Aberfan is in an area of relatively high rainfall, an average of a year. In 1960 it was , the heaviest of recent years in the run-up to the disaster. Between 1952 and 1965, there was severe flooding in the Pantglas area of Aberfan on at least 11 occasions. Residents complained that the flood water was black and left a greasy residue when it receded. Complaints had been made by residents to Merthyr Tydfil County Borough Council, who corresponded with the NCB between July 1963 and March 1964 on the topic of the "Danger from Coal Slurry being tipped at the rear of the Pantglas Schools". In early 1965 meetings were held between the council and the NCB at which the Board agreed to take action on the clogged pipes and drainage ditches that were the cause of the flooding. No action had been taken by October 1966 when the tip collapsed.