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China floods show need to prepare for more extreme weather events, experts say
- As the climate changes, long-term urban planning, including better drainage, and reliable weather forecasts seen as key to protecting lives and property
- Balance must be found between cost and level of flood protection needed, according to water and environmental engineering professor
Water and engineering experts say long-term urban planning, including better drainage systems, and reliable weather forecasts are key to protecting lives and property.
Big cities around the world could not have coped with the intensity of that rainfall, according to Chen Ji, a professor of water and environmental engineering at the University of Hong Kong.
“If rain of the scale seen in Zhengzhou happened in Hong Kong or Shanghai, it would be impossible to cope with,” he said, adding that Hong Kong sees average yearly rainfall of 2,400mm and has among the best flood control infrastructure in the world.
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As rains ease in central China, 3 million people still struggling with effects of deadly flooding
Chen said urban planners had to find a balance between the cost and level of flood protection needed, noting that “the cost would be huge” for a city to design and build drainage systems to handle rare events like a deluge of more than 200mm of rain in an hour.
Zhengzhou’s weather bureau described the rainfall as at a level seen only “once in a thousand years”. Forecasters describe extreme weather events like these using what is known as a return period to estimate the chance of it occurring in a particular year, according to the Hong Kong Observatory. For example, a 50-year event has a 2 per cent chance of occurring in any year, while there is an 0.1 per cent chance of a 1,000-year event.
In urban parts of Hong Kong, the flood protection system is designed to withstand a 50-year storm event, according to the Drainage Services Department.
“Major flood control facilities including stormwater drainage tunnels and flood storage tanks are generally designed to cope with a 200-year storm event, while the flood protection systems for village areas are designed for a 10-year event,” a department spokesman said, adding that stormwater in the coastal city could be quickly discharged into the sea.
He said a strategy of intercepting stormwater upstream, storing it midstream and improving drainage downstream “has proven to be effective in mitigating the risk of flooding arising from heavy rains”, but flood controls and emergency preparedness continued to be reviewed.
For developed cities like Hong Kong, he said flood carrying capacity could be increased, including building more underground stormwater storage tanks.
“But smaller cities with limited resources might find it difficult to build systems to deal with extremely rare weather events,” he said, adding that they could also build water storage infrastructure as a way to minimise casualties and losses.
Chen from HKU noted that weather forecasts “can save lives”. “The metro in Zhengzhou would have been shut down if the predictions were accurate enough,” he said.
“Zhengzhou’s weather forecaster had given repeated [rainfall] warnings but the local authorities didn’t pay enough attention, and residents didn’t take it seriously because they had never experienced such heavy rain before,” he said.
The provincial government was warned of possible extreme weather last Thursday, two days before it began, according to the deputy director of Henan’s meteorological bureau Su Aifang. But the focus was on Jiaozuo, a city about 66km away from Zhengzhou.
A warning was issued for Jiaozuo to expect up to 500mm of rain on Monday and residents living in low-lying areas were relocated. However, no pre-emptive measures were taken in Zhengzhou, home to 12.5 million people and the hardest hit. It took until Tuesday for the city to issue a red alert, the highest warning level, after which it experienced a record deluge.
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