Mark Davies was one of the eyewitnesses, as water cascaded over the wall of a car park after debris blocked a culvert.
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The 4 day event in Portmeirion has developed into one of the UK's best regarded small festivals.
More than 30,000 bees as well as 3 queen bees were stolen from a farm in Anglesey last month.
The Cob to Cardiff Challenge is part of 10 year celebrations at the Wales Millennium Stadium in Cardiff Bay.