Hydrological Risk of Artificial Floods Associated To Dam Operations

Past experience and statistical data show that extreme floods are an important factor for risk assessment of dams. In the last century, the methods to assess the hydrolog- ical safety of dams have considered principally the analysis of dam-break scenarios. For instance, one records four major reservoir disasters in Italy; the first one (1923) was due to structural dam failure immediately after dam completion, the second one (1935) occurred because dam overtopping due to a flood largely exceeding spillway capacity, the third one (1963) was produced by the largest recorded landslide into a reservoir without dam failure, and the fourth one (1985) was caused by poor mainte- nance of a small fill dam. Yet in the last years, dam failures have been significantly reduced. Berga (1998) reported that the percentage of failures before 1950 was 2.3%, while for dams constructed from 1951 to 1982 it reduced to 0.2%, and since 1982 is only 0.09%. This considerable reduction indicates that a significant progress has been achieved in dam safety. On the other hand, there is an increasing need in modern risk- averse societies to reduce the vulnerability of downstream river systems. This includes economic damage to properties and infrastructures, priority of human life protection, and environmental damages. Therefore new criteria to evaluate the hydrological safety of dams are required also considering the effects of artificial floods generated by dam gate operation. In several countries, the major hazard is indeed not that associated with potential dam-breach, but it deals with the artificial floods produced by dam gate op- eration in both emergency and maintenance modes. The increase of anthropization of riparian areas downstream dams constructed in the past has augmented the potential effects of such operations. In this respect, one can note that emergency gates of the dams may release discharges largely exceeding natural extreme floods. The role of reservoir operation during major floods has been deeply questioned for a number of destructive floods in recent years, because of the possible enhancement of flood sever- ity due to emergency gate operation. In the present paper, we compare the "Design Flood" of 551 dams located in Italy with the potential flood which could be artificially generated through dam openings operation. This indicates that artificial floods must be included in hydrological risk assessment of dams. Accordingly, we introduce new criteria for dam classification under hazardous artificial floods.