1. Introduction
Global warming has led to great changes in the cryosphere, including ice sheet melting [
1,
2], glacier melting [
3,
4], permafrost degradation [
5,
6,
7], land desertification [
8] and snow cover reduction [
9,
10], resulting in frequent disasters [
11,
12,
13]. As an important part of the cryosphere of the Qinghai–Tibet Plateau [
14,
15], permafrost has the characteristics of high ground temperature, poor hydrothermal stability and climate sensitivity [
16,
17]. The increasing air temperature, due to the climate change on the Qinghai–Tibet Plateau, warms the underlying permafrost, deepens the active layer, melts the underground ice and, finally, increases the depth of the permafrost table [
18]. Followed by the increasing air temperature, permafrost degradation triggers more frequently thermal hazards [
19], which greatly affects the ecological and hydrological environment in permafrost regions and threatens the safety of residents [
20,
21,
22] and engineering infrastructures built on permafrost [
23,
24,
25]. Similarly, the linear engineering of the QTEC also faces more serious permafrost thawing disasters [
26,
27,
28,
29,
30,
31,
32,
33,
34,
35,
36].
The current assessment of environmental disasters in permafrost areas is carried out qualitatively and quantitatively. The qualitative assessment mainly focuses on the evaluation of the engineering geological conditions on the Qinghai–Tibet Plateau and risk factors are selected on the basis of field surveys, monitoring data of permafrost and engineering buildings, and experimental data [
27,
37]. The quantitative evaluation mostly adopts the following eight methods. (1) Catastrophe progression method: risk factors are selected from the perspective of thermal stability of permafrost and natural environment for engineering geological evaluation [
38]. (2) Hazard degree analysis: according to the disaster environment, the characteristics of the disaster and the relevant literature, the disaster-causing factors of different geological disasters are selected [
39]. (3) The settlement index (Is): the active layer thickness and volume ice content are selected to analyze the thaw settlement hazard [
32,
33,
40,
41,
42]. (4) The risk zonation index (Ir): factors such as surface properties, volume ice content, soil texture and active layer thickness are selected to analyze the degree of thaw settlement disaster [
43,
44]. (5) The permafrost settlement hazard index (Ip): the effects of ecological factors such as soil texture, vegetation and organic content of soil on permafrost thawing disasters are considered [
45]. (6) The allowable bearing capacity index (Ia): the effects of soil type and mean annual ground temperature on the stability distribution of permafrost on the Qinghai–Tibet Plateau are considered [
46]. (7) The AHP-based Geo-hazard index (Ig): an analytic hierarchy process is used to select the corresponding thawing settlement risk factors [
18,
41]. (8) The combined index (Ic): the influence of three Geo-hazard indexes (Is, Ir and Ia) on permafrost thawing disasters is comprehensively analyzed [
47]. The main risk factors considered in the above methods include mean annual ground temperature, active layer thickness and volume ice content. At present, for the risk assessment of permafrost environmental disasters, the selection of risk factors usually adopts the traditional risk identification methods (expert investigation method, fault tree analysis method, checklist method, etc.) [
48,
49,
50]. The traditional method, mainly based on the qualitative and static thinking approaches, identifies and establishes a set of risk factors by considering the relevant normative data, the experts’ experience and the field survey. However, many influencing factors need to be considered in the process of risk identification and cannot be quantitatively described. These limitations restrict the application of the traditional methods, which can only simply identify and list the influencing factors of a certain risk and cannot analyze the interactions among the influencing factors. Therefore, traditional methods may cause one to miss the consideration of potential risk factors for a certain risk.
As the environment in the QTEC changes with time and climate, the environmental risk factors faced by the infrastructures in the Corridor are constantly changing. Therefore, it is necessary to dynamically identify the environmental risk factors and study their mutual influence [
51]. The objectives of this study are as follows: (1) to provide a dynamic engineering environment risk identification model which can trace the causes and analyze the consequences; (2) to figure out the degree of influence among dynamic risk factors through partial correlation analysis; (3) to identify the distribution characteristics of each factor in different static risk factor zones.
In order to achieve the research objectives of this paper, we selected the remote sensing data of five risk subsystems, i.e., regional geomorphology, climate change, ecological environment, permafrost environment and water environment, from 2003 to 2016 and put forward the following hypotheses: (1) climate risk is the root factor that leads to the dynamic change in engineering environmental risk factors at all times; (2) climate risk factors have a greater impact on other dynamic risk factors.
5. Discussion
Under the influence of climate change and human activities, the engineering environment in the cold region is a complex, open and dynamic risk system and the system has multiple input and output variables. The exchanges of material and energy, constantly occurring over time in the atmosphere–infrastructure–permafrost system, are affected by many factors (
Figure 2), so that the risk level of the system is always in a dynamic state, which not only increases the risk of the system, but also reduces the safety, stability and service performance of the project in the environment [
101].
Traditional risk identification methods are mainly divided into the expert investigation method and decomposition analysis method [
102,
103]. Their applications are largely limited by the following points: (i) it is easy to miss potential risks in changing risk factors; (ii) the expert investigation method is too one-sided and the cost is high; (iii) the decomposition analysis method can only simply list the risk factor matrix; (iv) the traditional method only looks at the risk of the system from a static perspective, which cannot reflect the dynamic nature of risk factors, that is, it simply combines the risks of each subsystem to analyze the risk of the whole system [
104]. The SD is centered on the feedback process of the system, focuses on determining the relationship between the influencing factors of the system from the overall perspective, pays attention to the overall change behavior of the system and then analyzes the causal relationships among various factors. Therefore, this method can find various hidden risk factors in practical applications. Furthermore, the process of engineering environmental risk in cold regions constantly changes and progresses. When the accumulated energy exceeds the bearing capacity of the system as the risk continues to evolve, it inevitably leads to the continuous occurrence of freeze–thaw disasters in cold regions, which, in turn, causes engineering safety accidents. Using system dynamics to establish a risk identification model with causal relationships not only can reflect the complexity and dynamics of cold region engineering environmental risk systems but can also analyze the conductivity, derivative, complexity and dynamics of various factors [
105]. At the same time, the model analyzes and identifies various risk factors from a macro perspective. Combined with the analysis of the influence degree of risk factors by partial correlation analysis, the risk system at any spatial scale can be further modified and analyzed. The SD regards the engineering environment as a system and divides it into different subsystems for analysis and research from the perspective of the whole. The combination of experts’ experience would be more conducive to our understanding of the engineering environment and other things and the system in which it is located can be more finely divided into multiple subsystems for analysis. Therefore, the combination of experts’ experience and SD can be used for overall understanding and detailed analysis of the system on a larger scale.
For the assessment of disasters caused by permafrost degradation, this is mainly divided into the qualitative evaluation of the environmental characteristics of permafrost engineering, focusing on the Qinghai–Tibet Plateau, and the use of different types of disaster risk index models to evaluate the disaster susceptibility on a large spatial–temporal scale [
106,
107]. The qualitative evaluation is based on the distribution characteristics of geological characteristic parameters such as permafrost type, MAGT, ALT, MAAT and soil type in different regions. The quantitative evaluation mostly adopts eight evaluation models [
18,
32,
33,
38,
39,
40,
41,
42,
43,
44,
45,
46,
47]. The main risk factors of permafrost degradation considered in quantitative assessments are volume of ice content, active layer thickness and mean annual ground temperature. On this basis, according to the characteristics of the permafrost environment in different regions and the needs of analysis, different risk factors are added for analysis. From the current research study, it was found that, in the current risk analysis of environment disasters, only the risk factors were statically identified based on the relevant literature and the interactions between the risk factors were ignored.
In this paper, the method of system dynamics was used to identify the engineering environmental risk in cold regions (
Figure 2) and the influence degree of each risk factor was analyzed by means of the partial correlation coefficient (
Figure 10f,
Figure 11f,
Figure 12f,
Figure 13h,
Figure 14h,
Figure 15h and
Figure 16g;
Table 7). According to the established risk identification model, the five main transmission paths of the engineering environmental risk system were analyzed and obtained, with the starting point of each transmission path being the climate change factor. Therefore, it could be inferred that the climate factor is the root factor in the transmission process of engineering environmental risk factors. According to the regional analysis and partial correlation analysis, it was found that the climate change factors affected the distribution characteristics of other dynamic risk factors in different static risk factor regions and that the climate change factors had a greater impact on the interaction degree of the dynamic risk factors. In general, there are still some shortcomings in this paper. (i) In order to keep the same resolution of remote sensing image data of various influencing factors, spatial interpolation was used in the process of basic data preprocessing, which made the accuracy of the calculation results of the partial correlation analysis among influencing factors have a certain deviation. (ii) The time limit for selecting influencing factors was short; follow-up work could expand the selection period of influencing factors and improve the accuracy of the partial correlation analysis. (iii) This paper used an empirical model to obtain basic data of some influencing factors and the accuracy of empirical models is affected by many parameters; therefore, follow-up work should strengthen the research and acquisition processes of the original data of the influencing factors. (iv) The engineering environment risk identification model for cold regions established in this paper was only analyzed by taking the QTEC as an example. However, the regional geological environments of permafrost in the world is quite different. Therefore, the research scope should be expanded in follow-up work and the risk identification model should be further revised according to the characteristics of the engineering environments in different regions.
6. Conclusions
In this paper, we present a novel method for the identification of engineering environmental hazards in cold regions and an engineering environmental risk identification model to dynamically identify the risk factors of engineering environmental hazards in the cryosphere. We successfully studied the environmental hazards in the QTEC. The following conclusions were drawn.
(1) The established model could describe the causal feedback relationships among the inter-influencing subsystems in the risk system, identify the interaction relationships among different risk factors and potential risks and clarify the structure of the whole system. Moreover, the model not only could trace the root cause and intermediate factors of a certain risk, but could also analyze the direct and indirect consequences, as well as forming the transmission path of the risk. The model identified five types of parallel risk paths in the QTEC.
(2) The static risk factors (elevation, slope, aspect, RDLS and VIC) were divided into different regions according to different classification standards and the spatial distribution characteristics of the dynamic risk in different regions were analyzed. In the areas with elevation below 4200 m and above 6000 m, the climatic factors mainly affected the spatial characteristics of other risk factors. The influence of the climatic factors on other risk factors was not related to slope change, but the influence relationship was different in different slope areas. In the southeastern, western and northwestern regions, the climatic factors mainly affected the spatial distributions of other risk factors. In the plain and moderate-relief mountain regions, the climatic factors mainly affected the spatial changes of other risk factors. In the zones of talik, ice-poor permafrost and ice layer with soil, the climatic factors were the main reason.
(3) Climate warming leads to the degradation of permafrost, which has a great impact on the permafrost environment and ecological environment of the QTEC. In the permafrost environment, the MAAT had a strong correlation with ALT and MAGT and the influence degree was significant; in 25.24% and 53.76% of the regions, the changes in MAST were affected by MAAT and MAP, respectively. In the ecological environment, the changes in ET in 11.80% and 12.46% of the regions were affected by MAAT and MAP, the changes in FVC in 45.09% of the regions were affected by SR, the changes in SM in 45.67% of the regions were affected by MAP and the ecological environment factors in other regions affected each other.
(4) Climate change and human activities have a significant influence on the allowable bearing capacity of permafrost in the study area, which further affects the stability of infrastructures. The dynamic risk drivers for the allowable bearing capacity were MAAT (strong correlation), ALT (45.88%) and SM (32.76%).
(5) Climate warming leads to the melting of underground ice and then forms thermokarst lakes of different sizes on the surface, which induces a variety of thermal thawing disasters and affects the safety of infrastructures. The area change of thermokarst lakes in the Corridor was affected by MAP (0.533), ALT (0.492), SM (−0.467) and MAST (0.417) and was significantly correlated with the first three factors.
Based on the above research results, follow-up work should carry out relevant engineering environmental risk identification and analysis in the Qinghai Tibet Plateau, Northeast China and the Arctic on a large scale according to the accuracy of remote sensing data.