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Bt Brown is part of the Marina Catchment

Singapore Catchment Map

Bt Brown is part of the Marina Catchment – a good digital elevation modal (DEM) should therefore be available by year’s end.

Report on Flooding in Singapore by expert panel in January 2012

–  This report documents the key findings and recommendations of the Expert Panel on Drainage Design and Flood Protection Measures.

–  Executive Summary

1 The Expert Panel on Drainage Design and Flood Protection Measures was appointed
by the Ministry of the Environment and Water Resources on 30 June 2011 to review all flood
protection and risk management measures that will be implemented in Singapore over the
next decade. Over the span of 6 months, the Panel reviewed the Public Utility Board’s (PUB)
drainage planning assumptions and parameters; identified innovative and cost-effective
solutions; and proposed improvements to ensure public resilience to floods. This Executive
Summary presents the key conclusions and recommendations of the Expert Panel Report.

(I) Singapore’s achievements in flood management and prevention
2 The Panel noted that much good work has been done by PUB in managing the
drainage and flood situation in Singapore over the past 30 – 40 years, despite the rapid
urbanization. In terms of storm drainage, Singapore compares well with other metropolitan

(II) Rainfall intensities have increased over the past few decades, and are likely to
increase in the future
3 In Singapore, heavy rainfall events impose varying constraints on its drainage
systems. Extreme discharges can result from events ranging from high intensity storms
lasting less than an hour to prolonged rainstorm events with moderate rainfall intensities.
4 Based on the rainfall intensity records over the past 30 years, there is strong
evidence of a trend towards higher rainfall intensities and frequency of intense rains. These
uptrends are consistent with the Inter-governmental Panel on Climate Change 4th
Assessment Report (IPCC AR4) and could add further strain on Singapore’s existing drainage
infrastructure. This evidence challenges past assumptions and, as such, there is the need for
PUB to conduct further studies and review its drainage design considerations to account for
these observed changes in rainfall trends.
5 However, the Panel recognises that the occurrence of 3 extreme events in the
Orchard Road area in an 18 month period is primarily part of the random nature of rainfall

(III) Impact of Urbanisation
6 Urbanisation has undoubtedly led to an increase in storm water runoff in Singapore.
There is therefore a strong argument for introducing measures to mitigate the effects of
such urbanisation.
7 However, the effects are often complex and require further modelling and analysis,
supported by higher resolution data. The additional analysis should include an assessment
of whether run-off coefficients traditionally used in Singapore are appropriate given the
high intensity of rainfall, compared with the countries where the run-off coefficients were

(IV) The Stamford Canal does not have the capacity to drain away the surface runoff
generated by the storms on 16 June 2010 and 5 June 2011
8 The Panel concluded that the floods at the Orchard Road area on 16 June 2010 and 5
June 2011 were mainly due to higher rainfall intensities leading to a volume of surface
runoff that overwhelmed the conveyance capacity of the Stamford Canal. The Panel noted
that the Stamford Canal had been designed to the standard in place at that time rather than
standards more typical of today.
9 From the 5 June 2011 event, it was also noted that the raising of Orchard Road has
reduced the flood risk for a large part of the Orchard Road area, although more detailed
studies are needed to determine whether the road raising has moved the flood risk from
one location to another.
10 The Panel does not believe that the whole-scale upsizing of the Stamford Canal is the
best long term solution to addressing flood risk in the Orchard Road area. A better approach
would be to reduce and delay runoff from the upstream catchment, complemented with a
diversion of any excess flow to an adjacent catchment.

(V) The Marina Barrage did not contribute to the recent floods at Orchard Road
11 The Panel noted that the Marina Barrage was designed primarily as a flood
alleviation scheme – to remove the influence of high tides on the low-lying areas of
Singapore, as well as release excess storm water from the catchment. From the evidence
provided, the Marina Barrage has not contributed to the flooding in Orchard Road in 2010
and 2011, as its influence does not go that far upstream.

(VI) Singapore now needs to move towards a more integrated risk-based approach
based on dynamic modelling and comprehensive monitoring
12 PUB should develop appropriate standards for future assessment and design that
reflect both the likelihood and consequence of flooding.
13 Modelling tools are essential in simulating flows and water levels in drainage
systems. With recent advances in instrumentation, information technology and modelling
capabilities, PUB should move comprehensively towards a dynamic modelling approach in
order to fully understand drainage system performance and the effect of future
14 This will require more flow monitoring and other data collection to verify that
models truly replicate actual system performance. This would include the comprehensive
collection of digital elevation data.

(VII) A wider range of interventions is required to help Singapore secure a more
adequate drainage system for the future
15 As part of the drainage planning process, PUB should consider a wider range of
drainage solutions, or interventions. By implementing a range of appropriate measures that
covers every spectrum of the drainage system from its source (e.g. local storage tanks and
ponds, green roofs, rain gardens, porous pavements, etc), pathways (e.g. drain capacity
improvements, diversion canals, regional detention, etc) and receptors (e.g. urban flood
plains, raised platform levels, flood barriers, etc), flood risk within the drainage catchment
can be more significantly reduced and effectively managed.
16 The Panel recognises that any drainage system, whatever the standards, has a finite
capacity. From time to time, intense rainfall will overwhelm the system, and there will be
residual risks that need to be managed. This applies not just to Singapore. Drainage planning
should be backed up by flood risk mapping so that any residual flood risk from extreme
events can be effectively managed.

(VIII) Improved engagement of stakeholders and the general public
17 There is an opportunity to further enhance public resilience towards floods through
active engagement. PUB should develop and implement a strategic public outreach
programme to educate and involve the general public proactively in its drainage and flood
management approaches, so as to enhance public awareness and preparedness towards
18 PUB should enhance its flood warning systems so as to provide the public with better
information and allow them to make informed decisions should a flood occur in their

(IX) Flexible and adaptable systems to manage future uncertainty
19 Singapore needs to plan for the consequences of future megatrends, e.g. climate
change, extreme storms, extended droughts, water scarcity, land scarcity, energy costs,
resource scarcity and food production. Drainage systems will therefore need to cope with
future uncertainty. Solutions that avoid high energy costs, deliver multiple benefits and can
be phased in over a period of time are likely to be more successful.
20 This will involve regularly reviewing design parameters, enhancing rainfall and
drainage performance modelling and monitoring capabilities, identifying new systems-level
interventions, as well as regularly checking on the adequacy and performance, as part of
drainage system master planning.


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