Small-scale sediment scouring and siltation laws in the evolution trends of fluvial facies in the Ningxia Plain Reaches of the Yellow River (NPRYR)

Abstract

This study presents an analysis of variable sediment erosion and deposition, and their the complex mechanism in natural heavy silt-carrying rivers in the Ningxia Plain Reaches of the Yellow River (NPRYR). The aims of this study are to examine the sediment scouring and siltation laws, along with the evolution trend of fluvial facies in the NPRYR. Four small-scale typical reaches (R1, R2, R3, and R4) were divided firstly based on river regimes and geological conditions of the river beds. And then with ground-truth data from 1993 to 2015, a cross-section morphology method was used to calculate the average fluvial facies coefficient ζa of the typical river reaches, and also the sediment siltation weight T during the different time periods. In this study, with application of mathematical statistics and regression analysis, the main channel, beach, and whole reach were used as the basic units in order to identify the relationships of the responses among the suspended sediment concentration per unit discharge (ξ), sediment deposition per unit water weight (W), and variations of the fluvial facies coefficient (△ζa) in different units. This analysis is for establishing the prediction method for the sediment erosion and deposition, along with the evolution trends of fluvial facies. Our results show a logarithmic mapping relationship between the ξ and W, and significant correlation between T and △ζa. Therefore, a quick analysis and prediction of the silt sedimentation, and the trends of fluvial facies variations in the small-scale river reaches under a single water and sediment condition are feasible with our approaches presented here. The critical incoming sediment coefficient ξcs for the scouring and siltation balance in the R1 to R3 reaches were 0.0051 kg·s·m−6, 0.0055 kg·s·m−6, and 0.0049 kg·s·m−6, respectively, while the R4 reach shows an overall continuous siltation trend. The fluvial facies coefficient of the R1 to R4 reaches show an overall decreasing trend. Our results provided robust theoretical support for analyzing the sediment erosion and siltation, and the river regime evolution in the NPRYR. Our study also indicates a need for future research with an aim to further understand the evolution of erosion and siltation in heavy silt-carrying rivers.