Drainage system under urbanisation stress?

Straits Times Forum 7 Jun 11;

I REFER to yesterday's report on Sunday's flood ('Govt to review drainage after year's worst flood').

With increasing urbanisation in Singapore, more areas of natural vegetation are replaced with impervious concrete. Instead of infiltrating into our soils, rainwater is now channelled as surface run-off.

This dramatically increases the pressure on our drainage system, which faces a peak discharge that is both larger in volume and faster in onset after every major storm.

While recent developments such as Ion Orchard and new condominiums in Bukit Timah may not have directly contributed to the flood, they may have accelerated the increase in run-off volumes that the drains must now cope with by occupying areas that were formerly parks and green spaces.

The traditional conveyance approach of channelling stormwater into large, concrete- lined canals has shown its limitations in times of intense, heavy rainfall. It is time that we considered alternative, storage-based approaches that would slow down the surface run-off that reaches our drains following rain. I would like to suggest the following:

Create bio-retention basins and rain gardens - pockets of green spaces that collect rainwater and release them into the drainage system at a slower rate;

Impose a quota on impervious surface coverage for new developments, insisting on sufficient green areas for rainwater infiltration;

Encourage green roofs and other innovative rooftop stormwater collection methods among developers; and

Review the quality of soils used on planted slopes and grass verges to ensure their water-retention capabilities.

Lau Kai Guan

Look at ways to slow flow from higher areas
Straits Times Forum 7 Jun 11;

I HAVE lived in the Upper Bukit Timah area for about 16 years and have witnessed how this previously undeveloped area has been paved over with new roads and residential developments. New canals have also been constructed.

I am not an engineer, but I cannot help but wonder if the effect of all this development, including better drainage at higher elevations, has been to increase the rate at which water now flows into the Bukit Timah/Dunearn Road canal during downpours. I would guess that previously more water could seep into the soil, and perhaps be absorbed by the trees.

I could still recall in my early years of living in this area that Upper Bukit Timah Road was often misty at night, due to trees transpiring on both sides of the road.

If my intuition is correct, then national water agency PUB should not only look at the flood-prone areas but also upstream, and consider how to slow the rate at which surface water drains from the higher areas to the lower areas of Singapore.

Ong Hui Guan

Green lungs to quell floods

Building boom is part of the problem; more sustainable development model is needed
Letter from Liew Kai Khiun Today Online 8 Jun 11;

IT SEEMS certain that the islandwide floods are getting more routine as last year's images of submerged roads, water-choked basement car parks and ankle-deep waters in malls returned to haunt us on Sunday morning.

In spite of the extensive drainage work undertaken over the decades, the authorities have conceded that no amount of preparation can stop such freak floods that are attributed predominantly to global warming and "acts of nature".

Nonetheless, I would also like to draw a correlation between the floods and the high growth rates of the past few years, rapid urbanisation resulting from the property boom and the spike in population. Hence, the problems are not only global and natural but also local and man-made.

Studies in the environmental sciences have indicated that building and transportation infrastructural projects are instrumental in displacing organically permeable soil and vegetation with impermeable concrete surfaces that have less capacity to store rainwater.

In the case of Orchard Road, the floods seem to coincide with the replacement of an open and relatively well-vegetated green space between Orchard Road and Paterson Road with the megamall Orchard ION that has probably the deepest basements in Singapore.

Along Bukit Timah Road, which is seeing the more severe floods, are the new condominiums complexes that are squeezed tightly into the previously quieter and spacious neighbourhood of bungalows with spread-out lawns and gardens.

Added to this, the current paradigm to tackling the problem seems to be largely technical, involving drainage systems and building codes. However, as Prime Minister Lee Hsien Loong stated recently, given the space constraints of Singapore, there is a limit to how wide and deep we can dig our canals.

In this respect, instead of seeing the problem as natural and the solution as pouring more concrete, there is a fundamental need for a more environmentally and socially sustainable development model. We have to see open spaces and natural vegetation not as potential exploitable land for property and industrial development but as green lungs and buffers with more intangible long-term benefits.

Increasingly too, trends in large-scale flood control worldwide are moving away from artificial canalisation and containment towards that of natural flood control management that entails the preservation of natural environments and natural water flows.

In Singapore, one such projects on the way to completion is the Waters@Kallang-Bishan Park project that involves the partial de-canalisation and the re-riverisation of water flows. Perhaps the Government should also start thinking of similar projects for Bukit Timah and Orchard roads with green lungs and corridor; and rethink development plans for existing green spaces like the current forested Bukit Brown Cemetery along Adam Road, as well as the lush stretch along the railway tracks from Tanjong Pagar to Woodlands.