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Flash Flood Risk Management – A Training of Trainers Manual Figure 16: Formation of a natural dam (left) and photograph (right) of river damming due to a landslide Rainfall Temporary lake Session 14 Landslide mass Photo source: WECS 1987 Figure 17: Types of river-damming landslides Type I Type II Type III Type IV Type V Type VI Source: Based on Costa and Schuster (1988) Table 10: Types of landslide dams Type Cause Effect I Falls, slumps Dams are small with respect to the width of valley floor and do not reach from one side to the other II Avalanches, slumps/slides Dams are larger and span the entire valley floor III Flows, avalanches Dams fill the valley from side to side and considerable distances upstream and downstream IV Falls, slumps/slides, avalanches Dams are formed by contemporaneous failure of materials from both sides of a valley V Falls, avalanches, slumps/ slides Dams are formed when the same landslide has multiple lobes of debris that extend across a valley floor at two or more locations VI Slumps/ slides Dams are created by one or more surface failures that extend under the stream or river valley and emerge on the opposite valley Source: Shrestha (2008) 94
Day 3 occurs). In most documented cases, the breach results from fluvial erosion of the landslide material; the head cutting typically originates at the toe of the dam and moves progressively upstream to the lake. When it reaches the lake, breaching occurs. The breach commonly does not erode down to the original level of the river bed, as many landslide dams contain some coarse material that is not swept away. Thus, smaller lakes can remain after dam failure. A landslide dams that have steep upstream and downstream faces and that also have high pore-water pressure are susceptible to slope failure. If a dam has a narrow cross-section or progressive slope failure, the crest may fail, leading to overtopping and breaching. Nearly all faces of landslide dams are at the angle of repose of the material or less; however, because they are formed dynamically, slope failures are rare. A special type of slope failure occurs when there is lateral erosion of the dam by a stream or river. Figure 18: Modes of failure of landslide dams % of all events 70 60 50 40 30 20 10 0 60 50 Overtopping Piping Slope failure Physically controlled (did not fail) Modes of failure Source: Based on Costa and Schuster (1988) Figure 19: Causes of landslides that form dams Session 14 RM 14.2: Process of landslide damming and factors that cause landslide dams Both natural and anthropogenic factors can initiate dam-forming landslides. The most important natural processes are excessive precipitation (rainfall and snowmelt) and earthquakes. Source: Based on Costa and Schuster (1988) Globally, about 50% of dam-forming landslides are caused by rainstorms and snowmelt, and about 40% are caused by earthquakes (Figure 19). In the HKH region, volcanic eruptions are rare, so most landslide dams are caused by rainfall, snowmelt, and earthquakes. % of all events 40 30 20 10 0 Rainstorm and snowmelt Earthquakes Modes of failure Volcanic eruptions Others Landslide dams are frequent in steep, narrow valleys bordered by high rugged mountains (Table 11). This setting is widespread in geologically active areas such as the HKH where earthquakes are common and mountain slopes are steepened by glacial activity. Steep, narrow valleys are dammed by a relatively small volume of material; thus, even small mass movements can cause landslide dams. Large landslide dams are caused by complex landslides that start as slumps of slides and become rock or debris avalanches. Table 11: Factors causing landslide dams Natural Anthropogenic High relief Undercutting of river banks Deforestation Improper land use Weak geology High weathering • agriculture on steep slopes • irrigation of steep slopes Intensive rainfall • overgrazing High snowmelt • quarrying Poor sub-surface drainage Construction activities Seismic activities Source: Shrestha (2008) 95
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Flash Flood Risk Management A Train
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Published by International Centre f
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Foreword The Hindu Kush-Himalayan (
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Acronyms and Abbreviations CFFRMC D
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Flash Flood Risk Management - A Tra
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Introduction Day 1 Session/Activity
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Day 1 Suggestions for the facilitat
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Day 1 Suggestions for the facilitat
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Day 1 Session 2 Flash Flood Hazards
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Day 1 Session 2 Resource Materials
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Day 1 RM 2.4: Flash floods in the H
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Day 1 in flash floods during the pe
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Day 1 Activity 3.2: Identifying the
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Day 1 Figure 4: Categorisation of f
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Day 1 Session 4 Flash Flood Hazard
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Day 2 Session/Activity Session 5: V
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Day 2 Activity 5.2: Flash flood ris
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Day 2 In a physically vulnerable zo
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Day 2 Step 2 Show the short video o
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Day 2 Session 6 Handouts Handout 6.
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Day 2 Figure 8: The four pillars of
Page 51 and 52: Day 2 • As a result of rapid cha
Page 53 and 54: Day 2 Session 7 Community-Based Fla
Page 55 and 56: Day 2 Step 4 Step 5 Discuss the dif
Page 57 and 58: Day 2 Figure 10: Framework for comm
Page 59 and 60: Day 2 • First-aid and health: Th
Page 61 and 62: Day 2 Session 8 Gender Perspectives
Page 63 and 64: Day 2 The CFFRMC also plays a role
Page 65 and 66: Day 2 Note to the trainer Clarify t
Page 67 and 68: Day 2 groups such as people with di
Page 69 and 70: Day 2 RM 9.3: Process of participat
Page 71 and 72: Day 3 Session/Activity Activity tim
Page 73 and 74: Day 3 Activity 10.3: Strategies for
Page 75 and 76: Day 3 RM 10.2: Various strategies f
Page 77 and 78: disseminated. The South Asian tsuna
Page 79 and 80: Day 3 Session 11 Resource Materials
Page 81 and 82: Day 3 Geospatial Stream Flow Model
Page 83 and 84: Day 3 Activity 12.2: Concept and co
Page 85 and 86: Day 3 perspective means looking for
Page 87 and 88: Day 3 their livelihoods. Watershed
Page 89 and 90: Day 3 Session 13 Hazard-Specific Fl
Page 93 and 94: Day 3 where Q = discharge A = area
Page 95 and 96: Day 3 There are two types of routin
Page 97 and 98: Day 3 Box 13: Exercise on flood rou
Page 99 and 100: Day 3 Activity 14.4: Measures that
Page 101: Day 3 Figure 15: Types of landslide
Page 105 and 106: Day 3 Figure 21: Tsatichhu landslid
Page 107 and 108: Day 3 Estimating downstream floodin
Page 109 and 110: Day 3 Strand (1977) uses the same e
Page 111 and 112: Day 4 Session/Activity Session 15:
Page 113 and 114: Day 4 Session 16 Hazard-Specific Fl
Page 117 and 118: Day 4 Figure 25: Development of the
Page 119 and 120: Day 4 RM 16.3: Impacts of GLOF even
Page 121 and 122: Day 4 Supra or englacial drainage.
Page 123 and 124: Day 4 Briefing for the Field Trip T
Page 125: Days 5, 6, and 7 Field Trip
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