Probable Maximum Flood Determination Method for Large Watersheds; Case Study Applied to Sobradinho Hydropower Plant in The São Francisco River, Brazil

The definition of the Probable Maximum Flood is an essential parameter for major hydraulic projects, as it involves critical decisions regarding safety and cost. The PMF is defined as the conversion of severe meteorological events into a flow and can be characterized as the physical limit of the flow in the basin.

The methods to determine the PMF are deterministic and endorse the establishment of a Probable Maximum Precipitation and its conversion into flow. The combination of this method with the probabilistic analysis of the frequency of hydrological variables, presented in this study, has achieved promising results. Its proposition establishes the use of observed hydrological data and physical limits of precipitation; this approach is recommended to be used in large watersheds.

The present study makes the estimation and validation of the PMF at Sobradinho HPP, the study is part of a project carried out by RHA Engenharia e Consultoria SS Ltda. and commissioned by CHESF. The case study area is located in the São Francisco River Basin, covering 506,000 km². São Francisco River has a strategic role in the Brazilian development, it has nine hydroelectric plants with an installed capacity of 10,367 MW, and is one of the main sources of water for irrigation and human supply in the Northeast Region of Brazil.

The study divided the basin into three sub-basins – Três Marias, Morpará and Sobradinho - and estimated the Probable Maximum Precipitation for each of the sub-basins. The PMP is obtained using the adapted Hershfield method (BURGER, 2014), despite being a statistical method of obtaining PMP, it takes the characteristics of the basin into account, therefore presenting more accurate results. With the PMP data, the spatial distribution started in one or more sub-basins simultaneously, because in large watersheds the consideration of a constant PMP is unrealistic. Hence, the spatial variability of precipitation must be considered to achieve more realistic results. In addition to the distributed precipitation, incremental flow values were added with a given return period. These combinations generated several scenarios, resulting in a data envelopment for the PMF in Sobradinho. This approach permitted the obtaining of a PMF consistent with the hydrological and meteorological characteristics of the basin. In order to validate the results of this study, they were compared with the project design flood. The study resulted in a method to be used in the São Francisco watershed, being suitable for use in other large watersheds.