Dr George Findlay, Cardiff and Vale University Health Board's Clinical Board Director for Children and Women, told ITV News that between six and 12 inches of water had flooded the top two floors of the Children's Hospital for Wales - and some ceiling tiles had come down with the force of the water.
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Local authorities said the move is necessary to "avert the growing number of deaths and increasing harm linked to psychoactive substances".
Sunny and bright spells for most of us this afternoon, feeling pleasantly warm.
North Wales Fire and Rescue Service say incidents of deliberate fires are more common during the school holidays.