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How do we hold back flash floods in Singapore?
Written by Ismail Weiliang and Yen Nie Ng, Senior Hydraulic Engineer with 11 years of experience in water resources .
ISMAIL WEILIANG
The Climatebender
YEN NIE NG
Co-author
Senior Hydraulic Engineer
Views are entirely ours
and not connected to any company
Challenge of Flood Resilience
With climate change, heavy localised sudden rainfall events with increasing frequency and intensity have overwhelmed Singapore’s waterways before. This has led to flash floods when drains have insufficient capacity.
Source – Pathway – Receptor Approach
A holistic approach for flexibility and adaptability is required for Singapore’s drainage system to be resilient to floods. Singapore adopts holistic “Source-Pathway-Receptor” approach to stormwater management. Through the “Source-Pathway-Receptor” approach, measures are not only carried out along the Pathway, but also implemented at the Source where stormwater runoff is generated and at the Receptor where floods may occur. This approach is sustainable as stormwater management is addressed at all parts of the drainage system, with building owners and developers playing a role in managing the impact of their developments on flood risks.
(Source: PUB)
Source control
Since January 2014, developers of new and redeveloped sites >0.2 hectares (2000 m2) are required to implement on-site measures such as detention tanks, green roofs, rain gardens or retention ponds to slow down the flow of stormwater discharged from their developments into the public drainage system. The target is to achieve 0.55 runoff coefficient, storm return periods of 10 years for storm durations of 4 hours.
Surface Source Controls: ABC Water Features:
ABC Water features are green features (named as SUDS/WSUD/Sponges cities in other regions) – which reduce runoff volume and discharge rates while serving active, beautifying and cleansing functions. Some examples include retention ponds, sedimentation basins, wetlands, green roofs, planter boxes, bioretention swales/ basins and gardens and porous pavements. One popular ABC Water features in Singapore would be the revamping of Bishan-Ang Mo Kio Park completed in 2011 where a large concrete canal was transformed into river teeming with life.
Underground Source Controls: Detention Tanks
Detention tanks collect and store stormwater runoff during a storm event, then release it at controlled rates to the downstream drainage system, thereby attenuating peak discharge rates from the site. With such systems in place, the drainage system as a whole can cater for higher intensity storms brought about by increasing uncertainties due to climate change. Detention tanks are usually located underground due to space constrains and to maximise storage volume. One popular detention tanks in Singapore would be the Stamford detention tank a mega detention tank which was completed in 2018. It has a 7000m2 footprint, extends 28m downwards and can hold up to 38,000m3 or 15 Olympic sized swimming pools of stormwater which can then be emptied in four hours to prepare the tank for the next heavy rainfall. The detention tank was built to tackle floods on orchard road area in the city.
ABC Water features detailed design
Water engineers go through the following process for ABC Water features design. Site investigation, especially soil property test and services detection, are to be carried out so that design suits the site condition. Flood control are considered with the use of the Rational Formula and Manning's Formula to size the overflow system, incoming and outgoing drains for ABC Waters design features to cater for small frequent storms (such as 1-in-3-month storms). Water Quality improvement is considered for removal of Total Suspended Solids, Phosphorous and Nitrogen. Plants considered for the filter media in the bioretention systems should suit the inflow conditions. Access for maintenance and design for public safety is also considered. More design considerations can be found here.
Detention tank detailed design
Water engineers go through the following process for detention tank design. Post-development peak run-off is calculated. If it exceeds the maximum allowable peak discharge for the site, detention shall be required. Site analysis may be carried out to determine if a single/multiple sub-catchment approach us best adopted for the detention system to meet the peak discharge requirement. Various detention system may be used to control the peak run-off discharged from the site based on design calculations. More design considerations can be found here.
Authors:
Yen Nie Ng is a Senior Hydraulic Engineer at Mott MacDonald Singapore. She has 11 years of working experience in hydrology and hydraulic study, water resources planning, water quality study and environmental management study and planning projects in Singapore, China, Macau SAR Cambodia and Trinidad & Tobago.
Ismail Weiliang is a climate resilience consultant with over half a decade of experience and specialises in flood risk advisory for Asia. His work involves advising governments and development banks on strategies to transform climate risks into resilience. He also founded “The Climatebender” a non-profit organisation that provides humanitarian relief to communities vulnerable to the climate crisis.
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