Lluest-wen Reservoir

In December 1969 there was a significant emergency when a local man discovered a large hole, measuring 5 ft (1.5 m) deep and 6 ft (1.8 m) long, after he and his horse fell into the crevice. The man alerted the emergency services who were able to free his horse and alerted the water department who sent experts to inspect the dam.[2] The officials discovered a large crack in a pipe used to drain water from the drawoff shaft,[4] which led to the evacuation of 350 houses, 7 schools and the relocation of miners from Maerdy Colliery.[2] 150 elderly residents were evacuated to higher ground and around 2,000 people also fled the valley.[6] As the reservoir did not feature any system to drain water which are utilised in more modern structures,[3] in order to relieve the pressure on the damaged area, a combined effort from the army, Royal Air Force (RAF), local volunteers and the fire brigade was required to divert water away from the reservoir with the assistance of an RAF helicopter. An estimated 200–300 workers were utilised in the efforts.[2]

To undertake repair work, the reservoir was drained by as much as 30 feet (9.1 m), leaving it at less than a third of its usual capacity.[2] The workers were hampered in their efforts by the poor access to the site that delayed the arrival of heavy water pumping equipment.[6] The work cost an estimated £1.5 million.[2] More than 450 tons of cement and 80 tons of bentonite were injected into the cores of the embankment during the repair work.[4] The dam was given the all clear by the end of January 1970,[2] although further work was carried out in the following months.[4] As part of the dam's new safety certificate, water was limited to 30 ft below the top watermark, the same level that the dam had been lowered to in order to avoid bursting.[7]

As part of an investigation into the incident, substantial work was carried out on the reservoir to ensure it met the current safety standards. This included reinforcing the embankments with a 600 mm concrete barrier and the installation of new drain-off pipework (nearly double the size of the original). The incident played a part in the introduction of the 1975 Reservoirs Act that required supervising engineers to monitor large, raised reservoirs.[4] The Secretary of State for Wales, George Thomas, 1st Viscount Tonypandy, ordered an inspection of all reservoirs in Wales over a certain age as a result,[8] also declaring that "lives have been saved" by the emergency work carried out.[7]

Two minor leaks were discovered at the end of the 20th century, although these were deemed to be of little threat to the structural integrity of the dam.[4]