Evolution of the Beaches in the Regional Park of Salinas and Arenales of San Pedro del Pinatar (Southeast of Spain) (1899–2019)

Coastal erosion is anissuewhich affects beaches all over the world and that signifies enormous economic and environmental losses. Classed as a slow phenomenon, the evolution of the coastline requires long-term analysis. In this study, old cartography and aerial photographs from various dates have been used to study the evolution of the coastline. The information has been processed with free software (QGIS) and for the calculation of sediment transport the Coastal Modeling System (SMC) software. The results show the accretion/erosion phenomena that occurred after the construction of the port in San Pedro del Pinatarin 1954 and which changed the coastal dynamics of a highly protected area. In some sectors, the beach has been reduced almost in its entirety, with retreat rates of up to −2.05 m per year and a total area loss of 66,419.81 m2 in Las Salinas beach and 76,891.13 m2 on Barraca Quemada beach.


Introduction
The coastline is a highly dynamic area with constant movement of sediments that alter the morphological features of the coast and in which continuous changes take place in various spatial and temporal areas [1][2][3]. One of the most sensitive alterations is that caused by coastal erosion, which constitutes one of the greatest threats to coastlines worldwide [4] and which is currently being aggravated by mean sea level rise caused by climate change [5][6][7].
The process of coastal erosion consists of the permanent loss of sand from a littoral system and its magnitude depends on, to a great extent, the type of coastline, taking into account characteristics such as exposure to waves and their levels, or the composition and size of the sediments and the slope of the beach [8].
The causes of coastal erosion have their origin both in natural processes and in anthropogenic activities that influence coastal dynamics [9], especially if they intervene over a long period of time. It is also common for changes in the coastline to result from a combination of factors [10], such as rising sea levels, winds and storms, extraction of submerged sands for beach regeneration, and hydrological regulation of hydrographic basins. The latter is especially due to the construction of reservoirs, which retain sediments and limit their transport due to fluvial dynamics, the main beaches nourishment. Likewise, changes in land use in small coastal watersheds (coastal watercourses in the case of the Mediterranean) and their occupation limit the sedimentary transport and lead to the erosion of beaches fed by these variable hydrological systems [11][12][13]. Furthermore, coastal construction, such as promenades, but especially coastal structures that interfere with coastal transport, such as dikes and outer ports, constitute one of the most common causes and have the greatest impact on beach erosion [14]. In fact, the enormous effect The coastal area studied runs from north to south between 37.79 • and 37.84 • north latitude and 0.75 • east longitude ( Figure 1). It has a total coastal length of 6.455 km (2.680 km are north of the port, where the beaches of El Mojón and Torre Derribada are located, and 3.275 km are south of the port, where the beaches of Las Salinas, Barraca Quemada and Punta de Algas are located). In addition, the port that separates both sections occupies 500 linear meters of coastline. The complex is part of the Regional Park of Salinas and Arenales of San Pedro del Pinatar, on the northern coast of the Region of Murcia (southeast of the Iberian Peninsula), which also includes all of the salt extractions area. The park can be considered to be made up of "hanging beaches" [25,26], formed by the accumulation of unconsolidated sediments on a rocky outcrop. These layers represent a limitation to the height of the waves that reach the shore of the beach. Considering their morphodynamic state [27], all the beaches in the park are dissipative.
industry, since the salt flats have prevented the urbanization of this sector of the coast, despite the pressure for construction of the Spanish Mediterranean coast. The beaches mainly receive waves from the NE-E sector, with the probability of occurrence of 13.72% for the NE cases, 17.39% for the ENE waves, and 15.64% for those of the E. In addition, the SSW waves also have an important probability of occurrence (11.88%).
The waves are not very energetic and the significant wave height is 0.8 m; in fact, the low energy of the waves is one of the keys that allows it to maintain a dissipative profile [28]. The coastline is oriented perpendicular to the ENE, in such a way that the waves coming from the ENE do not undergo hardly any refraction and are those that undergo The regional park covers an area of 856 hectares and belongs to the Natura 2000 Network and is classified as a Specially Protected Area of Mediterranean Importance (ZEPIM). Therefore, it is an area of great environmental relevance largely to the salt industry, since the salt flats have prevented the urbanization of this sector of the coast, despite the pressure for construction of the Spanish Mediterranean coast.
The beaches mainly receive waves from the NE-E sector, with the probability of occurrence of 13.72% for the NE cases, 17.39% for the ENE waves, and 15.64% for those of the E. In addition, the SSW waves also have an important probability of occurrence (11.88%).
The waves are not very energetic and the significant wave height is 0.8 m; in fact, the low energy of the waves is one of the keys that allows it to maintain a dissipative profile [28]. The coastline is oriented perpendicular to the ENE, in such a way that the waves coming from the ENE do not undergo hardly any refraction and are those that undergo the least energy reduction before the break. The submerged beach is characterized by the presence of Posidonia oceanica, and the meadows are notably wide due to the low slope of the bathymetry in this area of the coastline.

Software Used
Two software tools have been used for the treatment of images and generation of cartography as well as the digitization of the coastline and generation of transects. On the one hand, for the calculation of sediment transport, Coastal Modeling System (SMC) software has been used. This software was developed by the Institute of Environmental Hydraulics of the University of Cantabria [29], which contains a wide set of numerical models, database methodologies, and which allows issues related to the coasts to be addressed.
The QGIS Geographical Information System [30] has been used to process spatial information.

Sediment Transport Models and Active Depths
The models used for this purpose were the Coastal Engineering Research Center (CERC) model and a variation of it [31]. The CERC model is based on the assumption that the total sediment transport rate along the coast is proportional to the flow of energy along the coast: where: Sc is the transport of sand in m 3 /s; A is a coefficient dependent on the type of beach; c 0 , the speed of the wave in deep water in m/s; θ b , the angle between the depth contours and the crest of the wave in the surf zone; Hs 0 , the significant wave height in deep water in m; and K r is the coefficient of refraction. The Kamphuis model is a variant of the CERC formula that includes the effects of the wave period, known as wave slenderness [31,32], the slope of the beach, and the diameter of the grain. In the Kamphuis model, the longitudinal sediment transport rate can be expressed as: where: Q s is the sediment transport rate, expressed in kg/s; H, the wave height; T, the period of the wave; h, the depth; ρ, the density of water; µ, the dynamic viscosity of water; g acceleration due to gravity; x, y, z are directions; t is time; ρ s , the density of the sediment; and D the diameter of the sediment, which has been calculated using the sieve from the samples obtained from the beaches of the study area. In order to analyze the impact of the Port of San Pedro del Pinatar on sediment transport, the coastal depth of the beach profile (d l ) has been calculated from the wave height, which exceeds 12 h per year [33]. Its calculation is obtained from the expression:

Cartography and Aerial Photographs
Despite their limitations, archival topographic maps can be useful in the investigation of changes in the coastline [35]. There are potential errors associated with this mapping, including variations in scale, datum changes, projection failures, or different topographic standards. However, many of them can be resolved through different treatments and, on many occasions, as is the case of the beaches of the Regional Park of Las Salinas and Arenales of San Pedro del Pinatar, they constitute the only source of data available for certain dates.
To carry out the following study, the cartography of San Pedro del Pinatar has been georeferenced from the topographic studies carried out in August 1899 by the Geographical and Statistical Institute at a scale of 1:25,000.
Due to the microtidal character of this area and the fact that the aerial photographs used were taken in summer, the error due to seasonal sea level fluctuations can be considered insignificant.
Aerial photographs corresponding to the years 1956, 1981, and 2017 have been used for digitization. The coordinate reference system used was ETRS89 UTM30N.

Definition and Digitization of the Beach Surface and Coastline
Two proxies have been used with different criteria and with two objectives: the first was the calculation of the distances between the coastline distances at different dates, understood as the erosion or setback of each sector. The second was the calculation of the variations of the beach area.
To analyze the variability of the coastline, a definition is required, taking into account the techniques and available data [36]. For this study, the shoreline definition is the line that marks the boundary between the sea and the land, regardless of the criteria chosen for its representation and refers to the Mean High Water (MHW) shoreline.

Generation of Transects
QGIS Station Lines' plugin [37] has been used for transects generation. This plugin allows the creation of lines along polylines, from certain parameters (length, side, angle), and assuming a Cartesian coordinate system. In this study, transects have been generated every 15 m.

Areas, Distances, and Erosion Rates
As a preliminary step to calculating the rates, distances have been calculated with the Shoreline Change Envelope (SCE) and Net Shoreline Movement (NSM). The first is the existing longitude between the most distant coastlines for each transect, that is, without taking into account the dates. The NSM shows the distance between the oldest and newest coastlines, and since it does not use any other information between dates, the results of the NSM calculation are considered an end point measure.
The calculation of rates is one of the most used monitoring indicators in the transition zone between the marine and terrestrial environment. To calculate the rates of variation of the coastline from previously digitized data, it is common to apply various statistical methods, such as the end point rate, the average of the rates, the linear regression rate, or the rate of jackknife [38][39][40]. In this study, the annual rate of change, End Point Rate (EPR), has been used, which represents the rate of change that the coast has suffered, expressed in meters per year. For its calculation, the net distance is used, divided by the number of years that separate the dates of the coastlines obtained in the digitization.

Calculation of Errors in Digitization from Aerial Photographs
Due to the micro-tidal nature of the study area [41,42], tidal fluctuation has not been incorporated into this study. Nevertheless, it has been considered necessary to calculate three error components [42] for the digitized coast from aerial photographs: i.
Digitization error: error associated with the coastline digitization process. The digitization has been repeated several times by a single specialist. The error has been calculated from the standard deviation of them. ii. Error in geometric correction: expressed as the mean square error of the rectification process [43]. It is the distance between the control points established in the aerial photographs. iii. Scanning or pixel error: represented by the resolution of the different images [44]. The larger the pixels, the greater the uncertainty in the interpretation of the coastlines. For this purpose, the length of the pixel side is used.
Finally, once the three errors have been calculated, they have been computed as the square root of the squared sum of each component [15].

Drift, Sediments, and Depth
From the Coastal Modeling System (SMC) software, it has been possible to calculate the net transport of sediments from the littoral system. Without artificial obstacles, the sand would be distributed from north to south with volumes of 1,096,030 m 3 /year according to the CERC model, and 238,270 m 3 /year based on the Kamphuis model. There are seasonal differences: in the month of July transport is practically nil, however, transport in the opposite direction to general littoral drift is practically nil all year round. The calculation of the active depths has made it possible to analyze the shadow effect of the port facilities. In the case of the beaches in the study area, the coastal depth, in which huge changes in the profile occur due to both longitudinal and transverse transport, is located at 5.60 m, according to the Hallermeier model, which determines that the closure depth is 11.20 m. The Port of San Pedro exceeds the active depth (Figure 2), which represents a barrier with the capacity to interrupt the longitudinal transport of sediments.

Erosion and Accretion of the Beaches of the Salinas and Arenales Regional Park in San Pedro del Pinatar
From the digitization of the beaches' areas per year, the surface variations of the beaches in the study area and the inter-annual variations associated with each period (Ta- This interruption of the longitudinal transport of N-S sediments has had very negative consequences on the southern beaches, in addition to the degradation of the dune system; especially the one associated with the beaches of Las Salinas and Barraca Quemada, located immediately to the south of the port, in addition to being the beaches that register the greatest loss of sand. This intensity of use is transferred to the dune system, which has been affected by a dense network of trails that have caused intense erosion. In addition to both circumstances, there is also the rise in sea level as a consequence of climate change. In this sense, and taking into account the evolution of draughts in the Mediterranean area, the beaches and dunes of the Salinas Regional Park and San Pedro del Pinatar are in danger of disappearing in the coming decades.

Erosion and Accretion of the Beaches of the Salinas and Arenales Regional Park in San Pedro del Pinatar
From the digitization of the beaches' areas per year, the surface variations of the beaches in the study area and the inter-annual variations associated with each period ( Table 1) have been obtained. The greatest surface losses are related to the shadow effect of the port, the beaches of Las Salinas and Barraca Quemada have surface losses in all periods, but these soar after the construction of the dikes. Torre Derribada beach, despite lacking sediment feeding, increases its width due to the accumulation of sand in the Levante dike located south of the beach.

Sections Located North of the Port
On the beaches located to the north of the port, El Mojón and Torre Derribada beach (Figure 3 The total loss of sediment is related to the lack of supply within the system. The perimeter channel of Las Salinas, which connects the lagoon of the Mar Menor with the Mediterranean, has no supply capacity and the mouths of watercourses and rivers that were related to the original formation of the beaches have lost their connection with the littoral system, due to the increased sealing of the soil of the small sub-basins of the coastal watercourses and, above all, to the construction of the ports located in the south of the Province of Alicante and to the extreme regulation of the River Segura, whose mouth is 30 km north of the regional park.

Sections Located South of the Port
The erosive processes on the beaches of Las Salinas, Barraca Quemada and Punta de Algas ( Figure 5 Las Salinas beach expands towards the south from the western dock of Puerto de San Pedro, which is why, since the enlargement of the facilities, it has suffered intense erosion. For the analysis of this beach, there were 80 proxies (Figure 6a), which show, firstly, how the beach increased its width by 14.40 m in the first period (1899-1956). The western dike generates a transport of the waves towards the south and consequently a local retreat of the coastline. Furthermore, the eastern swell is reflected by changing the propagation of the wave fronts and the energy flow, causing the coastline to have undergone a rotation since the construction of the port.
The following period   Las Salinas beach expands towards the south from the western dock of Puerto de San Pedro, which is why, since the enlargement of the facilities, it has suffered intense erosion. For the analysis of this beach, there were 80 proxies (Figure 6a), which show, firstly, how the beach increased its width by 14.40 m in the first period (1899-1956). The western dike generates a transport of the waves towards the south and consequently a local retreat of the coastline. Furthermore, the eastern swell is reflected by changing the propagation of the wave fronts and the energy flow, causing the coastline to have undergone a rotation since the construction of the port. The 33 southernmost transects correspond to Punta de Algas beach (Figure 6c). In this sector, the width of the beach has increased since the 1980s, due to the accumulation of sand carried by the coastal drift towards the south, where they have run aground in the rocky outcrops, although much of the sediment leaves the system and is deposited in Las Encañizadas, a channel that connects the Mediterranean Sea with the Mar Menor, whose width has decreased due to clogging. The width of the beach decreased by 13.80 m in the first 50 years. In the period between 1956 and 1981, regression increased by 42.53 m, which corresponds to a rate of −1.70 m/year. The rate has become positive in recent decades (1.44 m/year), creating a situation of apparent equilibrium (EPR = −0.05 m/year) for the 120 years studied. The 33 southernmost transects correspond to Punta de Algas beach (Figure 6c). In this sector, the width of the beach has increased since the 1980s, due to the accumulation of sand carried by the coastal drift towards the south, where they have run aground in the rocky outcrops, although much of the sediment leaves the system and is deposited in Las Encañizadas, a channel that connects the Mediterranean Sea with the Mar Menor, whose width has decreased due to clogging. The width of the beach decreased by 13.80 m in the first 50 years. In the period between 1956 and 1981, regression increased by 42.53 m, which

Discussion
In generalterms, the Spanish Mediterranean coastline is in an erosive state, the main causes being of anthropic origin [45][46][47]. In the southeast of the Iberian Peninsula, the lack of feeding from the watercourses is the main cause of coastal erosion [12,48]. Changes in land use and in the hydrography of the littoral basins mean modifications important enough to change the direction of the sediments that feed the beaches.
The interruption of sediment transport caused by port facilities, in most cases gained from the sea [49], is usually one of the most common origins of beach erosion on the Mediterranean coast. Coastal construction, especiallythat of ports, due to their size, is one of the main causes of alteration of the coastal dynamics in the transport of sediments, as happens, for example, in the beach of Sa Ràpita, located on the south coast of Mallorca. The beaches in this area have suffered the shadow effect of the port built in 1977 [42].
In the case of the beaches of San Pedro del Pinatar, both the decrease in the contribution of sediments (for the reasons mentioned above) and the construction of the port have unbalanced the natural dynamics of the system, with great differences between the beaches located at Barlomar and Sotamar port.
The importance of maintaining some salt flats in this area is essential to try to control coastal erosion. One of the main objectives of the salt exploitation restoration plans is related to the creation of sustainable sites that facilitate continuous ecological and natural succession, a development that has been shown to be successful elsewhere since the 1990s [50]. In addition to the fundamental role as a support for marine fauna and its capacity to capture CO2 [51], one of the most forceful responses to coastal erosion is the adequate conservation of oceanic posidonia [22,52], due to its active role in coastal dynamics. The removal of dead leaves tends to become a common problem [49], since it means the disappearance of a defense against storms, which usually appear when the leaves fall and accumulate on the shore.
Updrift port of San Pedro del Pinatar, the LIFE-Salinas Project, funded by the European Union is being developed between 2018 and 2022 (LIFE17/ES/000184). It has carried out stabilization and reinforcement of more than 2 hectares of dune system along the first 600 m of beach, which includes a preferential restoration of priority conservation habitats in the European Union (1510 "Saline Steppes" and 2250 "Littoral dunes with Juniperus spp."). For this, a network of sand fences and sand collectors made of cane (Arundodonax), with an approximate length of 3000 m, arranged in an orientation perpendicular to the prevailing wind direction, have been installed, together with the elimination of the network of existing trails and the closure of access to people in the most vulnerable dunes [53].
Using open-source software has several benefits. Beyond being free software, free source Geographical Information Technologies have great versatility, which allows the modification of the software itself to adapt it to the specific needs of each project or study [54]. In addition, this type of software is continually improving due to contributions from the scientific community. The transects between coastlines from different dates with GIS software is a widely used method in studies of coastal erosion. Among the most outstanding tools available, the Digital Shoreline Analysis DSAS is worth mentioning. It issoftware developed by the U.S. Geological Survey which is often used in coastal evolution studies. However, this software is not compatible with QGIS, so, as an alternative, the QGIS Station Lines plugin has been used.
The use of aerial photographs is widely extended in coastal dynamics and coastal erosion studies [55][56][57][58][59]. Aerial photographs are compared and it is necessary to know if the profile corresponds to periods of fair weather or storms on each date. On sandy beaches, during fair conditions, low waves provide sediment that accumulates on the beach [60], while during high-energy and storm conditions there is a loss of sediment through the rip currents, which are generated in the area of breakers and compensate for the effect of over-elevation caused by the waves [61]. One of the advantages of using aerial photography on micro-tidal beaches is that the problem of defining the coastline is avoided.

Conclusions
The use of Geographic Information Technologies is essential in research studies for the analysis of coastal erosion, since it allows evaluation of the effects of coastal dynamics, especially through the study of the historical evolution of the position of the coastline.
The construction of the port of San Pedro del Pinatar represents a fundamental change in the natural sedimentary dynamics of the adjacent beaches, with accretion in the northern sector, which contrasts with significant erosion processes in the beaches located to the south.
Due to the lack of depositions and the shadow effect of the port, the current erosive state of the downdrift beaches of the regional park is serious enough that there is a need to adopt adequate management measures to prevent the disappearance of the beaches and the system of dunes.
In this sense, free software from Geographic Information Systems is deemed reliable and may successfully address any type of geographical study thanks to the open-source libraries developed by the scientific and professional community.