Increasing Rainfall Intensity on Singapore – Flood prevention will need major infrastructural upgrades
I was asked several questions in Parliament on the 3 floods in Orchard Road over the past 18 months.
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Topography of Orchard Road
1. Stamford Canal drains a catchment area of 631 ha, which starts upstream at the Botanic Gardens and Dempsey Hill. It extends downstream to Bras Basah and City Hall areas and ultimately drains into Marina Reservoir. Orchard Road is a valley in the mid-section of this catchment area. It is bounded by higher ground, including Mount Elizabeth in the north and Leonie Hill to the south. The gradient slopes from Tanglin to the Bras Basah. However, if you examine the slide carefully, you will notice that the gradient is punctuated by two smaller hills – Cuscaden in the west and Cairnhill in the east. Consequently, there is actually a basin between Cuscaden and Cairnhill Roads. This explains why this stretch of Orchard Road is of special concern.
Increasing Rainfall Intensity
2. There have been three episodes of flooding in this specific area over the past eighteen months. On 16 June 2010 some 100mm of rain fell in the area over 2 hours, causing Orchard Road between Cuscaden Road and Cairnhill Road to flood to a depth of 300mm. On 5 June 2011, some 124mm of rain fell over about four hours, and caused the Tanglin area to be flooded to a depth of 100mm. Most recently, on 23 Dec 2011, even heavier rainfall of some 153mm over 3 hours was recorded in the catchment.
3. These three episodes are part of a larger pattern of rainfall change in Singapore over the past decades. A recent analysis by the Meteorological Service Singapore indicates that rainfall over Singapore has become more intense over the past 30 years.
4. Increasing rainfall intensity and increasing urbanisation pose an on-going challenge for our drainage infrastructure. Nevertheless our major investments of more than $2 billion for drainage infrastructure through the past few decades have reduced low lying flood prone areas from 3200ha in the 1970s to about 49ha today.
5. This is why prolonged extensive floods that some of us may recall from our childhood no longer occur. Today, we are confronted by flash floods – transient localised episodes of about 30 minutes in areas where rainfall intensity has temporarily overwhelmed local drainage systems.
6. However, I want to stress that as far as I am concerned, a flood is a flood. As long as water accumulates in areas where it should not, and human safety or business operations are affected, then this is a problem that PUB and building owners must resolve.
Reduced Flooding Despite Heavier Rainfall on 23 December 2011
7. Let me return to the episode on 23 December 2011. Despite the even heavier rainfall, it is noteworthy that Orchard Road itself was spared from floods and remained passable to traffic. The completion of road raising works in June 2011 provided additional flood protection by preventing storm water from overflowing onto the road. PUB’s analysis shows that parts of the road would have been flooded had the road not been raised.
8. The flood protection measures that have been undertaken by building owners in Orchard Road over the past eighteen months were generally effective except for the basements of Liat Towers and Lucky Plaza that were overwhelmed by the heavy downpour on 23 December 2011.
9. PUB takes all flooding incidents seriously, and is already working very closely with the building owners at Liat Towers and Lucky Plaza to fine tune their flood protection measures, and review their operating procedures. For example, Liat Towers will now be building a 60cm-high wall to prevent rainwater from overflowing from their internal drain into their basement. Lucky Plaza is also in the process of installing flood barriers to prevent surface runoff from entering the basement.
Marina Barrage protects low lying areas in the City Centre
11. There has been much speculation on the role of Marina Barrage on floods. PUB had set the operating level of Marina Reservoir to below mean sea level throughout the month of December 2011. Even on 23 December 2011, despite the major inflow, we were able to keep the water level below mean sea level at all times. This is only possible because of the gates and pumps at Marina Reservoir. As designed, this removed tidal influence from the various drains feeding into the reservoir, including Stamford Canal, and helped keep low lying areas in the city such as Chinatown free from floods. The operations of Marina Barrage, however, do not have any effect in the upper reaches of Orchard Road which are higher, and beyond the ‘zone of influence’ on the reservoir.
Enhancing Drainage in Stamford Canal Catchment and Other Areas
12. In the long run, we need to increase the capacity of Stamford Canal to prevent similar incidents. To minimise the risk of flooding due to rainfall similar to or heavier than that on 23 December 2011, PUB estimates that Stamford Canal’s capacity would have to be increased by at least a third.
13. PUB is evaluating various options to achieve this, including storm water detention ponds in the upstream section or a new canal to divert storm water from upstream portions of the catchment to the Singapore River.
14. While there is urgency to implement the drainage enhancement, these options are very costly and technically complex in an intensely developed area. Therefore, detailed studies are required. A consultancy study on the feasibility of these options has already commenced and is expected to be completed by May this year. More details will be released in due course.
Cleansing and Inspection of Drains
15. In the meantime, it is important to ensure that existing drains are free flowing and perform according to specifications. PUB and its contractors have intensified the daily cleansing and inspection regime. CCTVs have also been installed at certain stretches to monitor canal flow on a continuous basis.
16. In preparing for the current Northeast Monsoon, PUB stepped up the frequency of cleansing and inspecting drains, especially in flood prone areas, litter prone areas and areas with heavy leaf shedding. The public can also assist in the monitoring of drain cleanliness, and we appreciate their feedback which can be given directly to PUB’s 24-hour call centre, posting messages on PUB’s Facebook or through PUB’s iPhone application.
17. Following the floods on 23 December 2011, PUB had checked the drains in Orchard Road, including those at St Regis Residences, Orchard Tower, and Liat Towers, and did not find any to be blocked.
Working with Building Owners
18. As part of PUB’s efforts to limit damage caused by floods, PUB will continue to work closely with the building owners at all flood prone areas -to recommend additional preventive measures, to review SOPs, and to ensure that building and renovation plans conform to drainage regulations.
Alert Systems – Keeping the Public informed
19. Last year, the free SMS alert system on water levels in drains and heavy rainfall was made available to the public. Many of the building managements, including those from Liat Tower and Lucky Plaza, have subscribed to the system. The alert system has been effective in providing subscribers with early warning and helping them to take action to protect their properties.
20. PUB will continue to enhance its warning system to provide earlier and more comprehensive flood warnings to building owners, businesses and the general public. It has recently increased the number of water level sensors across the island, and installed a system of CCTVs in flood prone areas which will help to keep the public informed and shorten response times. Flood warnings are also broadcasted on PUB’s website, Facebook and Twitter pages to provide up-to-date information on possible areas to avoid.
21. While PUB has embarked on these measures, I also welcome the recommendations by the panel of experts, which was appointed in the middle of last year to review our overall drainage design and flood protection measures. The panel is expected to complete its work by this week, and will share their findings and recommendations with the public. My Ministry, together with PUB, will carefully study its report and assess the feasibility for implementation in Singapore.
22. I would like to reiterate that the Ministry takes a serious view of every single flooding incident, and will take all reasonable steps to prevent flooding, mitigate its effects and keep the public informed of flood situations. We are prepared to undertake longer term investments and make significant infrastructural improvements to ensure the long term resilience and robustness of our overall drainage system in response to our development needs and changing climate patterns.