# Assessment of Flood Inundation by Coupled 1D/2D Hydrodynamic Modeling: A Case Study in Mountainous Watersheds along the Coast of Southeast China

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## Abstract

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## 1. Introduction

## 2. 1D/2D Real-Time Dynamic Coupling Flood Model

#### 2.1. 1D River Hydraulic Model

#### 2.2. 2D Flood-Routing Hydraulic Model

#### 2.3. Modified Models of Dry Water Depth and Wet Water Depth Theory

#### 2.4. Coupling of 1D and 2D Hydraulic Models

#### 2.4.1. Time Coupling of 1D and 2D Hydraulic Models

#### 2.4.2. Spatial Coupling of 1D and 2D Hydraulic Models

## 3. Data and Methods

#### 3.1. Research Domain

^{2}. Huazhou City is located in the south of the Tropic of Cancer and belongs to the subtropical monsoon climate zone. It has a long summer and warm winter, abundant light and heat, abundant rainfall and typhoons in summer and autumn. The average annual rainfall in Huazhou City is 1780 mm. It has entered the rainy season since the first 10-day period of April. The annual rainfall is mostly concentrated in April to September, accounting for about 85% of the annual rainfall. The watershed area in Huazhou was selected as the research domain, as shown in Figure 2.

#### 3.2. Data

#### 3.3. Model Pre-Simulation Processing

#### 3.3.1. Boundary Condition

#### 3.3.2. Selection of Roughness Value

#### 3.3.3. 2D Computation Mesh Construction

^{2}, and the average side length is 180 m. Both the calculation accuracy and efficiency are taken into account. In the process of meshing, the role of important water-blocking structures is taken into account, and important roads and dikes are used as control boundaries. After the meshing is completed, the digital elevation model, land use and high-resolution remote-sensing images are used to attach corresponding attribute values to the mesh, and the final mesh model is determined by trial calculation and optimization adjustment. The grid division and main parameter settings are shown in Figure 3.

## 4. Results and Analysis

#### 4.1. Flood Simulation under Typhoon ‘Chanthu’ in 2010

^{3}/s, which is equivalent to the peak flow of a 50-years’ flood. Huazhou Station of Jianjiang River Basin is the annual maximum flood with a peak water level of 13.11 m [36,37].

#### 4.2. Flood Simulation under Typhoon ‘Utor’ in 2013

#### 4.3. Simulation Prediction under Different Frequency Flood Conditions

#### 4.4. Flood Inundation Characteristics under Dike Conditions

## 5. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**Death toll from mountain torrents and the proportion of deaths caused by mountain torrents against those caused by floods (%).

**Figure 4.**Water level comparison of model calculation results and measured data under the typhoon “Chanthu” flood in 2010.

**Figure 5.**Water level comparing between the measured and calculated results of typhoon “Utor“ flood in 2013.

**Figure 7.**Simulated flood depth and extent of 1D/2D model at 12 h, 24 h, 36 h, and 48 h after the river dike breach.

**Figure 8.**Flow field distribution: flow field near the breach (

**a**), water-blocking and water-diversion effect of dike and local topography (

**b**).

Partial Roughness Values of Riverbed | ||
---|---|---|

River Features | ||

Type | Riverbed Morphology and Characteristics | Roughness Value |

I | The riverbed is made up of sand and its surface is smooth. | 0.020–0.024 |

II | The riverbed is a slab consisting of sand and gravel with a flat surface. | 0.022–0.026 |

III | The riverbed is sandy and the bottom of the river fluctuates greatly. | 0.025–0.029 |

IV | The smooth riverbed is composed of gravel. its terrain is relatively flat. | 0.025–0.029 |

V | The riverbed is composed of fine sand with a small amount of vegetation at the bottom. | 0.030–0.034 |

VI | The riverbed is composed of gravel and it is undulating. | 0.030–0.034 |

VII | There are pebbles and boulders at the bottom of the river, and the riverbed fluctuates greatly. | 0.035–0.040 |

VIII | The riverbed is pebble, the bottom of the river fluctuates greatly and its shape is irregular. | 0.04–0.10 |

Serial Number | Section | Measured Water Level (m) | Model Calculation of Water Level (m) | Absolute Difference of Water Level (m) |
---|---|---|---|---|

1 | Luojiang Estuary | 13.18 | 13.11 | 0.07 |

**Table 3.**Comparison between the measured and calculated results of the typhoon “Utor” flood in 2013.

Serial Number | Section | Measured Water Level (m) | Model Calculation of Water Level (m) | Absolute Difference of Water Level (m) |
---|---|---|---|---|

1 | Luojiang Estuary | 14.19 | 14.08 | 0.11 |

Flood Frequency | Maximum Flooded Area (km^{2}) | Maximum Flooded Depth (m) | Average Maximum Flooded Depth (m) |
---|---|---|---|

20-year flood | 56.58 | 7.617 | 2.58 |

50-year flood | 66.34 | 8.487 | 2.976 |

100-year flood | 79.24 | 9.511 | 3.39 |

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**MDPI and ACS Style**

Zhang, W.; Zhang, X.; Liu, Y.; Tang, W.; Xu, J.; Fu, Z. Assessment of Flood Inundation by Coupled 1D/2D Hydrodynamic Modeling: A Case Study in Mountainous Watersheds along the Coast of Southeast China. *Water* **2020**, *12*, 822.
https://doi.org/10.3390/w12030822

**AMA Style**

Zhang W, Zhang X, Liu Y, Tang W, Xu J, Fu Z. Assessment of Flood Inundation by Coupled 1D/2D Hydrodynamic Modeling: A Case Study in Mountainous Watersheds along the Coast of Southeast China. *Water*. 2020; 12(3):822.
https://doi.org/10.3390/w12030822

**Chicago/Turabian Style**

Zhang, Wenting, Xingnan Zhang, Yongzhi Liu, Wenwen Tang, Jan Xu, and Zhimin Fu. 2020. "Assessment of Flood Inundation by Coupled 1D/2D Hydrodynamic Modeling: A Case Study in Mountainous Watersheds along the Coast of Southeast China" *Water* 12, no. 3: 822.
https://doi.org/10.3390/w12030822