The three distinct sub-chapters in which the present chapter is fragmented practically represent analyzes on the past, present and future of a defining monument for the Romanian society. The investigations being centered on how the meteorological hazard from the past and the damage are caused, the assessment of the current geohazard risks, as well as the conservation methods. Thus, knowing the past, in the present the wooden church must be offered visible horizon in order to persist in the future.
A. Meteorological hazard
Meteorological hazards belong to the category of natural hazards and are divided into several categories according to the triggering speed. Tornadoes are perhaps among the most unpredictable fast-triggering weather phenomena. Precisely because of the speed of occurrence, the uncertainty in space and time, combined with the vulnerability of humans, the destructive potential of tornadoes is greatly amplified (Rauhala and Schultz 2009). Supercellular storms are the main factors accountable for the occurrence of most of the meteorological phenomena of risk; mostly all tornadoes from weak to violent being associated with these (Jones 2015; Spiridonov et al. 2010). This is also the case of the tornado of July 3, 1991 from Corbești, which was generated by a convective supercell responsible for producing a wide range of risk phenomena during the evolution in the region (Cristea 2004).
On July 3, 1991, at 12 o'clock, about three hours before the event that took place in Corbești, the synoptic situation in Europe was characterized by the presence of a deep barric depression over the Balkan Peninsula, while the northeast of the continent was under the influence of a dorsal of the Azoric Anticyclone (Fig. 6). The positioning of Romania at the connexion between the North and East Azoric Anticyclone, characterized by warm air masses (over 20°C) and the barric depression from southern Europe, with a cold air core (8-10°C) (Fig. 7) (Cristea 2004), created the ideal premises for the production of risky weather phenomena.
Around 16 o'clock, on the radar map from that date, a convective radio echo with all the characteristics of a super cell (elliptical shape, cumulonimbus clouds strongly developed vertically, high reflectivity, etc.) appears well defined in the south of Oradea (Cristea 2004), moving at a speed of about 50 km/h in the west-north-west direction. On the outskirts of Oradea there is also the village of Corbești, where the tornado appeared just before 16 o'clock. The tornado's trajectory was channeled on a valley that connects the villages of Corbești and Topa de Sus, leaving behind traces in which the effects were devastating, in one of them being the wooden church. The material losses following the event escaladed to 15 buildings destroyed, about 100 affected and over 1000 hectares of crops damaged and also registering a death.
In order to reconstruct the thread of the events, but also for a better understanding of the experiences of the locals, interviews were conducted in which the eyewitnesses participated. Thus, at the time of the event, most of the members of the local community were present at a funeral, on one of the hills bordering the village, at a considerable distance from the high intensity areas of the tornado. It is just sheer luck that the loss of human life is limited. Witnesses describe that before the tornado began, the sky "... had darkened, as if it were an eclipse or as it got dark ..." / "... it darkened. As if it was night ..." / "... it got dark, like a kind of fog", and later on the phenomenon would be accompanied by a ".... very strong sound", similar to the locals with the noise of a mill "... which grinds everything" . The witnesses' statement leaves no room for interpretations on the typology of the meteorological phenomenon, the vast majority of them resembling "... a whirlwind ...", others even identifying it as a tornado.
Despite the short time it took, "... about 6-7 minutes" up to "... 10 minutes", the tornado left behind a distressing view of the village, with houses completely destroyed or uncovered and trees removed from the roots (Fig. 9, b and d). Of all, perhaps most had to suffer the wooden church historical monument of the village, the primal point, the core of spiritual and social life of any Christian settlement. This was practically destroyed completely, only the bell tower remaining standing in a precarious balance (Fig. 9, a and c). The remnants of the sanctuary were the main subject of a film made by representatives of the Episcopate of Bihor, about eight hours after the disaster, with the purpose of immortalizing the damage caused by this unexpected phenomenon.
The feeling of repulsion stemming from the superstitions of the locals, as well as their need for a more spacious place of worship, to inspire more security, led the old wooden church not being rehabilitated. After two years when it was torn down, the opportunity arose to relocate it to the Monastery of the Holy Cross in Oradea Municipality. Therefore, like a phoenix bird, the once destroyed church was reborn from its own ash to become the first church of the newly established monastery in Oradea.
B. Geohazards - The current risks
The monastery of the Holy Cross, being located at the foot of the Oradia Hills, an area whose geology is a factor favoring the installation and development of the slope processes, thus creating the risk that both the monastic settlement in general and the monumental wooden church in particular to be affected by a new risk factor - landslides.
The hills of Oradea are presented as a low hilly component, which does not exceed 300 meters (max. 292 m in the Osorheiu hill) from a morphometric point of view. They have the shape of long ridges oriented predominantly east-west, which make the transition from the hill area to the plain of Western Romania. In general, their lithology varies vertically from marly deposits belonging to the Pontian with massive and compact appearance in the subsoil, to sandy-clay complexes with thicknesses of a few meters at the top. Along with the lithological formation, the inclination of the layers in a cuesta type relief, the rich precipitations especially in the spring-summer period, the massive deforestation in the last decades and the overloading of the hills with constructions, they collaborate in creating favorable conditions for the initiation of landslides in the region. These phenomena occur with a very high frequency, especially on the hills on the right side of Crisul Repede River in the perimeter of Oradea Municipality; area where the wooden church is located.
The index map of the landslide susceptibility, based on local conditions, presents the spatial probability of producing new landslides in the targeted area. This is a useful tool for managing the geohazards induced by these phenomena, through higher land use planning, as well as better decision-making in predisposed areas (Pham et al. 2016). The interpretation of the map indicates that the area has a great tendency with regard to the appearance of these slope processes. Evidence in this regard is also the multitude of such risk phenomena that cover the hills of Oradea. The analysis on the four value categories of the susceptibility of the area to landslides reveals a relatively homogeneous distribution. The areas with the highest risk (31.9%) have the largest extension, followed by those with medium risk (26.8%), no risk (20.8%) and small risk (20.5%). The area with medium susceptibility where the wooden church is positioned, is guarded at the top by a sector with a large spatial terrain which is defined by a high susceptibility of landslides.
In order to determine the influence that the lithological composition has in determining the occurrence of these processes, in the immediate vicinity of the monument a drilling was done in order to determine the nature of the underground deposits. The interpretation of the results obtained after the drilling (Fig. 10), indicates that above a dense-looking marly substrate there are successive layers that consist of different types of clays and sands with a consistency of the soils from soft to hard, of about 20 meters thick. The alternation of the different layers with distinct cohesion leads to the evolution of soil degradation processes, facilitating the landslides. The balance of the slope in the immediate vicinity of the monument church is also affected by the aggressive anthropic intervention, taking place mainly in the last two decades. In this sense, it can be seen in figure 10 that the surfaces built with buildings and road alignments occupy a large area, being scattered throughout the hills. The risk is even higher as the area is constantly expanding in terms of construction, being a residential area in demand by the population of the municipality.
The field surveys in the immediate vicinity of the monastic settlement, reveal the presence of the activities preceding the landslides, sign that the slope is active and presents a risk of geohazards. In this sense we can mention deep cracks in the soil that decrease the cohesion of the material, cracks in the network of roads and sidewalks etc. In Figure 11a we can see the inclination by about 8° compared to the right angle of an electricity pole, in the direction of the material movement from the slope. The case is not an isolated one, this aspect being ascertained at most of the electricity poles in the area. At the same time, the cracks in the concrete post on which the church is located (Fig. 11b) denote an increased instability of the slope.
The occurrence of the harbinger phenomena of the landslides must be an alarm signal for the decision-making bodies regarding the risk situations. The integrated action of the factors set out above may affect the safety of the citizens and their property; and in particular to the security of a national heritage asset, the wooden church.
C. Digitization for conservation and promotion
The position of the wooden church, set exactly on a sliding wave of land, greatly limits the preventive measures that can be taken to preserve the monument. But, through digital techniques, reactive measures can be provided, which prove to be of great help in restoring the church in case if it will be affected by geohazards in the future. In this context, photogrammetry is shown to be a key factor in obtaining three-dimensional models of heritage buildings, which can reconstruct the destroyed or damaged heritage element, first digitally, and then in reality,
The 3D model (Fig. 12a) obtained from the processing of the field photographs depicting the wooden Church "Saints Archangel Michael and Gabriel", can stimulate the way the edifice is kept, viewed, registered and reconstructed in case of need.
This approach facilitates the virtual presentation of the areas affected by the slope movement, in order to make the best intervention and reconstruction decisions. Monitoring the effects of landslides on the church can be done by making such models periodically, and by comparison the most susceptible parts can be displayed.
The advantages of photogrammetry include not only the fact that it allows access to objects at great distances, but also supports their analysis. Photogrammetry thus becomes a viable alternative to direct field measurement of object dimensions; an analysis of this kind is presented in Figure 12b. Following the introduction of a reference measurement (performed in the field) and resetting the computing algorithms, the model allowed with centimeter precision the measurement of the wooden church dimensions. These data prove to be of great value in the event of geohazards. The information thus obtained can be made available to the restorers. Moreover, based on the model created, the destroyed parts of the church can be scanned and inserted into the system, and with the help of the MeshLab program, they can be digitally reassembled by manual repositioning at the original location. At the same time, the replica allows an approximate estimation of the volume of material needed to perform certain repair interventions.
The opportunities offered by the three-dimensional models practically minimize the possibility that the main features of the church will be modified again following a restoration process. A very advantageous situation considering that through a comparative analysis of the three photographs that make up Figure 1, it can be seen that over time, through successive repairs, the authentic aesthetic aspect of the monument has been obsolete.
The model achieved mostly reproduces the original features of the monument. But in order to conserve, recover and rehabilitate, the limits of the model created must also be taken into account, which in the present situation is given by the absence of crosses on the roof. This limit was obligatory due to the material it was made of and the color of crosses (silver sheeting), which having a very high light reflectivity it caused the loss of the contour and finally the inappropriate reconstruction. Due to this fact, those elements were eliminated from the final model.
This approach not only represents a cheap and non-invasive opportunity for the preservation and rehabilitation of the monument, but also constitutes a way of promoting geotourism, taking into account the important scientific-didactic and spiritual valences of the area. This place could represent a first complex stop from an urban geotourism trail of Oradea Municipality, Romania.