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Sturzstroms) Generated by Rockfalls

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Sturzstroms) Generated by Rockfalls 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, landslides. 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 mudflows, lava flows, and glaciers. 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 Moon (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 "Landslide" 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 soil 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 friction" 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 mudflow 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 glacier. 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.
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