K. EMAMI - Chairman of the Board and President, Kurit Kara Engineers, Tehran, Iran

This paper describes the crucial importance of balanced and rational criteria for selection of the design flood for large dams to developing countries (where the main risk from dam failure may be from floods) and to the world as a whole from the viewpoint of both safety and economy. This is followed by a summary of key problems that arise in developing and establishing such criteria. Solution of these problems will above all require a holistic approach and dependence on risk based approaches and recognition of effectiveness and adaptability of nonstructural risk reduction measures. When plausible threats are examined with a broad view of possible ways to detect problems or warn the downstream population, then risk reduction may be accomplished with a holistic, balanced, and, therefore, cost-effective approach (As tragically indicated by the great Tsunami of Dec. 2004 with a death toll of more than 200000). Accordingly it appears that in many developing countries faced with grave social and economic challenges, the structural safety should be determined by Economic Risk Analysis. Minimizing the residual risk should be achieved by efficient nonstructural approaches and by increased structural resistance by low cost means (such as overtopping protection fuse plugs, fuse gates, unlined spillways, etc.). Considering the dynamics of numerous factors involved in dam safety, and the potential enhanced use and effectiveness of nonstructural options in coming years, the paper concludes that the overall risk accepted should be based on a shorter time step rather than 100 years commonly assumed for the life cycle of a dam (incremental risk acceptance). For example for a large and important project a total risk of 10-3 during its life cycle may be accepted. If a time step of 20 years is used, it means that the annual acceptable risk would be five times the one associated with the life cycle of 100 years. After 20 years, based on the latest information and technologies and hydrosystem and structure feedback, the most appropriate options for minimizing the overall risk in the next time step can be adopted.

Hydrological safety, Design flood, Dam safety, Criteria, Risk analysis, Spillway