Cities most prone to extreme rain and flood | Study reveals

By: Conrad Wasko, Professor Ashish Sharma, Professor Seth Westra,
Planning n Mitigation

Extreme rain and cities of the world

A study at the University of New South Wales, Australia have determined that cities face harsher and more concentrated rainfall as climate change not only intensifies storms, but draws them into narrower bands of more intense downpours. The findings have major implications for existing storm water infrastructure, particularly in large cities, which face higher risks of flash flooding.

The study titled Reduced spatial extent of extreme storms at higher temperatures by Conrad Wasko, Seth Westra and Professor Ashish Sharma that appeared in Geophysical Research Letters on April 25, 2016, shows the first evidence of storm intensification triggering more destructive storm patterns. ‘As warming proceeds, storms are shrinking in space and in time’, said Wasko, lead author of the paper. Storms are becoming more concentrated over a smaller area, and more rainfall is coming down with higher intensity over a shorter period of time. When the storm shrinks to that extent, there is huge amount of rain coming down over a smaller area and hence increasing the possibility of flash flood.

The study analysed data from 1300 rain gauges and 1700 temperature stations across Australia to see how air temperature affected the intensity and spatial organization of storms. They found that atmospheric moisture was more concentrated near the storm’s centre in warm storms than in cooler ones, resulting in more intense peak rainfalls in those areas. The storms were clearly shrinking in space, irrespective of the amount of rain that fell.

‘Although the data is sourced from Australia, this has global implications’, said Sharma. Australia is a continent that spans almost all the climate zones in the world—Mediterranean, tropical, temperate, subtropical, etc. except the Arctic or Antarctic. So, the results hold a lot of value. The pattern repeats itself over and over, around Australia and the rest of the world.

In the floods in Mumbai and Bangkok in 2015 and in Jakarta, Rome and many parts of Canada 2016, urban streets were submerged in huge amount of flood water. The study shows that it is because the storm water infrastructure cannot handle the rain, and rise in global temperatures have also increased rain in general.

‘Most urban centres have older storm water infrastructure designed to handle rainfall patterns of the past, which are no longer sufficient. The increase is especially noticeable in urban centres where there is less soil, unlike rural areas, to act as a dampener’, said Sharma. The drainage capacity is often overwhelmed due to the lack of space to contain the excess water.

Scientists have long suspected that the intensity of rainfall would be boosted by climate change, as the warming air raises the carrying capacity of moisture. But extreme rain events has been rising, while little was known about the mechanisms causing it. The latest study shows that storms are changing in spatial terms.

The latest study follows another study by the same authors published in Nature Geoscience in June 2015, showing that storms were also changing their ‘temporal pattern’- getting shorter in time, thereby intensifying. When it comes to flash flooding, the amount of rain that falls over a period of time is much more important than the total volume of extreme rain events a given storm delivers. This study was the first of its kind to show that climate change was disrupting the temporal rainfall patterns within storms themselves.

If both spatial and temporal changes in storms continue, as they are likely to do due to climate change, there will be more destructive flooding across the world’s major urban cities. In their Nature Geoscience study, the duo calculated that floods in some parts of Australia would likely increase by 40 per cent, especially in warmer places like Darwin. ‘If the spatial pattern from the latest study is added to the 40 per cent, that it may increase to 60 per cent in total’, said Sharma.

‘The storms are shrinking in space and shrinking in time in the storms will likely increase floods, on the assumption that the volume of water coming down is not changing,’ said Sharma. The assumption is very conservative though, because one would expect the air to hold more moisture in this situation. However, if factor is considered, it will be evident that there will be even more extreme rain events and floods.

Story by:
Conrad Wasko,
PhD Candidate,
School of Civil and Environmental Engineering,
University of New South Wales,
Sydney, Australia.

Professor Ashish Sharma,
School of Civil and Environmental Engineering,
University of New South Wales,
Sydney, Australia.

Professor Seth Westra,
School of Civil, Environmental and Mining Engineering,
University of Adelaide, Australia.

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