Typhoon triggered operation tunnel debris flow disaster in coastal areas of SE China

Abstract Typhoons have inflicted significant damage and loss of life to China, a large number of typhoon–rainstorm–debris flow–tunnel accidents occur in the southeastern coastal areas each year. Considering the disaster prevention and mitigation decision-making of disaster accidents in coastal areas and the reduction of regional economic losses, this Express Letter presents some typical accident scenes and rescue measures in recent years and analyses the hazard mechanism from three aspects. On this basis, we propose some suggestions such as the monitoring and early-warning system, which can provide a few references for reducing disaster losses and improving disaster treatments.


Introduction
Historically, the southeastern coastline of China is prone to suffer typhoon each year ( Figure 1) (Zhang et al. 2011Yap et al. 2015. Typhoons are often accompanied by strong winds, rainstorms and storm surges which have the characteristics of high frequency, sudden occurrence and wide range of influence and great intensity of disaster (Yin et al. 2013). In the vulnerable area of geological environment, the rainstorm caused by typhoons is easy to trigger geological disasters such as collapse, landslide and debris flow and bring about heavy casualties and property losses. A large area of mountainous region is located in the southeast coast, owing to the advantages tunnels have such as overcoming topography or elevation obstacles, improving the linear and shortening mileage that the demand is determined to build more mountain tunnels when building rail or highway projects in these areas (Vu et al. 2015;Luo et al. 2018;Qiu et al. 2018a,b). Typhoon disasters greatly increase the vulnerability of traffic facilities and the susceptibility of disaster chain in southeast coastal area Lai et al. 2016a;Yan et al. 2018a,b;). An increasing number of tunnels are constructed in disadvantageous environments (Lai et al. 2015(Lai et al. , 2016b(Lai et al. ,c, 2018aLuo et al. 2019;Li, Qi, et al. 2018;Qiu et al. 2018c,d), the typhoon-rainstorm-debris flow disasters constitute serious threats to mountain tunnels. The Express Letter briefly reports the tunnel debris flow accidents triggered by typhoons in southeast coastal area of China in current years and discusses the hazard mechanism of typhoon-rainstorm-debris flow-tunnel accidents Li, Xu, et al. 2018c;Qiao et al. 2018a,b).

Overview
In this paper, some typical tunnel debris flow accidents triggered by typhoons in southeast coastal area in last several years are presented ( Figure 2). To review the previous tunnel debris flow disasters triggered by typhoon, the occurrence period of accidents is more concentrated in June to October, which is the high frequency season of typhoon. Concretely, four typical accidents occurred in Zhejiang province (Table 1), one typical accident occurred in Fujian province (Table 2), two typical accidents occurred in Guangdong province (Table 3).

Rescue measures
After the dangerous omen occurred, the tunnel management unit immediately identified the accident from the monitoring system or the highway patrol, then controlled     Interval tunnel When the train was running during the tunnel between Lechang and Chenzhou, tunnel collapse accident occurred suddenly, mud rock flowed into the train, the train rocked violently, two men were buried in debris, two children were scratched by the window panes.
information boards and traffic lights to close the tunnel. Tunnel maintenance supervision unit immediately started emergency plan: (1) Carry out traffic control in the tunnel section and divert traffic; (2) Organize rescue personnel and emergency machinery to the accident site at once; (3) Set up on-site command and carry out on-site emergency command right away. After the barrier had been set up, rescue personnel transferred personnel and vehicles to a refuge urgently and sent injured personnel to on-site medical staff or nearby hospitals. Subsequently, the machinery and equipment were organized to rush to repair roads. To resume traffic as soon as  possible, large engineering vehicles and excavating machinery were invoked to clear the gravel, mud and water of accident section ( Figure 6).

Hazard mechanism
The necessary conditions leading to debris flow include topographic conditions, material reserves and excitation conditions. The geological structure and topography have provided favourable conditions for the development of debris flow in Southeast China,  such as a large amount of clastic loose sediments, and the place for debris flow formation, movement and accumulation. Rainstorm is the source and component of debris flow, and it is also the excitation condition and main driving force of most debris flow in Southeast China . Because of rainfall penetration, the soil on the hillside slope became unstable. Then debris flow is formed by mixture of the loose solid substances sliding down from the hillside slope and the water (Chen et al. 2013;Hungr et al. 2014).

Hazard factor
Typhoon and rainfall are considered to be hazard factors. The change of typhoon landing place and the impact area of disaster are closely related to moving path  Figure 8, typhoons of northwest path account for more than half, which need to be highly valued. Besides typhoon hazard factor, the critical rainfall that triggers debris flow can be determined by effective antecedent rainfall (R c ) and current day rainfall when debris flow occur (R d ). The formula for effective rainfall is shown as follow: In the formula, R c is the effective antecedent rainfall, R n is the rainfall at n days before the occurrence of debris flow, a is the attenuation coefficient that can be considered as 0.7 during typhoon period. The relationship between R c and R d is shown in Figure 9 (Feng et al. 2013). Samples of debris flow in the southeast coast of China are collected, F 1 and F 2 are threshold lines of rainfall. There is no debris flow in area 1, the possibility of debris flow increases in area 2 and lots of debris flow disasters occurred in area 3. If a single parameter of the critical rainfall is considered, the day rainfall when debris flow occurred can be counted as critical rainfall (Figure 10).  When the amount of precipitation is over 200 mm, the risk of disaster is extremely large. The critical rainfall can be based on 100 mm, when the amount of precipitation is over 100 mm, relevant departments need release early-warning and prediction.

Hazard-formation environment
The southeast coast of China is adjacent to the Western Pacific, tropical cyclones occur frequently, and with the significant impact of subtropical monsoon climate, heavy rainfall and storm surge will occur frequently. The mountains and hills are widely distributed especially in Zhejiang, Fujian, Guangdong and other coastal provinces and the geological conditions are complex with lots of clastic loose sediments. When typhoons land, then climb or round hills, the mountain has induced and forced effects. With feedback effect of typhoon rainstorm, low pressure is beneficial to form and develop, the mountain will strengthen the intensity of typhoon rainstorm. In the hilly areas of the southeast coast, the disaster chain of typhoon-rainstorm-debris flow is prone to be triggered.
Recently, the rapid expansion of urban land and industrial land results in a significant reduction in area of farmland and forest. Urban and engineering construction has gradually extended from plain to mountain area, especially in last several years, large-scale traffic construction, mine exploitation and other mountain engineering construction destroy the stability of mountain and slope, thus the potential risk of geological disasters is increased Qiu et al. 2017;Wang et al. 2018a,b). A large number of railway lines and mountain highway cross through the hilly areas, many tunnels have been constructed, which make the soil of fault fracture zone and the mountain with steep slope become more unstable (Lai et al. 2016d;Wang, Song, et al. 2018;Zhang, Li, et al. 2018). Thus, landslides and Figure 9. Relationship between effective antecedent rainfall (R c ) and the current day rainfall when debris flow occurred (R d ) in typhoon-debris flow disaster chain. Sources: re-drawn by authors (Feng et al. 2013).
debris flows occur more easily and frequently under the effect of heavy rainfall and gravity.

Hazard bearing body
With the rapid development of transportation, infrastructure construction is getting faster, at the same time, the destruction of mountainous areas caused by traffic construction leads to tunnels in the hill areas most at risk Wang et al. 2018e). To review the incident scene, tunnel debris flow will easily form the accumulation at the tunnel portal, and lead to water or mudflow rushing into tunnel. Seriously, people and traffic facilities will be washed away by the large mudflow at tunnel portal and traffic is prone to large jam. Disaster mechanism of typhoon disaster chain in coastal areas is shown in Figure 11.

Summary and perspective
Over the past several decades, the economic development has grown fast in the southeast coastal areas. When population and wealth are highly concentrated, a variety of natural disasters is frequently occurring. The period of high risk and profit has arrived (Liu et al. 2012(Liu et al. , 2018a. Heavy rainfall caused by typhoons can easily induce mountain landslides, debris flow and other geological disasters in southeast coastal area (Hong et al. 2010;Nie et al. 2018;Ren et al. 2018a,b). In view of current situation, it is vital to investigate disaster prevention further.
(1) The study on the distribution law of debris flow in coastal areas of SE China should be improved. For tunnel debris flow hazard-prone areas, Air-Space-Ground Stereo monitoring network can be carried out ( Figure 12). "Air" means applying oblique photography technology of low-altitude unmanned aerial vehicle (UAV). "Space" implies the application of GNSS satellite monitoring system that can implement dynamic monitoring of all-day and all-weather in the area of geologic hazard. "Ground" means using 3D laser scanning and measuring robot to monitor depth displacement of soil. With the application of Air-Space-Ground 3D monitoring network, accurate and reliable dynamic monitoring data will be provided for the implementation of emergency investigation and treatment project in disaster areas, a security network with high degree of informatization, strong sensitivity of reaction, and timely early warning of geological disasters will be constructed, the smooth progress of the geological disaster treatment project is ensured effectively.
(2) After improvement of monitoring system, the perfect early-warning mechanism of debris flow disasters ought to be established ( Figure 13). The study of the relationship between disasters and monitoring data such as rainfall or deformation should be strengthened and the rationality of forecasting methods and the timeliness of forecasting need to be advanced. Unified data acquisition platform needs to be established to analyse Figure 11. Disaster mechanism of typhoon disaster chain. Sources: drawn by authors. Figure 12. Air-space-ground monitoring system. Sources: http://www.cigem.gov.cn/auto/db/detail. aspx?db¼1006&rid¼37359&md¼15&pd¼210&msd¼11&psd¼5&mdd¼11&pdd¼5&count¼20 and re-drawn by authors.
the potential links between various data. Cloud platform should share and exchange information with various departments. After information has been summarized and processed, warning platform uniformly release to the masses through different ways.
(3) The land use structure can be adjusted reasonably. To evaluate the present situation of land use and the risk of geologic hazard. It is necessary to attach importance to ecological construction and human activities. The land use of urban construction and traffic construction should be controlled (Wang and Ling 2010;Wang et al. 2012;Zhou et al. 2017Zhou et al. , 2019Zheng et al. 2019a,b). Then, restoring vegetation in the area of debris flow in order to consolidate topsoil, reduce water loss and soil erosion.
(4) Comprehensive treatment of engineering measures and relocation can be carried out. Major engineering construction should avoid high risk areas of geologic hazard. Traffic engineering facilities that cannot be relocated could construct corresponding diversion, blocking, drainage and other projects.
(5) After disasters occur, all departments and related areas are supposed to establish advantageous cooperative linkage and information sharing mechanism, so as to ensure the efficient and orderly response.