Evolution Characteristics of Long Time Series of Secondary Perched River in Typical Reaches of the Lower Yellow River

Secondary perched rivers are extensively distributed in the lower section of the Yellow River, and their condition is grave, representing a significant peril to the flood control safety of this region. Consequently, conducting an analysis of their evolution characteristics holds immense engineering importance for ensuring the flood control safety of the lower reaches of the Yellow River. This study focuses on the downstream section of the Yellow River, specifically from Dongbatou-Taochengpu. This research is based on extensive data, including topographic measurements of large cross-sections and water and sand data from Huayuankou spanning from 1960 to 2022. The transverse slope of the beach, which indicates the level of development of the secondary perched rivers, was chosen as the calculation index. To analyze the trend and mutation of the transverse slope, statistical methods such as the Theil–Sen slope estimation, Mann–Kendall test, Pettitt test, and double cumulative curve method were employed. The findings indicate that the average transverse slope along the wandering section (Dongbatou-Gaocun) is 5.81‱, is significantly lower compared to the transitional section (Gaocun-Taochengpu), with an average transverse slope of 8.89‱. Furthermore, the range of fluctuation in the variation in the transverse slope along the wandering section (3.19–8.18‱) is considerably narrower than that observed in the transitional section (2.94–19.51‱). Prior to the implementation of Xiaolangdi, there was a significant increase in the transverse slope. Notably, the transitional section experienced a sudden change in 1975, while the wandering section experienced a sudden change in 1990. The abrupt alteration in the transitional section can be attributed to the substantial variation in the water and sand conditions. Conversely, the sudden change in the wandering section resulted from the insufficient flow rate of the flat beach. However, following the implementation of Xiaolangdi, the rapid increase in the transverse slope was effectively mitigated.