3.1.3. Case B

Both cases B and A extend the existing levees downstream by 1500 m, but the 10-year return period was chosen for the levee flood design standard in case B. The results simulated are as shown in Figure 8.

Inundation on the left bank still occurs, with the inundation area larger and inundation depth deeper for case B when compared to case A. The maximum water depth of inundated areas of the left bank was 0.3 m to 1.5 m for a return period of 5 years, 1.5 m to 2 m for return periods of 10 years and 25 years, and over 2 m for a return period of 100 years. Inundation of the right bank largely remained unchanged from the current situation.

**Figure 8.** Maximum inundation depth of case B. (**a**) 5-year return period flood; (**b**) 10-year return period flood; (**c**) 25-year return period flood; (**d**) 100-year return period flood.

#### 3.1.4. Case C

Case C is a concept proposed by the 3rd River Management Office. It proposed using the solution assembled from the interviews with the residents and stakeholders. The case consisted of building 600 m of discontinuous levees with a 25-year return period flood design standard on the left bank of the river, and adding a wildlife corridor to the discontinuous levees. The simulated results are shown in Figure 9.

It was found in the simulation that the inundation area of the left bank became smaller than the current situation after the implementation of case C. The inundation depth of the left bank also reduced. The inundation depth of the right bank, however, remained similar to the current situation. Areas not covered by the levees of the left bank had maximum inundation depths of 0.3 m to 1.5 m for 5 to 25-year return period floods, with the inundation area increasing with the length of the return period. When the return period was 100 years, the maximum inundation depth reached 1.5 to 2 m.

#### 3.1.5. Case D

Case D refrained from altering current structures, instead choosing to dredge 1 m of soil from the riverbed for 200 m both upstream and downstream of the bridge supports. The results of the simulation are shown in Figure 10, and show that main channel dredging makes no significant difference in inundation of the case area. The inundation area and maximum inundation depth of case D is similar to the current situation for all return periods.

#### 3.1.6. Case E

Case E proposed to plan a "Room for the River" using an area of about 36.35 hectares in size from the case area itself and areas in the immediate vicinity. "Room for the River" includes both main channel dredging and floodplain grading in this situation. The results of the simulation are shown in Figure 11, where the main channel elevation has been dredged and the flooding capacity increased.

In the simulations, no inundation occurred for floods with return periods below 100 years after the implementation of case E. When the return period was 5, 10, or 25 years, the floodwater distribution almost covered the "Room for the River" area entirely. The main channel depth was above 2 m for all return periods, with the floodplain inundation depth at 0.5 to 1.5 m when under a 5-year return period, 1.5 to 2.0 m when under a 10-year return period, and exceeding 2 m when under a 25-year return period. When the return period was 100 years, the inundation area exceeded that of the "Room for the River" area, and the maximum inundation depth reached 1.5 to 2 m.

(**a**) (**b**)

**Figure 9.** Maximum inundation depth of case C. (**a**) 5-year return period flood; (**b**) 10-year return period flood; (**c**) 25-year return period flood; (**d**) 100-year return period flood.

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

**Figure 11.** Maximum inundation depth of case E. (**a**) 5-year return period flood; (**b**) 10-year return period flood; (**c**) 25-year return period flood; (**d**) 100-year return period flood.
