A comprehensive Operations, Maintenance, and Surveillance (OMS) Manual, consistent with Canadian Dam Association (CDA) guidelines, has been developed for the Site C Clean Energy Project. A key component is the suite of Surveillance Response Plans (SRP), which define stakeholder actions when instrumentation readings reach certain Alert Levels. Establishing technically justified alert levels that are coherent with the expected behaviour of the dam is essential to ensure timely and proportionate responses to changes in performance. This paper presents the development and application of an advanced numerical model used to define alert levels for the Site C powerhouse buttress. The model integrates the roller-compacted concrete (RCC) buttress with the overlying generation structures and explicitly represents movement joints to capture realistic deformation and load transfer within the system. It also incorporates the large-diameter bored piles installed as part of the Right Bank Foundation Enhancements (RBFE). An interface shear strength reduction approach is used to simulate slip along a credible failure surface beneath the buttress, generating displacement fields representative of expected deformation modes. The corresponding increase in pile demand is used to define thresholds based on shear capacity: steel-only resistance for the Alert Level 1 and combined steel–concrete resistance for Alert Level 2. Model results are then used to extract predicted displacements at instrument locations—including inclinometers, extensometers, pendulums, and tiltmeters—allowing direct comparison with field measurements. This methodology provides a rational basis for defining alert levels within the OMS Manual and supports the interpretation of monitoring data to enable timely, risk-informed responses consistent with dam safety practice.