Flooding woes – the critical cause of poor soil quality

There is an old Geography adage that goes “earthquakes don’t kill people; buildings do.”

Of course, we have never met – and I hope never will meet – with earthquakes or natural disasters of such extent, but the recent wave of flooding and tree falls has struck worry and discontent into the hearts of Singaporeans. Making matters worse is MM Lee Kwan Yew, who drew upon the sacred phrase “act of God,” and prophesied that flooding would be unavoidable. The weatherman too, kindly informed us of more heavy rain and possible flooding ahead.

What a way to bring cheer to those affected by the floods. Instead of finding ingenious ways to mitigate further disasters, they give up and blame – of all things – the weather! What we are facing are not earthquakes, volcanic eruptions or tropical storms like our counterparts in almost every part of the world; surely if other countries can predict and mitigate their natural hazards reasonably well, what excuse do we have for ours? The time is short, the rains and December monsoon season are coming; it is imperative for us to act.

Flooding at Bukit Timah and Orchard areas has been blamed on the combined effect of heavy rains and the inability of the canals to channel the water away effectively. In the spotlight is Stamford canal at Orchard, which was found to be littered with debris and lacking in “design and capacity” . This, coupled with the “100mm of rain over two hours”, caused the canal to burst its banks and flood Orchard road on June 25^th .

What then can be done? The key issue here is drainage capacity, which concerns chiefly the areas at Orchard and further upstream to Bukit Timah; hence the opening of gates at Marina Barrage, which is the furthest downstream, would have no impact on flood prevention but can prevent floods from getting far worse. To cope with drainage density, one obvious method mentioned by MM Lee is to widen, deepen or build more canals. This he vetoed, citing the lack of space. Floods are unavoidable, “unless you want to lose half the roads and have canals”.

It is not hard to spot the false dilemma. There are plenty of ways to improve drainage capacity without digging deeper or wider holes in the ground. Here, I ought to point out what I feel is a critical cause of the flooding and tree falls: soil erosion.

Yes, soil erosion beyond the natural rate. You may immediately be thinking how there can be soil erosion in Singapore, a green garden city. This is a forgivable misconception that having greenery automatically protects against soil erosion. But just take a look at the canals, especially when it rains. The waters that rage past are hardly blue or clear, but murky and ‘dirty’. Dissolved or suspended in them are silt and sediment washed down from slopes or construction sites. Perhaps if we had eyes sharp enough we would notice the soil that mysteriously vanishes from our slopes, or from the bare heaps lying around in construction sites. During torrential downpours (or even lighter rains) these fine particles get washed into our canals, increasing the load and friction, effectively reducing the drainage capacity of the canals.

Thankfully, this problem of siltation in canals is reduced by NEA, which spends millions of dollars each year dredging and cleaning up the silt in our canals. Just days ago, the PUB awarded a $6 million contract to a company to desilt the Bukit Timah Phase II Diversion Canal. Such expensive maintenance of our waterways is probably well-spent if the occurrence of floods are reduced. Yet despite these efforts, when the thunderstorms come they bring along freshly eroded soils, slowing down the drainage capacity and causing floods.

Hence, addressing soil erosion would be a better bet. In Singapore, the problem starts and ends with two Cs: construction and compaction. We all know that construction in Singapore goes on perpetually. What we do not know is that soils have their own fragile structures and when disturbed, are weakened. Hardly anyone pays attention to soil quality – why should they when soils seem to be just a base to plant or construct things on? – but years of relentless construction have resulted in soil compaction. This means that the soils are compressed together, lowering their infiltration rate and increasing surface runoff. This increases the likelihood of flashfloods and soil erosion simultaneously.