*4.1. Adjustment of Cross-Sectional Shape and Distribution of Water and Sediment Factors before and after Construction*

Table 7 shows the comparison results of the asymmetric indexes of river section shape, velocity, sediment concentration, suspended sediment composition, and sediment-carrying capacity along with the transverse distribution before and after the construction project. (a) The asymmetry of each factor of the section at the bend top of the river channel before and after the construction project was significantly greater than that of the transition section. (b) compared with that before the construction project, the asymmetry of the river crosssection shape and water and sediment factors after the construction project was significantly increased, which made the river cross-section flow more concentrated and the river regime more stable. (c) Compared with that before the construction project, the overall increase in cross-sectional shape and asymmetry index of the river channel at the top of the bend after the construction project was 14–30%, and the overall increase in the cross-section shape and asymmetry index of the river channel at the transition section was 6–20%.


**Table 7.** Comparison of asymmetry indexes of various factors before and after construction.

## *4.2. Sediment Transport Capacity of River Channel before and after Construction*

Comparative analysis of Tables 2 and 6 revealed that in a large flow (5000 m3/s), regardless of the scouring and silting state of the river (i.e., scouring, silting, or scouring and silting equilibrium state), the sediment-carrying capacity of the river section was improved to a certain extent after the construction project. Taking the scouring and silting balance state of the river as an example, the average sediment-carrying capacity of a typical section of a downstream swing river without engineering constraints is 23 kg/m<sup>3</sup> , and after construction, the average sediment-carrying capacity of a typical section of a wandering river channel was 32 kg/m<sup>3</sup> , which showed that the construction of river regulation works did improve the sediment transport capacity of the river.

Tables 8 and 9 showed the small flow before and after construction (flow: 800 m3/s). The results showed that the sediment-carrying capacity of the river section slightly changed after the construction of the project, indicating that the construction of the river regulation project did not significantly improve the sediment-carrying capacity of the river channel at small flows.


**Table 8.** Verification of scouring and silting state of each section before the construction project (800 m3/s).

**Table 9.** Verification of scouring and silting state of each section after the construction project (800 m3/s).


## *4.3. Effect of Asymmetric Distribution of Water and Sediment Factors on River Bend Creep*

In natural rivers, the flow structure that affects and is affected by the riverbed form is often complex. In addition to the longitudinal flow, bend circulation occurs. Bend circulation is closely related to the transverse evolution of the riverbed and transverse sediment transport. The research showed that after the construction of the river regulation project, under the action of the bend circulation, the cross-sectional shape and the nonuniformity and asymmetry of the distribution of water and sediment factors at the bend were more prominent than those in the transition section, which improved the sediment transport capacity of the river to a certain extent. This is because, at the bend section, circulation is an important factor that promotes sediment transport on the concave bank. The circulation exerts a downward force on the concave bank to move the sediment from the concave bank to the convex bank. Under the influence of bend circulation along the horizontal axis, the surface flow with less sediment is inserted into the bottom of the concave bank riverbed, resulting in scouring of the riverbed; in the bottom layer with more sediment, the flow rises to the surface of the convex bank, resulting in siltation of the convex bank riverbed. Over time, under the influence of the bend circulation, the concave bank gradually retreats, and the convex bank extends forward and silts year by year. The whole bend showed a trend of moving slowly downstream, and the plane form showed that the river bend is creeping slowly.

#### **5. Conclusions**

(1) The construction of river regulation works had a stronger constraint on water flow and river regime, particularly due to the increase in the density of river regulation works. By comparing and analyzing the adjustment of river cross-section and water and sediment factor distribution before and after the construction project, it was confirmed that the construction of a river regulation project could enhance the asymmetry of the river cross-section and water and sediment factor distribution, making river flow more concentrated and river regime more stable.

(2) Compared with the river bend during the free development period, the asymmetry of the river cross-section and water sediment factor distribution after the construction project increased by 6–30% as a whole at the same flow (5000 m3/s). When the erosion and deposition of the lower channel were balanced, the sediment-carrying capacity of the flow increased by 39%, and the sediment transport capacity of the channel increased significantly.

(3) Compared with no engineering constraints, the asymmetry of the cross-sectional shape of the river under the action of the limited control boundary was stronger, which could block the sediment supply on one side of the river regulation project and easily cause a sudden change (significant increase) in the sediment gradient along the river at the junction of the soft and hard boundary of the river, resulting in a decline in the river regime.

**Author Contributions:** Conceptualization, E.J.; methodology, L.Z.; validation, L.Z.; formal analysis, L.X. and M.Z.; investigation, W.Z.; resources, E.J. and J.L.; data curation, L.X.; writing—original draft preparation, L.X.; writing—review and editing, L.X. and E.J.; visualization, W.Z.; supervision, E.J.; project administration, J.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by This research was funded by [Special Project of National Natural Science Fund], grant number [42041004,42041006], [Key Project of National Natural Science Fund], grant number [51539004], [National Natural Science Foundation of China], grant number [51809106], [Special Project of Basic Scientific Research Business Expenses of Central Public Welfare Research Institutes], grant number [HKY-JBYW-2018-03, HKY-JBYW-2020-15].

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** We would like to thank the potential reviewer very much for their valuable comments and suggestions. We also thank my other colleagues' valuable comments and suggestions that have helped improve the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

