometry of the Elm Sturzstrom (reproduced from Heim, 1932).

example, was sharply limited by well- ratio was considered an "equivalent the breakaway rim to the farthest tip of the defined lateral ridges and by a distal rim, coefficient of friction" by Shreve (1968a). sturzstrom. The slope of the line connecting which rose several meters above the im- The value of HIL for various rockfalls have those two points has been called fahrbo- mediate surroundings like an earth dam. been computed by those two authors. A schung by Heim. Physically this line is not There was no spraying or scattering of the more complete compilation was made by the line connecting the centers of gravity of debris beyond the rim. The movement of Scheller (1971), and his results, amended the about-to-fall block and of the the blocks was characterized by their sol- with supplementary data from the Huasca- sturzstrom deposit. The fahrboschung at idarity or togetherness (Buss and Heim, ran event (Browning, 1973) and from the Elm was, for example, 16° 54.5', (Fig. 2), 1881, p. 146; Heim, 1932, p. 100). The rim Moon (Howard, 1973a), are shown in whereas the line connecting centers of grav- of the sturzstrom deposit was slightly Table 1. In addition to the parameters of ity should have an inclination of about 23°. higher than the middle, forming a marginal fahrboschung introduced by Heim (1932) The assumption that the maximum drop of swell like the moraine of a glacier. Trans- and the equivalent coefficient of friction in- a sturzstrom divided by its over-all length is verse undulations, with convex side for- troduced by Shreve (1966, 1968a), I have the equivalent coefficient of friction is inac- ward (Fig. 1), marked the position of the calculated the excessive travel distance as a curate, to say the least. Nevertheless, arrival of successive waves of flowing de- measure of the mobility of the sturzstrom. fahrboschung does relate drop distance to bris. Flow structure was further delineated The excessive travel distance is the horizon- transport distance and allows an approxi- by the longitudinal trains of materials of tal projection of the travel distance beyond mate measure of the equivalent coefficient different composition (Fig. 1). what one expects of a rigid mass sliding of friction. The following observations by The Elm sturzstrom deposit on the flat down an inclined plane with a normal Heim and Shreve still hold: even if the line part of the valley had a length of 1.5 km, a coefficient of friction. connecting centers of gravity is adopted in- width of 400 to 500 m, and a thickness For small rockfalls with volumes less stead of the fahrboschung as a measure of ranging from about 50 m at Untertal to than 0.5 x 106 m3, the equivalent the friction angle, there would still be a sig- about 5 m at the distal end (Figs. 1 and 2). coefficient of friction is approximately nificant reduction of the equivalent The unexpected long travel on the flat equal to the predicted value of 0.6. Heim coefficient of friction for larger sturzstroms. course was responsible for the deaths of (1932) noted, for example, that the Airolo Heim postulated lubrication of the Elm many victims. rockfall data resulted in a coefficient of sturzstrom by wet mud. Shreve postulated Through a series of empircal observa- 0.64 and Schachental, a coefficient of 0.58. air lubrication. However, such large tions, Heim concluded that the travel dis- For larger rockfalls, however, the friction sturzstroms are now known to exist also on tance of a sturzstrom depends on the height coefficient is appreciably smaller than the the Moon where there is neither air nor wet of the fall, the regularity of the pathway, theoretical value. The Elm sturzstrom mud. The crux of the matter seems to be and the size of the fallen rock mass. shows an apparent angle of friction of 17° that most of the large rockfalls did not slide; they flowed. For flowing sturzstroms, If a mass slides, the travel distance L and and an equivalent coefficient of 0.3; larger the kinematics of the movement cannot be the height of rockfall H is related by rockfalls result in still smaller friction an- treated as a frictional sliding problem. I Coulomb's law of sliding friction: gles and lesser coefficients. Heim offered no explanation for this inverse correlation of propose that the moving debris be analyzed H = tan a L , (1) friction to volume of the fallen rock mass. as a flowing mass of cohesionless blocks. where tan a is the coefficient of friction of Shreve (1966, 1968a) speculated that the For such flows, the fahrboschung has a dif- the pathway and its value is commonly as- abnormally small coefficient of friction is a ferent physical meaning than the angle of sumed to be about 0.6. measure of the volume of the trapped air; sliding friction. The ratio of the height o: the highest the larger the fallen mass, the more air was point on the breakaway rim H and the trapped to lubricate the sliding process. Yet SLIDING OR FLOWING horizontal projection of distance from this the largest rockfall is the Tsiolkovsky slide point to the tip of sturzstrom L was called from the Moon. It has a volume of 1.2 x Heim (1882, 1932; also in Buss and 12 3 "Fahrboschung"2 by Heim (1932). This 10 m and an apparent friction angle of Heim, 1881) repeatedly emphasized in his 3.5°. There was no air on the Moon to lu- writings that large rockfalls did not slide — 2 One might translate the term "Fahrboschung" as the bricate such a slide. they flowed. Shreve (1966, 1968a, 1968b), angle of inclination of the course alonf; which a rockfall The application of equation 1 to calcu- on the other hand, asserted that such and its sturzstrom raced. The expression is much too rockfalls slid rather than flowed. What is long, and I shall keep Heim's original:-'nn as a technical late the equivalent coefficient of friction term, and write it with lower case, and without italiciza- implies that the center of gravity of the fal- the difference between a slide and a flow? tion. len mass shifted from the highest point on The definition of slide by the Oxford

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/1/129/3428976/i0016-7606-86-1-129.pdf by guest on 01 October 2021 CATASTROPHIC DEBRIS STREAMS GENERATED BY ROCKFALLS 133

TABLE 1. RELATION OF TRAVEL DISTANCE Concise Dictionary (4th ed., 1958) is TO SIZE OF FALLEN MASS "progress along smooth surface with con- tinuous friction on the same part of object Locality of event Fahrböschung Equivalent Excessive Volume (degrees) coefficient travel (10s m») progressing." The very fact that a rigid of friction distance block was converted into a debris tongue Le(km) should be sufficient argument that the fallen Airolo 33 0.64 0 0.5 block did not simply slide; there was not a Schächental 30 0.58 0.2 0.5 continuous friction on the same part of ob- Val Lagone 24 0.44 0.7 0.5 to 0.8 ject progressing; instead, more and more Mombiel 23 0.47 0.2 0.8 new surfaces on freshly broken debris came Huascaran 17 0.30 11.8 2 into contact with the valley floor as the Wengen (1) 24 0.45 0.3 2 to 3 sturzstrom surged forward. In contrast, Wengen (2) 23 0.42 0.45 5 to 6 Elm 0.31 1.15 10 flow has been defined as "glide along as a 17 Disentis 20 0.36 0.9 10 to 20 stream" (Oxford Concise Dictionary, 4th Corno d1 Dosdö 18 0.32 1.8 20 ed., 1958). A sturzstrom, which molded it- Madison 15 0.27 0.9 29 self to fit the geometry of the valley floor Voralpsee 18 0.33 1.6 30 and glided along as a debris stream, is thus Frank (Canada) 14 0.25 2.1 30 not a slide but a flow according to the nor- Sherman (U.S.A.) 12 0.21 4.1 30 mal usage of these words. Goldau 12 0.21 4.0 30 to 40 Why did Shreve insist then that the Gros Ventre (U.S.A.) 10 0.17 2.5 38 2.5 50 sturzstrom slid and did not flow? In his Dlablerets 19 0.34 Scima da Saoseo 15 0.27 3.1 80 publications and in his written communica- Obersee GL 20 0.36 2.1 120 tion to me, the single criterion cited to sup- Kandertal 11 0.19 6.9 140 port his contention was an observation that Poschivo 20 0.36 1.7 150 the original sequential order of the rock Apollo 17 (Moon) 11 0.20 ? 200 formations in a fallen block was preserved Silver Reef (U.S.A.) 7 1/2 0.13 ? 220 in the sturzstrom. Shreve reasoned that Vaiont (Italy) 19 0.34 0.7 250 such an order would have to be reversed if Blackhawk (U.S.A.) 7 1/2 0.13 7.6 280 the sturzstrom flowed viscously. There is a Deyen, Glarus 6 1/4 0.11 5.4 600 missing link in Shreve's logic: his argument Glarnisch 14 0.25 4.5 800 is invalid if the sturzstrom flowed but did Fernpass (Austria) 5 0.09 13.3 1,000 not flow viscously. Siders 8 0.14 13.5 1,000 to 2,000 Tamins 5 1/2 0.095 11.4 1,300 In fact, Heim was the first to observe a Pamir 14 0.24 3.8 2,000 preservation of the sequential order in a Engel berg 12 1/2 0.22 4.8 2,500 to 3,000 sturzstrom. He nevertheless envisioned a Fl ims 7 1/2 0.13 12.3 12,000 flowing, not a sliding process, and he gave Saidmarreh (Iran) 4 1/2 0.08 16.5 20,000 an adequate explanation of the preserva- Tsiolkosky (Moon) 3 1/2 0.06 7 1 ,200,000 tion of such sequential order during the flowage (Heim, 1932, p. 105): ical definition was given by Reiner (1958): nation of the gliding surface, but there was When a large mass, broken into thousand pieces, "Flow is the continuous and irreversible de- a continuous friction on the same part of and falling at the same time along the same formation with time of a body under a finite the object progressing. Such a mechanism is course, then the debris had to flow as a single force." indeed the frictional sliding of a flexible stream. The uppermost block at the very rear of Sturzstrom is a flow, whether one ob- sheet, but this model is very different from the stream would attempt to get ahead. It hurried serves the definition of Sander, that of the movement picture painted by the but struck the block slightly ahead, which was in eyewitnesses at Elm. the way. The kinetic energy, of which the first Reiner, or that of the Oxford Concise Dic- block had more than the second, was thus trans- tionary. Shreve was probably trapped by The difference between a flow and a slide mitted through the impact. In this fashion, the his own semantics when he described the can perhaps be best illustrated by a passage uppermost block could not overtake the lower sturzstrom flow phenomenon as the "slide quoted from Miiller (1964, p. 210), who block and had to stay behind. This process was of a flexible sheet." distinguished the movement of the Vaiont repeated a thousandfold, resulting eventually in The slide of a flexible sheet has been ex- slide from that of the sturzstrom generated the preservation of the sequential order in the perimentally demonstrated by Rengers and by other large rockfalls: debris stream. This does not mean that the Miiller (1970). They used aggregates of energy of falling blocks from originally higher small rectangular blocks to simulate a In this last phase, the major part of the sliding positions was lost; rather the energy was trans- jointed sliding rock mass (Fig. 3). They use masses probably fell down in a comparatively mitted through impact. The whole body of a very short time of about 30 to 45 seconds. The sturzstrom was full of kinetic energy to which a sliding surface with a small coefficient of entire masses then developed an almost transla- each single stone contributed its part. No stone friction (tan 14° or tan 23° instead of tan tory motion in the N. 8° E. direction with slight was free to work in any other way. 32°) to simulate a lubricated surface at the clockwise rotation, and some local distor- bottom of a slide mass. They could dem- tions. . . . This portrayal of the sturzstrom motion onstrate an "apparently" excessive travel The rock mass crossed the Vaiont gorge with- is fully in accord with the various technical distance of sliding because of the reduced out falling down into it and slid on the opposite definitions of "flow." For example, Sander friction on the sliding surface. However, the slope up to 140 m. (1948, p. 101) stated: "Flow is a continu- slide mass was displaced but not much de- The last phase of the phenomenon, the actual rock-slide was of a nature completely different ously arranged relative movement, and car- formed. Except for dilatation and slight ro- than could be envisaged from the great number ried out by sufficiently small (compared to tation and disturbance of individual blocks, of studies conducted from the start of the proj- the system under consideration) parts, as the slide mass has more or less retained its ect. ... In the last phase, with the substance long as those parts touch one another, so original dimensions. Flexibility of the quasi-plastic the entire mass could be expected to that finite forces were transmitted and an aggregate did permit the sliding block to flow. . . . We now have, however, certain proofs internal friction was expended." A rheolog- bend in conformity with a change in incli- of quite a different motion in which the rock

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/1/129/3428976/i0016-7606-86-1-129.pdf by guest on 01 October 2021 134 K. J. HSt)

mass was transported like a block across the seems that the flow motion of that This citation should suffice to dem- gorge. sturzstrom was propelled mainly by stress onstrate that Heim's postulate for the Larger interior deformations and partial dis- (T) transmitted by grains as a result of grain sturzstrom motion is similar to what Bag- placements such as generally observed on [other] collisions. The motion of such colliding nold envisioned for an inertial grain flow. large slides would have very quickly led to a grains has been referred to by Bagnold Flows of cohesionless grains can take filling of the gorge and to another equilibrium, so (1954, 1956) as the flow of cohesionless place on appreciably less steep inclines than much the more, as a large part of the kinetic energy would have been absorbed by internal grains or, simply, grain flow. Contrary to a that required for sliding. Bagnold (1954, p. friction and the deformation work. Contrary to fluidized bed in which fluid stress predomi- 62) described the phenomenon of "flowing this in the actual movement, which was essen- nates, grain flow has a negligible fluid gravel" as follows: "A river of gravel, esti- tially translation, very little energy was absorbed stress. Kinetic energy is transferred through mated as up to the size of hens' eggs at the by interior friction and deformation. Almost the grain collisions and dissipated by friction visible surface, was watched in movement entire kinetic energy was conserved and pushed during such collisions. It is interesting to along the floor of a mountain valley. The the mass as a block on the opposite slope [italics note that Heim wrote in 1932, more than gradient did not exceed a few degrees. The added]. 20 years before Bagnold proposed grain speed of the flow at the surface was esti- The Vaiont event, characterized by its collisions as a transport mechanism, yet mated as that of walking. No supporting translatory motion without much internal Heim's account of the Elm sturzstrom, as fluid was visible between the surface peb- deformation, is a large rockslide that slid. cited in the quotation above, appears to be bles." a good description of the Bagnoldian The Elm and many other rockfalls that gen- The gravitational stress Sg propelling the erate a sturzstrom were characterized by an grain-flow mechanism. movement is the same regardless if the mix- internal deformation that changed a more ture of gravel and mud slid or flowed (Bag- or less rectangular block into a sheet of de- FLOW OF COHESIONLESS GRAINS nold, 1954, p. 63): bris; the debris flowed and did not simply S = 1Pf + (Po - Pf)C] d g sin /3 , (3) slide. Bagnold (1954, 1956) regarded a mass of 0 That catastrophic rockfalls flowed was concentrated grains that were dispersed in where d is the thickness, C the volume con-

also postulated by Kent (1966) who pro- a flowing fluid medium to be a fluid, and he centration of the flowing gravel, ps — pf the posed the fluidization of a sturzstrom. treated it separately from the overlying cur- difference in the densities of the solid and Fluidization has been defined (Thrush, rent. The grains were not regarded as hav- fluid, respectively, and /3 is the slope angle 1968) as "A roasting process in which ing been transported by the fluid stress of the bed. finely divided solid is kept in suspension by exerted by the interstitial fluid. Instead, the The reason that gravel could flow down a rising current of air. ... A bed [of solid tangential force on the grains consists less-steep inclines than would be required particles] is fluidized when it is made to largely of a component of the effective for sliding has been attributed by Bagnold float by the upward movement of a liquid weight of the grains themselves. The (1954, p. 63) to the smaller resistance of or gas. In such a bed, friction between par- movement of such concentrated grains in flowing gravel; he traced this smaller resist- ticles is zero and they became highly dispersion3 was called grain flow by ance to the reduction of effective normal mobile." It should be pointed out that the Bagnold. pressure of dispersed solids in a fluid flow of mixtures of solid grains and fluid Bagnold (1956) reasoned that a static medium. involves a fluid-transmitted stress r and a grain mass cannot flow without some de- The normal dispersive stress in a grain grain-transmitted stress T (Bagnold, 1954, gree of dispersion. In the case of grain flow, flow is equal and opposite to the effective 1956). The total stress is the dispersion must be upward against the normal pressure of the sturzstrom on its bedward (or downward) body force. Bag- bed, namely (Bagnold, 1954, p. 63), T = T + T . (2) nold proved by experimentation the exis- P = (p.-p,)Cgd cos ¿3 . (4) tence of a dispersive stress normal to the In a fluidized bed, fluid-transmitted stress is motion of flowing grains (Bagnold, 1954). The shearing resistance to a grain flow is dominant and grain-transmitted stress is He further reasoned that this dispersive caused mainly by grain collisions, the fluid ideally zero, because of the absence of fric- stress must have originated from collisions stress being negligible, and this resistance is tion between fluidized particles. A fluidized of grains in a dispersion of high concentra- related to the dispersive stress by the for- sturzstrom would imply rockfall debris was tion (Bagnold, 1956, p. 240): mula (Bagnold, 1956, p. 240) actually suspended by and floating in a cur- T = P tan a' rent of rising air. It is highly improbable When the grains are but a diameter apart or less = (p -p,)Cgd cos/3 tana', (5) that blocks weighing many thousands of (volume concentration >9% for spheres) the s tons could be propelled by air, and it is even probability of mutual encounter, always finite for where the angle a' is associated with the less probable that rockfalls could have been concentration of a random grain array undergo- mean encounter conditions and tan a' ( = ing shear, approaches a certainty. The grains fluidized on the airless Moon. 0.6) is the dynamic analogue of the static must knock or push each other out of the way friction coefficient between grains. From eyewitness accounts at Elm, it according to whether or not the effects of their The initiation of a grain flow requires inertia outweigh those of the fluid's viscosity; and both kinds of encounter must involve dis- that placements of grains normally to the planes of S.5T (6) shear. So it seemed probable that the required normal dispersive stress between the sheared or grains might arise from the influence of grain on IPi + (P. - Pr) C}gd sin ¡3 grain (neglected by previous workers on this sub- > (ps ~Pf)Cgd cos 0 tana' . (7) ject). Further, the same encounters should give rise to an associated shear resistance (T) addi- Rearranging the terms, we obtain the criti- tional to that offered by the intergranular fluid cal angle 6f necessary for grain flow (Bag- (T). nold, 1954, p. 63):

3 Figure 3. A model of the sliding of a flexible Dispersion describes a mass of scattered solid parti- tan 6, = tan aj ~ Pf) C 1 (g) sheet (from Rengers and Miiller, 1970). cles in a fluid medium. L pf + (Ps ~ Pf) C J

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/1/129/3428976/i0016-7606-86-1-129.pdf by guest on 01 October 2021 CATASTROPHIC DEBRIS STREAMS GENERATED BY ROCKFALLS 135

3 The critical angle necessary for sliding 6S is, sequently, the kinetic energy of the a density 2.0 g/cm . Assuming a concentra- in comparison, sturzstrom begins to decrease, and the flow tion of the gravel of 55 percent (C = 0.55) begins to slow down. After point M, the tan 0, = tan a . (9) and a frictional coefficient of 0.75, Bagnold sturzstrom is flowing along a horizontal (1954, p. 63) calculated that such flowing As the ratio (p., - pf) Cl[pf + (ps - pr) C] is course, and there will be no more potential gravel can move steadily down an incline of always less than 1, and a is believed to energy loss. The sturzstrom nevertheless 6°, and the equivalent coefficient of friction equal a' (p. 240), we can conclude continues to surge forward on account of of the flowing grain mass is 0.106. the stored kinetic energy until point E' is What could be an interstitial fluid be- tan 6f < tan 0, ; (10) reached as the last of the kinetic energy is tween the blocks of a sturzstrom? Water- namely, the slope required for the flow of being dissipated by friction. saturated mud, as Heim suggested, might dispersed grains is always smaller than that It should be emphasized that the diagram indeed be present in some sturzstroms such required for frictional sliding of an undis- illustrates the energy transfer, not the dis- as the one generated by the Huascaran persed grain aggregate. We can thus explain placement of particles. It does not imply rockfall. Compressed air might also have the fact that the fahrboschung of that the block at the top of the breakaway played a role in some instances, but cer- sturzstroms is always smaller than the angle rim A is to be transported to the tip of the tainly not on the Moon. If a single element of repose of granular solids. sturzstrom E'. Instead, we should imagine is sought that might have been present at It was discussed above that the fahr- that the potential energy of the block at A Elm and Blackhawk, on the Earth and on boschung is, strictly speaking, not the angle has been transferred through numerous col- the Moon, that element could be the disper- of sliding friction. It is, of course, meaning- lisions to enable the foremost block at the sion of fine debris particles between collid- less to speak of the angle of repose of a frontal edge to move to the point £'. Simi- ing blocks, that is, the dense dust cloud en- flowing fluid. Heim's physicist colleague, E. larly, the potential energy of a block at T veloping a rampaging sturzstrom. Miiller, recognized this fact, and he com- has been transferred and has induced a The presence of a dense dust cloud above pared the fahrboschung to the energy line block in the sturzstrom to move to U'. We and within the sturzstrom was described by of a flowing liquid (Miiller, in Heim, 1932, might draw an infinite number of lines several eyewitnesses at Elm, who compared p. 145). Figure 4 is a diagrammatic analysis parallel to the fahrboschung AE' to illus- the cloud to black smoke from a steam of the changing energies of a flowing trate the transfer of energy from various locomotive or shot out of a cannon (Buss sturzstrom. AE' is the course of the rockfall points of the incipiently moving block to the and Heim, 1881). The dense dust cloud and its main surge. On the steep incline, the sturzstrom. From this fluid-mechanics point about other sturzstroms can be readily en- falling sturzstrom (path ATPQ) rapidly of view, the fahrboschung is a measure of visioned. Heim (1882) and Shreve (1966) loses potential energy while gaining kinetic the rate of dissipation of energy by internal further described the presence of small de- energy. For example, at point P of the friction; a small fahrboschung slope means bris cones, with graded bedding, which sturzstrom, a slight gradient of energy loss and a great were perched on top of large sturzstrom transport distance. OP = NP - NO blocks; the fine stones, sand, and dust must have settled out of a very dense suspension. DISPERSION OF BLOCKS One can easily visualize that large lunar (Kinetic energy gained) = IN A DUST CLOUD rockfalls should also be wallowed in dust, (potential energy loss) — (friction). especially if one has seen on television the Flowing grain masses can move down landing of the Apollo crafts. Between points A and Q, the kinetic energy gentle inclines because of the presence of an The common presence of small broken gained is positive, and the sturzstrom speeds interstitial fluid which reduces the effective debris and pulverized rock powder among up. After point Q, the sturzstrom is travel- normal pressure on grains and con- large blocks can be seen in dissected sec- ing on a gentle incline and the rate of energy sequently reduces the frictional resistance. tions of old sturzstrom deposits. In the dissipation by friction is greater than the In the case of the flowing gravel described Upper Rhein gorge near Flims, there are ex- rate of supply of potential energy. Con- by Bagnold, the interstitial fluid is a mud of cellent exposures of the Flims sturzstrom: large angular blocks are embedded in a ma- trix of rock flour, and the sturzstrom re- sembles a deposit of bombs and ejecta mixed in an airborne ashfall (see Heim, 1932, p. 125-127). Another excellent de- scription of the matrix of the Deyen-Wiggis sturzstrom in Glarus has been given by Oberholzer (1900, p. 73):

The debris pile represents a confusing mixture of large and small blocks with abundant brown- gray interstitial materials. Besides the predomi- nant Schrattenkalk blocks, there are Seewerkalk and Flysch rocks. Also present are Nummuliten- kalk and Gault. The interstitial materials are a mixture of small debris and pulverized rock powder of various colors, evidently derived from all the rock formations mentioned above. The whole mixture of blocks and matrix gives the ap- pearance of a moraine, except that the blocks have all been shattered and fractured. The inter- stitial materials tended to be washed away during the raining seasons, setting free larger blocks Figure 4. A diagrammatic analysis of changes of energies during the conversion from a rockfall to a which would then tumble down to the Lontsch sturzstrom (reproduced from Heim, 1932). Creek below.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/1/129/3428976/i0016-7606-86-1-129.pdf by guest on 01 October 2021 136 K. J. HSt)

If the effective normal pressure of a series of experiments with the aid of my as- onstrate the inverse relation of the sturzstrom is indeed reduced be:ause of the sistant, C. Siegenthaler. The experiments fahrboschung to the size of the sturzstrom. presence of interstitial dust (and small de- were suspended after Dr. Siegenthaler took In our first series of experiments, we at- bris), the density of such a dust cloud may another position while I was involved in tempted to find a material capable of be calculated with the aid of equation (8) of deep-sea drilling research. However, I am simulating the movement of the Elm this paper. At Elm, the fahrboschung was now in the process of continuing this sturzstrom. A scale-model analysis was car- about 17° and the tan a' was 0.305, a re- project. The ultimate goal is to enable one ried out for a model with a geometrical duction from the normal friction coefficient to predict the extent of flowage of a scale factor (KL) of 1:4,000. Assuming a (tan 32°) by a factor of 0.5. Using equation threatening rockfall. The details of the ex- rectangular shape for the Elm block before 8, perimental results will be included in a it fell (with dimensions di, /,, and and forthcoming paper. I am presenting herein volume Vj for the prototype), the dimen- C tan 0f = tan a' & ~ P^ =•• Vi tan a' , only the preliminary results which Dr. sions ¿2, l2, and w2 and volume V2 for the 1Pf + (Ps ~ Pf) C] Siegenthaler and I obtained that dem- model should be and thus

c (P. ~ Pf) = y„ = thickness ratio = KL = 1/4000 = 2.5 cm/100 m

[Pr + (P. - Pt) C] /2//, = length ratio = KL = 1/4000 = 8 cm/320 m Assuming a volume concentration of 50 w2lwi = width ratio = KL = 1/4000 = 8 cm/320 m percent (C = 0.5) and a rock density p of s 3 6 3 2.4, the computed density should be 0.8. V2/V, = volume ratio = dj2wdldlllwl = 160 cm /10 x 10 cm . Such a density represents a mixture of one- third dust and stones and two-thirds air or We chose a glass jar with dimensions 2.5 x tom. When the glass plate was suddenly vacuum. Considering the previously cited 8x8 cm3 and filled it with loose materials pulled aside, the loose materials or the fluid field evidence, the presence of interstitial or fluid to simulate the "about-to-fall" block inside the jar would flow out and race down dust-suspension of such density seems quite at Elm. A wood model with an approxi- the model course. probable in large catastrophic sturzstroms. mate geometrical similarity to the sur- If an experimental sturzstrom has One can easily fancy a stream of colliding rounding of Elm at a scale of 1:4,000 was achieved the geometrical similarity to the blocks swimming with terrifying speeds in a constructed to be the "race-course" prototype in nature, it should have the di- sea of small stones and dry rock powder. ("Fahrbahn") for the model sturzstroms. mensions Although interstitial mud was almost cer- The glass jar had a glass plate as a false bot- tainly absent in lunar sturzstroms and

probably absent in many terrestrial ones, its d2 = dtKL = 10 ir. x Kl = 0.4 cm = thickness of the model sturzstrom at the end presence in the catastrophic Huascaran w = wJi = 400 m x K = 10 cm = average width of the model sturzstrom sturzstrom is undeniable. The mud-soaked 2 L L = sturzstrom traveled some 14.5 km, at an hi b, = 1-5 km x Kl = 37.8 cm = length of the model sturzstrom on the flat course. average speed of 400 km/hr, or 3 to 4

times the average speed at Elm, Let Kt, Ku, Ka be the scale factors for bottom of the steep slope. However, in killed 80,000 persons en route and buried time, velocity, and acceleration, respective- none of our 50 trials did we obtain a model several villages; finally it was converted into ly. As the prototype and the experiments geometrically similar to a sturzstrom. a stream of slurry mud, which flowed were both taking place within the Earth's Viscous oils were also tried; they flowed

another 50 km down the valley at 15 km/hr gravitational field, a2!a, = Ka = 1. Fur- viscously, and they would continue to flow (Ericksen and others, 1970; Browning, thermore, wherever there was a slightly downward 1973). The interstitial suspension which slope. We were not able to bring the flow- provided the buoyancy for the dispersion of ing mass to a sudden and permanent halt as blocks in the Huascaran sturzstrom was cer- did the prototype at Elm. As we did not en- tainly not dry dust, but a wet mud. How- The scale factors Ku and K, must therefore vision a viscous-flow mechanism anyway, ever, the Huascaran sturzstrom may repre- be we stopped experimenting with oils after a sent a special case, because the fallen block few trials. Ku = VrL = V4/XK) = 63 included not only granodiorite but also a We were finally able to achieve geometri- block of glacier ice and a considerable K, = VrL = V4^SS = 63 . cal similarity by using a bentonite suspen- amount of water-saturated sediment. This A dry silt mixture was first chosen as the sion. Some 100 trials were carried out, and muddy sturzstrom was unusually mobile starting material. As we expected, the mix- not all of those reproduced geometrical for its size, probably because of the excep- ture failed to flow along the flat course as similarity, only a suspension of a certain tionally high density of the interstitial sus- the Elm prototype. Obviously, the consistency could do the trick. Once we pension; dust was mixed with water that coefficient of friction was such that the slid- found the right material, we could re- was derived in part from melted ice and in ing mass could barely get beyond the foot peatedly make the model sturzstrom run part from the wet sediment. of the steep 41° slope. The fahrboschung of the length of the 37.8-cm flat course and the model slide was about 33° and equal to thus simulate the Elm surge. MODEL EXPERIMENTS the normal friction angle. Our next task was to test whether a We then attempted a mixture of silt and geometrically similar model sturzstrom also If a sturzstrom generated by a rockfall dry ice, which gave off a vapor and tended simulated the Elm sturzstrom kinematical- flowed, it should be possible to find a suita- to increase the pore pressure of the silt. Our ly. The durations of the Elm event accord- ble fluid for a scale-model experiment to experiments failed to produce a coherent ing to estimates from eyewitness accounts simulate its geometry and kinematics. Dur- sturzstrom. Some of the silt, jetted by the and from theoretical considerations should ing the years 1966-1967, I carried out a gas pressure, did spray far out beyond the be about 50 sec (Heim, 1932; Hsii, 1975).

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/1/129/3428976/i0016-7606-86-1-129.pdf by guest on 01 October 2021 CATASTROPHIC DEBRIS STREAMS GENERATED BY ROCKFALLS 137

The total elapsed time for the kinematically strument to determine thixotropic viscosity. per hr stopped suddenly 10 or 20 sec later. similar sturzstroms should, therefore, be We used a conventional viscosimeter and This demonstration of the tremendous estimated the viscosity for the bentonite sus- braking power of a sturzstrom bears a t2 =tlK, = 50 sec x — = — sec. pension in run no. 5 to be about 3 cen- kinematic similarity to the sudden congela- 63 6 tipoises, when the speed of relative move- tion of a thixotropic flow. It is interesting to For this series of experiments, bentonite ment was in the range of a few centimeters note that clay slides with a volume of only a suspensions of different consistencies as per second. few thousand cubic meters can generate a well as a few wet silt mixtures were used. In order to test the preservation of thixotropic flow similar to a sturzstrom The experiments were filmed, using an stratigraphic sequence in a sturzstrom, col- (Terzaghi, 1950; Müller, 1964). Small clay 8-mm movie camera, with a shutter speed ored indicators were used. As Figure 6 slides might well be considered natural of 30 frames per sec. The time-distance re- shows, the colored materials at the bottom scale-model experiments on sturzstroms. lations were plotted on Figure 5. Of the 14 of the glass jar simulating the lower part of The success of our ability to kinemati- different materials studied, only the one the fallen block were pushed to the lateral cally model the Elm rockfall encouraged us used for run no. 5 traveled the correct dis- rim and to the distal end, as Heim observed to perform a second series of experiments

tance. It is comforting to note that this at Elm. again with KL = 1:4,000 to relate the vol- geometrically similar sturzstrom was also Although we found bentonite a suitable ume of a rockfall mass to its travel distance. the only one that gave an approximate experimental material through trial and A metal plate with one end bent to an angle kinematic similarity. Instead of the pre- error, we later recognized the kinematic of 40° from the horizontal was used. To dicted 5/6 sec, the model sturzstrom took 1 similarity of a sturzstrom to a thixotropic simulate surfaces of different roughness, the sec. Considering the uncertainties in es- flow. The rapid dispersion of the Elm plate was covered by a cloth fabric for some timating the Elm event, our result could be rockfall after the fallen block had disinte- of the runs. The starting materials were considered a good approximation. The grated is reminiscent of the spontaneous placed at three different heights to test the kinematic similarity in run no. 5 is sum- liquifaction of a thixotropic substance. It effect of maximum fall height on travel dis- marized by the data in Table 2. Bentonite will be recalled that several witnesses at tance. To our surprise, we found that suspension is a thixotropic liquid, and its Elm were surprised by the very sudden halt neither the surface condition of the plate viscosity is not a material constant, but ve- of the sturzstrom; the debris flowing at one nor the fall height had much effect on the locity dependent. We did not possess an in- moment at a speed of more than 100 km travel distance of the bentonite suspension

Simulated travel distance (meter) Figure 5. Kinematic analysis of experimental sturzstroms. Run no. 5 simulated approximately the time elapsed and distance traveled of the Elm event. The others ran short of the goal and at a slower simulated speed; they might be considered kinematic models of some sturzstroms that had the same volume but were less mobile than the one at Elm (see Fig. 8 on the question of mobility).

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/1/129/3428976/i0016-7606-86-1-129.pdf by guest on 01 October 2021 138 K. J. HSt)

TABLE 2. KINEMATICAL SIMILARITY OF A be the least mobile. The Vaiont slide, for sturzstroms, suggesting the kinematic simi- BENTONITE FLOW MODEL example, had a relatively large coefficient of larity of the mobile sturzstroms to our ben- Prototype Model run 0.34 for its large volume (Table 1); its mod- tonite models. according to no. 5 Heim, 1932 est height loss (550 m) helped to restrain 3. A number of sturzstroms did not go the slide from developing into a mobile the maximum distance that might be ex- Total distance of (719 + 1,513 m) 50 cm x 4,000 sturzstrom. travel (K^ = 4,000) = 2,232 m = 2,000 m pected on the basis of their size. Those Travel distance on 1,513 m 38 cm x 4,000 Our experiments revealed the possibility sturzstroms may have been just as mobile, flat course = 1 ,520 m that the parameter "equivalent coefficient but they lost too much energy on a tortuous (K = 400) l path (for example, Diablerets). More prob- Total elapsed time 50 sec 1 sec x 63 of friction" may not be the best indicator (Kt = 63) = 63 sec for the mobility of a sturzstrom. A new ably those sturzstroms may have achieved Maximum speed 83 m/sec 125 cm/sec x 63 parameter, called "excessive travel dis- only partial mobility and failed to develop (Kv = 63) = 83 m/sec tance," is introduced. This distance, Le, can fully into mobile sturzstroms because of an be defined as the horizontal displacement of insufficient height loss or a confinement by along the flat part of the course. The travel the tip of the sturzstrom beyond the dis- the topography (for example, Flims). Ben- distance was controlled mainly by the vol- tance one expects from a frictional slide tonite suspensions too thick to simulate ume of the bentonite that had to spread out down an incline with a normal coefficient Elm kinematically produced sturzstroms to make a tongue-shaped model sturzstrom of friction of tan 32° (0.62), namely, similar to those of the less mobile group. of a certain thickness: the greater the vol- 4. A most notable departure from the L = L - H/tan 32°. ume, the farther the bentonite suspension e norm is the famous Vaiont event. As we had to spread out and the greater was the Field observations of excessive travel dis- discussed previously, the Vaiont fallen mass distance of travel on the flat course. tances as well as the three experimental de- may be properly called a slide: the fallen The quantitative results of the experi- terminations are plotted on semilog graph mass moved by translation, not by flowage ments are shown by Figure 7. Three con- paper and shown by Figure 8. The follow- or internal deformation (Miiller, 1964), tainers simulating 1, 10, and 51) million cu ing generalizations may be made: and the slide largely retained its integrity m of rockfall volume were used. Those con- 1. The excessive travel distance is com- (Selli and others, 1964). The absence of a tainers were placed at three different ex- monly minor or negligible for rockfalls with jumping platform to shake loose the debris, perimental heights of 7.5, 15, and 25 cm less than 5 x 106 m3. the small height loss, and the confinement (simulating fall heights of 300, 600, and 2. One can postulate a semilog function of the valley apparently all contributed to 1,000 m). Because the sturzstrom of a given to relate the most mobile rockfalls larger prevent the generation of a sturzstrom. The volume would spread out to about the same than 5 x 106 m3, particularly the famous Vaiont fahrboschung was 19°, not much distance on the flat course, the apparent Elm, Sherman, Goldau, and Blackhawk smaller than the angle of repose of a cohe- coefficient of friction is smallest for the ex- rockfalls and the less well known Kander- sionless block perched on a water-saturated perimental rockfalls with the smallest tal, Fernpass, Tamins, and Siders rockfalls. sliding surface (Heim, 1932; Hsu, 1969a); height loss. This apparent dependence of All of those generated sturzstroms to ac- there was hardly any reduction in the slid- fahrboschung on height loss is actually quite count for the excessive travel distances. The ing friction of this massive slide. misleading. Rock slides with small height experimental sturzstroms made with the 5. At the other end of the spectrum is the loss in nature commonly had the largest bentonite solution to simulate the Elm extremely mobile Huascaran fallen mass. friction coefficient, because they tended to event fall close to the line for mobile The fallen mass did not generate a dry sturzstrom, but a muddy sturzstrom. Those fast-moving blocks, dispersed in an intersti- tial mud medium, did not stop suddenly; the mass was changed into a mudflow.

CONCLUSIONS

I have gone to a great length to support the claim that the sturzstrom flowed and to refute the hypothesis by Shreve that it slid on an air cushion. The mechanism en- visioned by Heim of stress transmittal by colliding blocks is similar to that defined by Bagnold as "grain flow." Applying the Bagnoldian analysis, I postulated that the apparently excessive travel distance of many sturzstroms might be related to a re- duction of frictional resistance of colliding blocks dispersed in a dust suspension; the effective normal pressure on such blocks is less because of the buoyancy force of the dust. Presence of interstitial watery mud might also be responsible for the mobility of some sturzstroms such as the Huascaran event; muddy sturzstroms might not halt Figure 6. Preservation of sequential order in a sturzstrom. These photos show the model used and the geometry of a model sturzstrom as it was photographed by a movie camera. This particular exper- suddenly as do most dry sturzstroms but iment shows that the thixotropic ben ronite suspension did not flow viscously. The colored fluid placed change into less rapidly advancing originally at the lower part of the about-to-fall "block" was pushed to the front and the side of a mudflows. Further developing a concept in- sturzstrom, somewhat in the fashion of the prototype at Elm. troduced by Heim and Miiller that the

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/1/129/3428976/i0016-7606-86-1-129.pdf by guest on 01 October 2021 Rockfall Volume (m3) Figure 7. Equivalent coefficient of friction related to rockfall (or rockslide) volume. The equivalent coefficient of friction has been defined as the maximum drop height divided by the maximum horizontal distance traveled. There is a general correlation between the coefficient and the size of rockfalls. This relation is demonstrated experimentally by three series of experiments simulating rockfall volumes of 1, 10, and 50 x 106 m3. To our surprise, the distance of travel on the flat course of a bentonite of a certain mobility is dependent only on its volume, and independent of the height of fall. Consequently, the one placed highest, simulating 1-km height drop, had the largest coefficient of friction for a given volume, as shown by the upper curve; the one placed lowest, simulating 300 m, had the lowest coefficient, as shown by the lower curve. Abbreviations for the rockfalls are A, Airolo; VL, Val Lagone; Hu, Huascaran; Sh, Sherman; Go, Goldau; Di, Diablerets; K, Kandertal; Bl, Blackhawk; F, Fernpass; Ta, Tamins; Si, Siders; A 17, Apollo 17 on the Moon; Va, Vaiont; En, Engelberg; Fl, Flims; Fp, Fernpass; Pa, Palmira; Sa, Saidmarreh; Ts, Tsiolkovsky on the Moon.

muddy sturzstrom rockslide + mobile sturzstroms Experimental sturzstroms (material simulating Elm) x less mobile sturzstroms Experimental sturzstroms • small rockfall (material not simulating Elm) 12 •

Rockfall Volume (106 m3)

Figure 8. Excessive travel distance (Le) related to rockfall volume (V). The excessive travel distance is the horizontal distance traveled by the tip of a sturzstrom beyond what is expected on the basis of frictional sliding (without lubrication). The distance Le is not only related to the rock volume but also to the mobility of the sturzstrom. Elm belongs to a more mobile group whose L,,/log V is represented by the slope of the line with a steeper gradient (line II). The bentonite suspension capable of simulating the Elm event produced experimental sturzstroms comparable to this group. A less mobile group of sturzstroms has a relation shown by the line with a gentler gradient (line I). Bentonite suspension too thick to simulate Elm kinematically gave experimental sturzstroms comparable to those less mobile sturzstroms. The least mobile is the Vaiont slide which failed to develop into a sturzstrom in spite of its large volume. Rockfalls with a volume of less than 5 million cu m commonly had a very small excessive travel distance. The notable exception is the truly cata- strophic Huascaran event which developed into a muddy sturzstrom more mobile than many larger sturzstroms. See explanation of Figure 7 for abbreviations.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/1/129/3428976/i0016-7606-86-1-129.pdf by guest on 01 October 2021 140 K. J. HSt)

fahrboschung of a sturzstrom is a measure don Proc., ser. A, v. 225, p. 49-63. ogy: San Francisco, Freeman, Cooper, and of the energy loss (by frictional dissipation) 1956, The flow of cohesionless grains in Co., 571 p. of the fluid-like, grain-flow motion, we fluids: Royal Soc. London Proc., ser. A, v. Kent, P. E., 1966, The transport mechanism in catastrophic rock falls: Jour. Geology, v. postulated that the kinematics of a 249, p. 235-297. Browning, J. M., 1973, Catastrophic rock slides, 74, p. 79-83. sturzstrom might be duplicated by a suit- Mount Huascaran, north-central Peru, Müller, L., 1964, The rock slide in the Vajont able fluid. In the second part of this paper, I May, 31, 1970: Am. Assoc. Petroleum Valley: Felsmechanik u. Ingenieurgeologie, have presented our preliminary attempts to Geologists Bull., v. 57, p. 1335-1341. v. 2, p. 148-212. model sturzstroms kinematically with ben- Buss, E., and Heim, Albert, 1881, Der Bergsturz Oberholzer, J., 1900, Einige prähistorische tonite suspensions. I recognize, of course, von Elm: Zurich, Wurster & Cie., 163 p. Bergstürze in den Glarneralpen: Beitr. Geol. the difference between the dynamics of a Ericksen, G. E., Pflaker, G., and Fernandez, J. V., Karte Schweiz., n.f., no. 9, 209 p. sturzstrom and that of a bentcnite suspen- 1970, Preliminary report on the geological Reiner, M., 1958, Handbuch der Physik: Berlin, sion: grain-transmitted stress is dominant events associated with the May 31, 1970, Springer, v. 6, p. 434-550. in the former, whereas fluid-transmitted Peru : U.S. Geol. Survey Circ. Rengers, N., and Müller, L., 1970, Kinematische stress is dominant in the latter. However, a 639, 25 p. Versuche an geomechanischen Modellen: Gary, M., McAffe, R., Wolf, C. L., 1972, Glos- Felsmechanik u. Ingenieurgeologie, Suppl., scale-model analysis indicated a surprising sary of geology: Washington, D.C., Am. v. 1, p. 20-31. degree of kinematic similarity between a Geol. Inst., 805 p. Sander, B., 1948, Einführung in die model of the Elm sturzstrom and the Elm Gilluly, J., , A. C., and Woodford, A. D., Gefügekunde der geologischen Körper: prototype. Apparently, the mobility of a 1968, Principles of geology (3d ed.): San Wien, Springer, 215 p. sturzstrom during its flowage is velocity- Francisco, Freeman and Co., 687 p. Scheller, E., 1971, Beitrag zum Be- dependent as is a thixotropic bentonite, Guest, J. E., 1971, Geology of the farside crater wegungsverhalten grosser Bergstürze: Ec- which has a velocity-dependent viscosity. Tsiolkovsky, in Fielder, G., ed., Geology logae Geol. Helvetiae, v. 64, p. 195-202. Experiments with bentonite suspensions and physics of the Moon: Amsterdam, Selli, R., Trevisan, L., Carloni, G. C., Mazzanti, suggested further that sturzstroms might be Elsevier Co., p. 93-103. R., and Ciabatti, M. 1964, La Frana del distinguished on the basis of their mobility Hampton, M. A., 1972, The role of subaqueous Vaiont: Gior. Geologia, Annali Mus. Geol. Bologna, ser. 2, v. 32, p. 1-154. which can be measured by relating a debris flow in generating turbidity currents: Jour. Sed. Petrology, v. 42, p. 775-793. Sharp, R. P., and Noble, L. H., 1953, Mudflow parameter called "excessive travel dis- Heim, Albert, 1882, Der Bergsturz von Elm: of 1941 at Wrightwood, southern Califor- tance" to the volume of a fallen rock mass. Deutsch. Geol. Gesell. Zeitschr., v. 34, p. nia: Geol. Soc. America Bull., v. 64, p. Finally, I would like to make a point that 74-115. 547-560. whether a sturzstrom flowed or slid is not a 1932, Bergsturz und Menschenleben: Shreve, R. L., 1966, Sherman landslide, Alaska: semantic question. Lives in areas threatened Zurich, Fretz & Wasmuth Verlag, 218 p. Science, v. 154, p. 1639-1643. by a potential rockfall may be saved be- Howard, A. D., 1972, Presentation of the Kirk 1968a, The Blackhawk landslide: Geol. Soc. Bryan Award to Ronald L. Shreve: Geol. America Spec. Paper 108, 47 p. cause of a correct diagnosis. If sturzstroms, Soc. America Bull., v. 83, p. xxiii. 1968b, Leakage and fluidization in air-layer as I believe, flowed, and if sturzstroms Howard, K., 1973a, Avalanche mode of motion: lubricated avalanches: Geol. Soc. America could be modeled by bentonite suspensions Implications from lunar examples: Science, Bull., v. 79, p. 653-658. as our preliminary results showed, we v. 180, p. 1052-1055. Terzaghi, K., 1950, Mechanism of landslides, in might have a powerful tool to make predic- 1973b, Lunar avalanches: Lunar Science IV: Paige, Sydney, ed., Application of geology tions on the maximum and minimum travel Houston, Texas, Lunar Science Inst., p. to engineering practice (Berkey volume): distances of a potential rockfa.l. Our cur- 386-387. New York, Geol. Soc. America, p. 83-123. rent experimental program is devoted to Hsu, K. J., 1969a, Role of cohesive strength in Thrush, P. W., 1968, A dictionary of mining, such a cause. the mechanics of overthrust faulting and of mineral, and related terms: U.S. Bur. Mines landsliding: Geol. Soc. America Bull., v. 80, Spec. Pub., 1269 p. p. 927-952. Zweifel, E., 1883, Der Bergsturz von Elm am 11. 1969b, Role of cohesive strength in the September 1881: Glarus, Buchdruckerei F. REFERENCES CITED mechanics of overthrust faulting and of Schmid, 183 p. landsliding: Geol. Soc. America Bull., v. 80, 7 American Geological Institute, 195 , Dictionary p. 955-960. MANUSCRIPT RECEIVED BY THE SOCIETY OC- of geological terms: New York, Doubleday 1975, Observations by Albert Heim on TOBER 18, 1973 & Co., 545 p. rockfalls and modern interpretations: In- REVISED MANUSCRIPT RECEIVED MAY 3, 1974 Bagnold, R. A., 1954, Experiments on a gravity- ternal. Cong, on Rock Mechanics Proc., CONTRIBUTION No. 57 OF THE LABORATORIUM free dispersion of large solid spheres in a Denver, 1974 (in press). FÜR EXPERIMENTELLE GEOLOGIE, ETH Newton fluid under shear: Royal Soc. Lon- Johnson, H. A., 1970, Physical processes in geol- ZURICH

Printed in U.S.A.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/1/129/3428976/i0016-7606-86-1-129.pdf by guest on 01 October 2021