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BIPARTITE ROCKSLIDES: GEOMORPHIC EVIDENCE...
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May 24 – 26, 2017, Barcelona, Spain JTC1 Workshop on Advances in Landslide Understanding JTC1 Workshop on Advances in Landslide Understanding Page 1 BIPARTITE ROCKSLIDES: GEOMORPHIC EVIDENCE OF PARTIAL EXTREME MOBILITY Alexander L. Strom* * Geodynamics Research Center – branch of JSC "Hydroproject Institute", Moscow, Russia Several typical examples of large-scale rockslides from the Central Asia region with bipartite deposits that have compact proximal and highly mobile avalanche-like distal parts are de- scribed. Such case studies illustrate partial involvement of rockslide debris in accelerated mo- tion resulting in its extra-mobility. Besides long runout many of them are characterized by the abrupt change of the direction of rock avalanche motion relative to the direction of the initial slope failure, so that they can affect areas that otherwise would be considered as safe. It is hypothesized that such bipartition is caused by the fluidization of the frontal part of rockslide body that reaches the valley bottom first or by the momentum transfer from the entire col- lapsed rock mass to its portion retaining possibility of further motion after its collision with an obstacle or after entering sharp valley constriction. Keywords: rockslide, rock avalanche, bipartition, fluidization, momentum transfer, mobility INTRODUCTION Large-scale bedrock landslides (rockslides) often convert into highly mobile rock avalanches that pose an exceptional threat, affecting vast areas extending up to 30-40 kilometers from the collapsing slopes (Kurdiukov, 1950; Crosta et al., 2015; Resnichenko, 2015; Robinson et al., 2015). Prediction of the reach of future slope failures that will occur in mountainous areas inevitably requires better understanding of motion mechanism(s) of such rock avalanches and of factors governing their origin and mobility. Two main approaches to reveal these regulari- ties can be utilized. One of them is based on the statistical analysis of various quantitative parameters such as rockslide volume, runout, affected area, drop height, fahrböschung (Shei- degger, 1973; Li, I983; Shaller, 1991; Kilburn & Sørensen, I998; Legros, 2002). Another ap- proach requires detailed case-by-case description of morphological and sedimentological pe- culiarities of highly mobile rock avalanches, both historical and prehistoric, their classifica- tion and identifying of factors predetermining such an abnormal behavior. Combination of both approaches allows statistical analysis of case studies featuring similar style of formation and motion that provides more rigorous quantitative relationships (Shaller, 1991). Compiling the rockslides database of the Central Asia region, which arid climate and lack of vegetation provide excellent morphological expressiveness of rockslide and rock avalanche bodies, we identified numerous case studies with distinct bipartition of the deposits that have compact and long runout (avalanche-like) parts. Study of their morphological peculiarities provides additional information on the processes evolving in catastrophic large-scale rock slope failures and governing their mobility. Several typical case studies classified as Jumping
Transcript of BIPARTITE ROCKSLIDES: GEOMORPHIC EVIDENCE...
M a y 2 4 – 2 6 , 2 0 1 7 , B a r c e l o n a , S p a i n
JTC1 Workshop on Advances in Landslide Understanding
JTC1 Workshop on Advances in Landslide Understanding Page 1
BIPARTITE ROCKSLIDES: GEOMORPHIC EVIDENCE OF PARTIAL EXTREME MOBILITY
Alexander L. Strom*
* Geodynamics Research Center – branch of JSC "Hydroproject Institute", Moscow, Russia
Several typical examples of large-scale rockslides from the Central Asia region with bipartite deposits that have compact proximal and highly mobile avalanche-like distal parts are de-scribed. Such case studies illustrate partial involvement of rockslide debris in accelerated mo-tion resulting in its extra-mobility. Besides long runout many of them are characterized by the abrupt change of the direction of rock avalanche motion relative to the direction of the initial slope failure, so that they can affect areas that otherwise would be considered as safe. It is hypothesized that such bipartition is caused by the fluidization of the frontal part of rockslide body that reaches the valley bottom first or by the momentum transfer from the entire col-lapsed rock mass to its portion retaining possibility of further motion after its collision with an obstacle or after entering sharp valley constriction. Keywords: rockslide, rock avalanche, bipartition, fluidization, momentum transfer, mobility INTRODUCTION Large-scale bedrock landslides (rockslides) often convert into highly mobile rock avalanches that pose an exceptional threat, affecting vast areas extending up to 30-40 kilometers from the collapsing slopes (Kurdiukov, 1950; Crosta et al., 2015; Resnichenko, 2015; Robinson et al., 2015). Prediction of the reach of future slope failures that will occur in mountainous areas inevitably requires better understanding of motion mechanism(s) of such rock avalanches and of factors governing their origin and mobility. Two main approaches to reveal these regulari-ties can be utilized. One of them is based on the statistical analysis of various quantitative parameters such as rockslide volume, runout, affected area, drop height, fahrböschung (Shei-degger, 1973; Li, I983; Shaller, 1991; Kilburn & Sørensen, I998; Legros, 2002). Another ap-proach requires detailed case-by-case description of morphological and sedimentological pe-culiarities of highly mobile rock avalanches, both historical and prehistoric, their classifica-tion and identifying of factors predetermining such an abnormal behavior. Combination of both approaches allows statistical analysis of case studies featuring similar style of formation and motion that provides more rigorous quantitative relationships (Shaller, 1991). Compiling the rockslides database of the Central Asia region, which arid climate and lack of vegetation provide excellent morphological expressiveness of rockslide and rock avalanche bodies, we identified numerous case studies with distinct bipartition of the deposits that have compact and long runout (avalanche-like) parts. Study of their morphological peculiarities provides additional information on the processes evolving in catastrophic large-scale rock slope failures and governing their mobility. Several typical case studies classified as Jumping
M a y 2 4 – 2 6 , 2 0 1 7 , B a r c e l o n a , S p a i n
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and Secondary rock avalanches (Strom, 2006, 2010) demonstrating different types of such slope failures are presented and analysed in brief. MORPHOLOGICAL TYPES OF BIPARTITE ROCKSLIDES Jumping rock avalanches originate when the sliding surface daylights well above the slope base so that its debris really jumps and then fell nearly vertically colliding with valley bottom almost at a right angle. Such rockslides form thick proximal accumulation with convex slopes and gradually thinning avalanche-like part. Besides several Central Asian case studies one of which is shown on Fig. 1, such morphology characterizes famous 1881 Elm rock avalanche originated on the slope undercut by slate quarry (Heim, 1882; Hsü, 1978).
Fig. 1. Overview of the Northern Karakungey rockslide of the Jumping type in Central Tien Shan, Kyrgyzstan. It
caved from the true right slope of the valley composed of granites (direction of motion is shown by the dashed arrow). Filling of the stream by avalanche-like portion of debris is shown in the inset
The most hazardous highly mobile Secondary rock avalanches originate when rapidly moving rockslide that gained its momentum during initial descend, either strikes with the slope base (not at an almost right angle as in the Jumping case) or with an opposite slope of the valley or enters sharp valley constriction, after which part of debris moves further demonstrating ab-normally high mobility. The characteristic features of the Secondary rock avalanches are a concave slope of the compact part above the avalanche-like part – the so-called "secondary scar", accumulation of large portion of mobile debris at its distal part – opposite to the along-way debris distribution of the Jumping cases, and more pronounced flow-like style of the ava-lanche debris motion (Figs. 2, 3). Rock avalanches of both Jumping and Secondary types can move either in the same direction as the initial slope failure or turn, practically up to the right angle. Thus they can affect areas that otherwise would be considered as safe even if we anticipate large scale slope failure from the source zone. While identified case studies of the Jumping type have relatively short mo-bile parts (the 2 km long 1881 Elm rock avalanche seems to be one of the longest) some of the Secondary rock avalanches demonstrate extreme mobility, often being 5-7 km long. Even length of the deflected cases that moved at a right angle to the direction of the initial debris
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motion can exceed 4 km. Sometimes one can identify several successive secondary scars re-flecting multistage interruption of debris motion (see Fig. 3).
Fig. 2. The Karasu Lake rockslide (Central Tien Shan, Kyrgyzstan) accompanied by the deflected secondary
rock avalanche that involved ~10% of the entire rockslide and moved 1.4 km down-valley at a right angle to the direction of the initial slope failure. Its well pronounced secondary scar is marked by yellow triangles. T – the
distinct right-side trimline. 3D Google Earth view
Fig. 3. The Big Dragon Lake rockslide about 2 km3 in volume (Eastern Tien Shan, China) with 370 m runup of
its frontal part accompanied by the 4.3 km long deflected Secondary rock avalanche that originated from the secondary scar # 1 and moved up to 2470 m a.s.l. with an intermediate secondary scar #2. 3D Google Earth view
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ASSUMED BIPARTITION MECHANSMS The bipartition of Jumping rock avalanches can be explained as follows. When the frontal part of sliding block had jumped and fallen down on the valley bottom, its following sections that originate from top of the ridge and, thus, have larger unit potential energy, are still sliding down and after jumping they fall down on top of debris that is already at place, compressing it. Moreover, such compression is not just static loading, but highly dynamic process that, considering giant dimensions of the study features, could fluidize the basal debris units ex-truding them from under accumulating tailing portion. This fluidized material flows down-stream as a dry viscous granular flow forming the avalanche-like part. Mechanism of the Secondary rock avalanches bipartition seems to be different. Rock mass collapsing catastrophically from the high slope gains enormous momentum. When it meets any obstacle it stops, but if some part of debris has "free face" it retains possibility of further motion. It is hypothesized that debris colliding with an obstacle deforms elastically and part of the overall momentum transfers to its portion that accelerates and ejects from the "free face" forming long runout rock avalanche. These phenomena can repeat several times as it can be seen on Fig. 3. REFERENCES Crosta GB, Frattini P, Valbuzzi E, and Hermanns RL (2015) The Cerro Caquilluco–Cerrillos Negros Giant Rock
Avalanches (Tacna, Peru). Engineering Geology for Society and Territory – Volume 2. G. Lollino, D. Giordan, G.B. Crosta, J. Corominas, R. Azzam, J.Wasowski, N. Sciarra. (eds.), Springer International Pub-lishing Switzerland, pp. 921-924.
Heim, A. (1882) Der Eergsturz von Elm. Z der Deutschen Geologishen Gesellschaf l34, 74-115. Hsü, K.J. (I978) Alberl Heim: observations on landslides and relevance to modern interpretations, in E. Voight
(Ed). Rockslides and Avalanches. V. I. Natural Phenomena Amsterdam: Elsevier, 71-93. Kilburn, C.R.J., Sørensen, S-A. (I998) Runout length of sturzstroms: the control of initial conditions and of
fragment dynamics, J. Geophys. Res. I03: No EB. I7877-I7884. Kurdiukov KV (1950) Ancient rockfalls in the Alay Range valleys. Problems of Geography, 21, 191-204. (in
Russian). Legros, F. (2002) The mobility of long-nmoul landslides, Engineering Geology 63, 301-331. Li. Tianchi (I983) A mathematical, model lor predicting the extent of a major rockfall, Z- fur Geomorphoiogie
N.F.. 27,473-IB2. Reznichenko N & Davies T (2015) The Gigantic Komansu rock avalanche deposit in the glaciated Alai Valley,
Northern Pamir of Central Asia. Engineering Geology for Society and Territory – Volume 2. G. Lollino, D. Giordan, G.B. Crosta, J. Corominas, R. Azzam, J.Wasowski, N. Sciarra. (eds.), , Springer International Pub-lishing Switzerland, pp. 895-898.
Robinson TR, Davies TRH, Reznichenko N & De Pascale GP (2015) The extremely long runout Komansu rock avalanche in the Trans Alay range, Pamir Mountains. Southern Kyrgyzstan. Landslides, 12, 523-535.
Shaller, P.J. (1991) Analysis and implications o/'large Martian and Terrestrial landslide... Ph.D. Thesis, ("alilomia Ins li lute of Technology.
Sheidegger, A.E. (1973) On the prediction of the reach and velocity of catastrophic landslides. Rock Mechanics 5,231 -236.
Strom, A.L. (2006). Morphology and internal structure of rockslides and rock avalanches: grounds and constraints for their modelling. In: Evans, S.G.; Scarascia Mugnozza, G.; Strom, A.; Hermanns, R.L. (Eds.) Landslides from Massive Rock Slope Failure. NATO Science Series: IV: Earth and Environmental Sciences, Vol. 49, 305-328.
Strom, A.L. (2010) Evidence of momentum transfer during large-scale rockslides’ motion. In: Williams AL, Pinches GM, Chin CY, McMorran TG, Massei CI (eds.) Geologically Active.Proc. of the 11th IAEG Con-gress, Auckland, New Zealand, 5-10 September 2010, Tailor & Frensis Group, London, 73-86.
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