DAMS AND FLOOD CONTROL Carlos E. M. Tucci Institute of Hydraulic Research University Federal of Rio Grande do Sul Porto Alegre – RS Brazil Tucci@if.ufrgs.br A bstract: Floods are a natural process in which the river spills over its banks. Hazards occur when the population occupy these areas during a sequence of small floods. In South America flood impacts have occurred mainly due to the lack of flood plain management. Flood hazards in some communities on the banks of major rivers have had major social and economic impacts which can be seen many years after the event. There is an explicit conflict between dams for energy production and their use to for flood control. In the Brazilian electricity sector, these dams have been operated since the end of the 1970s, with a waiting volume that aims not only at improving their safety but also at reducing the impact of floods. In this article, the potential impacts of dams are stressed, with operational features for flood control and the description of a conflict generated between a hydropower dam and the community of two cities. FLOODS Floods may occur due to the natural conditions or those generated by land use, such as urbanization or hydraulic structures. In this article we will discuss the natural floods and those related to dams for flood control or other Water Resources uses. The main flood in the regions occur on the banks of the major rivers of South America such as the Prata/ Paraná, Amazon, São Francisco and Tocantins. In Figure 1 the basin of the Prata river can be seen with the main reaches where flooding occurs and there are dams (hydropower dams). In the Paraguay river basin and in the lower reach of the Paraná river, floods are the result of prolonged rainy periods which affect large areas and produce flood levels for several months. In the reach involving Argentina, the areas are flat and usually flooded for several months. In the upper part of the Paraná river and in the Uruguay river floods are of short duration lasting only a few days. Flooding is a natural process in which the river floods its first bottom depending on the intensity and duration of floods and initial humidity conditions. When changes occur in the river, such as dam construction, flooding levels may change as a result of operational conditions in the dam. The main impacts on the population occur due to lack of: (i) knowledge regarding the occurrence of flood levels, and (ii) planning for space occupancy according to the risks of flood events. Levels can be forecasted for the short term (real time forecasting) or for the long term that is the statistical forecast or risk of occurrence of a given flood. The first type allows flood warning a few hours or even days in advance, reducing the resulting damages. The second type allows planning the occupancy of spaces at risk, or building protection works.