Hydrological safety criteria for existing dams

Hydrological Safety is one of the essential points in the field of Darn Safety. Experience and historical data show that overtopping is the most frequent cause of failure of dams, with almost 40% of the failures. So, in the last decades numerous countries have developed standards for the evaluation of the Design Flood which suppose substantial improvements and greater demands for the Hydrological Safety. This has given rise to the fact that the first generation criteria, in which the design Flood Selection was established on the basis of general and empirical concepts applicable to any dam in any situation, have evolved towards the second generation criteria in which the general principle is that of the Hydrological Safety of the dams should be a function of the downstream hazard. At the present rime, and in general, the basic criterion is that of classifying the dams in function of the downstream hazard, and demanding maximum safety in the high hazard darns, for example the PMF or high return periods (10,000 years). The application of these actual criteria supposes an important improvement for the design of new dams, but it also supposes a real problem in its application to the existing dams which were constructed with lesser Design Floods, and which in the greater part of the standards of different countries would oblige them to be upgraded for their accommodation within the new criteria. In this paper the philosophy of the hydrological safety and of the accommodation of the existing dams to the extreme floods are described. In general, the demands of hydrological Safety are the same for the new dams as for the existing dams, bur in many cases due to the fact that the necessary financial resources do not exist, these-criteria are very difficult to put into practice. For this motive, Organizations and Institutions in some countries are developing new criteria which permit the adoption of less arbitrary and more quantitative definitions of the downstream hazard with which the Selection of the Design Flood for the existing darns can be rationalized, and priorities given to the necessary requirements. The new criteria are based on the Risk Analysis and on the Socially Acceptable Risk Criteria. These new criteria could suppose for the high hazard dams lesser demands of Hydrological Safety than the existing standards and pen,lit some more discriminatory limits in the categories of the dams with potential loss of life downstream. For this reason, they represent a more rational concept for the adaptation of the existing clams and permit a new approach which makes more economically and socially viable the carrying out the necessary adaptations. Nevertheless, its legal application is not recognised in the majority of the countries and has only been established in the recommendations and guidelines of some few countries or organisations.