Crondall Friday Flash Flood
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25 July 2014
Extremely High Rain Event in Crondall 25th July 2014
(The Friday Flash Flood)
Summary
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At 14:28 on Friday 25 July 2014, a major storm crossed Crondall depositing over 1 month’s rain
(30mm) within 32 minutes. This caused some local flooding to a few houses, but also revealed some
weakness in the warning and drainage systems that need to be rectified before the winter rains, in
order to minimise and mitigate the floods risk that were so prevalent during the winter of 2013/14.
Weather
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According to the Crondall Weather station 30mm of rain fell within 1hr (data at 1 minute intervals is
available if required). Earlier suggestions that
60mm had fallen were erroneous and attributed
to a software problem in the weather monitor
caused by a suspected lightning strike at that
location. It is also noted that the storm crossed
southern England and caused damage over a
wider area than just Crondall.
Separately, lightening stuck a house off Church
Street causing considerable damage (as was
reported in the local papers).
Impact
The rainfall quickly pooled and caused minor local flooding (10cm of water was reported to have
collected on people’s patios). On several roads the water was unable to drain quickly enough and
deepened.
The Borough
The water flowed down Dippenhall Street and Church Street which collected in The Borough. Eye
witnesses report that the water in The Borough rose to ~30cm depth within 10-15 minutes. This
flowed into the gardens of houses on the lower-side of the street. A few managed to quickly fit their
flood guards and the impact was reduced by the quick actions of some local builders who warned
houses at risk. At least 2 houses were not able to install its FloodGuard and one was flooded to a
depth of about 12cm. The water drained almost as quickly as it has arrived and within about 10
minutes of the peak level, the street was only described as “wet”.
River
The Environment Agency depth monitors on the
river Hart showed how the level rose very
quickly as much of the land-water rushed off the
surrounding surfaces and into the Hart (and
associated feeder streams around the village).
Residents reported that it rose to the top of the
defined banks, but quickly flowed away.
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crondallweather.co.uk is a small but high-end weather station that also provides input data to the Met Office.
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1 August 2014
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Crondall Friday Flash Flood
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25 July 2014
Pankridge Street
A typical scene in Pankridge Street is shown below at the junction with Handcroft Close (outside the
Thames Water pumping station). The water has reached the level of the curbs (~10cm). The speed
of many passing vehicles was not reduced sending sheets of water onto the surrounding gardens.
Sewers
Some over-filling of the sewer was noted (evidenced by air backing up through sinks and toilets). But
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there were no reported instances of sewer related flooding.
Debris and Drains
Debris was swept down into several drains by the rapid rise in surface water. This was compounded
by some work on a hedge in Dippenhall Street, with clippings blocking the drain. Some of this can’t be
mitigated, as the cloud burst was so sudden and unexpected. But other drains failed to cope with the
water flow and there is a lingering suspicion that they have not been properly and regularly cleaned
out. Action over the late summer by HCC has cleared out many of the street drains previously ignored
by poorly-informed or inadequately tasked contractors. (Some will only do the drains directly on the
street, while surface water ignores such conventions and runs along the pavements, where the
additional drains have not been cleared!)
Power
There were brief power outages during this storm. This impacted the pumping of the sewers, but it is
not known how long the pumps were off-line.
Issues
Summer storms such as this are recognised as a very different type of event compared to sustained
winter rain. Some exceptional flow of water along streets and a modest inability of drainage systems
to cope are usual. However, the Friday Flash Flood has exposed some other issues, namely: No EA warning
 Drains
 Moderating traffic speed during such circumstances
 Reduce paved areas
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Given the sensitivity of the issue it would be wrong to categorically state that there were none, but to date no
residents have raised the issue, while they have been encouraged to report their experiences.
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