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significant; and how failure modes can be detected. However, it does provide clear advice on some of the behavioural features of significance to be considered in visual inspections and significant parameters to be monitored by instrumentation. 3. OPERATIONAL MODES AND FAILURE MODES At the outset, it is important to be clear as to the definition of a “Mode”, be it an operational mode or a failure mode by focusing first on the common use of the term mode: Mode: A state, condition, way of doing things (modus operandi), manner in which something occurs.  Common uses include “flight safe” mode; mode of transport (aircraft, car, railway, etc.) Mode of operation: A mode in which the operational states or objectives of the system manifest themselves.  In the context of dams, water retention and water conveyance are operational modes; at a more detailed level, the operational modes of a spillway gate system include a) dormant mode (during the dry season); in- service mode (closed to retain water or open to pass flows); and ready mode (just prior to initiation of gate movement). Failure Mode: The manner in which an element or component fails to achieve its operation (design) objective.  In the context of spillways, failure modes of a spillway gate include unintended closed state and unintended open state. Any owner, regardless of degree of sophistication can understand the operational modes of a dam being retention and conveyance when set out in terms of commonly understood terminology. With reference to well established experience in the nuclear industry, as well as first-hand experience of operational incidents at dams and reflecting on the Oroville Dam Spillway incident where the incident could not be attributed to a specific cause, it is proposed that attempting to define failure modes as sequences of events from initial cause to system end state should be avoided. Referring to the Fault Tree Handbook (Page II-1) [10] (Nuclear Regulatory Commission, 1981): “For systems that exhibit any degree of complexity (i.e., for most systems), attempts to identify all possible system hazards or all possible component failure modes-both singly and in combination-:-become simply impossible”. Concerning inductive techniques overall, such as Failure Modes Analysis and Event Tree Analysis, the Fault Tree Handbook points out that “due to constraints such as time, money and experienced staff, exhaustiveness in the analysis is a luxury that we cannot afford”. Thus, not only will it be impossible to 1556