# Characteristics of a Debris Flow Disaster and Its Mitigation Countermeasures in Zechawa Gully, Jiuzhaigou Valley, China

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## Abstract

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## 1. Introduction

## 2. Background

#### 2.1. Formation Conditions of the Zechawa Gully Debris Flow

^{2}and features five tributaries; the main channel is 2.57 km long and has a 61.1% longitudinal slope. The elevation difference of Zechawa Gully is approximately 1601 m, with a maximum elevation of 4040 m in the southwest of the watershed and a minimum elevation of 2439 m at the gully mouth near the scenic road. The topography of Zechawa Gully is steep, with 86.9% of the total area of the watershed having a slope exceeding 25°. The flow path of debris flow along Zechawa Gully can be divided into a formation zone, transport zone and deposition zone (Table 1). The formation zone is located in the upper reaches of Zechawa Gully (elevation above 3620 m), with an area of 0.26 km

^{2}and a channel length of 470 m. The transport zone is situated in the middle reaches, with the elevations ranging from 3620 m to 2600 m. The area of the transport zone is approximately 1.47 km

^{2}, and the channel length is approximately 1530 m. The deposition zone, with an area of 0.23 km

^{2}and a channel length of approximately 570 m, is located in the area below an elevation of 2600 m.

#### 2.2. Description of the Debris Flow Events in Zechawa Gully

^{4}m

^{3}of debris flow material [50].

^{4}m

^{3}. Some of the material was deposited on the debris flow fan with a deposit area of 0.77 × 10

^{4}m

^{2}, a thickness of 0.8–1.5 m and a volume of 0.89 × 10

^{4}m

^{3}. Additional material with a volume of 0.5 × 10

^{4}m

^{3}was transported to the scenic road. During this debris flow event, the pedestrian walkways were buried again, and the only scenic road from Nuorilang Waterfall to Long Lake was blocked, causing traffic disruption and serious property loss [37,51].

^{4}m

^{3}. The debris flow material volume trapped by the concrete check dam was approximately 0.48 × 10

^{4}m

^{3}(Figure 5). Some of the other debris flow material was trapped behind the retaining wall with a deposit area of 0.3 × 10

^{4}m

^{2}, a maximum deposit thickness of 4 m at the middle of the retaining wall and a deposit volume of 0.66 × 10

^{4}m

^{3}(Figure 6). The middle of the retaining wall was partially damaged, resulting in a breach with a width of 8.5 m, due to the high impact force of the debris flow. This breach allowed a portion of the debris flow material with a volume of 1.16 × 10

^{4}m

^{3}to be transported to the debris flow fan and scenic road (Figure 7). The material volume deposited on the fan was approximately 0.93 × 10

^{4}m

^{3}with a deposit area of 0.62 × 10

^{4}m

^{2}and an average deposit thickness of 1.5 m. The volume of the material blocking the scenic road was approximately 0.23 × 10

^{4}m

^{3}, with a deposit length of 180 m and an average deposit thickness of 1.8 m.

## 3. Calculation of the Debris Flow Peak Discharge

#### 3.1. Rain-Flood Method

_{df}is the blockage coefficient, whose value varies with the degree of blockage, namely, very serious blockage (D

_{df}= 3.0–2.6), serious blockage (D

_{df}= 2.5–2.0), normal blockage (D

_{df}= 1.9–1.5) and minor blockage (D

_{df}= 1.4–1.1); ψ

_{df}is the amplification coefficient of the debris flow peak discharge; γ

_{df}is the density of the debris flow (t/m

^{3}); γ

_{w}is the density of water (t/m

^{3}), usually taken as 1.00 t/m

^{3}; γ

_{s}is the density of the solid material (t/m

^{3}), usually taken as 2.65 t/m

^{3}; and Q

_{f}is the flood peak discharge under different return periods (m

^{3}/s), which is calculated by:

^{2}). Here, φ, S, t and n are calculated by the following empirical equations:

_{1}and H

_{6}are the 1-hour average rainfall and 6-hour average rainfall, respectively (mm), which are obtained from “The Rainstorm and Flood Calculation Manual of Medium and Small Basins in Sichuan Province” (published in 2010, with rainfall data from 1978 to 2004); K

_{1}and K

_{6}are the modulus coefficients corresponding to H

_{1}and H

_{6}under different return periods, respectively, which can be obtained from a Pearson type III distribution table; η is the runoff yield parameter, which reflects the average infiltration intensity (mm/h); t

_{0}is the runoff confluence time of the rainstorm when φ equals 1, which can be calculated by:

_{p}is the modulus coefficient when the variation coefficient is equal to 0.23, which is obtained from the Pearson type III distribution table; m is the runoff confluence parameter; and θ is the watershed characteristic parameter, which is obtained from:

#### 3.2. Cross-Section Survey Method

_{df}(m

^{3}/s) can be obtained by Ref. [54]:

_{df}is the area of the cross-section (m

^{2}), and V

_{df}is the average velocity of the debris flow (m/s), which can be calculated by:

_{df}is the roughness coefficient of the debris flow gully, R

_{df}is the hydraulic radius of the debris flow (m), and I

_{df}is the longitudinal slope gradient of the channel bed (m/m).

#### 3.3. Dam-Breaking Calculation Method

_{df}through:

^{2}/s); B

_{0}is the debris flow width before breakage (m); h

_{0}is the debris flow depth before breakage (m); B

_{m}is the breach width (m), and h

_{d}is the residual height of the dam.

_{df}by:

_{b}is the deposit length of the debris flow material behind the check dam (m); κ is the influence factor that accounts for residual height, which is obtained by:

#### 3.4. Maximum Boulder Size Method

_{f}is the fluid density (kg/m

^{3}); ρ

_{s}is the sediment density; β is the bed slope angle (degrees), and the value of β is usually between 15° to 25° when using Equation (19) [63]; φ

_{df}is the quasi-static friction angle (degrees); and Q

_{f}is the flood peak discharge (m

^{3}/s), which was estimated by the methods of Schoklitsch, Helley, Williams and Clarke.

#### 3.4.1. Method of Schoklitsch

_{f}(m

^{3}/s) by computing the unit width flux by Ref. [64,65]:

_{f}is the unit width flux; d

_{I}is the diameter of the boulder intermediate axis (m), and B

_{f}is the channel width (m).

#### 3.4.2. Method of Helley

_{f}(bed velocity) can be calculated by Ref. [66]:

_{b}is the maximum boulder density (kg/m

^{3}); d

_{L}is the diameter of the boulder long axis (m); d

_{S}is the diameter of the boulder short axis (m); C’

_{D}is the drag coefficient; MR

_{D}and MR

_{L}are the drag turning arm and lift turning arm, respectively; and α is the original imbrication angle of the deposited boulder. During the calculation process, Equation (22) uses English units of feet, and the units of critical velocity calculated by Equation (22) need to be converted into metres per second.

_{f}calculated by Equation (22) needs to be converted to the average velocity V

_{avg}[57]:

_{f}can then be calculated as the product of the average velocity, mean depth and channel width by:

_{f}is the mean flood depth (m). Given that the channel width was much larger than the mean depth of flooding, the hydraulic radius obtained by the Manning formula can estimate the average depth; thus, h

_{f}was obtained by the Manning formula:

_{f}is the roughness coefficient of a mountain stream.

#### 3.4.3. Method of Williams

_{I}is obtained through field investigation, and then the empirical relationship between the unit stream power w, bed shear stress τ, average velocity V

_{avg}and d

_{I}is established by Ref. [67]:

_{avg}, h

_{f}and Q

_{f}based on the shear stress can be determined by Equations (30)–(32), respectively:

_{avg}and h

_{f}based on the stream power can be obtained by:

_{f}in Equation (33) is obtained by Equation (32); then, Q

_{f}based on the stream power can be obtained by inserting the calculated values of V

_{avg}and h

_{f}from Equations (33) and (34), respectively, into Equation (26).

#### 3.4.4. Method of Clarke

_{f}(bed velocity) required to carry the maximum-sized boulder is solved by the following formula [68]:

_{D}is the lift coefficient of the boulder, which is dependent on the shape of the largest boulder, with C

_{D}= 1.18 for a cubic boulder and 0.20 for a spherical boulder; A

_{B}is the cross-sectional area of the largest boulder; and F

_{D}is the drag force, which is obtained by:

_{L}is the lift drag coefficient, which is dependent on the shape of the largest boulder, with C

_{L}= 0.178 for a cubic boulder and 0.20 for a spherical boulder; and F

_{C}is the critical force, which is calculated by:

_{B}is the boulder mass (kg). M

_{B}can be obtained for a cubic boulder and a spherical boulder by Equations (39) and (40), respectively:

_{f}can be obtained by inserting the calculated value of V

_{f}into Equations (25)–(27).

## 4. Results

#### 4.1. The Calculated Debris Flow Peak Discharge in 2016

^{3}/s and 36.69 m

^{3}/s. The values of A

_{df}, R

_{df}and I

_{df}were obtained through field investigation. The roughness coefficient of the debris flow gully (n

_{df}) is related to the properties of the debris flow fluid and channel characteristics, and the value in this case is 0.1 according to a field survey [54].

^{3}/s to 43.6 m

^{3}/s. The calculation result by Equation (14) was the lowest (36.5 m

^{3}/s), and the calculation result by Equation (15) was the highest (43.6 m

^{3}/s). The values of B

_{0}, h

_{0}, B

_{m}, h

_{d}, and L

_{b}were obtained by field investigation. Since the data inputs used in Equations (14)–(16) were the same, the differences among the results arose from the different combinations of data used for a given technique. The calculated values are reasonable and are similar to the debris flow peak discharge estimated by the cross-section survey method.

#### 4.2. The Calculated Debris Flow Peak Discharge in 2017

_{f}vary from 0.58 m

^{3}/s to 6.05 m

^{3}/s, and the calculated values of Q

_{df}range from 1.76 m

^{3}/s and 18.33 m

^{3}/s. The minimum permissible debris flow peak discharge of 1.76 m

^{3}/s is estimated through the method of Schoklitsch, and the maximum discharge of 18.33 m

^{3}/s is estimated through the method of Helley. ρ

_{f}is usually taken as 1150 kg/m

^{3}considering the turbidity of the flood waters [68]. ρ

_{s}is usually taken as 2650 kg/m

^{3}. Owing to the absence of information, a value of 36.5° was given for φ

_{df}based on previous studies [58]. The values of d

_{L}, d

_{I}, d

_{S}, ρ

_{b}, B

_{f}, β, and α were obtained through field investigation. The transported sediment concentration (C) is 0.67 by inserting the values of ρ

_{f}, ρ

_{s}, β and φ

_{df}into Equation (19). The roughness coefficient of a mountain stream (n

_{f}) is related to the channel characteristics, and a value of 0.05 was used here according to a field survey [69].

#### 4.3. The Calculated Debris Flow Peak Discharge under Different Occurrence Frequencies

^{3}/s, 32.73 m

^{3}/s and 48.27 m

^{3}/s respectively. In the calculation sections, the values of F, L and J are different, resulting in different debris flow peak discharges estimated by the rain-flood method.

^{3}. Under given conditions, the debris flow density is positively related to the debris flow peak discharge [54,71], thus the densities of the debris flows γ

_{df}under the three return periods (10-year, 20-year and 50-year) were 1.8 t/m

^{3}, 1.85 t/m

^{3}and 1.9 t/m

^{3}, respectively. According to the site investigation, the blockage degree of the channel was normal, and the values of D

_{df}were considered to be 1.8–1.9.

## 5. Discussion

#### 5.1. The Applicability and Limitations of the Calculated Debris Flow Peak Discharge

- (1)
- Due to the complexity of debris flows and the measurement limitation, the values of relevant parameters are usually obtained by field surveys and querying the specifications. In this study, the roughness coefficient of the debris flow gully (n
_{df}), the roughness coefficient of a mountain stream (n_{f}), the density of debris flow (γ_{df}) and the blockage coefficient (D_{df}) were obtained through field investigations and querying specifications. - (2)
- Considering the complexity of the debris flow and the operability of the calculation method, it is necessary to make certain assumptions and simplifications to obtain the peak discharge of the debris flow in the calculation process. The rain-flood method assumes that the occurrence frequencies of rainstorms, floods and debris flows are the same and that the calculated flood peak discharge is completely converted into the peak discharge of the debris flow [54]. Under such assumptions, important parameters such as debris flow peak discharge and total volume of debris flow material under different occurrence frequencies can be obtained, which provide important references for the design of engineering countermeasures. In addition, the breach in the check dam was idealized as a trapezoidal shape, and the average width of the breach was taken as the calculated value of B
_{m}in the dam-breaking calculation. - (3)
- Four methods were used to estimate the peak discharge of the debris flow based on the maximum particle size parameters (Table 3), and the related issues in the calculation are as follows: Both Clarke and Helley solved for the critical velocity required to move the largest boulder, obtained the flow depth through the Manning formula, and finally calculated the peak discharge. Differences in the critical velocity result in differences in the flow depth and peak discharge. The method of Clarke idealizes the largest boulder as either cubic or spherical for the shape-dependent parameters, and the calculated velocities are averaged to provide the critical velocity. By setting the critical force F
_{C}= 0, the downward gravitational component is balanced by the gravity-induced friction, and the extreme use condition of this method can be obtained. The limit bed slope angle (β) is equal to 34.1° for a cubic boulder and 12.7° for a spherical boulder when using the Clarke method; therefore, a spherical boulder is easier to move than the cubic boulder under the same conditions. According to the field investigation, β is equal to 19°, which exceeds the limit bed slope angle for a spherical boulder. Therefore, the selected boulder in this study was considered a cubic boulder, resulting in a calculated critical velocity that is higher than the actual value. Compared with the method of Clarke, the method of Helley neglects the bed slope, ignoring the downstream gravitational component. Generally, the bed slope of a stream is small; even for a stream with a channel longitudinal slope of 10%, the downstream gravitational component is negligibly small compared to the fluid drag and lift, so this component can be ignored [57]. However, the bed slope is 19° in this study, and neglecting the gravitational component results in a calculated critical velocity that is much higher than the actual value, ultimately resulting in a higher calculated peak discharge. The methods of Schoklitsch and Williams estimate the peak discharge by establishing an empirical correlation based on boulder size parameters without considering the influence of the boulder shape on the calculation results. In addition, the values of w, τ and V_{avg}in the method of Williams represent the lowest values, and the actual values are higher than the calculated value. - (4)
- In summary, certain assumptions and simplifications were made in the calculation process, causing the peak discharge of the debris flow calculated by a single method to exhibit low accuracy. Thus, multiple methods should be used to comprehensively obtain the peak discharge, further quantifying the scale of debris flow disasters. It is worth noting that the method for calculating the debris flow peak discharge proposed in this study is mainly based on the specifications in China, especially the selection of some parameters. When calculating the debris flow peak discharge in other countries, local specifications should be considered.

#### 5.2. The Scales of the Debris Flow Disasters in 2016 and 2017

- (1)
- The debris flow peak flow obtained by the cross-section survey method and dam-breaking calculation method are essentially the same and are generally equivalent to the peak discharge of the debris flow with a 20-year return period (Table 2 and Table 4). In addition, the total volume of the debris flow material W
_{df}is estimated by Ref. [54]:$${W}_{df}=0.264{Q}_{df}{T}_{df}$$_{df}is the duration time of the debris flow (s), and its value is approximately 1500 s based on the reports of patrol personnel. The value of Q_{df}is the average calculation result through the cross-section survey method and dam-breaking calculation method, and its value is 37.38 m^{3}/s. The total volume of debris flow material from Equation (42) is 1.48 × 10^{4}m^{3}, which is consistent with the value of 1.39 × 10^{4}m^{3}based on the field investigation. Thus, it is reasonable that the scale of the debris flow on 4 August 2016 is equivalent to that of a debris flow with a 20-year return period. Moreover, based on the study above, the debris flow peak discharges calculated by Equations (14)–(16) were similar to the values obtained by the cross-section survey method. Thus, we conclude that the debris flow peak discharge on 4 August 2016 was amplified by the failure of the check dam, causing widespread damage, and this aspect also explains why the magnitude of the debris flow on 4 August 2016 was large even though the accumulated rainfall and rainfall intensity were extremely low. Similarly, check dam failures have led to catastrophic disasters in other regions, such as the “8.13” Wenjiagou debris flow event [72] and the “8.8” Zhouqu debris flow event [73,74]. - (2)
- Based on the above analysis, the flood peak discharge estimated by the method of Helley is the largest, and is equivalent to that of a debris flow with a 10-year return period. Both of the peak discharges calculated by the methods of Clarke and Helley are larger than the actual value, while the value calculated by the method of Williams is smaller than the actual value. In addition, compared with the extensive destruction of the 2016 debris flow event with a 20-year return period, the destruction of the 2017 debris flow event was smaller, according to the field investigation. Therefore, it is reasonable that the magnitude of the debris flow in September 2017 was less than that of a debris flow with a 10-year return period.
- (3)
- In the remote mountain areas of China, rainfall data are difficult to obtain, and the rainfall throughout a whole catchment usually cannot be recorded by precipitation stations due to the influence of terrain, resulting in inconsistencies between the triggering rainfall and the scale of debris flow disasters. Thus, the relationships between the occurrence of debris flow disasters and the triggering rainfall are not researched in this paper.

#### 5.3. Mitigation Countermeasures in Zechawa Gully

^{4}m

^{3}of loose solid material was generated by the Ms 7.0 Jiuzhaigou earthquake and remains available as material for debris flows in Zechawa Gully in the near future [37,75]. Therefore, appropriate engineering countermeasures must be taken in a timely manner to mitigate post-earthquake debris flow disasters. According to the field investigation and calculation results above, the stone masonry check dam built in 2009 were broken, and the failure of the check dam amplified the debris flow peak discharge, resulting in a very large amount of damage during the debris flow event on 4 August 2016. Thus, the potential failure of a check dam should be fully taken into account during engineering design processes, and an integrated strategy including blocking measures and deposit stopping measures should be adopted for debris flow mitigation. On the one hand, the construction of deposit stopping structures (e.g., retaining walls) can increase the retention capacity of engineering structures; on the other hand, the debris flow material can be trapped by the deposit stopping structures even if the blocking structures (e.g., check dams) in the channel are damaged, thereby reducing the disaster risk downstream.

_{df}into Equation (42), and the resulting value is 1.91 × 10

^{4}m

^{3}(Table 4). Thus, the designed engineering structures are required to trap at least 1.91 × 10

^{4}m

^{3}of debris flow material. In addition, the control principles of prevention projects should not only control the debris flow itself but also operate in harmony with the landscape and reduce the harm to landscape resources, as required in Jiuzhaigou Valley [76]. Under the guidance of these principles, in conjunction with the specific characteristics of the Zechawa debris flows, a concrete check dam and a concrete auxiliary dam were constructed in the channel, and a concrete retaining wall was constructed on the debris flow fan. The concrete check dam, 42.6 m long and 6 m high, was built close to but downstream of the broken stone masonry check dam in order to reduce the peak discharge, stabilize the gully bed, minimize scouring along the bottom and sides of the gully, and stabilize the debris flow material trapped behind the broken check dam. The downstream concrete auxiliary dam, 38.1 m long and 3 m high, was constructed close to the concrete check dam to protect the latter’s foundation (Figure 5). Moreover, the reconstructed check dams were located somewhat upstream in the gully and were satisfactorily concealed. The retaining wall with a total length of 95.6 m was built 93 m away from the scenic road and is out of sight of tourists, and it can trap a volume of 2.27 × 10

^{4}m

^{3}of debris flow materials (Figure 6). In May 2019, new control works (the reconstructed check dam and the retaining wall) were finished.

#### 5.4. Effectiveness of Mitigation Countermeasures and Evaluation of Debris Flow Impact Force

^{4}m

^{3}of debris flow material was transported; this value was greater than the calculated total volume of debris flow material with a 50-year return period in Table 4. A volume of 0.48 × 10

^{4}m

^{3}of debris flow sediment was trapped by the concrete check dam (Figure 5), which contributed to stabilizing the gully bed and preventing entrainment of additional material. Moreover, a volume of approximately 0.66 × 10

^{4}m

^{3}debris flow sediment was trapped by the retaining wall (Figure 6), and a portion of material with a volume of 1.16 × 10

^{4}m

^{3}emerged from the breach in the middle of the retaining wall and was transported downstream. During the debris flow event on 21 June 2019, the prevention projects played a satisfactory role in controlling the debris flow disaster even though the flow magnitude exceeded the design standard.

_{peak}is the peak impact pressure (kN/m

^{2}); F

_{r}is the Froude number; and h

_{df}is the mean debris flow depth (m). Considering the large scale of the debris flow disaster on 21 June 2019, γ

_{df}is taken as 1.9 t/m

^{3}according to Table 4. Based on field investigation, the average velocity of the debris flow (V

_{df}) near the retaining wall was calculated through Equation (13), and related parameters are shown in Table 5.

^{2}[75], which is far below the calculated value of the peak impact pressure (80.39 kN/m

^{2}) in Table 5. The debris flow impact force was greater than the resistance of the retaining wall, causing partial failure of the retaining wall on 21 June 2019. Thus, the resistance of the retaining wall should be increased during the design processes. In general, considerable attention should be given to the post-earthquake debris flow disaster in Zechawa Gully in the future, and it is necessary to repair the broken retaining wall with a greater design resistance and remove the debris flow material deposited behind the retaining wall to prepare for the next post-earthquake debris flow in the near future.

## 6. Conclusions

- (1)
- In this study, the debris flow peak discharge was calculated using the rain-flood method, cross-section survey method, dam-breaking calculation method and maximum boulder size method. Based on our research, compared with previous results based on a single method, an accurate debris flow peak discharge can be obtained by comparing the results of each calculation method with each other, which increases the parameter accuracy for debris flow disaster prevention and risk assessment.
- (2)
- According to the classification criterion of the debris flow scale, the debris flows in Zechawa Gully can be classified as small-scale events (with a total volume of debris flow material less than 1.0 × 10
^{4}m^{3}) and medium-scale events (with a total volume of debris flow material between 1.0 × 10^{4}m^{3}and 10 × 10^{4}m^{3}) [81]. The scale of the debris flow event on 4 August 2016 was equivalent to that of a debris flow with a 20-year return period. After the Ms 7.0 Jiuzhaigou earthquake, at least one debris flow with a scale less than that of a debris flow with a 10-year return period was triggered in September 2017, and a destructive debris flow with a scale greater than that of a debris flow with a 50-year return period was triggered in June 2019. - (3)
- The debris flow peak discharge on 4 August 2016 was amplified by the failure of the stone masonry check dam, causing widespread damage. Due to the disaster risk caused by dam breach incidents, an integrated strategy including blocking measures and deposit stopping measures should be adopted for debris flow mitigation.
- (4)
- Based on the debris flow hazard characteristics of Zechawa Gully, optimized engineering countermeasures (including blocking measures and deposit stopping measures) with a design standard of a 50-year return period were proposed. Combined with the debris flow control principles for national parks, one satisfactorily concealed concrete check dam and one retaining wall out of view of tourists were constructed in Zechawa Gully in May 2019.
- (5)
- On 21 June 2019, a post-earthquake debris flow was triggered by heavy rainfall, and the engineering countermeasure, including blocking and deposit stopping measures, were effective in mitigating the debris flow disaster even though the debris flow magnitude was greater than the design standard of the reconstructed engineering projects. More attention should be paid to the post-earthquake debris flow disaster in Zechawa Gully, and it is necessary to repair the broken retaining wall with greater design resistance and to remove the debris flow material deposited behind the retaining wall in a timely manner to prepare for upcoming post-earthquake debris flows in the near future.

## Notation

A_{B} | Cross-sectional area of the largest boulder |

A_{df} | Area of the cross-section |

B_{f} | Channel width |

B_{m} | Breach width |

B_{0} | Debris flow width before breakage |

C | Transported sediment concentration |

C_{D} | Lift coefficient of the boulder, which is dependent on the shape of largest boulder |

C’_{D} | Drag coefficient |

C_{L} | Lift drag coefficient, which is dependent on the shape of the largest boulder |

D | Nominal diameter of the boulder |

D_{df} | Blockage coefficient |

d_{I} | Diameter of the boulder intermediate axis |

d_{L} | Diameter of the boulder large axis |

d_{S} | Diameter of the boulder short axis |

F | Watershed area |

F_{C} | Critical force |

F_{D} | Drag force |

F_{r} | Froude number |

g | Acceleration due to gravity |

H_{1} | 1-hour average rainfall |

H_{6} | 6-hour average rainfall |

h_{d} | Residual height of check dam |

h_{df} | Mean debris flow depth |

h_{f} | Mean flood depth |

h_{0} | Debris flow depth before breakage |

I_{df} | Longitudinal slope gradient of the channel bed |

J | Longitudinal slope of the channel |

K_{1} | Modulus coefficients corresponding to H_{1} under different return periods |

K_{6} | Modulus coefficients corresponding to H_{6} under different return periods. |

K_{p} | Modulus coefficient when the variation coefficient is equal to 0.23 |

L | Main channel length |

L_{b} | Deposit length of the debris flow material behind the check dam |

M_{B} | Boulder mass |

MR_{D} | Drag turning arm |

MR_{L} | Lift turning arm |

m | Runoff confluence parameter |

n | Attenuation index of the rainstorm |

n_{df} | Roughness coefficient of the debris flow gully |

n_{f} | Roughness coefficient of a mountain stream |

P_{peak} | Peak impact pressure |

Q_{df} | Debris flow peak discharge |

Q_{f} | Flood peak discharge |

q_{f} | Unit width flux |

R_{df} | Hydraulic radius of the debris flow |

S | Rainfall intensity |

T_{df} | Duration time of the debris flow |

t | Runoff confluence time of the rainstorm |

t_{0} | Runoff confluence time of the rainstorm when ϕ equals 1. |

V_{avg} | Average velocity |

V_{df} | Average velocity of the debris flow |

V_{f} | Critical velocity (bed velocity) |

W_{df} | Total volume of the debris flow material |

w | Unit stream power |

α | Original imbrication angle of the deposited boulder |

β | Bed slope angle |

γ_{df} | Density of the debris flow |

γ_{s} | Density of the solid material |

γ_{w} | Density of the water |

θ | Watershed characteristic parameter |

μ | Shape coefficient, which is dependent on the shape of the largest boulder |

ρ_{b} | Maximum boulder density |

ρ_{f} | Fluid density |

ρ_{s} | Sediment density |

τ | Bed shear stress |

η | Runoff yield parameter, which reflects the average infiltration intensity |

ϕ | Runoff coefficient of the flood peak, which is related to the convergence of runoff |

φ_{df} | Quasi-static friction angle |

ψ_{df} | Amplification coefficient of the debris flow peak discharge |

κ | Influence factor that accounts for residual height |

## Author Contributions

## Funding

## Conflicts of Interest

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**Figure 1.**Location of Zechawa Gully and its full view. (

**a**) Location of Jiuzhaigou Valley in Sichuan Province; (

**b**) Location of Zechawa Gully in Jiuzhaigou Valley; (

**c**) The full view of Zechawa Gully. The flow direction of the debris flow is perpendicular to the pedestrian walkways and the scenic road (from Nuorilang Waterfall to Long Lake).

**Figure 2.**Study area maps. (

**a**) Geologic map of the study area; (

**b**) Topographic map of the Tazang fault (TZF), the Minjiang fault (MJF), the Huya fault (HYF) and the blind extension of the HYF (modified from Zhao et al. [41]).

**Figure 3.**Images of Zechawa Gully debris flow in different periods: (

**a**)–(

**c**) 6 August 2016, (

**d**)–(

**f**) 16 August 2017, (

**g**)–(

**i**) 23 October 2017; (

**j**) large boulder transported by the debris flow that occurred in September 2017.

**Figure 7.**The debris flow that occurred on 21 June 2019 buried pedestrian walkways and blocked the scenic road (taken on 22 June 2019).

Zone Division | Formation Zone | Transport Zone | Deposition Zone |
---|---|---|---|

Elevation (m) | 4040–3620 | 3620–2600 | 2600–2439 |

Average gully gradient (‰) | 708 | 415 | 244 |

Gully length (m) | 470 | 1530 | 570 |

Gully characteristics | Steep slope (>50°), bare bedrock with severe frost weathering, low vegetation coverage and abundant collapsed regions | Steep slopes, a large number of landslides and high abundance of debris flow sediments on the gully bed | Gentle topography with no collapses or landslides |

**Table 2.**Calculation results of the debris flow peak discharge by using the cross-section survey method and dam-breaking calculation method.

Methods | Parameters | ||||||
---|---|---|---|---|---|---|---|

cross-section survey method | A_{df} (m^{2}) | R_{df} (m) | I_{df} | V_{df} (m/s) | n_{df} | Q_{df} (m^{3}/s) | |

6.45 | 0.75 | 0.391 | 5.16 | 0.1 | 33.29 | ||

9.58 | 0.85 | 0.182 | 3.83 | 0.1 | 36.69 | ||

B_{0} (m) | h_{0} (m) | B_{m} (m) | h_{d} (m) | L_{b} (m) | Q_{df} (m^{3}/s) | ||

dam-breaking calculation method | Equation (14) | 30.0 | 7.0 | 20.5 | 6.0 | / | 36.5 |

Equation (15) | 30.0 | 7.0 | 20.5 | 6.0 | / | 43.6 | |

Equation (16) | 30.0 | 7.0 | 20.5 | 6.0 | 44.0 | 36.8 |

Basic parameters | d_{L} (m) | 1.3 | B_{f} (m) | 6.5 | ρ_{s} (kg/m^{3}) | 2650 |

d_{I} (m) | 1.1 | β (degrees) | 19 | φ_{df} (degrees) | 36.5 | |

d_{S} (m) | 0.7 | α (degrees) | 6 | C | 0.67 | |

ρ_{b} (kg/m^{3}) | 2250 | n_{f} | 0.05 | |||

Parameters | ||||||

Method | V_{avg}(m/s) | h_{f}(m) | Q_{f}(m^{3}/s) | Q_{df}(m^{3}/s) | ||

Schoklitsch [64] | / | / | 0.58 | 1.76 | ||

Helley [66] | 4.26 | 0.22 | 6.05 | 18.33 | ||

Williams [67] | Shear stress | 2.16 | 0.03 | 0.61 | 1.85 | |

Stream power | 3.80 | 0.04 | 1.07 | 3.24 | ||

Clarke [68] | 2.49 | 0.10 | 1.59 | 4.82 |

Calculation Content | Parameters | Unit | Return Periods | ||
---|---|---|---|---|---|

10-Year | 20-Year | 50-Year | |||

The flood peak discharge | θ | // | 2.14 | 2.14 | 2.14 |

m | / | 0.26 | 0.26 | 0.26 | |

H_{1} | mm | 15 | 15 | 15 | |

H_{6} | mm | 25 | 25 | 25 | |

K_{1} | / | 1.72 | 2.10 | 2.58 | |

K_{6} | / | 1.66 | 1.99 | 2.42 | |

K_{P} | / | 1.31 | 1.42 | 1.56 | |

S | mm | 25.8 | 31.5 | 38.7 | |

n | / | 0.73 | 0.74 | 0.8 | |

η | mm/h | 4.26 | 4.62 | 5.07 | |

t_{0} | h | 1.52 | 1.43 | 1.34 | |

φ | / | 0.75 | 0.79 | 0.82 | |

t | h | 1.66 | 1.54 | 1.43 | |

Q_{f} | m^{3}/s | 6.37 | 8.58 | 11.55 | |

The debris flow peak discharge | γ_{df} | t/m^{3} | 1.8 | 1.85 | 1.9 |

D_{df} | / | 1.8 | 1.85 | 1.9 | |

Q_{df} | m^{3}/s | 22.27 | 32.73 | 48.27 | |

W_{df} | m^{3} | 0.88 × 10^{4} | 1.30 × 10^{4} | 1.91 × 10^{4} |

γ_{df} (t/m^{3}) | h_{f} (m) | R_{df} (m) | I_{df} | n_{df} | F_{r} | P_{peak} (kN/m^{2}) |
---|---|---|---|---|---|---|

1.9 | 1.55 | 1.11 | 0.19 | 0.1 | 1.20 | 80.39 |

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Gong, X.-L.; Chen, K.-T.; Chen, X.-Q.; You, Y.; Chen, J.-G.; Zhao, W.-Y.; Lang, J.
Characteristics of a Debris Flow Disaster and Its Mitigation Countermeasures in Zechawa Gully, Jiuzhaigou Valley, China. *Water* **2020**, *12*, 1256.
https://doi.org/10.3390/w12051256

**AMA Style**

Gong X-L, Chen K-T, Chen X-Q, You Y, Chen J-G, Zhao W-Y, Lang J.
Characteristics of a Debris Flow Disaster and Its Mitigation Countermeasures in Zechawa Gully, Jiuzhaigou Valley, China. *Water*. 2020; 12(5):1256.
https://doi.org/10.3390/w12051256

**Chicago/Turabian Style**

Gong, Xing-Long, Kun-Ting Chen, Xiao-Qing Chen, Yong You, Jian-Gang Chen, Wan-Yu Zhao, and Jie Lang.
2020. "Characteristics of a Debris Flow Disaster and Its Mitigation Countermeasures in Zechawa Gully, Jiuzhaigou Valley, China" *Water* 12, no. 5: 1256.
https://doi.org/10.3390/w12051256