3.2.3. MIKE FLOOD

MIKE FLOOD is used to couple the one-dimensional MIKE 11 model and the two-dimensional MIKE 21 model. By simulating the momentum transfer between the 1D river network and the 2D surface, the flood situation in the study area can be simulated. In this paper, a lateral coupling method was adopted; water above the river bank is exchanged with the two-dimensional surface model along the flow direction perpendicular to the river, and the exchange flow is approximately calculated by the weir flow formula [52]:

$$q = \mathcal{WC} (H\_{\text{ins}} - H\_{\text{ds}})^k \left[ 1 - \left( \frac{H\_{\text{ds}} - H\_{\text{w}}}{H\_{\text{las}} - H\_{\text{w}}} \right) \right]^{0.385} \tag{8}$$

where *q* is the exchange flow (m3/s); *W* is the width of the connection part (m); *C* is the coefficient of weir flow taken as 1.838; *k* is the weir index; *Hus* and *Hds* are the water levels in the upstream and downstream sections of weir (m), respectively; *Hw* is the elevation at the top of weir (m).

#### 3.2.4. Model Verification

On 7 May 2017, a flood event with a 20-year return period occurred in TieShan River basin, and was selected to verify the flooding simulation model.

(1) One-dimensional model verification

Choosing the water level in the upstream section of overflow weirs 7#–15# in TieShan River to compare with the observed water level, it is found that the differences between simulated and observed values are between 0.01 and 0.18 m (Table 2). Therefore, the one-dimensional model can be considered as accurate enough for flood simulation.


**Table 2.** Comparison of simulated and observed water levels.

#### (2) Two-dimensional model verification

The survey result shows that the culvert of SheGong River and the urban area in the south are the main flooding areas; meanwhile, the TieShan River is in a safe state. Comparing the actual flooding area and depth with the simulated ones, we found that the simulation results are basically consistent with the actual situation. Therefore, it can be concluded that the parameters in the 2D model are reasonable, and the model can be used to reflect the actual flooding situations in the study area [53].

#### **4. Results**

#### *4.1. Flooding Analysis*

We applied the flooding simulation model to simulate the situation of the current river channel when it encounters floods with different return periods. For a 2-year return period, the maximum flooding depth increases during the time interval of 2–6 h, and decreases from 6 to 17 h, reaching maximum value of 0.32 m at the time of 6 h; the flooding depth tends to be relatively stable after 17 h. In correspondence, the maximum flooding area increases from 2 to 9 h and decreases during 9–11 h, reaching the maximum value of 0.93 km<sup>2</sup> at 9 h, and remains unchanged after 11 h. For a 5-year return period, the maximum flooding depth increases from 2 to 5 h and decreases after that, with the maximum value of 0.59 m at 5 h; the maximum flooding area increases before 7 h and decreases from 7 to 14 h, with the maximum value of 2.32 km2; the flooding area remains almost unchanged after 14 h. When meeting the 10-year return period, the maximum flooding depth increases from 2 to 5 h, and decreases during 5–10 h, reaching the maximum value of 0.73 m; the maximum flooding area increases from 2 to 8 h, decreasing from 8 to 15 h, with the maximum value of 3.14 km<sup>2</sup> at 8 h. As for the 20-year return period, the maximum flooding depth increases over time before 8 h, but gradually decreases from after that, with the maximum value of 0.91 m and the flooding depth is relatively stable after 14 h; the maximum flooding area also increases over time before 7 h, decreasing from 7 to 16 h, reaching the maximum value of 4.03 km2 and remaining unchanged after 16 h (Figure 5).

The spatial flooding situations for different return periods are shown in Figure 6. In combination of the results from Figure 5, it can be found that the current flood bear capacity of SheGong River does not reach the standard with a 20-year return period; the flooding area increases from 0.93 km<sup>2</sup> to 4.03 km<sup>2</sup> with the return periods of 2, 5, 10, and 20 years, and the maximum flooding depths increases from 0.32 to 0.91 m. The inundated area is gradually increasing from midstream to downstream of SheGong River with the return periods of 2, 5, 10, and 20 years, respectively (Figure 6). When under the 10-year and 20-year return period scenarios, the flooding depth of the downstream of SheGong River is higher than 0.4 m. However, the TieShan River can bear flooding with a 20-year return period safely, and the downstream flood discharge capacity is still at a surplus. It can also be seen from Figure 6 that because the section width of culvert is too narrow, flood overflowed from river channel at this location, while the TieShan River can bear the flooding with a 20-year return period safely, and the flood discharge capacity of downstream is still at a surplus. Therefore, under this condition, how to solve the problem of insufficient flood control capacity in the section of the culvert should be the focus of regulation schemes.

**Figure 5.** Maximum flooding depths and areas.
