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Flash Flood Risk Management – A Training of Trainers Manual Figure 22: Schematic diagram showing the parameters used in calculating peak flow discharge: a) Isometric view; b) Cross-section view a Session 14 q in B b q in h Θ Source: Shrestha (2008) Mavis and Laushey (1949) calculate the velocity of bed flow, V b , in ft/sec as Vb 4 = 9 g 1 0.5d ( S −1) 2 where S g is the specific gravity of the boulder. The mean velocity, v , is then calculated as _ 1 V = V 3 b 100
Day 3 Strand (1977) uses the same equation for mean velocity, but calculates the velocity of bed flow, V b again in ft/sec), as 1 2 V b = 0.51d Williams (1983) suggests that V = 0.065d 0.5 Step 2: Depth calculation. The next step is to calculate the mean depth of the flow. Several methods are available for calculating the mean depth (D). Manning’s equation proposes _ _ D = ( V n √S ) 1.5 Session 14 where D is mean depth, V is mean velocity, n is the roughness coefficient, and S is the slope. Sheild (1936) suggests _ D =θd ( γ S −γ )/ S f where θ is dimensionless shear stress (use 0.02), g f = 1,070 kg/m 3 , g s = 2,700 kg/m 3 , and S is the slope. Williams (1983) suggests _ τ/ D = γ f S τ = 0.17d 1.0 where t is the shear stress (N/m²),d is the boulder diameter (mm), g is the specific weight of water, and S is the slope Step 3: Width calculation. The width is determined using an iterative method. For the cross-section, a straight reach (neither expanding nor contracting) is selected. The site should not be abnormally wide, narrow, steep, or flat. At least one, and preferably both, valley walls should be bedrock. The site should be close to the site where the deposition took place. At least two cross-sections are selected, these should be spaced about one valley width apart. No major tributaries should enter the main channel between the cross-sections. Once the cross-sections have been plotted, a line is drawn to represent the estimated top width of the cross section. The area of the cross-section is calculated using a planimeter; then the area is divided by the top width of the cross-section. If the value calculated in this way deviates from the estimated depth calculated in Step 2, the process is repeated by drawing a new top-width line and recalculating; the process is repeated until the two values agree. The cross-sectional area ‘A’ for the final top-width is then calculated. Calculation of discharge. Knowing the average velocity from Step 1 and the cross-sectional area from Steps 2 and 3, a single discrete estimate of flood discharge (Q) can be made using the equation: Q = V x A Land use regulations The increasing hazard and risk of LDOFs are a result of the unregulated use of land and the use of flood prone areas for infrastructure development such as buildings and roads. Regulations that encourage the proper use of land can help to prevent flood prone areas from being used for settlements or as sites for 101
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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
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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
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 and 102: Day 3 Figure 15: Types of landslide
Page 103 and 104: Day 3 occurs). In most documented c
Page 105 and 106: Day 3 Figure 21: Tsatichhu landslid
Page 107: Day 3 Estimating downstream floodin
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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