Case study: Risk analysis by overtopping of diversion works during dam construction: The La Yesca hydroelectric project, Mexico

A risk analysis-based methodology for the determination of the most economical layout dam–tunnel diversion works is introduced. The aim of the proposed procedure is to identify the least cost layout in terms of the diversion works overtopping risk. The methodology has been built upon the reliability theory advanced first-order second moment approach, and accounts for the probability of the maximum height reached by the upstream water elevation, associated with a design flood (as characterized by its return period), as well as for excavation and lining costs. The proposed procedure has been applied to the La Yesca hydroelectric project in Mexico, currently under operation. It is demonstrated that the use of composite roughness, which consists of lining the floor of the diversion tunnels with hydraulic concrete, while the walls and vault of the tunnels are lined with shotcrete, results in an increase in the discharge capacity of the tunnels, thus leading to a reduction of the overall risk of the project. The importance of economic risk assessments is emphasized and the flexibility of the proposed methodology to account for a suite of risk–cost combinations is shown.