Developments in the design and construction of the Usk Reservoir spillway

Usk Reservoir in South Wales is a large water supply reservoir formed by a 31m high earth embankment dam with a clay core wall and a concrete cut-off wall through the foundation. The spillway comprises a free overflow weir, leading to a concrete chute and stilling basin. In 2014, approximately 60 years after construction, leakage was noted adjacent to the spillway chute. The mass concrete forming the chute and stilling basin had cracked and required refurbishment.

This paper contrasts the original design approach and construction of the spillway with the adopted solution developed through modern investigation, analysis, design and construction techniques. It describes how a combination of modern and traditional methods of investigation, analysis and design were used throughout the feasibility and detailed design phases of the project to arrive at the final design solution. A suite of geophysical surveys was used to investigate the condition of the structure and to understand the mechanisms of deterioration. This informed the risk and value considerations in developing the design approach. Computational fluid dynamics were used to refine the design solution and to improve the performance of the stilling basin. The project showcases how modern technology - unavailable to the original designers - can be applied to extend the operational life of existing assets and improve their technical performance in a cost-effective manner.