Mon, Sep 11, 2017 - Page 6 News List

Where the storm water should go

By Chang Yen-ming 張炎銘

There have recently been many extreme weather events.

In July, Typhoon Nesat caused serious flooding in Pingtung County, with Jiadong Township (佳冬) alone receiving 600mm of rain in five hours and as much as 182mm in one hour. In August, Typhoon Hato caused tremendous damage to Macau — and even led to the territory’s weather bureau director stepping down. Next, Houston became seriously flooded after Hurricane Harvey brought the worst torrential rain in four decades to the US’ fourth-largest city.

The growing number of destructive floods is unsettling.

Flood control infrastructure is designed by looking at probability distributions and statistical analyses of past data. Because such planning is meant to find the ideal balance between acceptable cost and flood risk, it always deals with a certain level of risk.

For example, the flow of the Gaoping River (高屏溪) during the Japanese colonial period was 22,000m3 per second, while today it is 26,800m3 per second with a 100-year return period.

Should this latest data, including the heavy rainfall during Typhoon Morakot and other destructive typhoons in recent years, be considered when determining whether to raise the planned-for flood volume?

If left at the current level, the protection would not be adequate, as the existing infrastructure was built to contain less river flow. On the other hand, increasing the protection would involve building higher embankments and improving the drainage system and storm-water pumping stations, all at considerable expense.

Is that something the nation can afford, something Taiwanese are willing to pay for?

The problem is further complicated by the cost of urban land. The government will not tear down houses to clear room for better drainage systems, increased flood areas and more detention ponds.

Building effective urban drainage systems becomes increasingly difficult as populations become more concentrated in urban areas and as forested areas and green spaces decrease.

An expert who analyzed the flooding caused by Hurricane Harvey in Houston found that the area covered by the city’s roads had increased by more than 25 percent in the past 15 years, suggesting that human development was the flood’s main cause.

Economic development not only leads to increased vulnerability to natural disasters, but also results in higher economic loss and increased risk.

Underpasses and basements are major problems in urban flooding.

When Typhoon Xangsane caused serious flooding in 2000, many people drowned on the basement floor of a building in Keelung.

Other typhoons have led to people drowning in flooded underpasses after being trapped in their cars.

To prevent this, many underpasses have been constructed with gates that automatically lower, preventing cars from entering, when flooding is detected.

Underground exits at MRT railway stations are elevated to prevent them from flooding.

Basements still rely on water pumps. Although emergency power systems are frequently located in basements, the results are disastrous if the pumps do not work or fail to eject the water quickly enough.

The Water Resources Agency has implemented measures to control flooding, such as limiting sewer outflow and requiring land developers to take greater responsibility for storm-water runoff management by building detention ponds or other facilities.

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