Abstract
Reservoirs operation alters the natural flow regime at downstream site and thus has a great impact on the design flood values. The general framework of flood regional composition and Equivalent Frequency Regional Composition (EFRC) method are currently used to calculate design floods at downstream site while considering the impact of the upstream reservoirs. However, this EFRC method deems perfect correlation between peak floods that occurred at one sub-basin and downstream site, which implicitly assumes that the rainfall and the land surface process are uniformly distributed for various sub-basins. In this study, the Conditional Expectation Regional Composition (CERC) method and Most Likely Regional Composition (MLRC) method based on copula function are proposed and developed under the flood regional composition framework. The proposed methods (i.e., CERC and MLRC) are tested and compared with the EFRC method in the Shuibuya-Geheyan-Gaobazhou cascade reservoirs located at Qingjiang River basin, a tributary of Yangtze River in China. Design flood values of the Gaobazhou reservoir site are estimated under the impact of upstream cascade reservoirs, respectively. Results show that design peak discharges at the Gaobazhou dam site have been significantly reduced due to the impact of upstream reservoir regulation. The EFRC method, not taking the actual dependence of floods occurred at various sub-basins into account; as a consequence, it yields an under-or overestimation of the risk that is associated with a given event in hydrological design. The proposed methods with stronger statistical basis can better capture the actual spatial correlation of flood events occurred at various sub-basins, and the estimated design flood values are more reasonable than the currently used EFRC method. The MLRC method is recommended for design flood estimation in the cascade reservoirs since its composition is unique and easy to implement.
Highlights
Design flood estimation is often required by engineers, hydrologists, and agriculturalists for the design of hydraulic structures, such as dams, bridges, or culverts
This study aims to improve the flood regional composition method for design flood estimation of cascade reservoirs
The Equivalent Frequency Regional Composition (EFRC) method is described, as following: (1) If design flood volume zp with probability p occurs at downstream site C and the equivalent frequency flood volume xp occurs at upstream reservoir A site, the corresponding flood volume at interval basin B is given by y = zp − xp
Summary
Design flood estimation is often required by engineers, hydrologists, and agriculturalists for the design of hydraulic structures, such as dams, bridges, or culverts. When sufficiently long observed flow records (the annual maximum peak discharge or flood volume series) at the interest site are available, the design flood values are generally estimated from flood frequency analysis. The main modeling problem in flood frequency analysis is the selection of a probability distribution for flood magnitudes coupled with the choice of parameter estimation. In the United States, the log-Pearson type III (LP3) distribution is recommended as the candidate distribution for defining the annual maximum flood series, and the method of moments is used to determine the statistical parameters [6]. The Pearson type III (P3) distribution and curve fitting procedure are recommended by the Ministry of Water Resources of China as a standard procedure for flood frequency analysis [3]
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