Development Characteristics of Chada Debris Flow in Southeast Tibet and Its Influence on the Proposed Station

doi:10.19657/j.geoscience.1000-8527.2020.108

Abstract

Abstract:

In view of the influence of debris flow disaster on the site selection of Sichuan-Tibet railway station, based on field investigation and remote sensing interpretation, on the basis of ascertaining the current situation of debris flow, this paper analyzes the formation mechanism of debris flow from the material source, induced factors, dynamic factors and other disaster conditions, and calculates the dynamic parameters of debris flow. The velocity of Chada debris flow at the mouth of the gully is 7.63 m/s, the static and dynamic reserves are respectively 15.198,1 million m ³ and 38.08 million m ³ and the risk degree of debris flow is 0.5. It is determined that the debris flow is large-scale-viscous-rainstorm-gully-developing-medium-prone-moderate dangerous debris flow. In view of the fact that the mud-rock flow of Chada directly affects the site selection of Luolong station of Sichuan-Tibet railway, considering two different engineering settings of bridge and tunnel, in order to determine the appropriate route scheme, four schemes are selected. The comparison results show that there is no silting risk in the deck of DK scheme of bridge engineering, the risk of partially or completely blocking the bridge culvert aperture is low, and it is convenient to build protective works such as drainage channel, etc. the risk caused by debris flow is controllable, which is the optimal scheme. The research results of this paper can provide reference for the selection of railway station sites under similar conditions in this region.

Key words: Chada debris flow, formation mechanism, kinetic parameter, scheme comparison, disaster prevention and mitigation measure

CLC Number:

P642
U212.22

HUO Xin. Development Characteristics of Chada Debris Flow in Southeast Tibet and Its Influence on the Proposed Station[J]. Geoscience, 2021, 35(01): 83-91.

Figures/Tables 13

Fig.1 Full view of Chada debris flow gully mouth

Fig.2 Schematic diagram of route scheme and provenance distribution

Fig.3 Typical source photos of solid materials of Chada debris flow gully

Table 1 Source statistics of Chada debris flow gully

物源类型	数量/个	静储量/ 10⁴ m³	占比/%	动储量/ 10⁴ m³	占比/%
崩滑物源	7	65.21	4.29	5.84	15.34
沟道物源	10	564.91	37.17	8.09	21.24
冻融物源	31	77.82	5.12	7.91	20.77
坡面物源	10	811.87	53.42	16.24	42.65
合计	58	1 519.81	100.00	38.08	100.00

Fig. 4 Statistical chart of annual rainfall in Chada valley

Fig. 5 Schematic diagram of the longitudinal section of the main gully of Chada debris flow

Table 2 Statistics of debris flow morphology and elevation

形态分区	清水区	形成区	流通区	堆积区
高程/m	4 550~5 447	4 000~4 550	3 750~4 000	3 672~3 750

Table 3 Calculation results of debris flow velocity in each scheme

方案	1/n	H_c/m	V_c/(m/s)
D23K方案	12.5	1.50	7.63
DK方案	12.5	1.11	4.86
CK方案	8.8	0.50	2.28

Table 4 Calculation results of debris flow run-up in each scheme

方案	冲起高度ΔH/m	爬高ΔH_p/m
D23K方案	2.97	4.75
DK方案	1.20	1.93
CK方案	0.26	0.42

Table 5 Calculation results of impact force of debris flow in each scheme

方案	V_c	α/(°)	λ/γ	W/tf	F/(tf/m²)及F_S/tf
D23K方案	7.63	66	λ=1.47	3.27	F=14.82
D23K方案	7.63	66	γ=0.3	3.27	F_S=58.28
DK方案	4.86	66	λ=1.47	3.27	F=6.01
DK方案	4.86	66	γ=0.3	3.27	F_S=37.10
CK方案	2.28	66	λ=1.47	3.27	F=1.32
CK方案	2.28	66	γ=0.3	3.27	F_S=17.39

Table 6 Calculation of factor conversion in risk degree

类型	实际值	转换函数	转换值	权重系数	分项计算
泥石流规模m/10³m³	196.3	M=0,m≤1 M=lgm/3,1<m≤1000 M=1,m>1	0.76	0.29	0.22
暴发频率f/(次/百年)	1	F=0,f≤1 F=lgf/2,1<f≤100 F=1,>100	0	0.29	0
流域面积s₁/km²	23.8	S₁=0.2458× $s 1 0.3495$ ,0<s₁≤50 S₁=1,s₁>50	0.74	0.14	0.10
主沟长度s₂/km	7.8	S₂=0.2903× $s 2 0.5372$ ,0<s₂≤10 S₂=1,s₂>10	0.88	0.09	0.08
流域相对高差s₃/km	1.78	S₃=2s₃/3,0≤s₃≤1.5 S₃=1,s₃>1.5	1	0.06	0.06
流域切割密度s₆/km^-1	1.01	S₆=0.05s₆, 0≤s₆≤20 S₆=1,s₆>20	0.05	0.11	0.01
不稳定沟床比例s₉/%	75	S₉=s₉/60,0≤s₉≤60 S₉=1,s₉>60	1	0.03	0.03
危险度H					0.50

Table 6 Calculation of factor conversion in risk degree

类型	实际值	转换函数	转换值	权重系数	分项计算
泥石流规模m/10³m³	196.3	M=0,m≤1 M=lgm/3,1<m≤1000 M=1,m>1	0.76	0.29	0.22
暴发频率f/(次/百年)	1	F=0,f≤1 F=lgf/2,1<f≤100 F=1,>100	0	0.29	0
流域面积s₁/km²	23.8	S₁=0.2458× $s 1 0.3495$ ,0<s₁≤50 S₁=1,s₁>50	0.74	0.14	0.10
主沟长度s₂/km	7.8	S₂=0.2903× $s 2 0.5372$ ,0<s₂≤10 S₂=1,s₂>10	0.88	0.09	0.08
流域相对高差s₃/km	1.78	S₃=2s₃/3,0≤s₃≤1.5 S₃=1,s₃>1.5	1	0.06	0.06
流域切割密度s₆/km^-1	1.01	S₆=0.05s₆, 0≤s₆≤20 S₆=1,s₆>20	0.05	0.11	0.01
不稳定沟床比例s₉/%	75	S₉=s₉/60,0≤s₉≤60 S₉=1,s₉>60	1	0.03	0.03
危险度H					0.50

Table 7 Classification of debris flow types

序号	分类依据	分类指标	类型
1	激发因素	暴雨	暴雨型
2	地貌形态	山区沟谷	沟谷型
3	活动频率	易发程度评分102分	中等易发
4	发育阶段	斜坡稳定性较差	发展期
5	危险程度	危险度H为0.5	中度危险
6	流体性质	泥石流容重为1.703 g/cm³	黏性
7	暴发规模	百年一遇概率下一次泥石流的总量为19.63×10⁴ m³	大型

Table 8 Evaluation of debris flow impact on each scheme

名称	桥高净空/m	淤埋风险	冲击和掏蚀风险	风险评价
D23K	20.5	桥高净空小于泥石流的最低桥高建议值24.5 m,桥面存在淤埋风险	冲沟左侧3个桥墩位于泥石流的堆积区,面临泥石流冲击及块石冲击,此处泥石流整体冲击力14.82 tf/m²、单块块石的最大撞击力58.28 tf,泥石流冲击对桥墩的威胁大;冲沟右侧桥墩距离沟边仅11 m,存在侧向冲刷侵蚀的风险	风险较高,安全性较低,总体较差
DK	21.6	桥高净空大于泥石流的最低桥高建议值21.3 m,桥面不存在淤埋风险	3个桥墩位于泥石流堆积区后缘,此处泥石流整体冲击力6.01 tf/m²、单块块石的最大撞击力37.1 tf,泥石流冲击对桥墩的威胁较大	采取工程防护措施后,风险可控
CK	19.7	桥高净空大于泥石流的最低桥高建议值19.2 m,桥面不存在淤埋风险	有7个桥墩位于泥石流堆积区中部,此处泥石流整体冲击力1.32 tf/m²、单块块石的最大撞击力17.39 tf,泥石流冲击对桥墩的威胁较小	不利于工程防护措施的实施,风险难控
D17K	以隧道形式从泥石流流通区以下100 m穿越,泥石流对隧道无影响。隧道出口段540 m走行于冰碛层中,基础沉降难控制。采用明挖法施工,易扰动斜坡处的不良地质,引发次生灾害			风险点较多,危险性较大,方案最差

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