Recent years have seen many extreme weather events around the world, including intense rainfalls, floods, droughts, and wildfires.

In 2021 alone, intense rainfalls and floods have been reported from many regions, most spectacularly in Germany in July. Many of these floods have been termed ‘flash floods’ and have resulted in deaths and widespread damage to property and infrastructure.

Parts of south Wales have also experienced flash flooding in 2021. Communities in Skewen, Neath Port Talbot, have faced major disruption after intense rainfall. So far, most other parts of Wales have escaped major flash flooding this year, but the country has not always been so lucky.

READ MORE: Crickhowell - the village where flooding’s a way of life

In June 2012, flash floods occurred in Aberystwyth and surrounding villages following intense rainfall.

In 2020, Aberystwyth’s town centre was again hit by flash floods following thunderstorms, and flooding also occurred in other parts of Ceredigion, as well as Gwynedd and Rhondda Cynon Taf. Although no lives were lost, the damage to housing, social disruption and associated emotional trauma is still felt by many.

These events may be a portent. The August 2021 release of the Intergovernmental Panel on Climate Change’s sixth Assessment report states unequivocally that “many changes in the climate system become larger in direct relation to global warming”.

READ MORE: What is the UN's new climate report and why is it important?

The physics-based rule of thumb is that with every 1C of global atmospheric warming, roughly 7% more moisture vapour can be held. So when moisture-laden warm air is forced to cool – for instance, when rising over mountain ranges or undercut by colder, denser air – greater amounts of vapour will condense, perhaps generating intense rainfall.

As our atmosphere continues to warm to 1C or more compared to pre-industrial levels, the consequence will be more frequent, intense flash floods.

Key characteristics of flash floods include the rapid water level rises, with normally placid areas within and perhaps far beyond river channels being under many feet of fast-moving, debris-laden water within minutes.

Such floods have a high potential for erosion of channels, bridge pier supports, house foundations, and other infrastructure. Flash floods also have some of the highest fatalities as the unexpected, rapidly evolving events commonly catch people unaware.

So, in Wales and elsewhere, are we simply at the mercy of flash floods? Or can we manage flash floods better than we currently do?

READ MORE: Mark Drakeford again rejected a call for an inquiry into flooding in Wales

Some of the first steps in improved management are understanding the causes of flash floods. About 15 years ago, the Environment Agency commissioned a report to look at how people understand and respond to flash flooding incidents.

As part of the resulting 2009 report, a media article analysis of flash floods concluded that “… media reporting of the causative factors leading to flash flooding - from which people might learn about the precursors of flash flooding, and which might make them more able to predict, assess and identify an increasing risk of flash flooding - is not wholly accurate”.

My research with colleagues has been addressing the causes of flash floods globally, including emphasis on improving public communication of the findings. We have focused particularly on arid parts of Argentina and Jordan, but our findings have implications for closer to home.

In arid regions, intense rainfall commonly falls on sun-hardened ground with little or no vegetation. Rather than allowing rainfall to soak into the ground, these surface conditions promote surface run-off down hillsides and into channels, which then experience rapid water level rises.

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Importantly, similar surface conditions may be replicated in more humid catchments subject to land uses involving vegetation cover decreases, particularly if this coincides with droughts, or in some forest areas that have experienced severe wildfires.

In urban settings, the widespread use of concrete or asphalt on surfaces also limits the extent to which rainfall can soak into the ground and instead promotes rapid run-off and flood generation.

The mantra for improved management of flash floods needs to be “slow the flow, spread the flow”.

Slow the speed at which water runs off the land to decrease the rate of floodwater rise. Spread the water over time to reduce flood peaks. In other words, reduce the flashiness, erosion potential and danger of floods.

Luckily, there are many land management approaches that can help. These approaches include protecting and restoring wetlands such as peat bogs, tree replanting schemes, and incorporating more porous surfaces into new housing developments.

Examples from arid and more humid parts of the world demonstrate how different combinations of approaches can be moulded to local circumstances.

For instance, near Las Vegas, USA, artificial wetlands have reduced flash flood erosion, and the increased surface water and constructed trails have provided ecological and recreational benefits in the desert.

In Pickering, Yorkshire, a ‘slowing the flow’ scheme involving moorland restoration and tree planting helped to reduce floods by 15 to 20% and prevented flooding of homes and the local museum.

Nevertheless, such approaches are not a panacea.

Regardless of any international climate agreements arising from the forthcoming Cop26 Glasgow Climate Change Conference, we now have a degree of ‘lock in’ to atmospheric global warming and extreme weather.

READ MORE: Climate change: Eight areas where urgent action is needed

Floods will still happen in future, some of them flashy. But case studies worldwide show how past community experiences of dealing with floods may also prove instructive.

In Argentine Patagonia, for instance, devastating floods struck the embryonic Welsh settlements in the late 19th and early 20th centuries.

Our ongoing research has shown how community memories of these floods, including use of photographs and eye-witness accounts in museums, help to raise awareness of flooding potential and show how recovery from floods was enhanced by strong community bonds.

In Patagonia, just as in Wales and other parts of the world, widespread use of cameras and social media now offer many creative ways to record, curate, and share 21st century flash flood experiences.

Doing so may contribute to improved public understanding of flash flood characteristics and risk, and also help improve flood management under our changing climate.

Stephen Tooth is a professor in the Department of Geography and Earth Sciences at Aberystwyth University. His research and teaching interests focus on landscape development, environmental change, and science communication.

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