*4.2. Deposition Distribution*

The simulated deposition at the end of the 30 years is presented in Table 5. From the numerical modeling, it can be found that the section from Wangjiaba to Sanhejian is dominated by erosion, with an amount of 1.05 million m<sup>3</sup> . The section from Runheji to Wangji shows an equilibrium state. The other sections in the middle reach of the Huai River have a major deposition with an amount of 7.01 million m<sup>3</sup> of sediment, which accounts for 74% of the total deposition. In the transverse direction, all the sections have erosion in the main channel and deposition along the bank. The average erosion depth is around 30 cm, while the deposition height is 15 cm. In Scenario 2, with repetitive series, the main reach of the Huai River shows erosion in the upper reach and deposition in the downstream of Sanhejian station. Further analysis indicates that the upstream has a larger reduction in deposition, especially from Wangjiaba station to Sanhejian station. The middle reach of the Huai River shows dominated erosion behavior in the main channel under the continuous series of Scenario 1. Although the changes are not significant, the impacts of incoming sediment are very clear. Overall, the main reach of the Huai River shows increasing erosion and decreasing deposition from the simulation.



### *4.3. Recommendations*

The recommendations for flooding mitigation in the middle reach of the Huai River are presented. The Wanglin section is used as the test site. For flooding mitigation, it is important to maintain stabilization of the bank. From the simulation, it is found that the overall trend of the Huai River is erosion. Some recommendations are provided, as follows:

(1) Main channel dredging

The simulation indicates that the Wanglin section will be dominated by erosion. However, the main channel in this section is around 250 m in width. The flow capacity in the main channel is around 1500 m3/s, which is less than 1/5 of the bankfull value. The main channel is relatively small, which presents a larger threat to overland flooding. Dredging the main channel is thus recommended. The present study indicates that dredging the channel is preferable to widening the main channel.

(2) Floodplain protection

The beach bank along this section is close to 1500 m in length and has many functions, including water drainage, water storage, and sediment deposition. From the simulation, less incoming sediment results in a reduction in sediment deposition, which may impact

the stability of the banks. The recommendation is to construct bank protection to mitigate the threat of bank failure. Additionally, sediment mining should be well-managed to avoid the potential impacts on bank stability.

### **5. Conclusions**

The present study was conducted to investigate the flow rate and sediment relationship in the middle reach of the Huai River. The Wanglin section was used as an example for numerical simulation. The model is well validated and calibrated. In total, 30 years of data were used for modeling sediment behavior. Due to the special geophysical characteristics of the Wanglin section, the overall trend in this section is erosion in the main channel. Two scenarios were used to investigate the flow–sediment behavior at the Wanglin section. It was found that in the flow–sediment type of rich to medium, an equilibrium sediment transport could be reached. Based on the simulation results, recommendations were provided for flood mitigation in this section, including main channel dredging and bank protection.

**Author Contributions:** Conceptualization, J.N., B.Y., P.W.; methodology, J.N.; writing—original draft preparation, J.N.; writing—review and editing, P.W. and J.N.; supervision, B.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Key Research and Development Program of China, grant number 2017YFC0405602 and the Hydraulic Science and Technology Program of Anhui Province, grant number SLKJ202001-07, and NSERC Discovery Grant.

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

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors are thankful for the financial support provided by the National Key Research and Development Program of China (2017YFC0405602), the Hydraulic Science and Technology Program of Anhui Province (SLKJ202001-07), and NSERC Discovery Grant.

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

### **References**


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