Flash Flooding from Intense Rainfall

The Flash Flood! simulation was commissioned and funded by the NERC-FFIR programme. That’s the Natural Environment Research Council, and their research programme, Flash Flooding from Intense Rainfall. The funds come from the Knowledge Exchange programme and Flash Flood! is designed to be a tool to allow the researchers to communicate their science more effectively to a range of audiences.

“To reduce the risks of damage and loss of life caused by surface water and flash floods through improved identification, characterisation and prediction of interacting meteorological, hydrological and hydro-morphological processes that contribute to flooding associated with high-intensity rainfall events” – NERC-FFIR project aims.

But, what do we mean by Flash Flooding from Intense Rainfall?

In the warm summers in the UK it is possible for intense convective thunderstorms to form. These are difficult to forecast. We know the conditions in which they can form, so meteorologists can say with confidence general areas and times where their formation might be likely. However, these storms can be small and very localised (less than a few kilometres wide) and short lived (just a few hours), and predicting when and where exactly the storms will form is much more difficult.

This is problem, as the large amounts of rain produced by these storms over a short period can overwhelm the ground’s capacity to absorb it, so the excess water flows along the ground quickly into rivers – we call this Hortonian overland flow.

In the right conditions, these rapid flows of storm water into river systems can quickly swell the river and cause water levels to rise and fall over a short space of time. This is a flash flood. These rapid flows can also carry lots of sediment (mud and stones) and debris (rocks, tree, plants, cars), and these increase the potential amount of damage the flood can do. The movement of this material can also change the shape of the river, which in turn will change the future flood risk in the area.

In some extreme examples, a sudden influx of sediment and debris into the river channel can slow the movement of the flood wave down the river. Here, the flood backs up behind it before cascading rapidly downstream in what is often described as a ‘wall of water’ – these have been witnessed in the UK, but are rare and never been caught on film.

Geomorphologists seek to understand the changing shapes of the Earth’s surface, and flash flooding can form a key component in the development of any river system. Flash Flood! shows how a river can be shaped by just a single event, and you can read more about the event in the Research section.

Predictions for climate change suggest that the occurrence and intensity of convective thunderstorms in the UK will increase, and in turn so will the likelihood of flash flooding. It is important that we can improve our capability to predict flash flooding from intense rainfall events, but also understand the geomorphic impacts they have on our landscape.

 

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