As Typhoon Fanapi swept across eastern and southern Taiwan on Sept. 19, it brought extremely heavy rainfall of 500mm to 800mm or more in six hours to Gangshan (岡山) and Ciaotou (橋頭) townships in Kaohsiung County and Nanzih (楠梓) and Zuoying (左營) districts in Kaohsiung City. That is well in excess of the 480mm deluge that can be expected to occur once every 200 years. In fact, we could expect this kind of volume to occur in these areas only once every 500 years. Such heavy rainfall is bound to cause flooding, whatever city it falls on.
Various media pundits and academics, however, have failed to judge this objectively. Instead, for their own political purposes, they have chosen to lash out at the Kaohsiung City Government. Such emotional criticism is hardly reasonable. It would have been better to have taken a more scientific approach.
When rain falls on a plain, it has to run through a drainage system, converging in streams and rivers before finally flowing into the sea. On this occasion, the flooded area included the aforementioned four townships and districts and spread over half of Kaohsiung City, about 210km2 in total. The drainage system for this lowland plain consists of just four rivers — the Agongdian (阿公店溪), Dianbao (典寶溪), Houjin (後勁溪) and Love (愛河) rivers. The discharge capacity of these four rivers, measured in cubic meters per second, is about 500 for the Love River, 600 for the Houjin River, 340 for the Agongdian River and 250 for the Dianbao River, adding up to 1,690m3 in all. If the average rainfall over six hours for the 210km2 that were flooded was 600mm, then the total amount of rain that fell on that area in that time must have been 126.3 million cubic meters.
Over the same period, all the drainage rivers in that area could only discharge 36.5 million cubic meters of floodwater — just 29 percent of the total — while the remaining 71 percent (89.8 million cubic meters) had nowhere to go. It could only sit there, flooding the area to an average depth of 43mm, or flow into low-lying areas where the flood would then be even deeper.
Another factor to be considered is that, as the center of Typhoon Fanapi moved out to sea near Tainan in the afternoon, powerful winds blowing in a counterclockwise direction drove a storm surge at least 1m high onto the coast of Kaohsiung. That, combined with the high tide that came in at dusk, blocked the floodwater from discharging into the sea.
With such a heavy rainfall and given the accompanying conditions, there was going to be a flood no matter who happened to govern Kaohsiung city and county at the time. The city and county governments must bear some responsibility for failing to dredge the drains beforehand and to provide plenty of sandbags to keep floodwater out of basements, as well as extractor pumps to pump out water when it got in. But other than that, it was a matter of waiting calmly in the emergency operations center, ready to handle any situation as it arose.
Indeed, at 6am on the day after the typhoon, the Kaohsiung City Environmental Protection Bureau’s garbage collection teams were fully mobilized to clear away fallen branches and shop signs. That task kept them working late into the night and they spent three days clearing trash until the job was done. They should be commended for their perseverance and efficiency.
In view of all this, consider the opinion of a professor who once served in a senior post at the forerunner of today’s Water Resources Agency. This professor claimed that seven flood detention ponds could have prevented the flood. It should be kept in mind that the most each detention pond could hold would be about 100,000 to 110,000 tonnes of floodwater. With tens of millions of tonnes of floodwater to deal with, that really would be like trying to put out a fire with a squirt gun.
Meanwhile, a candidate standing in the area that will become Greater Kaohsiung’s year-end election claimed that if she were elected, she could remove the threat of flooding in Kaohsiung in less than a year.
Such empty talk is mere electioneering and suggests that the candidate in -question really doesn’t have a clue.
Given that Kaohsiung’s urban drainage system can only handle the kind of rainfall that occurs every five to 10 years or so, and the rivers in the area can only discharge a downpour that happens once every 50 years, there was no way flooding could have been avoided under the conditions described above. What’s more, global warming may cause extra-heavy downpours to happen more and more often.
The only way to reduce the impact of heavy rainfalls is to designate wider reservations around streams and rivers, in which only low-intensity development should be allowed. When their drainage profile is widened, rivers can discharge floodwater more effectively.
Additionally, urban drains should be dredged before typhoons arrive. Sandbags should be stored or floodgates installed at the entrances to basements of large buildings so that residents can use them stop the basements from being flooded. Plenty of extractor pumps should also be available at all district offices so that they can be dispatched to pump water out of inundated basements.
Such measures could mitigate the damage caused by flooding and give less cause for know-it-all commentators to pontificate after the damage is done.
Shern Jian-chuan is a professor of marine environment engineering at National Kaohsiung Marine University.
TRANSLATED BY JULIAN CLEGG
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