A two-stage method of quantitative flood risk analysis for reservoir real-time operation using ensemble-based hydrologic forecasts

Quantitative analysis of the risk for reservoir real-time operation is a hard task owing to the difficulty of accurate description of inflow uncertainties. The ensemble-based hydrologic forecasts directly depict the inflows not only the marginal distributions but also their persistence via scenarios. This motivates us to analyze the reservoir real-time operating risk with ensemble-based hydrologic forecasts as inputs. A method is developed by using the forecast horizon point to divide the future time into two stages, the forecast lead-time and the unpredicted time. The risk within the forecast lead-time is computed based on counting the failure number of forecast scenarios, and the risk in the unpredicted time is estimated using the reservoir routing with the design flood hydrographs. As a result, a two-stage risk analysis method is set up to quantify the entire flood risks by defining the ratio of the number of failure scenarios (excessive the critical value) to the total scenarios number. The China's Three Gorges Reservoir is selected as a case study, where the parameter and precipitation uncertainties are conducted to produce ensemble-based hydrologic forecasts. Two reservoir operation schemes, the historical operated and scenario optimization, are evaluated with the flood risks and hydropower profits analysis. The derived risk, which units with yearly scale, associates with the flood protection standards (described with return periods) that can be used as the acceptable level to operate reservoir. With the 2010 flood, it is found that the proposed method can greatly improve the hydropower generation with acceptable flood risks.