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Hydrological frequency calculation method of inconsistent flood series based on wavelet analysis

Massive embankments construction on the West River basin has leaded to flood returning to main channel not only, but caused significant changes also in flood channel storage relationship of the river, and hence the flood sequences for risk assessment of flood disaster after this project must be inconsistent with or different from those for flood control planning. The previous studies of flood returning to main channel focus on a recovery computation from the channel overflowing state back to the original one through modification. They lack a sufficient consideration of the encountering of mainstream and tributary floods and the interval flood, and a systematic approach to the causes of flood variation and the frequency analysis method of inconsistency flood. This article presents a frequency analysis method of such inconsistent floods based on wavelet analysis for calculation of flood frequency distribution in a river, with a case study of the Wuzhou station of the West River. First, by assumption of an inconsistent annual maximum discharge series, this method decomposes the flood series into a relatively consistent random component and an inconsistent deterministic component. Second, it fits the trend components with several nonlinear functions(power function, logarithmic function, index function, polynomial function, etc.) to obtain the consistency and certainty of the flood in time domain. Third, it determines the randomness of the flood sequence in frequency domain by fitting the frequency distribution of random component with a P-Ⅲfrequency curve. Forth, it predicts the deterministic component and the design value of random components with a distribution synthesis technique, extracts the random component by the theory of decomposition and composition in time series analysis, and finishes the frequency analysis with a constrained and weighted fitting method. The last step is to synthesize the deterministic forecasting value and stochastic design value and thus obtain the past and present frequency distributions and a forecast of the future distributions of the annual maximum discharge series.