Managing Urban Runoff DRAINAGE HANDBOOK Managing Urban Runoff DRAINAGE HANDBOOK Managing Urban Runoff – Drainage Handbook 1st Edition: June 2013 © PUB, the national water agency. This handbook is jointly produced by PUB, the national water agency and The Institution of Engineers Singapore (IES). No reproduction is permitted in whole or in part without written permission from PUB and IES. Foreword With our abundant rainfall and relatively low-lying land, flood management is an ongoing challenge in Singapore. Over the last 30 years, the Government has invested more than $2 billion to upgrade Singapore’s drainage infrastructure. These measures have been effective in relieving Singapore of widespread and prolonged floods, with significant reduction in flood prone areas today. Moving forth, with the challenges of increasing urbanisation and climate change effects, we will adopt a holistic and catchment-wide approach in drainage management. A diverse range of interventions – covering every spectrum of the drainage system from the source, pathways and receptors – will help us manage flood risks more effectively. This handbook explains the concepts behind the Source-Pathway- Receptor approach in drainage management. It also highlights innovative “Source” and “Receptor” measures that developments can implement to reduce peak flows, while creating aesthetic value and benefits from these measures. The contents of this handbook will be continually refreshed to showcase more ‘best practices’ and innovative ideas from the development and engineering communities. I would like to take this opportunity to thank the public agencies and organisations who have contributed to the development of the handbook. This collective effort is a critical step to improving Singapore’s flood resilience and creating a more sustainable environment. Chew Men Leong Chief Executive PUB Engineering plays a key role in building Singapore’s resilience against the impact of climate change. In the face of intensifying rainfall and growing flood threats, The Institution of Engineers, Singapore (IES), as the national society for engineers, will build upon our long-standing relationship with PUB, as well as various government agencies to increase protection against floods for Singapore in two major ways. Firstly, we will continue to represent the engineering community at large to provide feedback on professional engineering matters to the government and relevant authorities. Secondly, IES will keep our members and the engineering community abreast of the latest technological developments in the industry and policy requirements from government agencies through courses, seminars and talks. IES fully supports PUB’s holistic ‘source-pathway-receptor’ stormwater management approach, as it provides a comprehensive set of measures to enhance our drainage systems. We believe that the Handbook on Managing Urban Runoff will become a vital reference on PUB’s approach when engineers seek to create and implement engineering solutions to deal with the increasingly challenging environment. The fight against climate change is long-term. IES is committed to work in partnership with PUB, government agencies, the industry, our members and the engineering community to develop innovative, sustainable and cost effective strategies and solutions to achieve desired levels of flood protection for Singapore in the long run. Chou Siaw Kiang President The Institution of Engineers Singapore Contents 1. INTRODUCTION 2. RESOURCES FOR DESIGNING 3. SOURCE SOLUTIONS TO 1.1 Background 7 STORMWATER DRAINAGE MANAGE STORMWATER SYSTEMS ON-SITE 1.2 PUB’s Stormwater Management Strategies 8 2.1 Overview 13 3.1 Where is the Source? 19 1.3 The Need for Holistic 2.2 The Code of Practice on 3.2 The Need for Managing Stormwater Management 9 Surface Water Drainage 15 Runoff at Source 19 1.4 Benefits of Holistic 2.3 ABC Waters Design 3.3 Strategies for Planning, Stormwater Management 10 Guidelines 15 Designing and Implementing Source Solutions 21 1.5 Goals of the Handbook 11 2.4 Other Resources 17 3.4 General Design Considerations for Stormwater Detention and Retention 23 3.5 Options for Detention and Retention within the Development Site 24 3.6 Interfacing between Source Elements and Public Drains 31 4. RECEPTOR SOLUTIONS TO 5. SAFETY, OPERATIONS 7. FREQUENTLY ASKED PROTECT DEVELOPMENTS AND MAINTENANCE QUESTIONS FROM FLOODS CONSIDERATIONS 7.1 General 75 4.1 Where is the Receptor? 33 5.1 Safety Considerations 51 7.2 Source Solutions: Tools 4.2 The Need for Implementing 5.2 Operations and to Manage Stormwater Receptor Solutions 33 Maintenance On-site 75 4.3 Planning Receptor Considerations 54 7.3 Receptor Solutions: Solutions 33 Flood Protection Strategies 76 4.4 Designing Structural 6. CASE STUDIES Solutions 36 6.1 Waterway Ridges, 59 GLOSSARY 77 4.5 Non-Structural Receptor Singapore Solutions 48 6.2 Tanglin Mall, Singapore 63 REFERENCES AND RESOURCES 78 6.3 Wisma Atria, Singapore 66 6.4 The Prisma, Germany 70 ACKNOWLEDGEMENTS 79 06 DRAINAGE HANDBOOK Introduction 1 MANAGING URBAN RUNOFF 07 Rainfall (mm) 1.1 Background 300 As a tropical island located 1.5° north of the Equator, 1 250 Singapore experiences a hot and wet climate, with about INTRODUCTION 2400 millimetres of precipitation annually. Storms come 200 in the form of monsoon surges, Sumatra Squalls and 150 sea breeze-induced thunderstorms. December is usually the wettest month of the year in Singapore (Figure 1.1). 100 Singapore is relatively flat, with pockets of low-lying areas 50 located along the southern and eastern coastal fronts, and some further inland (Figure 1.2). These areas face 0 JanFebMarAprMayJun Jul Aug Sep Oct Nov Dec higher flood risks, especially when heavy rains coincide with high tides. Figure 1.1 Mean monthly rainfall in Singapore. Low-lying areas Medium elevation areas Higher elevation areas Waterbodies Figure 1.2 Topography of Singapore. Figure 1.3 View of the Marina Reservoir, Singapore’s first reservoir in the heart of the city. With a catchment area of 10,000 hectares, or one-sixth the size of Singapore, the Marina catchment is the island’s largest and most urbanised catchment. 08 DRAINAGE HANDBOOK Post-development At the same time, like many other Asian cities, Singapore hydrograph Pre-development Runoff Peak runoff from has undergone rapid urbanisation over the last few hydrograph decades, with the population increasing from 1.6 million a developed site people in 1960 to 5.31 million people in 20121. Over time, the development of high-density satellite towns, residential and commercial developments, has resulted in an increase in paved (impervious) areas and a reduction Peak runoff from pre-development in green spaces. During a storm event, this results in an site increase in peak flows where more runoff is generated and flows faster into the drainage system over a shorter period of time instead of being regulated by infiltration into the soil and through evapotranspiration (Figure 1.4). Time Time to Peak Figure 1.4 Storm hydrograph showing the difference in peak runoff between an urbanised area and a pre-development, or greenfield site. The greater the degree of urbanisation, the higher the peak runoff over a shorter period of time. 1.2 PUB’s Stormwater Management Strategies PUB manages flood risks in the following ways: Providing adequate drainage Implementing flood protection Improving drainage in 123ahead of new developments. measures by stipulating flood prone areas continually requirements such as by widening or deepening minimum platform levels drains, and/or by raising and crest levels in the Code low-lying roads. of Practice on Surface Water Drainage. Figure 1.5 Summary of PUB’s stormwater management strategies. These strategies have been effective in reducing the extent and duration of floods in Singapore such that floods experienced today mostly occur in a small locality and subside within an hour. Note 1 Data from Department of Statistics, Singapore. MANAGING URBAN RUNOFF 09 160 1.3 The Need for Holistic Stormwater Management 1 140 Rainfall data from 1980 to 2012 has shown a trend towards higher rainfall intensities (Figure 1.6) and an increasing INTRODUCTION 120 frequency of high intensity rain events. In addition, climate change effects of sea level rise and increases in rainfall intensities make it necessary for drainage infrastructure to 100 Rainfall (in mm) Rainfall be upgraded and drainage requirements to be raised in order to protect developments from flood risks. 80 However, widening drains to increase drainage capacity is 60 challenging in land-scarce Singapore. Rapid urbanisation 1980 1984 1988 1992 1996 2000 2004 2008 2012 over the last few decades due to population and economic Year growth has resulted in competing land uses and limited land Figure 1.6 Annual maximum hourly rainfall intensities in available for expanding our drainage systems. Singapore (based on records from 28 stations, from 1980 to 2012). A wider range of interventions is thus necessary to help Singapore secure a more adequate drainage system for the future. This includes implementing higher drainage design standards and holistic solutions, building new capabilities and working with stakeholders to improve preparedness. SOURCE Detention Tank Green Roof SOURCE Porous Pavement SOURCE Detention/ Retention Pond PATHWAY Diversion Canal SOURCE Rain Garden PATHWAY Canal Improvement RECEPTOR RECEPTOR Minimum Platform Flood Barrier and Crest Levels SOURCE PATHWAY RECEPTOR The location where The means or routes Where stormwater stormwater runoff through which flows may propagate is generated, i.e. origin stormwater is to and affect of the stormwater flows conveyed infrastructure Figure 1.7 Examples of Source, Pathway and Receptor solutions to slow down and reduce peak runoff and to protect developments (receptors) from flood hazards. 10 DRAINAGE HANDBOOK Recognising that expanding canals and drains will not be sufficient, especially for areas that are more developed and have site constraints, PUB will go beyond implementing pathway solutions (e.g. drain capacity improvements, diversion canals, centralised detention tanks and ponds, etc.) to working with developers to put in place source solutions (e.g.