The Risk Map of Cross-Regional Cultural Heritage: From a Perspective of Slow Degradation

Abstract

The Cross-Regional Cultural Heritage (CRCH) is a continuous, large-spanned, and comprehensive system with a large number and diversity of components. To support the integrated conservation of CRCH, an analytical approach with simple structure and easy operation is needed. Therefore, this paper aims to conduct risk maps to interpret, understand, and manage the slow degradation of built heritage in the daily environment. The hazard factors of slow degradation in the environment are extracted and translated into meteorological data for mapping with the Geographic Information Systems (GIS). A risk map to evaluate the risk level of the heritage is obtained by overlapping the vulnerability and hazard factors. With the case study of the Chinese Eastern Railway (CER), the risk maps revealed the risk condition and spatial distribution pattern of CRCH, based on which a network-connected working platform is developed, and suggestions for solutions are proposed. In terms of the operational plan for conservation works, the graded architectural risk sections are delineated. The priority and extent of intervention are proposed according to the degree of urgency and preventive conservation measures can be implemented in advance by using weather forecast data. In terms of social management strategy, through a level-by-level penetration education model, the social awareness of heritage conservation can be cultivated, the technical methods for restoration and maintenance will be trained, and the operating system for monitoring and inspection can be established. Relevant stakeholders, such as government officials, experts, users, and visitors can participate together in the preventive conservation process of the heritage.

1. Introduction

Cultural heritage is a sustainable resource with significant value and potential [1]. For economically disadvantaged or geographically remote areas, the attractiveness of cultural heritage can help stimulate tourism development, promote regional industrial transformation, and boost economies [2]. Therefore, the preservation and utilization of cultural heritage is a necessary long-term and continuous strategy, both at the national level and at the level of cities, towns, and villages, especially for those areas that are still in the process of modernization and urbanization.

The conservation of cultural heritage, especially architectural heritage, is a comprehensive and complex task that involves political, capital, and cultural aspects, and includes a game of interests at all scales, from the state to the city, the community and the individual [3,4]. Spatial scale is an important factor that increases the requirements and difficulties of heritage conservation. The heritage concepts related to large spatial scale include Heritage Area [5], Cultural Route [6], and Heritage Corridor [7]. Such cultural heritage becomes Cross-Regional Cultural Heritage (CRCH) when its spatial scale spans more than two different cultural regions, geographical regions, or administrative regions. CRCH contains a large number of elements, mainly immovable monuments, buildings or relics, but also covers landscapes and natural environments. As a result, CRCH has a strong impact on the regions in which it is located, making its preservation and reuse pivotal to social and ecological sustainability and to economic development as well [8,9]. In contrast, the cross-ethnic, cross-regional, and cross-cultural protection of heritage complexes also makes the process encounter more and more difficult problems [10].

There is an ongoing paradigm shift within the cultural heritage discourse, both for individual monuments and for CRCH, the thrust of which is, on the one hand, the preservation of cultural heritage in parallel with the regional development, on the other hand, the theoretical and operational integration of cultural heritage into the concepts of sustainability and resilience [11]. In this context, the concept of preventive conservation of architectural heritage has evolved, which can be defined as the periodic condition assessments, early damage detection, and planned interventions to minimize deterioration and enable long-term resource efficiency [12]. During the preventive conservation project, the first step is the diagnosis of the cultural heritage, with the aim of knowing the damage and degradation of monuments to design appropriate interventions and maintenance projects, to support the analysis of threats and vulnerability [13]. Preventive conservation measures include social awareness, skills training, documentation activities, and the organization and management of periodic inspections and maintenance for architectural heritage, with the purpose of enhancing the resilience of buildings against threats, reducing vulnerability or alteration agents to minimize risks [14]. Therefore, preventive conservation is fundamentally a form of risk management that addresses the characteristics of cultural heritage. The definition of risk can be summarized as a measure of the possibility of occurrence of a threatening event and of its potential negative consequences [15]. The common denominator between preventive conservation and risk management is a perspective shift from reaction to prevention [16], by utilizing innovative and cost-effective approaches to promote sustainable development. Research related to cultural heritage risks is now well established, with issues such as establishing risk models, conducting risk assessment, carrying out risk analysis and developing risk management [17,18,19]. A risk model is a basic system that includes formulae, constants, and variables together with unambiguous definitions of the constants and variables, follow which the assessment and analysis can be conducted. Risk assessment starts with a formal and structured identification of the generic and specific risks, based on which the magnitude values of association between affecting factors and the risks can be calculated. Then, consequent analysis is conducted by qualitative or quantitative methods. With all the results, risk management consists of the proposal of different ways of mitigating the risks and the guidance on the application of available resources [20].

From a methodological point of view, risk can be seen as the product of the hazards of the surrounding environment for the vulnerability of the exposure [21]. Hazard is the probability that a phenomenon may occur in a certain area during a period, which will cause a negative effect when it exceeds the threshold [22]. Three types of hazards are mentioned in the protection of architectural cultural heritage, which are static-structural hazards, environmental-air hazards, and anthropogenic factors. Among all the types, the immediate dangers caused by earthquakes, floods, fires, or wars, are most concerned [23]. This type of hazard is characterized by a lower frequency of occurrence, but their effect is severe and therefore receives more attention. In the everyday environment where the built heritage resides, there are hazards that accelerate or intensify the decay and deterioration of the buildings, and although the severity of the consequences is low compared to disaster, they are constant presences. For the reuse of architectural heritage, the risk caused by this kind of hazard is more worthy of attention, and it is the main work to be dealt with in preventive protection.

Hazard is brought by the external environment, while vulnerability is for the object itself that is affected or threatened, which can be defined as the degree of loss of elements as a consequence of the occurrence of hazard [24]. Vulnerability is the interaction of sensitivity, exposure, and adaptability to the capacity of policies, environments, and society to the hazards [25]. In the study or practice of cultural heritage risk management, vulnerability assessments are mandatory for a comprehensive assessment of risk, which can be examined with either engineering or social approach [26]. The former is concerned with the physical state of the heritage elements, while the latter is dealing with the characteristics of the community or population that lead to differential impacts of natural hazards [12]. In concrete practice, vulnerability often needs to be quantified into specific values, which can be defined as vulnerability index calculated by vulnerability matrix. According to the vulnerability index distribution, the vulnerability of the heritage can be divided into sequential levels to provide corresponding protection strategies in the risk management stage [27,28].

When it comes to the risk of CRCH, whether it is faced with a sudden disaster or a daily environment, it generally involves a large number of objects in a large-scale spatial scope with geographic information, natural environment, climate situation, and social conditions. Therefore, technologies, methods, and tools of Geographic Information System are widely utilized to overlap the hazard, vulnerability, and related elements to create risk maps. The Risk Map of Italian Cultural Heritage is a project initiated in Italy in the last century [23], which implemented data collection [29], spatial analysis, and the construction of a digital information system [30], to display the degrees of the potential impacts on cultural heritage and explore ways to mitigate risks. A mainstream pattern which can be found in the literature review is that risk map studies generally focus on natural hazards in a single scenario, such as seismic [31], flood [32], and landslide [33]. On top of that, risk maps under multi-scenario are derived, in which multi-hazard risks are superimposed in a specified area [34]. By appraising the level of hazards according to their effects, risks will be clarified and ranked to support proposals of preventive strategies.

As environmental issues become a growing worldwide problem, the environmental-air hazards of architectural heritage are receiving more attention. On one side, climate change and pollution are exacerbating common environmental conditions and increasing the degradation rate of cultural assets [35]. On the other hand, the slow cumulative damaging process caused by daily environment changes is also considered as a progressive risk. The innovative concept of heritage climatology is induced to associate the traditional different climate types with potential deterioration of buildings [36,37]. As a methodological system for cultural heritage risk response, one of the key advantages of a risk map is its spatial scalability. A risk map can be used across cities or nations and can also be extended to a larger spatial scale, such as the intercontinental scale or even the global scale, making it an ideal method for the preventive conservation of CRCH.

For most countries, it is hard to bear the financial pressure of carrying out a multi-hazard risk analysis for CRCH with a large number of assets [15]. Architectural heritage is at greater risk in areas with small populations and poor economic conditions, which makes conservation frequently unavailable [38]. This article focuses on the evaluation and management of the slow degradation risk of CRCH in the daily environment and aims to develop an intensive, sustainable, and integrated preventive conservation strategy for areas with relatively backward economic conditions and few disasters. To address this need, a simplified methodology is developed to perform the qualitative risk assessment of numerous heritage assets with limited resources, which will be easy to use for non-technical user groups to work with experts in preventive risk reduction strategies. The paper consists of four main sections. First, a brief introduction to CER cultural heritage was given. Secondly, the work content and creation method of a risk map for slow degradation are discussed. Thirdly, a risk map was produced for CER illustrating the risk status of its heritage. Finally, based on the risk analysis, recommendations are made for taking advantage of risk maps for preventive conservation.

2. Materials and Methods

2.1. Chinese Eastern Railway as an CRCH

Located in northeastern China, the Chinese Eastern Railway (hereafter CER) was built by Tsarist Russia in the late 19th and the early 20th century. The total length of the railway is 2489.2 km which spans four different Chinese provinces. The main line is 1514.3 km long runs from Manchuria to the Suifenhe, and the southern branch goes from Harbin to Lüshun which is 974.9 km long. In the course of decades, thousands of transportation, industry, military, public, and residential buildings, as well as various railway projects and municipal facilities, have been developed along the railway. After a hundred years of history, the architectures preserved along the Chinese Eastern Railway are undoubtedly a valuable cultural heritage with far-reaching significance and unique features. The CER has undergone a complex change in ownership and designation. In this paper, CER refers to the entire railway route during the period from 1898 to 1905 when Russian dominated, and the main line during the period from 1905 to 1935 apart from the southern branch controlled by Japanese. Cultural heritage of CER covers the architecture and engineering facilities along the railway during these periods (Figure 1).

In the planning of CER, according to the distance, geographic environment, and resource conditions of each location on the line, 92 stations were set up, which were classified into five classes considering the functions and importance (

Table 1. The number of CER stations of different classes.

Classes of Stations	First	Second	Third	Forth	Fifth
No.	2	9	8	34	39

). In addition, there are 95 passing loops to meet the demand of railroad driving. At the time of the construction of CER, most of the areas along the main line were uninhabited; however, since the railway started operation, the areas where the stations from grade 1 to grade 3 were located developed into cities and towns, which to a certain extent constitute the distribution of the administrative hierarchy of the northeastern region (Figure 2).

The architectural heritage of CER includes two major categories, Engineering Facilities and Architecture. Railway engineering facilities mainly include railway tracks, bridges, tunnels, and culverts. The railway industry has a comprehensive demand on large numbers of buildings with different functional types along the railway. According to statistics from our survey of 1912 existing architectural heritage sites, there are four functional categories in general and 14 specific categories within them (

Table 2. Categories of CER built heritage.

General Category of Buildings	No.	Specific Category of Buildings	No.
Railway traffic and industrial architecture	559	Stations	85
		Work areas	224
		Railway industrial buildings	19
		Railway annexes	207
Buildings for military and police stations	117	Military buildings	84
		Police stations	5
Public buildings and integrated service facilities	89	Office buildings	54
		Hospitals	20
		Educational buildings	11
		Entertainment buildings or sanatoriums	26
Railway community residential buildings	1147	Residences for staffs	1132
		Dormitories for staffs	15

). Each specific category contains specific building types (Figure 3). In addition, the function of some buildings on the surveyed sites could not be identified. The buildings along the CER are mainly made of brick, stone, and wood, among which the number of masonry buildings is 1647, accounting for more than eighty percent. Therefore, masonry buildings are taken as examples in the study of risk map in this paper, while wood structures are not considered for the time being.

CER is a typical linear CRCH with a huge spatial span and obvious diverse and regional characteristics of the natural and social environment. Geographically, CER crosses regions, such as mountainous areas, forest areas, plains, and coastal areas. Climatically, it crosses warm temperate zones and mid-temperate zones, and also goes through humid zones, semi-humid zones, and arid zones, which leads to large differences in temperature and humidity and obvious gradients of changes. The daily natural environmental risks faced by architectural heritages also correspond with these distribution characteristics. In terms of administrative scope, CER traverses four provinces, which are Inner Mongolia, Heilongjiang, Jilin, and Liaoning, meanwhile, the regional scale in which the heritage is located includes district, village, town, city, provincial, and inter-provincial scopes.

2.2. The Risk Map of Cross-Regional Cultural Heritage

The risk map will be created by overlapping the vulnerability index of the cultural heritage and the hazard map of the environment [39,40] (Figure 4). The indexes of damage and loss of adaptive capacity can be obtained through a series of measurements and calculations [41,42,43], on the basis of which the vulnerability index was derived. The environmental hazard factors of the area where the building is located can be recognized by field survey and building pathology analysis [44,45]. Through datafication and graphical representation of the hazard factors, hazard maps were created.

2.2.1. Vulnerability Assessment

Vulnerability assessment is a measurement of the ontological state of architectural heritage, measured by sensitivity, exposure, and adaptive capacity. The slow degradation hazard factors that accelerate the aging of built heritage are widespread in the daily environment, so the exposure of heritage is a constant, ever-present state that can be disregarded in the calculation. The sensitivity of built heritage is measured by its current damage level. Vulnerability index is the superposition of Damage index and the loss of adaptive capacity of the building heritage.

• Damage index

The damages of masonry-built heritage affected by the daily environment can be divided into many categories depending on their manifestations. In this paper, the damage identification and diagnosis are based on the integrated results of damage types, manifestations and basic pathogenic mechanisms of brick and stone materials in the Online Damage-Expert for Monumental Buildings (https://mdcs.monumentenkennis.nl/damageatlas/1/material, accessed on 1 December 2021), which was derived from the Monument Damage Diagnostic System (MDDS) developed under the European Environment Programme [46]. It is a support tool for the evaluation of damage to historic buildings that can help define and recording damage types in visual inspection. In order to simplify the technical process of damage identification while covering the main problems occurring in the heritage, five main types of damage prevalent in CER building heritage were summarized in conjunction with the field survey, including disintegration, structural cracks, deformation, mechanical damage, and settlement.

Observation and recording of the degree of damage to architectural heritage, are mainly conducted through on-site visual inspection. Damage assessment is conducted by valuing observed factors which are easy to be observed and can be evaluated quantitatively. Observed factors are indicators/indexes that can be used to measure the degree of damages. For example, disintegration can be used as an observed factor, then its area and the maximum depth are the observed values (

Table 3. Specification of the damages.

Damage Type	Damage Form	Damage Description	Observed Factors
Disintegration		The integrity of a building material is lost. Disintegration may occur inside a material or between two materials that supposed to be attached to each other and forming a whole. One of the most common damages in CER buildings.	Area; Maximum depth
Structural cracks		Brick or stone is fissured or broken, and can be clearly observed on the wall, not just limited to the surface of the material unit. One of the most common damages in CER buildings.	Maximum Width; Maximum length
Deformation		The original form of the masonry is altered. Deformation may include an increase in volume or a deviation from the original form in various ways, which may involve the whole wall or part of it. Deformation is a serious structural damage, which is relatively rare in CER buildings	Partial deformation; Structural deformation
Mechanical damage		Mainly caused by anthropogenic factors and has a weak relationship with the natural environment. Mechanical damage is prevalent in CER buildings and may spawn or accelerate other damages, also may lead to structural problems.	Metal perforation; General damage
Settlement		Caused by the slow natural subsidence of the building and is mainly influenced by the geological factors of the area where it is located. It occurs in some areas along CER.	Depth; Type

). The scoring criteria of observed factors is determined by the construction characteristics and current state of the building heritages of CER [47], and a score of 0 was assigned when the factor is not present (

Table 4. Scoring criteria of observed factors of damages of masonry building heritages of CER.

		Scoring Criteria of Observed Values
Damage Type	Observed Factors	1 Point	2 Points	3 Points	4 Points
Disintegration	Area	0 < s ≤ 1/8S	S/8 < s ≤ S/4	S/4 < s ≤ S/2	S/2 < s ≤ 3S/4
	Maximum depth	0 < d ≤ 10 mm	10 mm < d ≤ 30 mm	30 < d ≤ 60 mm	60 < d ≤ 130 mm
Structural cracks	Maximum width	0 < w ≤ 5 mm	5 mm < w ≤ 15 mm	15 < w ≤ 25 mm	w > 25 mm
	Maximum depth	0 < w ≤ 5 mm	5 mm < w ≤ 15 mm	15 < w ≤ 25 mm	w > 25 mm
	Maximum length	0 < l ≤ L/4	L/4 < l ≤ L/2	L/2 < l ≤ 3L/4	3L/4 < l ≤ L
Deformation	Partial deformation	—	—	occurrence	—
	Structural deformation	—	—	—	occurrence
Mechanical damage	Metal perforation	occurrence	—	—	—
	General damage	scratch	perforation	splitting	Chipping
Settlement	Depth	Threshold slightly higher than outdoor ground	Threshold as high as outdoor ground	Threshold slightly lower than outdoor ground	Threshold is well below the outdoor ground
	Type	—	—	—	Uneven settlement

Note: “s” stands for the area of disintegration, “S” stands for the area of the wall, “d” stands for maximum depth of disintegration, “w” stands for with of structural cracks, “l” stands for maximum length of structural cracks, “L” stands for height of the wall.

).

The threats posed by damages are different, and the intensity of observed factors for each damage is also dissimilar, so, in the process of index calculation, it is necessary to determine the weighted values of each damage and its observed factors. The weighted values of the damages and their observed factors were calculated by Analytic Hierarchy Progress (AHP), and a questionnaire survey of 22 experts was conducted. To ensure that the results fit better with the characteristics of CER architectural heritage, the experts were selected to meet two conditions: firstly, their research interests are directly related to CER cultural heritage, and secondly, they have conducted field research on the CER architectural heritage, and the number of surveyed or measured buildings is greater than 100. The results are shown in

Table 5. The weighted values of the damages and the observed factors of CER architectural heritage.

Damage Type	Disintegration		Structural Cracks			Deformation		Mechanical Damage		Settlement
Weighted Value (WV)	0.1596		0.2765			0.2979		0.1422		0.1238
Observe factors	Area	Maximum depth	Maximum Width	Maximum depth	Maximum length	Partial deformation	Structural deformation	Metal perforation	General damage	Depth	Type
Weighted value (WV)	0.0708	0.0889	0.0878	0.0881	0.1006	0.1256	0.1722	0.0562	0.0860	0.0462	0.0776

.

Damage index (DI) is a synthetic indicator obtained by superimposing the indexes of all damages. In the case of a building, for a certain damage, the cumulative sum of product of the observe values (OV) and their corresponding weighted values (WV) is the DI of the damage; and the cumulative sum of the DI of all the damages is the DI of the building [48].

The calculation formula can be expressed as [49]:

$$\mathrm{DI}= \sum OVn\times WVn$$

Based on the above indexes and formulas, damage indexes of all CER buildings can be calculated. According to the numerical distribution of damage indexes, the damage level of the buildings can be divided into four grades: mild (0–0.62), general (0.63–1.19), serious (1.20–2.14), and endangered (2.15–3.71).

2.

Loss of adaptive capacity

Adaptive capacity is the ability of a system to modify its behavior to cope better with changes of its living environment, which stresses the shifting philosophy from potential to feasible adaptation [25]. For architectural heritage, various factors affect the adaptive capacity, including the identity of the building owner, local government policies, state of utilization, and functional condition of protective constructions.

The architectural heritage management policies of the four provinces in the region through which CER passes do not differ significantly. Although most of the architectural heritage is on the conservation list of each province, no actual conservation measures or reuse programs have been implemented, and overall, the situation of building conservation is a vacuum. Through extensive site surveys, the adaptive capacity factors that influence the risk level of CER architectural heritage can be summarized in three main aspects: whether the building is in use, the functional status of the protective construction, and the presence of inappropriate repair treatments. Although the spontaneous use of built heritage by residents without technical guidance and rules may cause some problems, in general, as long as the building is still in use, it can be maintained to some extent, which is important for the preservation of the buildings [50]. On the other hand, in severe cold regions, domestic heating in buildings in winter can prevent or slow down the occurrence of freezing damage to some degree. Hence empty buildings are generally more vulnerable [38]. This conclusion is consistent with the survey results, and, moreover, many vacant CER buildings are in a state of abandonment and at greater risk. Intact protective building structures, such as roof design, apron slope, and downspouts, are important factors in resisting further damage to the building in negative environments [51]. The CER buildings that are still in use are mainly utilized as residences by inhabitants of towns or villages along the route, who do not have the knowledge and skills for heritage maintenance and restoration, and, therefore, have taken inappropriate restoration measures when the buildings were damaged or decay. For example, when there is material loss caused by weathering on the wall, they generally use cement to cover it directly. Although the building is repaired in a short time, it will lead to more serious material shedding and accelerate decay. In the field inspection, the conditions of the three adaptive capacity factors were observed and recorded, and according to their severity levels, scores were given to obtain the value of loss of adaptive capacity. The scoring criteria is shown in

Table 6. Scoring criteria of loss of adaptive capacity.

Type of Loss of Adaptive Capacity	1	2	3	4
Utilization condition	In use	—	—	Vacant
Protective construction	intact	Minor damage	Medium damage	Heavy damage
Inappropriate restoration	None	Slight	Medium	Severe

.

The vulnerability of a building is the superposition of its damage level and level of loss of adaptive capacity. For the building damage levels discussed above, the four grades of mild, general, serious, and endangered, are assigned 1 to 4 points, respectively. The vulnerability index is calculated by the cumulative summation of the damage index and the values of loss of adaptive capacity. The weighted value of each factor is also calculated by AHP (

Table 7. The weighted value of vulnerability index.

Vulnerability Assessment Factors	Damage Level (DI)	Utilization Condition (UC)	Protective Construction (PC)	Inappropriate Restoration (IR)
Weighted values	0.4030	0.2529	0.2164	0.1277

).

The vulnerability index calculation formula of CER architectural heritage is:

VI = DI × WV_DI + UC × WV_UC + PC × WV_PC + IR × WV_IR

The vulnerability indexes of all CER buildings can be calculated and divided into four classes, which are Class A, with a vulnerability index between 0.8723 and 1.631, Class B, vulnerability index between 1.7081 and 2.1501, Class C, vulnerability index between 2.1617 and 2.6418, and Grade D, vulnerability index between 2.6534 and 3.5282, the larger the value the more vulnerable the building is. Figure 5 shows an example of the basic information collected in field inspection, and the recording and calculation process of various indicators.

2.2.2. Risk Map: Overlapping the Hazard Map and Vulnerability Index

Hazard factors for the slow degradation risk of built heritage are the daily environmental factors that accelerate the development of building damages. CER buildings are mainly constructed with brick, stone, and mortar, which are porous materials. Except for the mechanical damage caused by special activities such as urban construction and anthropogenic destruction, the occurrence and deterioration of building damage are mainly caused by two processes: the volume change in the water in the material caused by the freeze–thaw cycle and the crystallization process of the salt in the material caused by the dry-wet cycle [52,53,54,55]. Both phenomena lead to the deterioration of material properties by destroying the microstructure of porous materials, and ultimately the building will be destroyed.

Freeze–thaw cycles are the key reason for the degradation of physical and mechanical properties of porous building materials in cold regions [56,57,58,59]. In cold and humid environments, materials with high capillaries and porosity are rapidly saturated with water. The volume of frozen water will increase by about 9% when the temperature drops below 0 °C, putting pressure on pores and cracks in the surrounding material. The increase in pressure enlarges pores and cracks and leads to the creation of new microfractures [60]. As the temperature rises, the new cracks absorb more water, creating more pressure when it freezes again. As the freeze–thaw cycle progresses, the material will endure unnecessary weathering, and the intensity of the damage is positively correlated with the number of freeze–thaw cycles happened. When the building material is soluble and is in a cold pluvial area, the degradation effects of freeze–thaw phenomena are more critical [61,62,63].

Salts are contained in building materials, and the moisture adsorbed in the pores of the materials will also dissolve the salt ions in the air and soil to generate salts. The crystallization process occurs when the salt solubility is being exceeded [64]. The entry of the soluble salts into the building material promotes the crystal growth in pores and the subsequent crystallizations in existing fractures, which gives rise to tensions promoting the enlargement of the cracks or pores. The crystallization produces certain stress within the pores or existing fissures of the building material [65], proportional to the natural logarithm of the supersaturation degree [66]. If this physical stress exceeds the tensile strength of the material, deterioration occurs. With the fluctuation of external temperature and humidity, water flows in the material in the process of evaporation, and the salt will be transported to the part close to the surface where crystallization occurs, resulting in delamination [67,68,69]. Most of the weathering forms that appear in CER buildings are associated with salt crystallization.

In these two decay processes, changes in temperature and moisture in the environment are the most critical hazard factors, which directly cause or accelerate the disintegration and structural cracks. Therefore, the magnitude and frequency of changes in temperature and relative humidity are the key indexes to measure the level of hazard factors. In addition, when continuous rainfall occurs, the soil-bearing capacity will be reduced, which will cause or accelerate building settlement [70]. The continuous deterioration of the above damages will lead to the deformation of part or the whole structure of the building.

The hazard factors in the slow degradation risk of built heritage in the daily environment that this paper aims at can be summarized as temperature change, humidity level, precipitation intensity, and freeze–thaw cycle intensity. The hazard factors need to be measured with data to facilitate statistical and visual expression when drawing the risk map of CRCH. The temperature change level is expressed as the variance of the monthly average temperature, the humidity level is expressed as the mean of daily relative humidity throughout the year, and the rainfall level is expressed as the number of days with daily precipitation over 0.1 mm. The freeze–thaw cycle occurs when the temperature fluctuates around 0 °C. The number of days in which the daily mean temperature is between −2 °C and 2 °C during a year is selected as a parameter to measure the frequency of freeze–thaw cycles. At last, use GIS to graphically express the value of hazard factors and superimpose them with the vulnerability indexes of the buildings to obtain the risk map of CER.

A notification should be proposed here that the acceleration of the built heritage deterioration caused by environmental factors is a complex process. To facilitate practical operation, the correspondence between hazard factors and building vulnerability has been simplified in this paper.

3. Results

3.1. Mapping the Vulnerability of CER Cultural Heritage

3.1.1. Vulnerability Index Distribution

Based on the proposed building damage identification and evaluation methods, a comprehensive on-site census of the CER building heritage had been carried out. In addition to the inspection of building damage performance and the calculation of the vulnerability index, the basic information of the building was also recorded and entered into GIS, including geographic coordinates, conservation level, property ownership, and usage status. Among the 1604 CER masonry-built heritage sites that had been inspected and registered, nearly three-quarters have a vulnerability index at level A and level B, which is an ideal preservation state overall. The number of buildings at Level C and that of Level D are close to each other, and they both have a high risk from the viewpoint of vulnerability (Figure 6).

All the data were included in a GIS to make it possible to conduct spatial analysis by using Kernel Density Estimation (KDE) [42]. The KDE color depicts the building density in different statuses, and the red color refers to the highest density. The distribution of buildings in level A are close to that of all the masonry buildings of CER, both of which are widely distributed on the entire railway line, with a higher density in high-grade stations. Buildings at level B are relatively evenly distributed, while a high concentration could be found at Angangxi Station. The density distribution of buildings at level C has a similar form to that at level A, but the concentration at several high-level stations is more prominent. Buildings of level C and level D are close in quantity, but the level D buildings are distinctly concentrated in Harbin (Figure 7).

By observing the spatial distribution status of VI levels, it can be found that cities or towns located in high-grade stations, i.e., first grade, second grade, and third grade, are with a large number of built heritages, as well as a high number of buildings in poor VI status. This phenomenon is the result of the combined action of social factors. First, the early understanding of built heritage conservation was imperfect, lacking the concept of holistic conservation of group heritage, and only cognizing or judging the single building as object of protection. Second, since a large part of CER buildings is standardized design construction, the architectural forms are close to each other, and many are even duplications. It can be found in the field investigation of CER that this phenomenon has weakened the awareness of building preservation and reduced the level of protection, leading to considerable destruction and even demolition in the expansion construction of cities or towns in the region. Third, during the process of regional development, uptowns are constructed outside the historic district in urban areas, meanwhile, it is difficult for historical buildings to provide convenient living conditions, so residents move out to new residences one after another. Within the region where CER is located, urban development has absorbed the population of the surrounding towns and countries; at the same time, in terms of population movement throughout the whole country, a large number of people from the Northeast are migrating to other regions in recent years. All these factors have contributed to the vacancy and abandonment of built heritage of CER, and the subsequent deterioration.

What is more noteworthy is the spatial distribution of built heritage with vulnerability index at level D, which shows that the largest number of the buildings in poor condition are located in Harbin, the capital city of Heilongjiang Province, that is one of the most urbanized cities in the whole region, and theoretically has the financial and technical capacity for reasonable conservation of buildings. However, in reality, due to the lack of conservation strategies and the absence of proper restrictions on the use of buildings, the reuse functions of historical buildings in Harbin are mostly individual workshops or warehouses for handicrafts, which have a great negative impact on the buildings. On the other hand, due to the urban development and construction, in the historic districts designated as conservation areas, the government has mobilized the residents to move out early. However, due to financial and management problems and delays in implementing further conservation or utilization measures, the buildings have been left vacant for a continuous and long period of time in the harsh environment, and, as a consequence, the deterioration has been deepened. In contrast, Angangxi is a town with relatively slow development, and the buildings were originally used for residential purposes. Although most of the buildings are no longer in use, the residents moved out later and the buildings were left vacant for less time so that a larger percentage of buildings are at level B and level C, the situation is much better than that of Harbin. As can be seen, the early process of rapid urbanization in China, and the deficiencies in the management presented, can greatly exacerbate the risks of built heritage, which are more immediate and intense than those posed by the natural environment, but it is also easier to control.

3.1.2. Damage Index Distribution

The occurrence, development, and acceleration of building degradation are related to various hazard factors, and the frequency and extent of each damage in a given environment are spatially distributed in patterns that can be interpreted, which allows for targeted strategies in field restoration and conservation management to be made. In the survey, each damage was recorded, measured, calculated, and graded as a basis for spatial analysis. In addition to the original scoring principle, if the damage was not present in the building, the score was recorded as 0 (Figure 8).

From the perspective of damage status, structural cracks, disintegration, and mechanical damage are the most common types of damages, which take place in more than 95% of the buildings. Among them, the scores of structural cracks mostly concentrate between 1 and 2, which represent relatively minor degrees; the number of buildings with disintegration at 2 points and above is higher, the damage status of which is relatively serious; the degree of mechanical damage of 3 and 4 points accounts for more than 50%, and the actual impact is more serious than expected. Since the formation of mechanical damage is mainly caused by human factors, more attention should be paid to the adverse effects given by the users on the building in future conservation. Observable deformations are not common in buildings, and even less so to a serious degree. So, although it is the most destructive damage type, it actually requires special monitoring and salvage repair of only individual buildings and has relatively little impact on the integrity conservation of CER heritage. The deep or uneven subsidence that can cause other serious damages is rare.

In terms of macroscopic spatial distribution, the extent of structural cracking damage in masonry buildings on the main line and the southern branch line is similar (Figure 9), while the level of this damage is significantly stronger in the buildings in Harbin, which, as mentioned before, is mainly due to human factors. The overall proportion of the damage disintegration is similar throughout the entire CER, but the western part of the trunk line has a significantly higher percentage of buildings with severe disintegration. Mechanical damage is closely related to the status of utilization and maintenance of the buildings. It is not serious on the western line, partly because of programs of restoration and renovation of built heritages conducted by governments of the cities and towns there, such as Manzhouli, Suifenhe, and Zhalantun, in recent years. The eastern and southern branches of the line are dominated by mechanical damage at the level of 3 points and 4 points, mainly because the buildings in these two areas are generally private residences and no systematic restoration and maintenance strategy has been implemented, and the users mostly make improper restoration, minor alterations or additions to the buildings based on their living needs, which have caused more damage to the integrity of the buildings. Due to the small number of the appearance of deformation and settlement, the spatial distribution analysis will not be expressed here.

3.2. The Risk Map of CER

The Risk map of slow degradation of the built heritages of CER is the overlapping of vulnerability index and hazard factors. In this study, the meteorological data of the whole year of 2019 were chosen as the data source for all the hazard factors of the risk map, as an example to express and illustrate. The data were gathered from the Resource and Environmental Science and Data Center, Chinese Academy of Sciences (http://www.resdc.cn, accessed on 10 September 2021). The meteorological data were collected from 121 meteorological stations distributed in the four provinces, including 32 in Heilongjiang Province, 27 in Jilin Province, 23 in Liaoning Province, and 39 in Inner Mongolia autonomous region. All parameters were entered into the GIS system, and he spatial distribution of hazard factors was mapped by the Inverse Distance Weighted method. In this paper, to simplify the analysis process, only the built heritages in poor condition with the vulnerability index at level C or level D are extracted for the risk analysis, which is based on four hazard factor maps including temperature change, humidity level, precipitation intensity, and freeze–thaw cycle intensity (Figure 10). The change from warm to cool colors in the graph represents the change from low to high of each hazard index.

The higher the value of the hazard factors index, the higher the risk of the spatial environment. As seen from the risk map, taking the main stations of CER as the reference points, the environmental risk areas can be classified into three levels: high, medium, and low, as detailed in

Table 8. Levels and spatial distribution of environment risks of CER.

Hazard Factors	High Risk Segment	Medium Risk Segment	Low Risk Segment
Temperature change	Zalantun to Manzhouli increment	Zalantun to Hengdaohezi decrease; Harbin to Tieling decrease	Hengdaohezi to Suifenhe decrease; Tieling to Lvshun decrease
Humidity level	Acheng to Hengdaohezi, decreasing from the center to both ends	Angangxi to Acheng; Mudanjiang to Suifenhe; the southern branch except for the section from Haicheng to Liaoyang	Angangxi to Manzhouli decrease; Haicheng to Liaoyang.
Precipitation intensity	Acheng to Hengdaohezi, decreasing from the center to both ends	Angangxi to Acheng; Mudanjiang to Suifenhe; the southern branch except for the section from Haicheng to Liaoyang	Angangxi to Manzhouli decrease; Haicheng to Liaoyang.
Freeze–thaw cycle intensity	Boketu to Anda, decreasing from the center to both ends; Haicheng to Lvshun increment	Harbin to Suifenhe increment, Harbin to Haicheng	Boketu to Manzhouli, decreasing from the center to both ends

. The four hazard factors were overlaid in GIS to form a risk map that can represent the comprehensive environmental risk (Figure 11). The integrated risk map shows that the middle part of the western line has the highest value of integrated hazard factors, and the density of buildings with high vulnerability index level is also relatively high, which means that the buildings are at great risk. The value of integrated hazard factors of the eastern line takes the second place, while the number of vulnerable buildings is the largest. The buildings in the remaining areas, except Harbin, have low integrated environmental risk.

Disintegration and structural cracks are the most typical damages in masonry buildings of CER, and the freeze–thaw cycle is the most influential hazard factor in the frigid climate environment of the region. Therefore, the analysis and application approach of risk maps are illustrated taking the superposition of these two damages together with the level of the freeze–thaw cycle and the vulnerability indexes as an example. As can be seen from the maps, the buildings with severe disintegration and structural cracks are obviously concentrated in the areas with a higher degree of freeze–thaw cycles (Figure 12). This confirms the conclusion that the freeze–thaw cycle is an important pathogeny of causing and accelerating the two damages, and on the other hand, it will be a reference for the future management policy of maintenance and restoration for these two damages.

4. Discussion

As a typical CRCH, CER faces comprehensive difficulties in risk response, including both natural ones, such as the decay of buildings, the invasion of animals and plants in forests, the influence of climate change, the impacts of meteorological factors; and social ones, such as the economically backward in Northeast China [71], population loss caused by aging and out-migration [72,73,74], weak heritage protection concepts, backward official protection strategies, and conflicts between urban construction land expansion and heritage locations [74,75], and the low level of construction technique. CRCH conservation is not only an architectural project but also a development issue influenced by the interaction of multiple factors. Although many of these factors are irreversible and uncontrollable, it is still possible to avoid or mitigate various risk events targeting the goal of preserving the heritage. For such heritage with complex content, the overall conservation objective is to maintain its integrity, continuity, and diversity. Guided by this principle, risk response strategies should be considered at both the technical operational and managerial aspects. The technical operation level mainly deals with the risk of aging and deterioration of built heritage in the natural environment, focusing on the restoration and maintenance of individual buildings; the management level mainly deals with the influence of human factors in the social environment, targeting mainly on coordination and control. Difficulties at these two aspects are inevitable in all architectural heritage conservation work. For the CRCH, like CER, which has a huge geographical span, diverse contents, and complex natural and social environments, the conservation work is feasible only if specific measures were taken, which address different problems from macro to micro.

In terms of technical operation, according to the results of the regional environmental risk assessment of the built heritage of CER, buildings in different status are given appropriate disposal accordingly. The risk map contains three types of information: vulnerability indexes that evaluate the state of the building’s body, which also includes human factors; maps of hazard factors that describe the degree of risk posed by the environmental hazard factors; and the superposition of the two, which reveals the distribution of the risk level of the building in the environment. In the specific implementation, the priority hierarchy of interventions should be clarified for different types and levels of risks, emphasizing the right remedy according to the severity of the existing problems. First, the comprehensive risk of the building in the area is divided into three levels, and three corresponding types of measures, from the highest level to the lowest, respectively, are salvage restoration, general restoration, and maintenance. Within the same priority, buildings with high vulnerability index, high damage index, and structural damage should be focused on in the first place. Through this rank, given the limited capacity within each region, the time for the intervention of each work is determined reasonably and the amount of resources invested is allocated in an integrated manner (

Table 9. Disposal recommendations for CER buildings based on risk maps.

Priority	Risk Conditions	Intervention	Principles	Content
Fist level	High hazard indexes + High vulnerability indexes	Salvage restoration and reinforcement	Under the same risk state, priority disposal is given to buildings with highly integrated damages, vacant buildings with structural hazards may be subject to temporary reinforcement and support measures	Structural deformation, uneven settlement, severe structural cracks
	Medium hazard indexes + High vulnerability indexes
	High hazard indexes + Medium vulnerability indexes
Second level	Low hazard indexes + High vulnerability indexes	General restoration and reinforcement		Partial deformation, general structural cracks, deep disintegration, deep settlement
	Medium hazard indexes + Medium vulnerability indexes
	High hazard indexes + low vulnerability indexes
Third level	Medium hazard indexes + low vulnerability indexes	Routine repair and maintenance	Prioritize inspection and disposal of vacant buildings	Removal of improper restoration measures, repair of protective constructions
	Low hazard indexes + Medium vulnerability indexes
	Low hazard indexes + Low vulnerability indexes

).

In practice, special attention should also be paid to the forecast and warning of dynamic meteorological information to take proactive protective measures in advance. For example, in areas where continuous short or medium term heavy rainfall is forecasted, it can be expected that the soil bearing capacity will be significantly reduced, so attention should be paid to areas where serious settlement occurs on the buildings, and preventive conservation measures need to be taken for buildings with settlement or structural deformation, such as repairing the apron slope, cleaning the surface drainage system or reinforcing the foundation to reduce the serious damages caused or developed under such condition.

At the management level, the social issues in CER conservation can be grouped into three main categories. First, the government agencies need to respond to the risks posed by the construction needs through multi-level regulation and rational resource allocation; second, it is necessary to reshape the conservation concepts of managers, building owners, users and visitors to reduce the possible risks of architectural heritage in the daily production and living environment; third, periodic inspections and maintenance of CER architectural heritage need to be conducted to counter to the risks posed by economic decline and population loss in Northeast China. In view of the characteristics of CER and the different management needs, we need to develop specific strategies and use appropriate methods and tools to deal with various problems.

Firstly, basing on GIS, a platform connected by a local network covering the whole region was developed to support a collaborative mode of hierarchical control, which is in conjunction with the current official administrative system (Figure 13). On the one hand, the accounts of the platform are registered for the personnel directly related to the architectural heritage in the region, and the operation authority is given to people for the work they are responsible for. On the other hand, the CER heritage display and virtual roaming system on the internet should be developed and opened to the whole society, and an interactive communication channel is prepared for public communication. This semi-open management mechanism can operate from two directions. From the perspective of managers, they can give notification, guidance, and instruction from the top down, and conduct inspection and evaluation based on the updated real-time condition of the heritage; from the perspective of heritage users and visitors, they can report abnormalities of the buildings and violations by local governments, enterprises, or individuals from the bottom up. The network-based collaborative system with universal supervision and wide communication is expected to minimize the additional risks that may be brought to build heritage during the construction of cities and towns.

Secondly, according to the development status of the areas along CER, a large part of the buildings are still in use, so it is necessary to gradually cultivate the awareness of heritage conservation, related knowledge, and technological skills among various groups of people through official training, policy incentives, education and media publicity [76,77,78,79]. Based on the established multi-level management system, it is still possible to gradually penetrate the relevant concepts, notions, and knowledge to the entire population throughout the region by means of cascading education. Due to the dynamic nature of the risks, understanding the daily condition of built heritage is an important part of efficient prevention and control of risk events, so education of building users is particularly important. The training work to be accomplished includes operational procedures for inspection, recording, and reporting of architectural heritage, maintenance methods, precautions, and basic restoration techniques. This type of training is simple and straightforward in dealing with the risks in the daily environment. For example, during the snow period, the occurrence and development of material loss caused by freeze–thaw cycles and humidity changes can be largely reduced by heritage users through not piling snow on the lower part of the walls; avoiding wedging metal or wooden elements in the walls can effectively reduce the generation of structural cracks; timely repair of protective constructions such as gutters, apron slope, and downspouts can effectively reduce moisture erosion and avoid unnecessary deterioration. After building owners and users are trained to establish the concept of heritage conservation and are capable of dealing with simple technical issues in a reasonable manner, they can be given a certain amount of authority to restore or renovate their buildings to ensure more quality living conditions. Through these measures, the original inhabitants will be retained as much as possible, and a historic community atmosphere will be formed through the preservation of collective memory.

Finally, for uncontrollable macro-social issues such as regional economic and demographic changes, human intervention is needed for the affected architectural heritage. According to the data of recent years, most of the areas along CER will continue to face the problem of a shrinking population for a long time to come. Within Northeast China, the uneven nature of regional development will also lead to cities and towns adsorbing populations from neighboring villages and settlements. Therefore, the vacancy and abandonment rates of buildings of CER, especially in the main line that carries majority of built heritage, will gradually increase, and the risks to the buildings will subsequently increase significantly. In the face of this situation, a systematic inspection and guardianship system should be set up, taking the railway station management model of the CER during its operation period as reference. Specifically, the first step should be to divide the whole line into four levels of management sections regarding the regional administrative hierarchy, development level, initial station grade, and distance. The centers of sections at the first level are prefecture-level cities or locations of first-class stations, such as Harbin, Dalian, Qiqihaer, etc.; the second ones are second-grade or third-grade stations or towns of a certain scale, such as Hengdaohezi, Yimianpo, and Zhalantun; the third ones are fourth-grade or fifth-grade stations or villages of a corresponding scale, such as Shanshi, Yuquan, and Zhalainuoer; the last ones are small colonies including passing loops and work areas, such as Xinganling, Yiliekede, and Hongfangzi. At the first and second levels, “monitoring points” are established for resource allocation and management, while at the third and fourth levels, “inspection points” are established as working points for heritage survey and maintenance (Figure 14). With these base points as the operational units, the vacant architectural heritage of CER can be regularly inspected and maintained, and the building status will be reported in the risk management platform. Although this kind of conservation only protects the architectural heritage as exhibits, it is relatively a feasible way to maintain the holistic and diverse characteristics of CER heritage in the current socio-economic state.

5. Conclusions

Cross-Regional Cultural Heritage is a heritage system with a huge spatial scale, containing a large number of heritages in multiplex types. Beyond the traditional principles of cultural heritage conservation, continuity and wholeness are important goals for the conservation of such objects. The CRCH has great potential to contribute to regional development, but also to face more difficult and more complex extra problems. In addition to the complexity of CRCH itself, the management capacity, financial level, and social and natural environment of different regions vary greatly in the process of conservation. Thus, although facing the same object, there are huge gaps between regional agents, making it difficult to cooperate. The problem is even more acute when marginalized areas are economically degraded, ideologically staled and technologically backward. Within this context, starting from the risk of the slow deterioration of built heritage in the daily environment, this paper aims to propose a simplified, easy-to-operate, and heritage-focused approach that can be adapted to different operational and management needs of all the regions, with the lowest technical threshold and minimal resource consumption.

The content of the risk map is mainly composed of the vulnerability of the building and the hazard factors of the environment. The vulnerability index is the superposition of the damage index and the level of loss of adaptive capacity of the building. In the research, four major types of damages in CER buildings, namely, disintegration, structural cracks, deformation, mechanical damage, and settlement, were used to calculate the damage index; utilization condition, protective construction, and inappropriate restoration were chosen as the measurement factors of the level of loss of adaptive capacity. Through a series of calculations, the quantitative data reflecting the vulnerability level of the buildings were obtained, which were analyzed and visualized through GIS. Hazard factors are environmental elements that cause or accelerate the damage. Four meteorological parameters, temperature change, humidity level, precipitation intensity, and freeze–thaw cycle intensity, are selected for hazard mapping.

The vulnerability index and level of hazard factors were overlaid in GIS to obtain the risk map of CER architectural heritage to support the subsequent analyses. Buildings in poor vulnerability index condition and located in areas with high hazard factors are of high risk, follow this principle, hierarchical building risk sections were defined, and corresponding disposal plans were proposed. Based on the risk map, a risk management platform was developed to support the preventive conservation strategies and methods, which has taken into account the natural and social characteristics of the areas along CER. The strategies and methods include taking proactive defensive measures in advance in response to the information provided by weather forecasts, cultivating social awareness of heritage conservation through education at each social level, training techniques for restoration and maintenance, and establishing an operational system for periodic monitoring and inspection that considers the spatial characteristics of CER.

The work in this paper took visual inspection and brief measurements as the main methods of investigation, aiming to establish a working framework containing brief information of all heritage with risk index to support subsequent more precise and in-depth studies, such as non-destructive testing, laboratory inspection, and sensor monitoring. The meteorological data used were macroscopic data at the regional scale, while in the future, when targeting more specific heritage objects, micro environmental measurements and analyses should be included. Moreover, for built heritage, the hazard factors are more than just the four mentioned above, and the correlation with damage is more complex, requiring in-depth studies that more detailed and specific monitoring, maintenance or restoration projects need to be implemented. Meanwhile, in addition to masonry buildings, there are many buildings along the CER that are constructed of wood, concrete, and metal, as well as engineering facilities such as bridges, tunnels, and tracks. Therefore, further work on the investigation and analysis is still obligatory.