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Flash Flood Risk Management – A Training of Trainers Manual Step 2 Step 3 Step 4 Describe the empirical scoring system for moraine-dammed glacial lake outburst hazard. Discuss the methods that can be used to estimate the magnitude of a potential flood in order to estimate downstream flooding and implement appropriate mitigation measures downstream. Discuss GLOF risk mapping. Explain how estimating the magnitude of past floods can be used as a basis for preparing for future hazard and risk. Discuss the methods that can be used to calculate the parameters of past floods such as velocity, depth, width, and discharge. Note to the trainer Remember to briefly remind the class that measures such as landuse regulations and early warning systems, which were covered in previous sessions, also apply in the case of GLOFS. session 16 106
Day 4 Session 16 Resource Materials RM 16.1: Glaciers and formation of glacial lakes Glaciers A glacier is a huge mass of flowing ice. Since in any given year, more snow falls than melts, snow layers pile up year after year and are compressed under their own weight. As the compression progresses the density increases and the compacted layer, now called ice, becomes impermeable to air. The density of ice ranges from 0.83 to 0.917 g/cm 3 . When the thickness of the snow and ice exceeds a certain critical value, the ice mass of the lower layers undergoes a plastic deformation, flows down to a lower elevation, and becomes thinner. Moraine The debris from both sides of the steep slope settles on the glacier surface and is carried downslope. The debris that piles up on both sides and at the end of the glacier are known as moraines. ‘Terminal’ or ‘end moraines’ are formed at the foot or terminal end of a glacier. Lateral moraines are formed on the sides of the glacier. Medial moraines are formed when two different glaciers, flowing in the same direction, coalesce and the lateral moraines of each combine to form a moraine in the middle of the merged glacier. Less apparent is the ground moraine, also called glacial drift, which blankets the underneath surface of much of the glacier below the equilibrium line (the imaginary line separating areas of positive and negative mass balance). Glacial lakes Glacial lakes (Figures 23 and 24) are the ponds which form when glaciers melt; they are usually found near the ends of glaciers. Since the structure of these lakes is not consolidated, they can be unstable. These lakes are created when glaciers surge; they can broadly be classified into three categories based on the composition of the dam: • moraine dammed lakes; • glacial ice-dammed lakes; • ice-core moraine dammed lakes. session 16 Most of the glacial lakes in the HKH are moraine dammed. When the glacial dams break, huge amounts of water from the glacial lake are released downstream in a sharp flood wave. This flood wave is usually released with so much force that it can wash away infrastructure found along the river banks. This destructive flood is known as a glacial lake outburst flood (GLOF). The formation of glacial lakes The formation and growth of glacial lakes is closely related to deglaciation. Shrinkage of glaciers is a widespread phenomenon in the HKH region at present and is closely associated with climate change. The glacial retreat that has been observed in recent decades is generally attributed to an anthropogenic increase in the concentration of greenhouse gasses in the atmosphere and the overall warming that is attributed to it. Valley glaciers are typically dotted with supra-glacial ponds; in a warming environment these grow bigger and merge with each other. This process is accelerated by the rapid retreat of glaciers. As glaciers retreat, they leave large voids behind, and the melt water that is trapped in these depressions forms lakes. Figure 25 shows the rapid growth of the Tsho Rolpa Lake in Nepal as an example. 107
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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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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
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Day 2 • As a result of rapid cha
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Day 2 Session 7 Community-Based Fla
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Day 2 Step 4 Step 5 Discuss the dif
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Day 2 Figure 10: Framework for comm
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Day 2 • First-aid and health: Th
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Day 2 Session 8 Gender Perspectives
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Page 69 and 70: Day 2 RM 9.3: Process of participat
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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 91 and 92: Day 3 Session 13 Resource Materials
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 and 102: Day 3 Figure 15: Types of landslide
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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
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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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