Mutation Analysis of Runoff–Sediment Combination and Profitable Frequency of Wetness–Dryness Encounter in the Middle Yellow River

By using the long series data of annual Runoff-Sediment (R-S) recorded at the Tongguan hydrological stations in the period 1961–2019 to clarify the variation rules in the mid & up reaches of Yellow River, the dynamic changes and the runoff–sediment relationship trend was analyzed. The findings were as follows: (1) The long sequence have been declining, with an abrupt discontinuity point in 2003 and a remarkable decrease, particularly beyond 2003. A more obviously reduced tendency in the sediment data is also observed. (2) The joint distribution of the R-S load changes significantly after a mutation of the R-S load relationship occurs. When the probability is lower than 90%, both the R-S load decrease, and the reduction in the sediment load is more obvious. (3) The analysis revealed that the probability of encountering an asynchronous runoff decrease and a sediment load increase is higher than that of having a synchronous runoff reduction and an increasing sediment load. The eigenvalues changed significantly at the design frequency. Detailed suggestions and basic data for the engineering design, as well as a flood mitigation and prevention design based on a joint distribution model are presented.