**3. Results and Discussion**

*3.1. Hydrologic Assessment for Each Case*

3.1.1. Current Situation

To compare the current situation with the flood mitigation of each case, the hydrography simulation under each return period was carried out based on the current situation of the case area. The results of the simulations are shown in Figure 6.

(**c**) (**d**)

**Figure 6.** Maximum inundation depth of current situation. (**a**) 5-year return period flood; (**b**) 10-year return period flood; (**c**) 25-year return period flood; (**d**) 100-year return period flood.

> When the return period was 5 years, the maximum inundation depth of the main channel was between 1.5 and 2 m. When the return period was greater than 5 years, the maximum inundation depth of the main channel exceeded 2 m. The topography of the left bank in the case area is relatively lower, so under current conditions the inundation area on the left bank increases with increases in upstream flow. There were some inundation areas on the floodplains of the right bank, with the maximum inundation water depth being 0.3 to 0.5 m when the return period is 5 years. As the return period increased, the inundation area and the maximum inundation depth increased, with maximum inundation depth on

the floodplains of the right bank exceeding 2 m when the return period was 100 years. The floodplains of the left bank had lower elevation, and thus had worse flooding. The maximum inundation depth of 5-year return period floods was 0.5 to 1.5 m. When the return periods were 10 and 25 years, the inundation area of the left bank increased further, and maximum inundation depth reached 1.5 to 2 m. When the return period was 100 years, the maximum inundation depth exceeded 2 m. The topography of the left bank in the case area is relatively lower, so under current conditions, with the increase in upstream flow, the inundation areas of the left bank also increase.

#### 3.1.2. Case A

Case A proposes to extend the existing levees downstream by 1500 m to meet the 100-year return period flood design standard. According to the results simulated, after the existing levees were extended downstream, the inundation of the left bank can be significantly improved, as shown in Figure 7.

Inundation only occurred in low-lying parts of the left bank when the case area was subject to a 100-year return period flood. The water depth of the main channel rises slightly for all return periods when compared to the current situation, while the inundation of the right bank largely remains unchanged for all return periods.
