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Western Dam Engineering Technical Note August 2016 7 Instrumentation Monitoring: Piezometers: Monitoring pore pressures within the dam and foundation can provide useful information about potential changes in stress, development of cracks and internal erosion, and potential changes in uplift pressures. Surface Survey Monuments: Monitoring monuments along the crest of the dam can provide useful information about settlement of the dam or foundation and the development of slope movement over time. Modern GPS receivers make regular monitoring of surface movement less expensive and thus feasible for virtually any dam owner. Conduit Inspections: Remote conduit inspections can detect deterioration of the pipe and joints that may eventually lead to erosion of embankment material through the conduit. While all pipe has a finite life span, metal pipe and especially CMP has an even shorter lifespan, especially when placed in chemically reactive soils. The condition of the interior of a conduit is best inspected by directly viewing it. However, it may be possible to indirectly determine the trend of a pipe’s deterioration through regular chemical analysis of the outflow through the pipe. Maintenance Proactive maintenance of changing conditions observed through regular inspections can arrest several of the long-term deterioration processes. It is much less expensive to periodically address developing issues before they become a major incident. Periodic maintenance may include: • Backfilling animal burrows and root holes • Maintaining uniform crest elevations through grading and fill placement • Managing vegetation, especially large, woody vegetation both on the embankment and within earthcut spillways • Exercising gates and valves • Performing maintenance on instrumentation • Clearing debris that may block flood passage Risk-Informed Decision Making Because of the high cost of fixing or rebuilding dams, many owners are using risk analysis to prioritize what to fix first. Risk analysis is a systematic approach to estimating the likelihood of various failure modes progressing to a dam breach and coupling that likelihood with the consequences of the breach. Through this process, an owner, or regulator, can compare various deficiencies of a single dam, as well as various levels of risk posed across a portfolio of dams. Risk analysis begins with brainstorming all of the ways in which the dam could potentially fail—known as Potential Failure Modes (PFMs). The PFMs are then evaluated for the factors that influence the likelihood that the PFM will occur. These factors may include the likely loading the dam may experience, physical attributes of the dam and its foundation, operating procedures, past performance, and ability to detect a developing PFM and intervene before its full progression. Based on these factors, the PFMs can then be ranked or categorized to allow focus on the PFMs of most significance. Formal risk analyses have been used by the Bureau of Reclamation in the U.S. since the mid-1990s; and the risk analysis process continues to expand to other federal and state dam safety agencies, as well as private dam owners and engineers. Risk-informed decision making is a shift in process, and mindset, from a strictly “standards-based” or “criteria- based” process. In the standards-based process, dams are evaluated in regard to whether they meet minimum factors of safety or modern design criteria. All dam safety agencies and engineers still use this process. However, because so many aging dams do not meet today’s standards, it can be overwhelming to determine what is most important to fix. Risk analysis is a tool more and more dam safety engineers, owners, and regulators are using to supplement the standards- based requirements to accomplish the following objectives: • Identify potential deficiencies of dams and appurtenant structures • Prioritize a portfolio of dams or a suite of deficiencies at a given dam to decide which are the most important to fix • Understand the urgency of deficiencies • Identify effective risk reduction actions • Evaluate the effectiveness of rehabilitation alternatives Risk-based decision making does not replace modern standards-based criteria, but provides a means to