Recommendations for flood prevention by the Expert Panel for Drainage Design and Flood Protection Measures 2012

Quick fixes and better data to fight floods

Expert panel suggests both short- and long-term measures

By Feng Zengkun & Grace Chua, The Straits Times, 11 Jan 2012

ROOFTOP rain gardens and porous roads are some of the immediate ways to alleviate flooding problems in Singapore, says an expert panel on floods.

But in the longer term, the Government needs to gather better data about how much rain falls and how it flows and pools across the island.

It also needs to upgrade its computer modelling systems so that its flood predictions are more accurate.

The panel, made up of 12 local and foreign experts, made these and other recommendations yesterday after wrapping up a six-month study.

It had been appointed by Minister for the Environment and Water Resources Vivian Balakrishnan after a flood in June last year inundated the Tanglin area.

The thrust of the report was that solving Singapore's flood woes will require managing the entire chain of rainwater flow. While some of this will mean embarking on big infrastructural projects, other measures can be implemented quickly.

The quick fixes: mandating that rooftop rain gardens or green roofs be built on top of new buildings and retrofitting old ones; redeveloping roads to make them porous; installing flood barriers and raising buildings' thresholds.

But while these can be achieved in the short term, what is more crucial is the collection and study of topographic and weather data, they said.



Professor Chan Eng Soon, head of the panel and dean of the engineering faculty at the National University of Singapore, said most of the relevant drainage data collected by agencies here has been limited to water movement within drains and canals.

The Government needs to create a digital map of the country's landscape and ground surface types, and build a computer model that can better predict where water will go above-ground and which areas may be flooded, he said.

The panel said that currently, only some areas in Singapore are very well-mapped, such as the Orchard Road area. 'You must understand the entire catchment to know how water is going to flow,' said Prof Chan.

The experts also noted that the latest technology can measure the height of the land to an accuracy of 10cm. But Singapore is still relying on topographical data which was estimated - in some parts - using aerial photographs which can be inaccurate by whole metres, not centimetres.

Prof Chan said installing more rain gauges to measure rainfall - a project which national water agency PUB has already embarked on - will also produce more granular data on rainfall in specific areas.

But he noted that it would take time for such data to support any conclusions about changes in weather patterns. The panel had access to only 30 years' worth of consistent data, which it said was not enough.

Ms Elena Pison San Pedro, a meteorologist at research organisation DHI Singapore, said 30 years is the minimum for a climate change study, and phenomena that affect rainfall patterns, such as El Nino, can occur in cycles that take five to 20 years.

'Rainfall is still one of the most difficult variables to model due to its high... variability, especially in Singapore's tropical weather,' she said.

More robust data aside, the panel agreed with PUB that a detention pond and a diversion canal could be longer-term solutions.

The water agency is studying the possibility of building these, but Dr Balakrishnan said in Parliament on Monday that the pond would require land the size of two to three football fields, while the canal, which will re-route water from the maxed-out Stamford Canal to the Singapore River, will cost between $300 million and $400 million.

Mr Kam Yim-Fai, a panellist and chief engineer of the land drainage division at Hong Kong's drainage services department, said the Hong Kong government built an underground water storage tank in Kowloon in 2004 and reduced floods there. The project cost HK$285 million (S$47 million) and the tank was hidden under football and rugby fields.

'This could be useful in Singapore if the tank is installed in a low-lying area, so water flows there naturally. It should also be located close to the flood-prone area,' he said. The panel declined to comment on where such a tank could be located in Singapore, saying it needed more information from a modelling system.

The experts also suggested creating smaller storage tanks along Stamford Canal's route to relieve its burden. Such tanks are traditionally sited above ground to reduce cost and for easier maintenance, but underground versions have been used in Hong Kong and Chicago.

Mr Veera Sekaran, managing director at Greenology which installs green roofs, said the panel's suggestion to build rain gardens will be effective only if they cover 5 to 10 per cent of the catchment area.

Ms Lee Bee Wah, chairman of the Government Parliamentary Committee for the Environment and National Development, said the drainage solutions need to be aligned with the country's land-use master plan and global weather patterns.

Panel's suggestions

COLLECT WATER
Create rain gardens to capture and retain rain
Build green roofs
Build local storage tanks and ponds at ground or underground level
Improve drain capacity
Design temporary storage space such as basement amphitheatres - for recreation but can be used to hold rain water.

DRAIN WATER AWAY
Build porous pavements to soak up rain water
Build a diversion canal or canals

KEEP WATER OUT
Raise platform levels
Install flood barriers

BETTER PREDICTION
Enhance flood warning system for the public
Invest in modelling tools

Expanding Stamford Canal 'not a long-term option'

By Kezia Toh, The Straits Times, 11 Jan 2012

THE panel of specialists set up to look into the flooding problem has concluded that upsizing the Stamford Canal is not the best way to fend off flash floods in the long run.

It suggested that a better way would be to reduce and delay the flow of rainwater into the canal, and to divert the excess flow elsewhere.

This could be done, for example, with small storage tanks along the canal's route, or by building a diversion canal upstream of Orchard Road, leading to the Singapore River.

The 4km-long Stamford Canal, which runs along Orchard Road, has been fingered as the culprit of floods in the shopping belt for its failure to drain away the runoff from heavy downpours.

The floods of June 2010 and last December caused damage to goods in the basement shops of Liat Towers and Lucky Plaza as well as inconvenience to shoppers.

National water agency PUB had previously estimated that the canal's capacity would have to be increased by 30 per cent to prevent a repeat of such floods.

Minister for the Environment and Water Resources Vivian Balakrishnan also acknowledged in Parliament this week that widening and deepening the canal was not feasible.

The panel said that while the move to raise a 1.4km stretch of Orchard Road between Orange Grove Road and Cairnhill Road after the 2010 flood has reduced the flood risk for a large part of the Orchard Road area, more in-depth studies are necessary to determine if the road-raising 'has moved the flood risk from one location to another'.

As for speculation that the Marina Barrage contributed to the floods in Orchard Road, the panel dismissed the idea.

It said the barrage removed the impact of high tides on low-lying areas and released excess storm water from the catchment.

It did not cause the floods in Orchard Road in 2010 and last year, as its influence did not extend that far upstream, said the panel.

PUB 'has done well managing floods for 40 years'

The Straits Times, 11 Jan 2012

DESPITE the recent flood episodes, national water agency PUB has done well to manage the drainage and flood situation in Singapore in the past 40 years, acknowledged the expert panel.

This, despite rapid urbanisation, which makes it harder for rainwater to seep into the ground.

Singapore's storm drainage, said the panel, also compares well with other metropolitan areas.

Civil engineering professor Yong Kwet Yew, who is vice-president of the National University of Singapore and one of the panellists, noted that the number of flood-prone areas has been reduced dramatically.

From 3,200ha in the 1970s, such areas have been reduced to just 56ha today.

This will be further reduced to 40ha next year.

Over the last 30 years, the Government has invested $2 billion in upgrading drainage infrastructure.

PUB continues to spend about $150 million each year improving existing infrastructure in a bid to fend off floods.

But the panel added that the likelihood of more intense rain will burden the network of canals and drains here.

This refers to high intensity storms lasting less than an hour, to prolonged rainstorms with moderate rainfall intensities.

It cited rainfall intensity records over the past 30 years, showing a trend towards higher rainfall intensity, and the frequency of intense rain.

The data came from the meteorological services under the National Environment Agency (NEA).

These rainfall trends are also consistent with the United Nations' Intergovernmental Panel on Climate Change's Fourth Assessment Report.

In view of this, PUB needs to conduct further studies and review the design of its drains, said the panel.

It also attributes the three flooding incidents in Orchard Road over 18 months as part of the 'random nature of rainfall patterns'.

Two malls - Liat Towers and Lucky Plaza - at the premier shopping belt were hit by flash floods on Dec 23. Last June, water flowed into Tanglin Mall and St Regis Residences, and in 2010, large swathes of the Orchard and Scotts Road junction were submerged.

Cheap and quick green roofs
Rooftop gardens can be relatively inexpensive to install
By Jessica Cheam, The Straits Times, 13 Jan 2012

ROOFTOP rain gardens are relatively inexpensive and can hold large amounts of water. And if the authorities accept a recommendation by an expert panel on floods to mandate that all buildings have these green roofs, they can be quick to install, too.

The 12-man panel, tasked by the Government to look into solving Singapore's flooding woes in the long run, has recommended that building owners be required by law to build green roofs. These rooftop gardens, traditionally installed to beautify the skyline and reduce the heat around a building, can help absorb rainwater and reduce the speed of water flow.

Local contractors The Straits Times interviewed yesterday said these gardens would cost from $20,000 to $180,000, depending on their size.

Property developer City Developments, a leader in green buildings here, spends $150 to $400 per sq m for a green roof for a new building, and $105 to $150 per sq m to retrofit an existing one. For a residential project with an extensive green roof, installation generally does not exceed 1 per cent of total construction cost, it said.

Contractors say such gardens can store anything from about six to 34 litres per sq m. The size can range from 200 sq m for a commercial building, to 1,200 sq m for the entire roof of a multi-storey Housing Board carpark.

The National Parks Board (NParks) said the cost of such gardens is usually between $100 and $150 per sq m, which means a commercial green roof costs between $20,000 and $30,000 and can hold 6,800 litres. An HDB carpark roof costs $120,000 to $180,000 and can hold 40,800 litres of water.

Mr Andy Chew, director of local firm Elmich, which designs, builds and installs green roofs, said the idea of rooftop gardens to help alleviate flooding could work for Singapore as large amounts of rainwater can be stored in the garden's water retention system.

This comprises soil-like material, membranes and storage trays. The water is then eventually absorbed by the plants as they grow. He added that the soil-like material also helps to regulate the flow of water; therefore, the speed of any excess water that flows down to ground level is reduced. The system is also light and can typically be installed in an average building.

Elmich, which has been in the business for 26 years and has installed gardens such as the one atop Orchard Central mall, offers systems that can store between six and 28 litres per sq m.

Another firm, Prince's Landscape & Construction, offers a proprietary solution whose water reservoir feature can store up to 34 litres of water per sq m.

Its manager Eugine Spicer said its roof gardens can help alleviate flooding as 'the sudden flow of water is minimised'. Depending on the size of the project and whether there is easy rooftop access, installing a green roof of about 400 sq m could take a month. Growing the plants takes two to three months before that, said Mr Spicer.

Prince has installed green roofs for properties such as Marina Bay Sands, and typically installs gardens 500 sq m in size at a cost of $150 per sq m.

NParks deputy director of horticulture and community gardening Ng Cheow Kheng told The Straits Times that to date, 36ha of skyrise greenery - greenery planted on rooftops or vertically on walls - have already been installed in buildings across Singapore.

NParks has a Skyrise Greenery Incentive Scheme which pays for half of installation costs, up to a maximum of $75 per sq m of green roof and $750 per sq m of green wall. Since the scheme launched in 2009, it has seen 40 buildings get fitted with 1.1ha of green roofs and 0.1ha of green walls.

Under the Building and Construction Authority's green building rating scheme Green Mark, buildings that feature such green roofs get extra points.

HDB has also piloted green roofs in existing housing blocks in recent years to reduce heat build-up and slow down stormwater. Its first eco-friendly residential project, Treelodge@ Punggol, features a rain-harvesting system where water collected is put to uses such as washing common areas.

PUB said it is studying the recommendations of the expert panel and will respond at a later date.

Benefits of rooftop gardens

A GREEN roof, also known as a rooftop garden or rain garden, is a roof that is partially or completely covered with plants and provided with an irrigation system.

Such greenery reduces the 'urban heat island' effect which makes a built-up area significantly warmer than its surroundings.

Green roofs also:

Conserve energy use in the building by keeping temperatures down so less air-conditioning is needed;

Improve air quality;

Reduce noise pollution;

Enhance a building's aesthetics;

Store rainwater and reduce the amount and speed at which water flows to the ground.

Dempsey is prime spot for detention pond: Experts
By Kezia Toh, The Straits Times, 13 Jan 2012

THE Dempsey area is the most plausible place to build a detention pond to solve Orchard Road's flood woes, say property and construction experts.

It fits the bill on several counts: It is in a low-lying area, which means water can flow down naturally to the pond; it still has relatively large, undeveloped parcels of land; and it is near the start of the Stamford Canal.

'Elevation-wise, it is lower than the rest of Orchard, so water can just flow down using gravity,' said property firm SLP International's head of research Nicholas Mak.

A detention pond stores excess stormwater temporarily, releasing it at a controlled rate to protect downstream areas. The idea for it was first mooted last year, together with the building of a diversion canal to relieve the strain on Stamford Canal, which serves Orchard Road.

Minister for the Environment and Water Resources Vivian Balakrishnan said in Parliament on Monday that the pond or ponds would require land the size of two to three football fields, and would be sited upstream.

The 4km-long Stamford Canal starts from Dempsey Hill and the Botanic Gardens, and passes through Orchard Road to the Marina Reservoir.

National water agency PUB is doing a consultancy study which will be completed in May.

While an expert panel set up to study flood prevention has endorsed the idea of detention ponds, it could not say where the best site for them would be, saying it needed more information from a modelling system.

But property pundits say Dempsey is the most ideal spot, despite it lying on a prime cut of real estate. The authorities also have the option of choosing to build the pond either above or below ground.

A pond could serve double duty as a recreational spot, with plants to beautify the area, said Dr Ho Nyok Yong, president of the Singapore Contractors Association.

But while building it above ground would cut costs as it does not require major digging, it also takes up valuable space in land-scarce Singapore.

Mr Alan Cheong, head of research at Savills Singapore, said an underground pond is more practical as land covering it can be sold.

'For that part of town in districts 9, 10 and 11, wherever you turn, there is a huge opportunity cost if you were to build something that huge over it,' he said.

Also, the Dempsey area - known as a lifestyle hot spot - is peppered with former army barracks that house restaurants and furniture shops, making an underground detention pond feasible.

Said Mr Mak: 'If it is underground, you cannot have high buildings on top of it because you cannot drive the foundation pillars that far in. So the Dempsey site may fit this criterion because it is mostly low-rise.'

But this is likely to change, said Mr Li Hiaw Ho, executive director of valuation at property consultancy CBRE.

He said: 'In the next 20 to 30 years, it is likely that the Dempsey area will be due for redevelopment to more intensive commercial or residential use.'

The area is state-owned and managed by the Singapore Land Authority (SLA). Under the Urban Redevelopment Authority's Master Plan 2008, it is marked as a reserve site, which means a specific use for it has yet to be determined.

The cost of building a pond will depend on factors such as how deep the excavation will go and the area of land use, explained Mr Mak.

In 2004, the Hong Kong government built an underground water storage tank in Kowloon and reduced floods there. The project cost HK$285 million (S$47 million) and the tank was hidden under football and rugby fields.

There is also the opportunity cost of real estate that will be foregone, especially since the area is highly valued, said Mr Mak.

But it is a small price to pay, as long as the detention pond works, said real estate brokerage firm PropNex's chief executive Mohd Ismail, adding that regular floods could depress Orchard's land value by 10 to 15 per cent.

'Even if it is prime land, we have no alternative. By not doing it, the entire stretch becomes less valuable if floods become a perennial problem there,' he said.

Annual rainfall is increasing here: Experts
Panel's conclusion differs from NEA's view that there's no clear pattern
By Grace Chua, The Straits Times, 12 Jan 2012

NATIONAL Environment Agency (NEA) officials, who briefed a panel of drainage experts probing recent floods, were of the view that Singapore's rainfall will show no discernible pattern in the future.

But the experts did not quite believe the weatherman.

'It didn't seem physically plausible,' said panel member Lui Pao Chuen, 69, of the NEA's climate vulnerability study, which was first released in 2010 and is meant to project the climate in the future.

So the panel asked to see the rainfall data. And when they pored over the figures, looking at how much rain fell in different parts of Singapore, and which areas experienced the most intense rainfall in an hour, a different picture emerged.

Professor Lui found that there are significant differences in annual rainfall in different parts of Singapore - as much as 50cm between regions. For instance, Changi got 215cm of rainfall a year on average between 1980 and 2009, but the Central Catchment area received 265cm of rain.

Annual rainfall increased, on average, by 15mm a year between 1968 and 2008.

The number of days when there was at least 40mm of rain per hour - a relatively high intensity - went from 50 in 1980 to 65 in 2010.

And the number of days when there was at least 70mm of rain per hour went from five in 1980 to 13 in 2010.

Prof Lui is adviser to the National Research Foundation and advises several other government agencies. The physicist by training was Singapore's chief defence scientist for 22 years till his retirement in 2008.

He and others on the panel were briefed on the study on July 8 last year, which stated that local rainfall till the year 2100 would show 'no discernible trends'.

The 12-man expert panel, formed last June, was tasked with reviewing Singapore's drainage and flood-prevention measures, after the upmarket prime shopping belt of Orchard Road and other areas suffered floods in 2010 and June last year.

Even updated drainage codes may have to be changed further to account for future rainfall predictions.

'The bible may no longer be valid because things have changed,' Prof Lui said, referring to drainage planning parameters.

In news reports immediately after last June's floods, the NEA had also said that, based on its long-term records, its analysis of the rainfall patterns in Singapore showed no significant trend.

But from its data analysis, the panel believed that weather patterns have already been changing and will continue to do so.

It concluded that higher rainfall intensity, increased urbanisation, and a too-small Stamford Canal contributed to the flooding.

In its recommendations, which were released on Tuesday, it said Singapore needs to tackle rainwater at various points along its flow chain and improve monitoring and data collection to better predict possible floods.

It suggested building detention ponds, green roofs and porous roads, among other things.

While the work of national water agency PUB has cut Singapore's flood-prone areas to 56ha today, Prof Lui raised another concern - that that figure might have bottomed out.

That means that flood-prone areas would increase in future if rainfall intensity keeps growing.

Asked to explain the contradiction between the data provided to Prof Lui and its analyses, and if the data was shared with and used by drainage planners, the NEA did not respond by press time.

The PUB was also asked if it collected its own rainfall data, if this data was used by drainage planners, and if planners were aware that rainfall intensity was increasing.

For its part, the PUB said it will study the recommendations of the expert panel and respond at a later date.

Related
Key conclusions and recommendations of the Expert Panel on Drainage Design and Flood Protection Measures
Report on Key Conclusions and Recommendations of the Expert Panel on Drainage Design and Flood Protection Measures
PUB to spend $750 million on drainage projects

Full Report by the Expert Panel on Drainage Design and Flood Protection Measures