*1.1. The Concept of Coastal Vulnerability: Causes and Assessment*

The concept of coastal vulnerability presents a very broad field of study that has been analyzed from different points of view for some time [1]. There are several approaches that focus on, for example, their analysis of the environmental [2], economic [3], physical [4], or social impact [5], and some that even consider the combination of several of them [6,7]. This problem can affect various environments such as large cities [8], natural areas [9], tourist beaches [10], developing countries [11], or small islands [12], among others, and is often associated with phenomena such as urbanization and the construction of infrastructures [13,14], hazards [15,16], and in recent years, the global climate change of the planet [8,17]. However, there are many other less common reasons that may bring about this problem, such as land subsidence [4], hydrology [18], or soil erosion [19] for example, with a complex set of these causes ultimately being the final origin of the vulnerability of a coastal territory in many cases.

Monitoring this phenomenon for the performance of strategies for its mitigation is not straight forward [20,21]. The scientific bibliography provides several analysis tools such as the classical study of sedimentary transport [22], the development of risk management matrixes [18] or the analysis of the rise in sea level [23] of current scenarios. In this field, it is evident that the physical transformations of the territory derived from human activity inevitably modify the coastal vulnerability of an area. That is why it is very important to understand the negative repercussions of human activities in order to avoid undesirable tendencies that accentuate its coastal vulnerability. Nevertheless, how can we determine the extent to which the constructions or land transformations increase the coastal vulnerability of a large-scale territory? The effects appear slowly and are very often difficult to measure [1]. Furthermore, there is usually no clear interconnection between causes and effects in this matter. The question becomes even more complex when several variables are interwoven in the analysis.

A habitually complex context in this matter is the determination of coastal vulnerability in urbanized areas. This phenomenon is particularly challenging if we seek to determine the origin of the current effects in areas that have been subjected to fast urbanization processes, because of the difficulty of evaluating multiple long-term impacts in short periods of time [24]. In this case, integrated analysis such as the DPSIR (Drivers forces, Pressures, State, Impact & Response) method [25] or multi-level nested frameworks for socioecological systems [26,27], for example, are also recommended. Due to that, despite being a more recent technique in this field, the implementation of multivariable GIS analysis is today becoming more and more widespread, thanks to the progressive improvement in the method's accuracy [28]. This tool allows the spatial variable in the analysis to be introduced and is transversal to the diverse origins of the issues.

A problem usually faced by this methodology is the difficulty in modeling the different physical phenomena that contribute to impacts in the coastal area [20]. It should also be noted that current issues associated with coastal vulnerability require a precise analysis of subjects whose time frame in reality extends over a period of years or even decades [1,29]. The effects of buildings and infrastructures, hazards or climate change in coastal dynamics, the transformation of the territory, or the alteration of the soil usually require a precise evaluation over 10, 30, or even 50 years to be reliably appreciated.

In this context, GIS analysis can prove very interesting if we have the necessary geo-referenced information [30–32]. In this field, several interesting studies are available that parameterize the coastal vulnerability of a territory through some spatial indicator [8,33,34]. Nevertheless, the retro-historic GIS type of analysis in this matter from an integrated perspective is a very scarcely studied field of research [35]. The implementation of GIS indicators through historical spatial information allows modeling the phenomenon from a numerical approach that can additionally be analyzed from a statistical perspective [36]. This approach becomes even more infrequent in scientific studies if we are faced with territorially complex areas in which there are several interrelated causes whose effects feed on each other. In this context, the use of geo-statistical tools can be very useful and innovative since it enables spatially cause-effect relationships to be correlated in these complex environments.

#### *1.2. The Mar Menor Case Study: A Coastal Complex and Antropized Territory*

In this study, the relationship between coastal transformations and the current vulnerability for a complex territory such as the Mar Menor, a coastal lagoon located in the Southeast of Spain, will be evaluated. This is a coastal territory of high environmental value [37–39] and with a 20 km long dune cord that separates two seas with very different characteristics (Figure 1). The area has been subjected to the impact of an intense human activity linked to tourism [40,41] in recent decades, which has resulted in massive urbanization, the construction of marine infrastructures, and the land transformation of its coastal perimeter [42].

**Figure 1.** Scope of the study (source: SITmurcia [43]).

Several studies have examined the impact of these and other activities in the area at a biological and ecological level [44–46]. However, the assessment of the physical vulnerability of the coastal territory as a result of human activity remains scarcely investigated since it is a very long-term impact that is not easy to visualize at a glance, requiring large-scale and long-term analysis. In this sense, the extent to which the different constructions and transformations of the territory have affected its current coastal vulnerability is not known, and has been the subject of social controversy on different occasions. Consequently, through the development of a retrospective GIS analysis, the physical impact of different phenomena in the coastal perimeter will be evaluated from the 1970's (moment when mass tourism began in the area) to the present day. This impact will be correlated at a spatial level with the different current coastal risks by using geo-statistical tools in order to determine the extent to which human actions have contributed positively or negatively to the coastal vulnerability of the area during the last decades (analyzed area included in a KML file as supplementary material).

#### **2. Materials and Methods**

The selected territory is a very interesting case in which to propose this new methodology of long-term GIS retro-historic geo-statistical analysis of coastal vulnerability for several reasons. In the first place, it is a complex case of evaluation in which the different parameters of analysis may have certain interrelationship of feedback, generating a framework of difficult identification of the cause-effect relationships. Secondly, it deals with a series of impacts and physical transformations in the coastal perimeter whose overall effects are not easy to evaluate, since they are phenomena whose incidence emerges over decades. Finally, it is a territory in which geo-referenced spatial information is available for almost 90 years (the first aerial photographs were taken in 1929), thus enabling a wide retro-historic GIS analysis with numerous data.

The analysis of its coastal vulnerability will be carried out from an evaluation perspective of its physical support against the different impacts of direct and indirect human activity. This evaluation will focus on how the human transformations of the territory during the last decades have increased, or not, its current coastal vulnerability. It should be noted that this territory has been subjected to human action since Roman times; since then some small fishing population settlements have existed for example [47]. Nevertheless, it was with the arrival of tourism from the 1960's [40] that the current main coastal impacts started (urbanization, construction of ports, and marine infrastructures, artificial widening of the communication channels between the Mar Menor and the Mediterranean, land fillings of beaches, etc.).

Therefore, based on all the above, a diagnosis of the long-term impacts of physical transformations on this coastal area will be performed using GIS retro-historic indicators; the following three global phenomena are used as evaluation variables: The urbanization of the coastal perimeter, the incidence of ports and marine infrastructures, and the alteration of coastal edges uses. Later, the geo-statistical correlation between these GIS indicators and current coastal vulnerabilities will be spatially assessed. Below, the modeling criteria of the retro-historic GIS indicators, vulnerability evaluation, and geo-statistical methodology are presented.

#### *2.1. Retrohistoric GIS Indicators of Impact*
