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I I I I I I I I I I I I I I I I II I i I \ I 1. INTRODUCTION 1.1. Objective. The objective of these Guidelines is to provide a basis for consistency throughout Australia in the assessment of the hydrologic safety of dams, having regard to the consequences of failure as a result of a flood, and within the context of overall risk assessment, integrated with the traditional deterministic approach. These guidelines supersede the 1986 ANCOLD Guidelines on Design floods for Dams in accordance with the ANCOLD aim for integration of risk assessment into guidelines. They include mllior changes from the 1986 Guidelines which provided prescriptive standards for spillway design flood, ranked with consequences, but in isolation from other dam safety aspects. The background to these guidelines is given in Appendix 2, Commentary, sub-section A2.L It is important to note that the risk procedure does not provide a specific solution for an Acceptable Flood Capacity (AFC) and spillway provisions. Instead, it provides options for consideration, in parallel with traditional standards within the total asset risk management context. In the 1986 guidelines recommended design floods were proposed for adoption. The risk process now requires the owner, or other decisiomnaker, to take the responsibility to set the risk management criteria and then to make the decisions on the overall management, political and legal issues, using the basic information provided by risk and traditional studies, The risk criteria and risk reduction options proposed by the owner will be subject to regulations and approval of regulatory authorities in States where those apply, and should be made transparent to the community at risk. An important point reiterated in this paper is that hydrologic safety should be assessed ,vithin the total load context, and not as a separate case. This is essential to identify all . critical safety issues for prioritisation in the development of an optimum solution for an appropriate safety level. 1.2. Scope. These guidelines have been prepared for application by dam engineering practitioners, and guidance of owners. They relate specifically to the selection of an Acceptable Flood Capacity (AFC) and associated spillway provisions for all proposed and existing dams that pose a potential threat to the community; they are' not restricted to dams within the Referable Dams classification. Such structures would be expected to include all potentially hazardous stmctures which are designed to store water, or other liquids, including flood retarding basins, service basins and tailings dams, but not in general levees and road and railway embankments. The major revisions to the ANCOLD Guidelines on Design Floods for Dams, 1986, are summarized in section 2, They take into consideration that the application and methodology of risk assessment and criteria are still evolving from the ANCOLD Guidelines on Risk Assessment 1994 (RA94), and the Position Paper on Revised Criteria for Acceptable Risk to Life, August 1998 (ANCOLD, 1998), These guidelines relate specifically to flood safety, and do not repeat detailed procedures and general references from complementary and associated, guidelines. It is therefore important that these guidelines are read in conjunction with the current ANCOLD Guidelines on Risk Assessment and ANCOLD Guidelines on Assessment of the Consequences of Dam Failure. It is noted that the traditional deterministic approach is used in many countries either as the sole basis, or in conjunction with a risk approach. Traditional standards are also assessed as part of the risk process. The guidelines therefore include references to deterministic standards, as well as the probabilistic risk procedures, Key sections ANCOLD Guidelines on Selection of an Acceptable flood Capacity for Dams 1