Mutability and Periodicity of the Characteristic Flow Level of Bifurcated Reaches in the Middle Reaches of the Yangtze River

Abstract

Systematic analysis of the riverbed evolution regularity in the bifurcated reaches in the middle reaches of the Yangtze River (MRYR) found that under different flow levels, the main stream line usually locates at three main plane positions, which has obvious influence on riverbed evolution. The flow levels corresponding to specific riverbed locations, called the characteristic flow levels, should be divided into three grades: initial migration discharge, floodplain discharge, and translocation discharge. The river evolution has a close relationship with the duration of the characteristic flow level (DCF) acting on the corresponding riverbed position. When the DCF is longer than the critical value, the corresponding riverbed position will suddenly deform. Wavelet analysis shows that, taking the duration series of characteristic flow level as an example, the sudden change year when the river evolution and adjustment occurred suddenly is consistent with the peak years of the wavelet transform coefficients’ real parts of this sample; because the sample is periodic, the corresponding riverbed deformation also is periodic. Before the operation of the Three Gorges reservoir (TGR), the main cycle of the DCF from initial migration discharge to floodplain discharge (FD) was 4–16 years, the cycle from FD to translocation discharge (TD) was 5–16 years, and the cycle above TD was 4–14 years. Influenced by the operation of TGR, the main cycle of the three series had variation tendencies of extension, reduction, and extension, respectively. This study fills the gap of the dynamic mechanism between the runoff process and the riverbed evolution process, and has great significance for predicting river evolution trends and scientifically governing river channels.