Cobb Dam Sinkholes

Author: Bruce Walpole (TrustPower Limited), Craig Scott (Riley Consultants Limited)

Presented/Published: NZSOLD/ANCOLD Conference, Queenstown, 2007.

Keywords: Dam Safety, Embankment, Sinkholes, Foundations, Dam Drainage, Geophysical Investigations.

Abstract:

Monitoring and surveillance is crucial to managing the ongoing performance of dam structures. The true value of appropriate monitoring, surveillance and review processes is only realised when potential dam safety issues arise. TrustPower’s civil safety monitoring and surveillance program includes nineteen hydro schemes throughout New Zealand and incorporates structures with Potential Impact Classifications (PIC) ranging from Low to High.

TrustPower promotes a continual improvement policy on its management of safety issues and conducts inspections on a regular basis. Routine and periodic independent inspections of the key components within a scheme are paramount to the viability of the safety management system. The importance and purpose of these inspections has recently been highlighted by the discovery of two sinkholes on the face of the earth dam associated with the Cobb hydro electric power scheme.

This paper provides an example of the need for continual monitoring and surveillance, vigilance of observations, good archiving systems and documentation. It discusses the broader issues surrounding the subsequent response processes to potential dam safety deficiencies, and the success (or otherwise) of investigative methods. It also highlights that an adequate dam safety compliance system has commercial value as there is a measurable reduction in dam performance uncertainty and hence greater efficiency in the speed at which accurate resolutions can be drawn.