# Risk Assessment and Control of Geological Hazards in Towns of Complex Mountainous Areas Based on Remote Sensing and Geological Survey

^{1}

^{2}

^{3}

^{4}

^{5}

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^{†}

## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

#### 2.1. Overview of the Geological Environment of the Study Area

^{2}. Approximately 11,550 people and 9 administrative villages were in the town area. Because the climate of the town is temperate continental monsoon with a mild and humid climate, the average annual rainfall in the area is 499.4 mm, and the rainfall is concentrated from June to September, often in the form of heavy and continuous rain.

_{2}

^{2}xh

_{5}and D

_{2}

^{2}xh

_{6}) and includes light gray shale and slate with a small amount of chert and siltstone, etc. The rock mass in the study area is broken, weathered highly and weaker competency that is affected by the surrounding active fault. The features of rock mass are collapse and landslide, which includes the black carbonaceous shale and schist fragments with significant rheological properties.

#### 2.2. Study Data Sources

## 3. Methodology

#### 3.1. Town Risk Assessment Process

- Geological hazard risk identification.

- Research on the formation mode of geological hazards.

- Geohazard Risk Analysis.

- Vulnerability assessment of potential disaster-bearing bodies.

- Geological hazard risk assessment in the township area.

- Recommend countermeasures for risk control.

#### 3.2. Geohazard Risk Identification and the Disaster Generation Model

^{2}, accounting for 8.4% of the total area of the study area, and the density of hazard development is 1.85 places/km

^{2}. The total area of provenance area of landslide, landslide and debris flow development is 1.67 km

^{2}, and the area of their corresponding provenance areas are 7.2%, 57.5% and 35.3%, respectively.

#### 3.3. Town Risk Assessment Methods and Models

#### 3.3.1. The Slope Stability Evaluation Model

_{s}:

_{s}is the natural weight of the geotechnical body (kg/m

^{3}). t is the potential slip thickness (m). θ is the slope inclination (°). h is the slope groundwater level (m). ρ

_{w}is the weight of water (kg/m

^{3}).

^{2}). T is the hydraulic conductivity of the saturated soil (m

^{2}/d). b is the width of the considered water flow cross-section (grid accuracy) (m).

_{s}= 1, the critical rainfall for rainfall-induced slope initiation can be obtained as

#### 3.3.2. The FLO-2D Fluid Model

_{ox}and S

_{oy}are the streambed slope drops in the x-direction and y-direction (%), S

_{fx}and S

_{fy}are the frictional slope drops in the x-direction and y-direction (%). FLO-2D provides dynamic wave mode and diffusion seeding mode to simulate the process of movement and accumulation. Equation (2) is the continuity equation, which is the volume mass conservation equation. Equations (5) and (6) are the equations of motion of the force balance. In this model, the expression of the shear stress gradient of the fluid:

_{f}is frictional decline (%), S

_{y}is yield decline (%), S

_{v}is viscous decline (%), S

_{td}is turbulent-dispersion decline (%), τ

_{y}is yield stress (MPa), γ

_{m}is specific gravity of fluid (t/m

^{3}), K is laminar drag coefficient, $\eta $ is fluid viscosity coefficient, n is Manning coefficient, and v is flow velocity (m/s). The parameters ${\tau}_{y}$ and $\eta $ are calculated from the equation $\eta ={\alpha}_{1}{e}^{{\beta}_{1}\cdot {C}_{v}}$ and ${\tau}_{y}={\alpha}_{2}{e}^{{\beta}_{2}\cdot {C}_{v}}$, ${\alpha}_{1}$, ${\alpha}_{2}$, ${\beta}_{{}_{1}}$ and ${\beta}_{{}_{2}}$ are set by rheological tests or table setting.

#### 3.3.3. The River-Flow 2D Rheological Model

^{3}). a is the slip surface inclination angle (°). ϕ is the internal friction angle (°). a

_{c}= v

_{i}

^{2}/R is the centrifugal acceleration of the curved slip surface. r

_{u}is the cavity pressure coefficient, the ratio of the cavity pressure to the normal stress at the bottom of the calculation unit. ξ is the turbulence coefficient (m

^{2}/s).

_{b}is the slip stress (MPa), τ

_{b}= gρhcosθtanθ

_{b}, τ

_{y}is the yield stress (MPa), τ

_{y}= 0.181 ∙ exp(25.7C

_{V})/10 is the yield stress (MPa), τ

_{u}is the viscous stress (MPa), τ

_{u}= 0.036 ∙ exp(22.1C

_{V})/10. ρ = ρ

_{w}(1 + 1.65C

_{V}) and θ is the slope of the landslide (°). θ

_{b}is the internal friction angle of the landslide (°), and ρ is the fluid density of the landslide debris (kg/m

^{3}). ρ

_{w}is the water weight (kg/m

^{3}). C

_{V}is the volume concentration.

## 4. Model Validation and Results

#### 4.1. Model Validation

#### 4.1.1. Town Slope Hazard Analysis

#### 4.1.2. Typical Evaluation Demonstration

^{2}, the relative height difference in the area is 693 m, the main channel length is 2.53 km, the average longitudinal ratio drop of the ditch bed is 273.9‰, and the total amount of loose solids source in the watershed reaches 242.38 × 10

^{4}m

^{3}. The field investigation data show that the debris flow dynamic process is: soft and hard lithology combination of medium and shallow landslide start → blockage body instantaneous collapse flow amplification → along the course of channel erosion → more intense bend wash silt → stop silt accumulation or blockage of the river, is a typical collapse—channel erosion mixed debris flow. In the process of movement, the flow is immediately amplified 3~10 fold after experiencing the blockage and collapse of loose accumulation source or coarse and large particle source, forming a super large-scale mudflow.

^{3}, 1.89 t/m

^{3}and 1.97 t/m

^{3}. In the actual investigation, the upstream channel in Quanjia Bay is severely blocked, and the formation after the collapse of the middle and shallow landslide weir will produce a certain amplification effect, so the input of the FLO-2D model is calculated multiplied by the volume expansion coefficient. Finally, the parameters and rate were input of the FLO-2D model (Table 4), and the calculation results were more reliable and realistic without human intervention during the whole process.

^{4}m

^{2}, 1.36 × 10

^{4}m

^{2}, 3.26 × 10

^{4}m

^{2}, 4.35 × 10

^{4}m

^{2}. The areas of the very high-risk zone, high-risk zone, medium-risk zone and low-risk zone under 50-year (2% frequency) precipitation are 2.80 × 10

^{4}m

^{2}, 1.10 × 10

^{4}m

^{2}, 1.96 × 10

^{4}m

^{2}, and 2.95 × 10

^{4}m

^{2}. The areas of very high-risk zone, high-risk zone, medium-risk zone and low-risk zone under 20-year (5% frequency) precipitation are 1.79 × 10

^{4}m

^{2}, 0.42 × 10

^{4}m

^{2}, 0.98 × 10

^{4}m

^{2}, 2.04 × 10

^{4}m

^{2}.

#### 4.1.3. Typical Landslide Evaluation Demonstration

^{2}, a main slide direction is 154°, and a total slope is 27°. The field investigation shows that the disaster mode of the landslide is a small avalanche slip collapse overburden damage under the unloading effect of the back wall of the old landslide.

^{4}m

^{2}, 1.62 × 10

^{4}m

^{2}, 1.79 × 10

^{4}m

^{2}, 1.02 × 10

^{4}m

^{2}. The areas of very high hazard zone, high hazard zone, medium hazard zone, and low hazard zone under the 50-year (2% frequency) precipitation condition are 1.36 × 10

^{4}m

^{2}, 1.24 × 10

^{4}m

^{2}, 1.17 × 10

^{4}m

^{2}, 1.42 × 10

^{4}m

^{2}. The areas of very high hazard zone, high hazard zone, medium hazard zone, and low hazard zone under the 20-year (5% frequency) precipitation condition are 0.54 × 10

^{4}m

^{2}, 1.11 × 10

^{4}m

^{2}, 1.07 × 10

^{4}m

^{2}, 1.83 × 10

^{4}m

^{2}.

^{2}, 4.64 km

^{2}, 13.97 km

^{2}and 17.43 km

^{2}, respectively. The areas of very high hazard zone, high hazard zone, medium hazard zone and low hazard zone under 50-year (2% frequency) precipitation are 1.19 km

^{2}, 1.78 km

^{2}, 7.28 km

^{2}, and 28.14 km

^{2}, respectively. The area of the very high-risk zone, high-risk zone, medium-risk zone and low-risk zone under 20-year (5% frequency) precipitation condition is 0.53 km

^{2}, 0.75 km

^{2}, 4.13 km

^{2}, and 32.99 km

^{2}, respectively.

#### 4.2. Results

#### 4.2.1. Assessment of the Vulnerability of Towns to Geological Hazards

_{d}is the vulnerability index of the disaster-bearing body. H

_{d}is the burial depth of the debris-flow siltation (m). H

_{c}is the effective height of the building or structure (m); if (H

_{d}/H

_{c}) ≥ 1, it means that the building or structure has been completely buried by the landslide, and its value is 1.

#### 4.2.2. Risk Assessment of Urban Geological Hazards

_{i}is the probability of risk occurrence under different precipitation working conditions. Before calculation, all kinds of indices in Equation (13) must be normalized, and the normalization method is as follows:

_{max}and H

_{min}are the maximum and minimum hazard values, respectively. V′ is the normalized value of vulnerability. V is the normalized value of vulnerability. V

_{max}and V

_{min}are the maximum and minimum vulnerability values, respectively.

_{i}calculation method takes the previous geological hazard statistical samples of the study area as an example. A logistic regression statistical model is used to determine the spatial and temporal probability of geological hazard risk in Longlin Town based on the completion of regional geological hazard risk zoning. The P

_{i}is 0.72 for 100-year rainfall conditions, 0.23 for 50-year rainfall conditions, and 0.08 for 50-year rainfall conditions, respectively. After normalizing the risk and vulnerability of geohazards in Longlin Town, the calculation method of Equation (13) is used to calculate the risk probability of geohazards under different precipitation conditions. The comprehensive risk degree of geohazards in Longlin Town is obtained by multiplying the normalized data of hazard of geohazard chains and the normalized data of vulnerability of disaster-bearing bodies. The risk evaluation results classification was conducted used by the method of the natural breakpoint and characteristic points according to the risk characteristics of geological hazards in Longlin Town and the objectives of risk control. The risk degree is divided into four levels: very high risk (0.697~1), high risk (0.538~0.697), medium risk (0.356~0.538), and low risk (0~0.356). The blocks of each risk level were indicated by different spots based on the risk level classification standard, and the raster units of the same risk level were combined to draw the geological hazard risk zoning map of Longlin Town (Figure 9).

^{2}, 4.54 km

^{2}, 7.02 km

^{2}, and 24.93 km

^{2}, respectively. The area of the very high-risk zone, high-risk zone, medium-risk zone and low-risk zone under 50-year (2% frequency) precipitation conditions are 0.64 km

^{2}, 1.41 km

^{2}, 5.14 km

^{2}, and 31.21 km

^{2}, respectively. The area of the very high-risk zone, high-risk zone, medium-risk zone and low-risk zone under 20-year (5% frequency) precipitation conditions are 0.15 km

^{2}, 0.64 km

^{2}, 3.32 km

^{2}, and 34.29 km

^{2}, respectively.

## 5. Discussion

#### 5.1. Disaster-Forming Pattern Identification Markers

#### 5.2. Development and Evolution of Disasters under Different Rainfall Scenarios

^{2}under 100-year rainfall conditions, which are 3.14, and 8.16 multiple more than that in under 50-year and 20-year rainfall conditions, respectively. The area of high risk is 7.0 km

^{2}under 100-year rainfall conditions, which are 2.36- and 5.47-fold higher than in under 50-year and 20-year rainfall conditions, respectively. Under different rainfall frequencies, 75.23% of the area always remained low risk, 24.38% of the area’s risk level increased with decreasing rainfall frequency, and 0.39% of the area always remained very high risk. Our research obtains the mapping of geological hazards and risk regionalization in the study area under different precipitation frequencies. The results show that rainfall can not only scour the loose accumulation of rock and soil bodies on the slope of landslides but also exacerbate the deformation and damage of landslides by the formation of high dynamic water pressure on the potential slip surface separated from water relatively. The area of very high and high risk reaches the biggest under 100-year extraordinary rainstorm conditions, it is indicated that rainfall has a very significant role in the induction of geologic hazard. The geological conditions of the study area also impacted the occurrence of geological disasters. The region is characterized by active neotectonics movement, discordant valley landforms by the denudation and cutting strongly, complicated and broken rock structure, and developed weak rock. The formation of soft and fluid plastic soften belt on the contact surface between the soil and rock due to the strength of the soil and the lower part of the soft rock is greatly reduced, which are induced landslides by reducing the stability of the slope.

#### 5.3. Suggestions for Geological Hazard Risk Control

^{2}in the catchment area, we will improve the disaster prevention knowledge and awareness of the residents in the affected areas through policies, propaganda training and social management. Carrying out some work enhanced the ability of group measurement and monitoring and emergency avoidance, which are strengthening propaganda and training, professional guidance, inspection and control, and emergency drills in flood and key areas. Further, the level of disaster prevention and mitigation can be improved by summarizing the experience and lessons learned in disaster prevention and mitigation and revising the behaviors, habits, and guidelines for disaster prevention and mitigation continuously.

## 6. Conclusions

## Author Contributions

## Funding

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**Geological background map of study area. (

**a**). The location of the West Hanshui Basin in China. (

**b**). The structural outline map of the West Hanshui Basin. (

**c**). Development of weak rock layers in Longlin Town.

**Figure 2.**Flowchart of geohazard risk assessment and control for cities and towns in complex mountainous areas.

**Figure 5.**Simulation results of debris flow risk in Jianyuwan under different precipitation conditions. (

**a**). Longitudinal profile of the main channel. (

**b**). Blockage of the main channel downstream of formation area. (

**c**). The situation of the main channel downstream of the circulation area. (

**d**). A total of 100 hazard zones of debris flow under the condition of one rainfall. (

**e**). A total of 50 risk zones of debris flow under one rainfall condition. (

**f**). A total of 20 hazard zones of debris flow in case of rainfall.

**Figure 6.**Simulation results of landslide hazard in Panping Village under different precipitation conditions.

**Figure 8.**Vulnerability zoning map of geohazards in the Longlin Town Collective Area under different conditions.

Basic Data | Data Source and Production | Data Format |
---|---|---|

Geohazard data | From the Longnan West Hanshui Basin Disaster Geological Survey (2019–2021) project database | 1:10,000 precision vector data |

DEM | Geospatial data to extract slope, gully density, debris flow gully bed ratio drop, etc. | 5 m × 5 m raster data |

DOM/DLG | Land use type data | 5 m × 5 m raster/vector data |

Remote Sensing Data | Interpretation for risk source identification, carrier types, etc. | P-star and UAV data, raster data |

Rainfall information | Lanzhou Central Weather Station, Longnan town geohazard Professional Monitoring Network | Vector data |

Geological data | Lithological zoning, fracture structure | 1:200,000 regional geological map, vector data |

Survey and test data | Physical and mechanical indicators such as geotechnical density/capacity, water content/permeability coefficient, and angle of internal friction, cohesion, etc., for model calculation and analysis | Text Data Format |

Type | Initial State | Ageing Deformation Stage | Progressive Deformation Damage Stage | Post-Damage State | Description of Model Elements |
---|---|---|---|---|---|

Type I | |||||

Type II | |||||

Type III |

Grading | Landslide Stability Calculation Concerning Surface Macro Deformation | Landslide Development Rate Evaluation Reference | ||||
---|---|---|---|---|---|---|

Surface Macro Deformation Characteristics | Stable State | Reference Value of Stability Coefficient | Developmental Status | Landslide Development Characteristics | Fertility Reference Values | |

Extremely high | Signs of overall landslide sliding can be clearly observed on the surface, and the slide body can be separated from the slide bed | Landslide initiation | <0.9 | Full developmental maturity | Landslide has been initiated and overall sliding is highly probable | 0.9~1 |

High | Landslides can be initiated when there is localized damage to the ground surface, and overall sliding precursors appear | Unstable | 0.9~1.00 | Developmental maturity | Slippery slope can be started, the overall sliding possibility is high | 0.7~0.9 |

Medium | Signs of surface deformation begin to intensify and the landslide progresses rapidly toward the initiation phase; or the surface shows significant local deformation, but the rate of deformation is slow | Critical state or less stable | 1.00~1.10 | Developmental immaturity or onset of development | Accelerated deformation of the landslide, with the possibility of overall sliding; or local deformation of the slope, with the possibility of forming a landslide | 0.3~0.7 |

Low | There are only local signs of minor deformation on the surface, and there is no development trend for the time being, or no signs of deformation are observed on the surface for the time being | Basically, stable or stable | 1.10~1.20 | Not yet developed or not developed | The slope deformation range is very small and the possibility of landslide formation is minimal; or no landslide | <0.3 |

Projects | Frequency of Rainstorms | Simulation Parameters | Value | ||
---|---|---|---|---|---|

P = 5% | P = 2% | P = 1% | |||

Watershed area/(km^{2}) | 1.26 | Calculation grid/(m) | 5 × 5 | ||

Total material sources/(10^{4} m^{3}) | 242.38 | Manning roughness coefficient | 0.15/Residential district | ||

Debris flow capacity/(t/m^{3}) | 1.77 | 1.89 | 1.97 | 0.05/Road | |

Debris flow peak/(m^{3}/s) | 6.49 | 10.38 | 12.97 | 0.22/Cultivated land | |

Sediment correction factor | 0.89 | 1.17 | 1.44 | 0.2/Bare ground | |

Sediment blockage factor | 3.5 | 0.8/Woodland | |||

Debris flow discharge/(m^{3}/s) | 42.94 | 78.8 | 110.82 | laminar flow friction factor K | 2280 |

Volumetric concentration | 0.47 | 0.54 | 0.59 | ${\alpha}_{1}$ | 0.811 |

Debris flow amplification factor | 1.89 | 2.17 | 2.44 | ${\alpha}_{2}$ | 0.00462 |

Simulation flow/(m^{3}/s) | 81.21 | 170.93 | 270.45 | ${\beta}_{1}$ | 13.72 |

Simulation time/(h) | 0.3 | 0.8 | 1.5 | ${\beta}_{2}$ | 11.24 |

Simulation accuracy/(%) | 81.38 | 75.53 | 86.74 | Sediment specific gravity/(t/m^{3}) | 2.65 |

Projects | P = 5% | P = 2% | P = 1% |
---|---|---|---|

Internal friction angle θb/(°) | 14.4 | 12.96 | 11.6 |

Slip density ρ/(kg/m^{3}) | 20.2 | 23.23 | 25.05 |

Volumetric concentration C_{V} | 0.618 | 0.802 | 0.912 |

Slip body yield stress τy/(MPa) | 0.886 | 1.422 | 1.886 |

Slip viscous stress τμ/(MPa) | 0.141 | 0.212 | 0.270 |

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## Share and Cite

**MDPI and ACS Style**

Ding, W.; Wang, G.; Yang, Q.; Xu, Y.; Gao, Y.; Chen, X.; Xu, S.; Han, L.; Yang, X.
Risk Assessment and Control of Geological Hazards in Towns of Complex Mountainous Areas Based on Remote Sensing and Geological Survey. *Water* **2023**, *15*, 3170.
https://doi.org/10.3390/w15183170

**AMA Style**

Ding W, Wang G, Yang Q, Xu Y, Gao Y, Chen X, Xu S, Han L, Yang X.
Risk Assessment and Control of Geological Hazards in Towns of Complex Mountainous Areas Based on Remote Sensing and Geological Survey. *Water*. 2023; 15(18):3170.
https://doi.org/10.3390/w15183170

**Chicago/Turabian Style**

Ding, Weicui, Gaofeng Wang, Qiang Yang, Youning Xu, Youlong Gao, Xuanhua Chen, Shenglin Xu, Lele Han, and Xinru Yang.
2023. "Risk Assessment and Control of Geological Hazards in Towns of Complex Mountainous Areas Based on Remote Sensing and Geological Survey" *Water* 15, no. 18: 3170.
https://doi.org/10.3390/w15183170