Urbanisation contributes the most to worsening flood conditions

~by: C L Cheong~

In the article posted on TOC on January 19, I have attempted to provide, and to share, a perspective of flood problems. 

To recap, there are several fundamental aspects to it.  First, floods are caused by two types of heavy storms with different characteristics, one giving rise to localized flash floods and the other widespread, prolonged flooding.

Second, urbanisation is perhaps the most significant factor that contributes to the worsening of flooding conditions as it results in a significant increase in surface runoff in a storm event.  (Since writing the article, I read of the news in the January 11, 2012 edition of The Straits Time that rainfall intensity has been increasing over the last three decades.  This, I think, has also contributed to an increase in surface runoff to aggravate flooding conditions.)  

Third, space constraint and prohibitive cost would preclude constructing drainage systems capable of completely preventing the occurrence of floods; a range of known measures can be applied in conjunction with existing canals and drains to mitigate floods and to minimize flood damage, and some typical examples of these are cited. 

Lastly, effective measures can be designed based on a good understanding of flood behaviors. Modeling techniques can be employed to simulate flooding conditions due to large and extreme storm events, or resulting from urban development and urban renewal activities.  The simulated results can then be used as the basis for evaluating options of flood mitigation, which can be engineering measures or non-engineering measures or both.     

It would be useful to clarify that these aspects are discussed in generalized, conceptual terms.  The discussion is meant, for instance, to throw light on the underlying causes of flooding, and what measures are generally employed to mitigate floods and their effects.  There is therefore no pretence to suggest solutions for specific flood problems.  A sound engineering solution will be found through a feasibility study going through the process and rigor of engineering survey and investigations, engineering analyses, comparison and evaluation of alternatives.

I would like to make two other brief comments. 

Mr Peter Sellers asked: what is the pump capacity at the Marina Barrage and how this compares with peak runoffs during heavy storms?

As I understand it, there are seven pumps installed at the barrage, each having a pump capacity of 40 cubic meters per second and an aggregate of 280 cubic meters per second for the seven pumps.  To give some physical idea of its size, one pump is capable of evacuating, in one minute, a volume of water equivalent to that of an Olympic swimming pool (Also see information posted by Mr Ethen Jin-Chew on January 23, 2012). 

I would not be able to comment on the second part of the question as I do not have the information and data.  It was reported that the size of the detention storage required for resolving the flood problems at Orchard Road is of the order of 40 to 50 Olympic-size pools (January 10 2012 edition of TODAY).  This would indicate the volume of water spilling out of the Stamford Canal during a flash flood.  Using this as a basis for comparison, the entire body of water of the detention storage can be evacuated in about seven minutes with all seven pumps running simultaneously.  The pumps are said to be the largest of their kind in the world.  

There was a question asked (Comment No 17): What about the afflux caused by the completion of the Marina Barrage?

When a drain discharges into a pool of water, the flow is retarded, and water level at the lower end of the drain will be ‘pushed up’ resulting in a backing up effect.  This effect will extend upstream up to a point where the condition of continuity of flow (i.e., ‘what flows in flows out’) is attained.  This phenomenon of the backing up of water level is termed afflux. 

Let us first consider the ‘without barrage’ scenario.  At high tide a drain will discharge against a high water level, while at low tide, a low water level.  Next consider the ‘with barrage’ scenario in which the reservoir is maintained at a water level above the low tide but below the high tide so as to avoid inundation of the low lying areas in the city centre (See information posted by Mr Ethen Jin-Chew on January 23, 2011).  The drain will now discharge against a water level intermediate between high tide and low tide. This would suggest that the extent of backing up effect in the ‘with barrage case’ would be intermediate between the extent of backing up effect due to high tide and that due to low tide in the ‘without barrage’ case, all things being equal.

In passing it would be of interest to note a piece of news, posted in the October 7 2011 edition of The Straits Times, that a panel of experts ‘used a computer program to recreate the storm conditions during the Orchard Road floods but removed the barrage from the picture.  It found that the floods would still have occurred even without the barrage.’  This means that the backing up effect is not felt at Orchard Road, which correlates with the observation that flood waters here recede within a short space of time. 

Finally on a personal note (as a response to a comment by Mr Peter Sellers), I am a visitor to Singapore, and have no connections with PUB or any other governmental bodies. 

I had worked in the field of water resources engineering since the beginning of 1960s, starting as an apprentice engineer learning how hydraulic structures are constructed.  I then progressed to the level of operating and managing water resources systems, and later to planning and review of water resources projects, along with the review and upgrade of engineering standards and procedures.  In the later part of my career, I did short-term engineering assignments for international organizations, which added more continuing learning and excitement in my working life.  Six years ago, I retired completely from engineering practice, spending time with my family with a bit of travel.  

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