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Farringdon rainfall and groundwater flood risk - Updated 1st March 2020

The Environment Agency’s warning about flood risk highlights the link between the rise in groundwater levels to recent rainfall. The big question is how much rainfall is needed to raise the level of groundwater to create flooding and how quickly groundwater levels respond to long periods of heavy rainfall, such as we experienced with Storm Dennis?

Real-time groundwater levels at Farringdon are available on-line but it is hard to see longer term trends in the 5-day snapshots.

I have started to compare our rainfall data with groundwater levels as a step towards understanding this complex relationship, which may help to predict the probability and timing of groundwater flooding. For example the graph below shows rainfall in February and that Ciara and Dennis rain may have taken about a week to affect groundwater levels.

February Rainfall

After a rapid rise these are now levelling off and whether groundwater will breach the surface remains to be seen, depending critically on rainfall amounts over the coming weeks.

To put this in context, while the Met Office announced today that February was the wettest month on record across the UK, our local records show that rainfall in February 2014 was much greater.

Prior to the 2014 flooding the December, January and February rainfall amounts were 178, 235(!) and 183 mm respectively (representing 97% of the average annual rainfall within three months).

This year the equivalent amounts are 116, 80 and 149 mm (57% of annual average) almost half what we experienced in 2014.

I’d like to look at this in more detail and have requested all the data the EA hold for the Farringdon borehole since 2000 to compare to local rainfall records. I would be very interested to know If anyone has their own records of flooding, say when cellar flooding began, or when fields began to flood along the Lavant watercourse? This would help build a much more detailed picture of groundwater behaviour around the village and its relationship to changing climate.

Dave Mattey

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Images & Text ©Dave Mattey MMXX