Urban flash floods - what could've gone wrong

The flash flood that occurred on Feb 18, 2013 along Km21.36 and Km23.50 of the Damansara-Puchong Highway (LDP) caused considerable damage and inconvenience to local businesses, motorists and the public.

The main cause of the flood cannot be attributed to heavy rainfall or the low ground level of the affected areas. The Drainage and Irrigation Department’s (JPS) telemetric rainfall station in the vicinity recorded 68mm of rain in the late afternoon of that day, which is not exceptionally heavy and statistically, such rainfalls could be exceeded several times in a normal year.

However, it is heartening to note that the Subang Jaya Municipal Council (MPSJ) was prompt in responding to the matter, and had called for a meeting of the parties involved, namely  Linkaran Trans Kota Sdn Bhd (Litrak), Malaysian Resources Corp Bhd (MRCB), Syarikat Prasarana Negara Bhd (SPNB) and JPS to identify the causes of the flood.

It was established that one of three underground culverts was not functioning properly, and was therefore unable to drain off the excess rainwater. Why this could have happened needs to be investigated thoroughly.

There could be many reasons why the culvert ‘malfunctioned’. For instance, rubbish is being conveniently thrown into drains every day by irresponsible people, and the rubbish would get trapped in the culvert.

The public must be informed that the water drainage system is not a rubbish disposal system. There are other facilities for proper rubbish disposal, which should be provided for and used, whilst enforcement against indiscriminate dumping of rubbish must be carried out. It is time everyone understands that rubbish and other wastes can contribute to floods as well as other negative environmental effects.

In addition, the local authorities and road concessionaires should also ensure that periodical and continuous clearing of rubbish and silts from flood prone areas are carried out efficiently.

Another possible reason for the flood in this case, could have been the ongoing construction and excavation works near and around the flood affected areas. During recurring rainfall, silts and debris from the construction sites could have been washed and deposited at the culvert. The earth deposits on the bottom of culverts would reduce the cross-sectional area of the culvert, and impend the capacity of the culvert to carry away water.    

Ongoing construction works had been the culprit in causing many flash floods. Contractors often wrongly take shortcuts in temporary diversion works, thereby taking risks and reducing costs. Contractors must adopt a more professional approach and use the appropriate rainfall data in the design of temporary works. Best management practices on silt traps, culverts and drains must also be incorporated. Again, enforcement by the authorities is necessary.

The Institution of Engineers Malaysia (IEM) is keen to assist the authorities in resolving any recurrence of such flood incidents, and if there is a need, we would be pleased to nominate our members to offer their expertise.


IR PROF DR JEFFREY CHIANG CHOONG LUIN is honorary secretary of the Institution of Engineers Malaysia.