Rockslides and Rock Avalanches of Central Asia

1 INTRODUCTIONScope and objectives of this book (see the 'Background and purpose' section above).

2 OVERVIEW OF THE STUDY REGION Brief overview of the geology, geomorphology, neotectonics, seismicity and climate of he study region – factors that predetermine rock slopes instabilities and trigger cata-strophic landslides will be presented.

3 SPATIAL AND SIZE DISTRIBUTION OF BEDROCK LANDSLIDES3.1 REGIONAL DESCRIPTION 3.1.1 TIEN SHAN 3.1.2 PAMIRSIn this chapter we will provide regional description of bedrock landslides distribution within the Tien Shan and Pamirs mountain systems. Database of large-scale bedrock landslides (shapefiles) can be presented as the ancillary materials. Areas with numerous landslides and those lacking such features will be described and compared with the geomorphic, geological, neotectonic and seismological conditions of the study region. Special emphasis will be placed on the relationships between bedrock landslides and active faults. Combination of factors most favourable for bedrock slope failures will be revealed. 3.2 PROBLEM OF SEISMICALLY TRIGGERED LANDSLIDES DISCRIMINATIONSpecial section will be focused on the important, though still controversial problem of seismically triggered landslides discrimination. Some promising ways allowing such discrimination will be discussed and exemplified by case studies from the Central Tien Shan mainly.3.3 BEDROCK LANDSLIDES IN GLACIATED REGIONSSpecial section will be devoted to rockslides that fell on the glaciers. Frequency of large-scale slope failures in glaciated regions where traces of such catastrophic events can disappear within decades will be analysed and exemplified by case studies from the heavily glaciated Pamirs and Central Tien Shan.3.4 SIZE DISTRIBUTION OF BEDROCK LANDSLIDES – ANALYSIS AND COMPARISON WITH OTHER MOUNTAINOUS REGIONSAnalysis of size (volume, area) distribution of rockslides in the Tien Shan and Pamirs will be performed. Magnitude-frequency distributions of bedrock landslides in those parts of the study region featuring high modern seismic activity (e.g. Northern and Southern border zones of the Tien Shan) and those with much lower present day seismicity (Central Tien Shan) will be compared. The comparison with such distributions in other mountainous regions (Alps, Apennines, etc) will be performed as well.

4 BEDROCK LANDSLIDES CLASSIFICATION BASED ON THE DEPOSITS MORPHOLOGY4.1 MORPHOLOGICAL TYPES OF ROCKSLIDES AND ROCK AVALANCHESVarious types of rockslides and rock avalanches will be described and exemplified by numerous case studies from the Tien Shan and Pamirs. 4.1.1 "PRIMARY ROCK AVALANCHES" Type of rock avalanches where the entire debris is involved in the avalanche-like motion and accumulates at the distal part of the transition-deposition zone. Primary rock avalanches form both in unconfined and confined conditions. 4.1.2 "JUMPING ROCK AVALANCHES"Rock avalanches with compact proximal and gradually thinning avalanche-like parts that occur when sliding surface comes out on the slope much above its foot so that debris really jumps from the slope like from the ski-jump. 4.1.3 "SECONDARY ROCK AVALANCHES" (CLASSICAL AND BOTTLENECK SUBTIPES)Rock avalanches that are also characterized by dual morphology with compact proximal bodies and highly mobile avalanche-like parts. The latter are characterized by debris distribution typical of the “Primary” rock avalanche. Two subtypes of “Secondary” rock avalanche have been identified. Those of the “Classical” subtype originate when the collapsing rock mass collides with opposite valley slope or valley bottom. They have a distinct concave slope of the compact part. Rock avalanches of the “Bottleneck” sub-type originate when rapidly moving debris passes through a sharp valley narrowing4.2 MINOR MORPHOLOGICAL FEATURES TYPICAL OF ROCKSLIDE DEPOSITS CROMORPHOLOGIES (MOLLARDS, TRIMLINES, BRANDUNGS)Conical hills on rock avalanche surface (the so called mollards), the trimlines, brandungs and other micromorphological features indicative for rockslide origin of the deposits will be described and their role in reconstruction of debris motion mechanism will be highlighted.

5 INTERNAL STRUCTURE AND GRAIN-SIZE COMPOSITION OF LARGE-SCALE BEDROCK LANDSLIDESUnique case studies with kilometres-long outcrops of the completely dissected rock-slide dams up to 400 m thick allowing detail analysis of the internal structure of large rockslide dams and long runout rock avalanches will be described.5.1 PRESERVATION OF THE INITIAL LITHOLOGICAL SEQUENCESeveral illustrative rockslides that had originated on slopes composed of variable and multicolour lithologies allowing comparison of their mutual position in the source zone and resultant deposits will be described.5.2 DUAL GRAIN-SIZE DISTRIBUTION – COARSE CARAPACE AND INTERNAL COMMINUTED "CORE"Analysis of deeply incised past rockslide dams identified in the study region demon-strate that the dual grain-size distribution of debris with coarse carapace and giant in-ternal (lower) "core" composed of the intensively comminuted debris is typical of large-scale bedrock landslide deposits.5.2 EFFECT OF THE DEPOSITS INTERNAL STRUCTURE ON ROCKSLIDES' MOTION AND STABILITY OF NATURAL BLOCKAGESIn this section the effects of the rockslides deposits' characteristic peculiarities on the stability of the existing and future blockages will be discussed.

6 ROCKSLIDES' (ROCK AVALANCHES') EMPLACEMENT MECHANISM MODELS BASED ON THE DEPOSITS MORPHOLOGY AND INTERNAL STRUCTURE ANALYSISClose relation of the emplacement mechanism(s) with the overall rockslide debris mor-phology and minor morphological features exemplified by numerous case studies from the Tien Shan and Pamirs will be analysed. It will be shown that the momentum transfer within rapidly moving dry granular flow, intensive rockslide debris comminution and transition from sliding to flow-like motion are the basic mechanical processes acting during the rockslide emplacement, which determine their deposits morphology. The universality of the proposed classification criteria and their applicability outside the study region will be demonstrated by the selected case studies from other parts of the World.

7 ROCKSLIDE DAMS – ORIGIN, EVOLUTION, AND FAILURERockslides that form natural dams pose major threat and, thus, are of special interest from the practical point of view. Examples of their formation, evolution and breach from the study region as well as discussion on some controversial issues will be presented. 7.1 ROCKSLIDE vs. MORAINE ORIGINThe controversial issue of the natural barriers' origin in the formerly glaciated river val-leys of Pamirs (rockslides vs. moraines) critically important for hazard assessment will be discussed. The preferred opinion on the rockslide origin of most of the existing and past blockages in this region will be substantiated based on the detailed analysis of nu-merous case studies. 7.2 HYSTORICAL CASE STUDIES FROM THE CENTRAL ASIA REGIONThe historical case studies of the formation and failure of rockslide dams in the Pamirs and Tien Shan mountains (the 1911 Sarez, 1963 Issyk, 1964 Aini, 1966 Yashinkul) will be described. Special emphasis will be paid to the examples of the "delayed" breach of rockslide dams that had existed for centuries and millennia being considered as safe features. It is important for the assessment of the possibility of catastrophic breach of several existing rockslide-dammed lakes in the study region. 7.3 EVIDENCE OF PREHISTORIC CATASTROPHIC OUTBURST FLOODSBesides the historical examples of catastrophic outburst floods evidence of similar pre-historic events will be presented and analyzed. 7.4 ROLE OF ROCKSLIDE DAMMING IN RIVER VALLEYS' EVOLUTIONSeveral case studies where rockslide damming have resulted in the disturbance regimes of the affected river valleys (concept proposed by K. Hewitt, 2009) and, in some extreme cases, in the bypass valley formation.

8 EVIDENCE OF SLOPE INSTABILITIES – POTENTIAL OF FUTURE LARGE-SCALE SLOPE FAILURESThough main attention in the proposed book will be paid on large bedrock landslides that had been formed already and, thus, do not pose a threat except for the existing landslide dams, areas with evidence of slope instability will be described and possibility of the future catastrophic rock slope failures will be discussed.

9 CONCLUSIONSRemaining problems and perspectives of future studies will be discussed in the conclu-sive part of the proposed book.

10 REFERENCES

11 INDEX