**3. Results and discussion**


As a result of the InfoWorks ICM simulations conducted in the study area, the inundation depths were compared for a variety of rainfall return periods and rainfall ephemerides. Figure 5 illustrates the distribution of the simulated maximum water depth on the ground in the study area. In different rainfall return periods and calendar periods, water accumulated on the ground to a depth of usually less than 0.3 m, followed by 0.3∼0.5 m. The depth of flood inundation increased with the increasing rainfall return period for the same rainfall duration. The depth of flooding increased with the increasing rainfall duration for the same rainfall return period. Overall, the ponded water was primarily concentrated in areas adjacent to rivers and low-lying roads, mainly because the water level of the rivers is higher than the drainage inlets or the terrain of the ponded areas is lower than the surrounding terrain resulting in inundation.

**Figure 5.** Simulated inundation depth map.

#### 3.1.2. Analysis of the Duration and Extent of Flood Inundation

In addition to the inundation depth, the duration of inundation and the inundation extent are essential indicators for evaluating inundation hazards. The amount of precipitation, the type of subsurface, the drainage capacity of the pipe network, and the drainage capacity of natural watercourses are the main causes of persistent flooding. When precipitation is low, the type of subsurface and the drainage capacity of the pipe network control the extent and duration of flooding. Conversely, when rainfall is relatively high, the water level in the river rises and the stormwater pipe network is unable to re-drain or the water in the river can reverse its flow into the stormwater pipe network. At this point, the drainage capacity of the river becomes an important factor in the occurrence and duration of flooding. Analysis of the flood inundation duration and flood extent, therefore, has an important role to play in flood risk assessment. Figure 6 shows the inundation duration and inundation extent of the study area simulated under different scenarios. The simulation results showed that the inundation duration of the study area was mainly from 0∼3 h. At the study area scale, the extent of inundation increased with the duration of rainfall, whereas the extent of flood inundation tended to increase with increasing rainfall return periods. The elevation range of the study area according to the DEM data is 85.05 m 100.34 m. The areas with longer inundation durations are mainly concentrated on the banks of rivers and low-lying areas of the terrain, mainly because the areas with longer inundation durations tend to have deeper inundation, and the areas with deeper inundation generally have lower terrain, which makes it difficult to drain the accumulated rainwater within a short period.

According to the characteristics of precipitation in the study area, normal precipitation can recede within two hours, and precipitation with an inundation time of less than two hours does not affect our normal activities or pose too much of a threat to people's safety. An inundation time of greater than four hours affects people's travel activities. In view of the above, we subdivided the inundation time into five ranges [41]. For each inundation calendar time classification, the inundation area was calculated based on the simulation results for the different scenarios of rainfall recurrence. According to Table 3, under the 1 h rainfall calendar, the inundated areas for the rainfall recurrence periods 5 a, 20 a, and 50 a were 4283.11 ha, 4977.61 ha, and 5238.07 ha, respectively, and the total inundation area increased by 954.96 ha from 5 a to 50 a. The inundated areas under the 2 h rainfall duration

were 4453.39 ha, 5079.15 ha, and 5527.19 ha, respectively, with a total increase of 1073.8 ha from 5 a to 50 a.

**Figure 6.** Simulated inundation durations and inundation ranges.



Under the scenarios with rainfall recurrence periods of 5 a, 20 a, and 50 a, the inundated areas with a rainfall duration of 2 h were compared with those with a rainfall duration of 1 h. The simulation results showed that the inundated areas with an inundation duration <1 h decreased by 236.95 ha, 320.04 ha, and 451.65 ha, respectively; the inundated areas with an inundation duration of 1∼2 h increased by 146.41 ha, 110.89 ha, and 170.26 ha, respectively; the inundated areas with an inundation duration of 2∼3 h increased by 100.06 ha, 125.78 ha, and 190.85 ha, respectively; the inundated areas with an inundation duration of 3∼4 h increased by 355.41 ha, 428.08 ha, and 444.54 ha, respectively; and the inundated areas with an inundation duration >4 h decreased by 194.65 ha, 243.17 ha, and 64.88 ha, respectively. The inundated areas with inundation times <1 h and >4 h showed a decrease with increasing rainfall calendar time, indicating that the longer the rainfall calendar time, the faster the drainage of the study area.

The modelling results showed that the inundated areas increased with increasing rainfall return period for the 1–2 h inundation time scenario. For the 1–2 h inundation time scenario, the inundated areas increased by 146.41 ha, 110.89 ha, and 170.26 ha. For inundation durations of 2 to 3 h, the inundated areas increased by 100.06 ha, 125.78 ha, and 190.85 ha. Inundation areas with inundation durations of 3 to 4 h increased by 355.41 hectares, 428.08 hectares, and 444.54 hectares. Inundated areas with inundation

durations <1 h and >4 h decreased with increasing rainfall duration, with inundation decreasing by 236.95 ha, 320.04 ha, and 451.65 ha at inundation durations <1 h. The inundated areas with an inundation duration >4 h decreased by 194.65 ha, 243.17 ha, and 64.88 ha. The above results indicate that the longer the rainfall duration, the faster the drainage rate of the study area.

#### 3.1.3. Flood Flow Rate Analysis

Flood flow velocities are an important indicator of urban flood risk conditions [31,42,43]. As shown in Figure 7, the flood flow velocity distributions under different scenarios are shown for the study area. In the study area, the flood flow velocities were usually between 0 and 0.5 m/s for different return periods of rainfall. The flood flow rates tended to increase with increasing rainfall return periods, and increasing rainfall durations also contributed to the increase in the flood flow rates. The areas with a high flood flow velocity are located near the main drainage network's outlet and in densely populated areas. The magnitude of the flood flow can affect the travel and safety of people in the study area. Excessive flood velocities can impede traffic and float objects on the ground, posing a threat to human life. This is particularly important for the subsequent flood risk analysis.

**Figure 7.** Simulated flood flow map.
