9m wide canal at Woodlands
Singapore sees heavy tropical storms that deposit large amounts of rain within short minutes. Any drainage design system has to cope sufficiently with such large flows. Over the years, global warming has further increased the need for systems to manage sudden stormwater discharges.
Drainage design involves a good understanding of the topography of the site, hydraulics, structural and geotechnical design, as well as maintenance issues.
In general, for such a tropical environment with high intensity of rainfall, box drains are used instead of pipes. The top slabs of these drains also normally serve as the footpaths along the road side.
Internal view of a large box drain
To reduce maintenance and choking of drains, flow velocities within drains have to be designed to achieve self cleansing velocity. Based on the local regulations, this is between 1 m/s to 3 m/s. A higher velocity scours the concrete in the drains and is to be avoided. Typical drain gradients will range between 1:200 to 1:1000 for small drains to larger drains respectively. Sufficient freeboard of at least 15% of the normal flow depth should be ensured, and Froude number of the flow kept below 0.8.
In addition any stagnant water is a major contributor of mosquito breeding and has to be avoided at all costs. Hence filled systems used in colder climates are not applicable in equatorial Singapore. To prevent settlement of the drains and water stagnation, most drains in Singapore are piled which requires proper site investigation, supervision and submissions to BCA.
RC Piling for drains to prevent settlement
The use of box drains and culverts requires structural design to the Eurocodes to ensure their load bearing capacity and longevity. Box drains and culverts have to be designed to SS EN standard to withstand onerous loading in various combinations of earth and water pressure. Typically these structures have a design life of more than 50 years.
Installation of reinforcing to box drain wall
Aside from the hydraulic and structural aspect of drainage design, the interaction with existing services is a major consideration requiring close coordination with service agencies. Some services will require diversions and some will require to under cross the drain at distance specified by PUB. In rare circumstances with PUB approval, the services may over cross the drain
In addition to the the basic drainage design, storm water detention and treatment via Active Beautiful Clean (ABC) features need consideration and integration to the system to ensure a sustainability in the long term. ABC features may be in the form of swales, raingardens or wetlands.
Example of a stormwater detention tank constructed using precast methods
Example of a vegetated swale
Proper design of drainage facilities are invaluable to prevent flooding situations and disamenity to the public. This involves both a clear understanding of hydraulics, hydrology, maintenance requirements as well as structural design. PUB’s Code of Practice on Surface Water Drainage provides invaluable resources for such designs in Singapore and has ensured a reliable drainage system in tropical Singapore.
Author Er Kumara Govindar Rasu with over 20 years experience in drainage design, ABC features and detention systems in Singapore and New Zealand.