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Millions of slum residents across the world suffer the hazards and misery of frequent flooding of their streets and homes, which may result in savings of a lifetime being washed away in a few hours, damaged property, loss of work and time, and higher risks of diarrhoea, worm infection and other health problems. Storm Drainage is written to help engineers, aid and agency workers understand drainage problems more clearly in the developing world, so that they can work towards finding practical solutions. It focuses on three questions of particular relevance to low-income urban areas in developing countries: what is drainage performance? how can we evaluate a drainage system, to access how best to improve its performance? what are the effects of solids in drains upon performance? Storm Drainage is the outcome of two-and-a-half years of fieldwork in the city of Indore, in Madhya Pradesh, India and can be used as a practical aid by municipal engineers, consulting engineers and engineering instructors and students, as well as development and aid workers involved in drainage systems.
Acknowledgements ix
1 Introduction 1
Drainage and surface water drainage 1
Surface water drainage and public health 1
The purpose of the manual 2
Who this manual is for 3
The structure of the manual 3
Origins of the manual 3
2 Drainage systems, flooding, and performance 5
Wet weather processes: what happens when it rains? 5
Rainfall becomes runoff 5
Runoff transports solids 9
Runoff enters the drain 10
Runoff flows through the drain 10
Flooding takes place 11
Flood waters are contaminated 12
Dry weather processes: solids deposition and drain maintenance 12
Solids deposition 12
Drain cleaning 15
Community perceptions of flooding 16
Priority 16
Predictability 16
Expectations 17
Drainage performance and evaluation 17
3 Factors that affect performance 19
Types of drainage system 19
Major and minor drainage 19
Types of minor drainage systems 19
Hydraulic capacity 21
Frequency of flooding 21
Depth of flooding 25
Area of flooding 25
Duration of flooding 28
Street grading 28
Inlets 29
Catchment surface and storage 30
Effect on runoff volume 30
Effect on timing of flow 31
Annexe 3-A: Derivation of frequency and capacity relationships 33
Annexe 3-B: Performance aspects of inlets 35
4 Drainage evaluation: general approaches 38
System-wide evaluation 38
Gather background data 40
Perform field work 41
Analyse the data 42
Write up the findings 44
Evaluating a specific catchment 45
Gather background data 45
Perform field work 46
Analyse the data 46
Write up the findings 47
5 Studying the catchment 48
Topographic survey 48
Level of accuracy 48
Data to collect 49
Analysis 49
Defining a catchment 51
Surface cover survey 52
6 Assessing flooding as a problem 54
Resident surveys 54
Avoid 'leading' questions 55
Ask more than one person 56
Try to be specific 56
Retrospective flood surveys in Indore 57
Direct observation 57
Resident gauges 59
Chalk gauges 59
Electronic level gauging 62
Summary 63
7 Flow estimation 65
Catchment area and cover type 65
Rainfall intensity 66
Using IDF curves to estimate flows 67
Limitations of IDF curves and simplified methods 69
Annexe 7-A: How to develop IDF curves 71
Developing curves from continuous data 71
Developing curves from limited data 72
8 Assessing drainage capacity 81
Concepts of capacity 81
Three types of capacity estimation 82
Examples of the three levels of analysis 84
Design capacity 84
As-built capacity 85
Actual capacity 85
Drainage network surveys 86
Level of network analysis 87
Annexe 8-A: Using software for drainage analysis and design 88
Introduction 88
Classification and characteristics of software 89
Choosing between packages 92
Why invest in a software package? 93
What are the pitfalls to avoid? 93
9 Drainage network structural survey 94
Conduit measurements 94
Dimensions 94
Levels 95
Condition of conduits 100
Open conduits 100
Closed conduits 100
Condition of inlets 105
10 Maintenance surveys 106
Drain solids surveys 106
Visual observation 106
Solids levels 106
Solids build-up surveys 107
Solids sampling and size distribution 110
Inlet solids surveys 112
Blockage of inlet mouths 112
Solids levels 113
Drain cleaning observation 113
Removing solids from the drain 115
From the drain to safe disposal 115
Solid waste monitoring 116
11 Studying drainage systems in action 118
What to look for in wet weather 119
Catchment and subcatchment boundaries 119
The nature of flooding in flood-prone areas 119
The hydraulic performance of the total drainage system 119
The surface flow routes followed by runoff during floods 120
The nuisance, hazard, and damage of flooding 120
How to manage wet weather tasks 120
Organizing a team 120
Organizing specific tasks 121
Summary 125
12 Summary and conclusions 126
Themes of preceding chapters 126
Why flooding matters 126
Performance 126
Factors that affect performance 126
General approaches to drainage evaluation 127
Studying the catchment 128
Assessing flooding as a problem 128
Estimating flows from runoff 128
Assessing drainage capacity 129
Drainage network structural survey 129
Maintenance survey 129
Studying drainage systems in action 129
Final conclusions 130
Implications for improving system performance 130
Implications for improving drainage design 130
13 References 132
Pete Kolsky is Senior Water and Sanitation Specialist in Latin America and Caribbean Region at The World Bank, Washington D.C. Metro Area
