Flash Flood Risk Management â A Training of Trainers ... - ReliefWeb

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Flash Flood Risk Management: A Training of Trainers Manual Session 13 Activity 13.4: Flood routing and calculating hydrographs at different locations Time: 15 minutes Step 1 Discuss the concept of flood routing and differentiate between hydrologic and hydraulic routing. Explain why flood routing is important for flash flood management. Step 2 Discuss the various types of routing methods. Assign a simple exercise on flood routing where the participants have to calculate flow values at different times and locations. (See Box 13 in RM 13.4 for the exercise.) Activity 13.5: Flood frequency and return period Time: 10 minutes Step 1 Step 2 Define flood frequency and return period, and discuss the method used to determine return periods. Discuss why return period is an important parameter for flash flood management. 82
Day 3 Session 13 Resource Materials RM 13.1: Rainfall measurement and catchment rainfall Rainfall measurement Rainfall is a main source of water supply; other forms of precipitation include snow, hail, sleet, mist, dew, and fog. It is important to measure rainfall in order to be able to forecast and prepare for flash floods. In the case of riverine floods, the total amount of rainfall during a period of time is important. The total amount of precipitation can be measured using simple rain gauges. However, for flash floods, the total amount is less important than the intensity of the rainfall, since high-intensity rainfall for even for a short period of time can cause a flash flood. The intensity of rainfall cannot easily be determined by manual rain gauges. More appropriate are recording-type rain gauges that give a continuous record of rainfall which can be resolved into desired time intervals; examples of these are tipping bucket or siphon-type gauges. Session 13 A rain gauge gives a point measurement at a particular location, but intense rainfall can be also spatially variable, particularly in mountain areas. A dense network of rain gauges is needed to obtain a reliable spatial survey of rainfall in a catchment. If it is not possible to deploy a large number of gauges then, it is important to identify the key locations in the catchment that can provide the most crucial information for flash flood risk management. Catchment rainfall For flash flood risk management measures such as forecasting or modelling, point data from rain gauges alone are not sufficient. The data must be transformed into spatial data or area-averaged data for the catchment. Several methods can be used to calculate area averages. The simplest is to calculate the arithmetic average of rainfall in each rain gauge; however, this method cannot capture spatial variability and is seldom used. A simple method is the Theissen polygon method, which uses an average based on the assumption that a gauge best represents the rainfall in the area nearest to it. The resulting polygons represent the areas closest to each gauge. The average rainfall derived from the Theissen polygon method is remarkably similar to the arithmetic average in this case, but generally these two methods give different results. A more accurate method for calculating catchment rainfall is the isohyetal method. In this method, isohyets or lines of equal rainfall are drawn in the same way that contour lines are drawn on an elevation map. There are various computer software models available that use sophisticated algorithms to generate isohyets. Some incorporate terrain characteristics in generating the map. Raster maps of rainfall distribution over an area can also be generated. Raster maps represent continuous rainfall fields over the area of interest. As needed, average rainfall at different spatial scales can be calculated from the raster maps. RM 13.2: Runoff and factors that affect it Runoff The rainfall in a catchment is stored either on the surface or as moisture in the soil; part of it is lost by evaporation and part by transpiration. Only some of the rainfall, known as excess rainfall or effective rainfall, contributes to runoff from the catchment. In order to estimate the size of the flood that can be generated by a given amount of rainfall, the runoff must be calculated. 83
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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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Page 43 and 44: Day 2 In a physically vulnerable zo
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Page 47 and 48: Day 2 Session 6 Handouts Handout 6.
Page 49 and 50: 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
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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: 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
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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 113 and 114: Day 4 Session 16 Hazard-Specific Fl
Page 115 and 116: Day 4 Session 16 Resource Materials
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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