The Expert Panel on Drainage Design and Flood Protection Measures was appointed by the Ministry of the Environment and Water Resources on 30 Jun 2011 to review 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 Utilities Board’s' (PUB) drainage planning assumptions and parameters; identified innovative and cost-effective solutions; and proposed improvements to ensure public resilience to floods.
The Executive Summary presents the key conclusions and recommendations of the Expert Panel Report.
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EXECUTIVE SUMMARY
(I) Singapore’s achievements in flood management and prevention
1. 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 areas.
(II) Rainfall intensities have increased over the past few decades, and are likely to increase in the future
2 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.
3 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 Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report 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.
4 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 patterns.
(III) Impact of Urbanisation
5 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.
6 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 areas where the run-off coefficients were derived.
(IV) The Stamford Canal does not have the capacity to drain away the surface runoff generated by the storms on 16 Jun 2010 and 5 Jun 2011
7 The Panel concluded that the floods at the Orchard Road area on 16 Jun 2010 and 5 Jun 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.
8 From the 5 Jun 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.
9 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
10 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 on 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
11 PUB should develop appropriate standards for future assessment and design that reflects both the likelihood and consequence of flooding.
12 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 interventions.
13 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
14 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.
15 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
16 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 floods.
17 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 vicinity.
(IX) Flexible and adaptable systems to manage future uncertainty
18 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.
19 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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