First underwater installation of a geomembrane system in a UK dam: Llyn Teifi spillway

Llyn Teifi reservoir, owned by Dŵr Cymru Welsh Water and used for water supply, is formed by a series of small embankment dams. One of these has a concrete gravity section acting as overflow, with adjoining draw-off tower and wing walls. The concrete of the spillway was in poor condition and leaking. In the interests of safety, to improve water tightness and stability, the designer MWH selected a geomembrane system that could be installed without dewatering or lowering the reservoir water level, due to water resource constraints. The system lines the upstream face of the spillway, the joint with the southern wing wall, and part of the draw-off tower. A drainage system behind the geomembrane allows the watertightness to be monitored. The challenges were difficult boundary conditions, uneven surfaces, difficult geometry involving multiple changes in inclination in restricted areas, complicated drainage discharge and poor underwater visibility. Waterproofing works started on 26 October 2016 and were completed on 1 December 2016, after completion of downstream concrete works to increase the spillway’s stability.

The paper outlines precedents and the design of the geomembrane system; addresses difficulties and measures adopted to overcome them; gives some installation details; and information on performance of geomembrane systems installed underwater on upstream faces, cracks and failing joints.