Following the club’s AGM on 18 January, member Nick Platt gave an entertaining short talk recalling his first job, as a junior site civil engineer at Clywedog dam during 1963-67. Work had hardly started when he arrived. Numerous old black & white photos showed a muddy Clywedog valley floor. Soon they were blasting out deep foundations for the dam and the buttresses which support it. Nick recalled how that was a time before surveying was supported by GPS and before calculations were done on a computer. Clywedog is the tallest concrete dam in Britain so everything was surveyed with great care and results laboriously calculated. An early task was to build a temporary bridge across the neck of the valley so work could be undertaken on both sides.
The river was supposed to flow around the workings through a 10 ft (3m) pipe specially laid for the purpose. Unfortunately, when it rained very hard one winter of construction , the laying of a single 10 ft diameter pipe, instead of a double pipe, proved to have been an inadvisable cost saving. The floods which Llanidloes town residents remember that year, are also remembered by workers on the washed out construction site! The dam is now one of the beauty spots of Mid Wales but in those days it caused offence to some minorities. Cranes provided just the target the Welsh Nationalists needed to try interrupt the work, but such was the hectic pace of activities that progress was only delayed a few weeks when one of the crane masts was blown up by activists.
After the talk, questions included the curious shape of the dam, bulging downstream from the reservoir instead of upstream into the reservoir. But Clywedog is a buttress dam, in which the water pressure is transferred to buttresses buried in the hard rock of the river downstream of the dam rather than outwards through the curvature to the softer valley sides.