~by: C L Cheong~
From time to time a spate of flash floods would trigger off public concern and debate for a while.  While newspaper reports and commentaries on floods tend to be fragmentary in nature, a perspective of the flood problems can be gained by piecing together relevant published information and data on the subject.
The salient aspects are:

• What cause floods?

• How does urbanization affect flooding?

• How severe are the flood problems?

What cause floods?

Floods are essentially caused by heavy storms. There are two types, and they have almost diametrically opposing features.
Type 1 – Long-duration storms.  These are associated with the northeast monsoon, when incessant rain of low to moderate intensity occurs over an extensive area.  As a consequence, a huge volume of rainwater is produced to cause wide-spread flooding and, in extreme cases, island-wide flooding.
Two major storms had occurred in recent decades.  One, the largest on record, occurred in December 1978 with 512 millimeters of rain falling in a 24-hour period, an average of 21 millimeters (0.83 inches) an hour.  Another, next largest in magnitude, occurred in December 1969 with 467 millimeters of rain recorded in a 17-hour period, or on average 27 millimeters (1.1 inches) an hour.  Both storms caused island-wide flooding of catastrophic proportions, the flooding situations having been aggravated by the flood events coinciding with the occurrence of very high tides.  Extensive damage and loss was sustained.  In addition, the December 1978 flood claimed seven lives while the December 1969 flood five.
Type 2 – Short-duration storms.  As opposed to long-duration storms, short-duration storms, such as thunder storms, occur in localized areas with rain of high intensity (intensity that can be several times that of long-duration storms).  Notwithstanding the high rainfall intensity, because of its short duration and localized nature, relatively small volume of rainwater is generated in a short-duration storm.  Consequently the resulting flood is transient, hence the term flash flood.  Flash floods may be of nuisance value or they may cause varying degrees of damage and loss such as were witnessed in the Central Business District not too long ago.
How  does urbanization affect flooding?

Urbanization is perhaps the most significant factor that contributes to the worsening of flooding conditions.  This is because urbanization leads to a dramatic increase in surface runoff, i.e., the volume of rainwater which directly flows into rivers and drains during a storm.  Figures obtained elsewhere with similar climatic and physiographical conditions show that the surface runoff generated in a forested/vegetated catchment is typically of the order of 30% of the total volume of rainwater, the other 70% being made up of different amounts due to interception by foliage, detention on ground depressions and infiltration into the ground.  For a moderately urbanized catchment, the ratio can be reversed, i.e., the surface runoff can now be 70%.  The proportion is even higher for a highly urbanized catchment due to the near absence of foliage and ground depressions and the fact that a higher percentage of land is covered by surfaces impervious to the infiltration of rainwater.
How Severe are the flood problems?

To appreciate the current status of the flood problems, it helps to take a look of measures taken to deal with the problems in recent decades.
In the 1960s there were some 7,000 ha (70 square kilometers) of flood-prone areas, about 10% of Singapore’s land area.  The need to resolve the flood problems became even more pressing in the 1970s and 1980s during which accelerated urban development took place.  Against this backdrop major projects to upgrade and expand drainage works were implemented across the island from the mid-1960s through the early 1990s.
These include:

• widening, deepening and concrete-lining of existing drains

• construction of new drains

• construction of flood diversion canals

• construction of tidal gates

As a result the island’s flood mitigation capacity and capability was strengthened considerably.  Simply explained, concrete-lining of an existing drain would double its discharge capacity.  By also widening and deepening the drain, the discharge capacity would be increased several folds.  As another example, major flood diversion works had served to overcome the problem of chronic flooding which plagued Bukit Timah areas in the past.
The efficacy of the upgraded drainage works was put to test when a significant storm occurred in December 2006.  This is the third largest long-duration storm on record with 366 millimeters falling in a 24-hour period, an average of 15.3 millimeters (0.6 inches) an hour.  No wide-spread flooding had been experienced; there were pockets of localized flooding totaling some 15 ha.  This storm is appreciably smaller in magnitude compared with the two major rainstorms that preceded it.  However judging from the fact that only minor flooding took place in this event, it is reasonable to expect that the extent and severity of flooding will be much reduced should a storm of the same magnitude as the 1978 storm recur.
*Marina Barrage.  With the completion of the barrage and estuarine reservoir at Marina Bay in late 2000s an important item has been added to the array of drainage works.  Tidal gates serve to stop the ingress of sea water and prevent inundation of low-lying city areas at high tides.   However when a heavy storm occurs, inundation of these areas will still take place if the gates have to be kept open during high tides for storm waters discharge.  The Marina Barrage is equipped with large gated openings and also huge drainage pumps which enable the water level of the reservoir to be regulated for unimpeded storm waters discharge during high tides without the low-lying city areas being inundated.  It therefore enhances the flood mitigation capability of the drainage system.  In passing, this fashion of hydraulic engineering may be replicated to advantage in other drainage basins with an estuarine reservoir.
Flash floods still occur from time to time despite all the major drainage projects implemented, the reason being that a short-duration storm occurring with very high rainfall intensity generates a surface runoff rate easily exceeding the storm waters discharge rate of the drainage system.
Space constraint and prohibitive cost preclude the construction of drainage systems with capacity large enough to completely prevent the occurrence of floods.  On the other hand, in short-duration storms the surface runoff rate peaks rapidly and falls equally fast to within the discharge capacity of the drainage system.  Consequently, the volume that overflows to cause flooding is generally small.  It then boils down to this: how best it is to deal with the overflow volume in order to minimize flood damage and loss.
A range of measures are known and applied to minimize flash flood damage.  For instance, the excess storm waters is diverted to a lake or an open field for temporary detention and released back to the drainage system when the peak flow is over.  Or, as was built at Opera Estate, an underground storage would serve the same purpose.  Installation of flood barriers at commercial complexes to prevent ingress of flood water and damage to property, and raising road surfaces above flood levels, as recently implemented in the Central Business District, are two other examples.  Such measures are site specific: it depends on the constraint and opportunity at each location.  Innovation and ingenuity helps in the evolving of simple yet effective solutions.
After considerable efforts over the years and more than S$2 billion spent, the flood problems are now within manageable proportions.  According to Ministry of the Environment and Water Resources, flood-prone areas have been reduced by more than 95% since the 1960s.  Compared with about 7,000 ha at that time, the flood-prone areas were estimated at 130 ha at the end of 2007.  While the latter figure is not absolute, it is clear that the scale of the flood problems has been greatly reduced.
Three other pertinent points are worth mentioning.
1.  Limitation of drainage works.  This limitation is not confined to short-duration storms.  It is equally true of long-duration storms.  A drainage system may justifiably be constructed at high cost to cope with a large storm, flood will still take place when a yet larger storm occurs.
With the view to mitigating flood damage and loss non-engineering measures, like flood forecasting and early flood warning, can also be employed.  Flood forecasting and early flood warning is useful where the flood peaks slowly, such as during a long-duration storm, so that there is sufficient lead time to enable safe evacuation of people and property.  Flood risk mapping, another non-engineering measure, serves to identify vulnerable locations in a flood-prone area and to facilitate advance preparation for protection and evacuation of lives and property.
2.  Small catchments. Singapore with a land area of 680 square kilometers is drained by a dozen or so of rivers and streams.  Its largest drainage basin, the Singapore River/Kallang River Basin, has a catchment of only 100 square kilometers or so.  This is an advantage in terms of the magnitude of flood problems to be dealt with.
3.  Seeking out flood mitigation opportunities. Urban development and urban renewal activities have been and will continue to be an on-going feature.  All these activities impact on existing flooding conditions.  Converting a park land into a commercial complex, for example, will increase the runoff with possible adverse effects.  Land fill to raise a building platform above flood levels may result in transferring the problem to another flood-prone location.
But urban development and renewal also afford an opportunity for taking a fresh look of existing flood problems and incorporating measures in the development/renewal project not only to mitigate its adverse effects but also to improve flooding conditions.
A closing remark
Floods cannot be completely controlled.  But floods can be mitigated, and measures can be taken to minimize flood damage and loss.
Effective measures can be designed and executed based on a good understanding of flood behaviors.  In this regard using modeling techniques to simulate flooding conditions and to assess the effects of flooding due to large and extreme storm events as well as urban development and renewal, as a basis for evaluating options, which can be engineering measures or non-engineering measures or both, merit consideration.
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* The completion of the Marina Barrage marked the culmination of the successful implementation, over a period of three decades commencing with the cleaning up of heavily polluted rivers in the SingaporeRiver/Kallang River Basin in late 1970s, of a singularly unique multi-purpose water resources project.  Apart from drainage and flood mitigation, it also provides other benefits: water supply, recreation and environmental enhancement.  The reservoir and its surrounds is a vibrant entertainment hub and the venue of major national events.
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Mr Cheong is a civil engineer who has spent most of his working life looking into drainage and irrigation issues.