Monitoring Retreat of Coastal Sandy Systems Using Geomatics Techniques: Somo Beach (Cantabrian Coast, Spain, 1875–2017)

remote sensing

Article Monitoring Retreat of Coastal Sandy Systems Using Geomatics Techniques: Somo Beach (Cantabrian Coast, Spain, 1875–2017)

José Juan de Sanjosé Blasco 1,* , Manuel Gómez-Lende 2, Manuel Sánchez-Fernández 1 and Enrique Serrano-Cañadas 2

1 Departamento de Expresión gráﬁca, Escuela Politécnica, Universidad de Extremadura, 06006 Badajoz, Spain; [email protected] 2 Departamento de Geografía, Facultad de Filosofía y Letras, Universidad de Valladolid, 47011 Valladolid, Spain; [email protected] (M.G.-L.); [email protected] (E.S.-C.) * Correspondence: [email protected]; Tel.: +34-927-25-70-00 (ext. 57546)

Received: 25 August 2018; Accepted: 16 September 2018; Published: 19 September 2018

Abstract: The dynamics and evolution of a coastal sandy system over the last 142 years (1875–2017) were analyzed using geomatics techniques (historical cartography, photogrammetry, topography, and terrestrial laser scanning (TLS)). The continuous beach–dune system is a very active confining sand barrier closing an estuarine system where damage is suffered by coastal infrastructures and houses. The techniques used and documentary sources involved historical cartography, digitalizing the 5-m-level curve on the maps of 1875, 1908, 1920, 1950, and 1985; photogrammetric flights of 1985, 1988, and 2001 without calibration certificates, digitalizing only the upper part of the sandy front; photogrammetric flights of 2005, 2007, 2010, and 2014, using photogrammetric restitution of the 5-m-level curve; topo-bathymetric profiles made monthly between 1988 and 1993 using a total station; a terrestrial laser scanner (TLS) since 2011 by means of two annual measurements; and the meteorological data for the period of 1985–2017. The retreat of the sandy complex was caused by winter storms with large waves and swells higher than 6 m, coinciding with periods demonstrating a high tidal range of over 100 and periods with a large number of strong storms. The retreat was 8 m between December 2013 and March 2014. The overall change of the coastline between 1875 and 2017 was approximately 415 m of retreat at Somo Beach. The erosive processes on the foredune involved the outcrop of the rock cliff in 1999 and 2014, which became a continuous rocky cliff without sands. To know the recent coastal evolution and its consequences on the human environment, the combined geomatic techniques and future TLS data series may lead to the improvement in the knowledge of shoreline changes in the context of sea level and global changes.

Keywords: coastal geomorphology; monitoring; morphology; geomatic techniques; sandy systems

1. Introduction Sandy complexes of shallow coasts are highly dynamic systems deriving from the interrelationships between marine and atmospheric agents. Their morphogenesis varies constantly and at different rhythms, and changes are, therefore, spatial and temporal, alongshore and cross-shore [1]. The multiple factors that affect the coastal dynamic have been subjected to theoretical quantitative analyses since the end of the 1960s by means of reference modeling for the study of sandy systems [1]. Several geomatics studies were conducted in recent decades on the coastal sandy system of El Puntal de Somo [1–10], and other research analyzed several of the world’s beaches using diverse geomatic techniques [11–18]. In this case, the geomorphological context in which the sandy systems

Remote Sens. 2018, 10, 1500; doi:10.3390/rs10091500 www.mdpi.com/journal/remotesensing Remote Sens. 2018, 10, 1500 2 of 18 developed not only depend on their exposure to waves, astronomical tides, storm surge, run-up, winds, or fluvial runoff, but also on the energy received and the alteration or acceleration of morphogenetic processes. To these factors, their estuarine configuration must be added, forcing the consideration of tidal currents [19] and geomorphological elements such as sand bars [20], both influential on the coastal dynamic as a natural barrier against the storms of the open sea. Human action is the most influential factor in the morphodynamics of beaches and sand strips in estuary systems on the coast of the Cantabrian Sea [21–29] and Santander Bay. The elimination of the submerged sand bar known as Las Quebrantas due to dragging in the mouth of the bay led to an increase in the effect of waves on the sand banks. The geomorphological context and human action also affect the increase or attenuation of the risk of erosion and flooding, both now and in the future [2,3], as well as the potential retreat of the shoreline. The nature of the interphase of coastal areas, together with land use, leads to the need for knowledge in order to provide suitable territorial management in tourist and leisure areas such as the one we are dealing with here. The need to monitor, analyze, and model processes is recommended to determine suitable interventions on the coast in the present and immediate future [5,18,19] in a place such as Santander Bay, which underwent great urban and socio-economic transformation. The main aim of this study was to analyze storms in combination with geomatic observations of the beach–dune system in El Puntal de Somo, one of the largest on the Cantabrian coast [30]. The retreat of the sand bar between El Puntal and Loredo Beach was analyzed using geomatic techniques and historical maps in combination with the analysis of the influence storms had on it during the period of 1985–2017.

2. The Study Area The sandy system of beaches and dunes of El Puntal in Somo, Loredo is located in the bay of Santander, one of the most important estuarine inlets of the Cantabrian coast, and has an area of 23.46 km2 and a perimeter of 97 km. The estuary belongs to the bay of Biscay and is represented by two subsystems, the Estuary of Santander itself, a diapiric basin formed in clays and gypsum of the Triasic age (Keuper facies) with fractures in the NW–SE direction where small streams flow out (Tijero, Boo, and Solía); and the Estuary of Cubas, the fluvial valley of the Miera River with an infill of alluvial sediments [20,31,32]. The exterior of the estuary is made up of a continuous beach–dune system within a confining sand barrier associated with a low coastal environment prolonged with a sand strip (Figure1). There are two large morphosedimentary areas: the barrier strip of El Puntal–Punta Rabiosa, ~3 km in length and protected from the waves of the open sea, and the beach of Somo–Quebrantas–Loredo, ~2 km in length and exposed to the most active swells from the NW [1]. The barrier strip and beaches close the Santander Estuary and are boxed within the estuarine subsystem of the Cubas Estuary [33,34]. Taken as a whole, it makes up one of the most extensive sandbanks of the Cantabrian Sea with a total of 1,047,190 m2 (Puntal de Somo: 308,149 m2 and Somo–Quebrantas–Loredo dune complex: 739,041 m2)[30] and forms the mouth of the Miera River through the Cubas Estuary. Both systems are the result of the divergence of the NW-dominant swell by diffraction and reflection on the Magdalena Peninsula and in the sandy bar of Las Quebrantas toward Punta Rabiosa, a sheltered area with NE exposure, and toward the beaches of Somo and Loredo, exposed beaches with N and NW exposure, respectively [1,3,10,30,35]. The evolution of the entire sandy system shows a longitudinal tendency toward the west, its apex stretching and turning toward the interior of the bay, and a layout of the whole sand bar toward concavity through its central area [5,10,19,20] (Figure1). The mean annual height of the significant swell (Hs) is 1 m, and 5 m for swells in winter storms of a NW component with peak periods of 16 s, with short small waves in the interior of the bay of a SW component generated locally, and semi-daytime tides averaging 3 m and 5 m in spring [7,10]. Nevertheless, the Magdalena Peninsula implies intense variations in swell, reaching all the studied sand system [3]. Remote Sens. 2018, 10, x FOR PEER REVIEW 2 of 17

developed not only depend on their exposure to waves, astronomical tides, storm surge, run-up, winds, or fluvial runoff, but also on the energy received and the alteration or acceleration of morphogenetic processes. To these factors, their estuarine configuration must be added, forcing the consideration of tidal currents [19] and geomorphological elements such as sand bars [20], both influential on the coastal dynamic as a natural barrier against the storms of the open sea. Human action is the most influential factor in the morphodynamics of beaches and sand strips in estuary systems on the coast of the Cantabrian Sea [21-29] and Santander Bay. The elimination of the submerged sand bar known as Las Quebrantas due to dragging in the mouth of the bay led to an increase in the effect of waves on the sand banks. The geomorphological context and human action also affect the increase or attenuation of the risk of erosion and flooding, both now and in the future [2,3], as well as the potential retreat of the shoreline. The nature of the interphase of coastal areas, together with land use, leads to the need for knowledge in order to provide suitable territorial management in tourist and leisure areas such as the one we are dealing with here. The need to monitor, analyze, and model processes is recommended to determine suitable interventions on the coast in the present and immediate future [5,18,19] in a place such as Santander Bay, which underwent great urban and socio-economic transformation. The main aim of this study was to analyze storms in combination with geomatic observations of the beach–dune system in El Puntal de Somo, one of the largest on the Cantabrian coast [30]. The retreat of the sand bar between El Puntal and Loredo Beach was analyzed using geomatic techniques and historical maps in combination with the analysis of the influence storms had on it during the period of 1985–2017.

Both systems are the result of the divergence of the NW-dominant swell by diffraction and reflection on the Magdalena Peninsula and in the sandy bar of Las Quebrantas toward Punta Rabiosa, a sheltered area with NE exposure, and toward the beaches of Somo and Loredo, exposed beaches with N and NW exposure, respectively [1,3,10,30,35]. The evolution of the entire sandy system shows a longitudinal tendency toward the west, its apex stretching and turning toward the interior of the bay, and a layout of the whole sand bar toward concavity through its central area [5,10,19,20] (Figure 1). The mean annual height of the significant swell (Hs) is 1 m, and 5 m for swells in winter storms of a NW component with peak periods of 16 s, with short small waves in the interior of the bay of a SW component generated locally, and semi-daytime tides averaging 3 m and 5 m in spring [7,10].

Nevertheless, the Magdalena Peninsula implies intense variations in swell, reaching all the studied Figure 1. Geographical location of Somo Beach. sand system [3]. Figure 1. Geographical location of Somo Beach.

Erosion rates rates within within annual annual cycles cycles were were also also modeled modeled and highlighted and highlighted depending depending on the different on the protectiondifferent protection of beaches of with beaches respect with to the respect diffraction to the of diffraction the swell. In of this the way, swell. the In more this exposed way, the beaches more exposedhave a greater beaches erosion have ratea greater and a erosion marked rate annual and erosion a marked cycle annual as a result erosion of thecycle greater as a frequencyresult of the of greatererosion frequency during the of autumn erosion months during [the3]. autumn months [3]. The dunes of of El El Puntal Puntal Beach Beach and and the the Miera Miera Estuary Estuary are are sites sites of ofcommunity community importance importance due due to theirto their high high ecological ecological value value in spite in spite of the of theintense intense human human activity activity deriving deriving from from urban urban expansion expansion and intensiveand intensive extractions, extractions, dragging dragging,, and andinfills infills of theof the bay bay since since the the 18 18thth century century [31,35,36] [31,35,36].. Its Its dune systems werewere alsoalso the the subject subject of restorationof restoration programs, programs, mainly mainly following following damage damage by large by storms large storms [19,37], [19,37]and research, and research into the influenceinto the influence of human of activity human on activity the risk on of the floods risk and of floods erosion and by theerosion entrance by the of entrancethe sea shows of the a sea flood shows level a onflood the level beach on of the Loredo beach of of 10 Loredo m [3]. of 10 m [3].

3. Methodology

3.1. Analysis of Historical Mapping (1726 (1726–1860)–1860) For the historical analysis of the dynamic of the El Puntal Beach Beach,, the first ﬁrst maps of the city of Santander and its surroundings can be used. These These were were drawn drawn up up by by Isidro Isidro Próspero Próspero Verboom Verboom in 1726 (Army Geographical Service). These These maps maps represent represent the the barrier strip that emerges in the neap low tides (Figure 2 2a).a). Later,Later, onon thethe 18601860 mapmap drawndrawn upup byby FranciscoFrancisco dede CoelloCoello (National(National GeographicGeographic Institute), the sandy bar disappeared disappeared,, and a sand barrier was generated perpendicular to the area of Las Quebrantas (Figure 2 2b).b).

(a) (b)

Figure 2. (a) Map of the beach of Somo by Isidro Próspero (1726) with the sandy bank; (b) map by Figure 2. (a) Map of the beach of Somo by Isidro Próspero (1726) with the sandy bank; (b) map by Francisco de Coello (1860) with the sand bar. Francisco de Coello (1860) with the sand bar.

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The maps of 1726 and 1860 were not used in a cartographical study of the evolution of the beach of El Puntal since it was not possible to georeference them, i.e., transform the map into the geodesic system of official reference in Spain (ETRS 89). As seen in Figure2, there are no cartographic details in which ground control points can be located (crossings of paths, buildings, field fencing or walling, etc.). Despite not using these maps for the metric analysis of the evolution of Somo Beach, these historical maps give a good idea of how the coastal changes are continuous and are not due to punctual processes. The most influential factor in the retreat of the coastline at Somo Beach may possibly be large storms; thus, a detailed study was made of the last 32 years (1985–2017), since reliable information is available for this period.

3.2. Analysis of Sea Storms (1985–2017) Sea storms were studied using the daily swell data of the Cabo Mayor lighthouse (4 km to the north of the beach of El Puntal) at 9:00 a.m. and 3:00 p.m. (information recorded by the State Meteorological Agency and available since 1985). Days with waves of over 5 m were selected, as well as those with waves of over 6 m (very rough seas). In addition, high tides from 1985 were scrutinized to find those with a range of over 80, as well as those with a range of over 100. In order to analyze the influence of sea storms on the coast, the coincidence between high tides and large swells was considered. We think that, if these two factors do not coincide in time, the retreat of the shoreline will not be as significant. Table1 shows the number of days and number of storms (there are storms that last more than one day) distributed in five-year periods. Table1 reveals two coincidences: swells of between 5 m and 6 m with a tidal range of over 80 (medium storms), and swells of over 6 m with a tidal range of over 100 (large storms).

Table 1. Number of days that coincide in time (timetable): large waves and large tidal range (high tide).

Swell between 5 m and 6 m Swell >6 m Period Tidal Range >80 Tidal Range >100 1985–1989 2 days (2 storms) 0 days 1990–1994 2 days (2 storms) 0 days 1995–1999 9 days (8 storms) 3 days (2 storms) 2000–2004 2 days (2 storms) 2 days (2 storms) 2005–2009 3 days (2 storms) 3 days (2 storms) 2010–2014 4 days (3 storms) 6 days (3 storms) 2015–2017 0 days 0 days

The information obtained from the lighthouse of Cabo Mayor was checked against the library of headlines in local newspapers, which described the rough state of the sea; in all cases, there was a coincidence between the storms recorded at the lighthouse of Cabo Mayor and the newspaper headlines. The dates taken from the records of Cabo Mayor were also checked against the statistical data of swells recorded by the Environmental Hydraulic Institute of Cantabria and against those of the swell statistics of REDCOS (Coastal Network of Swells at State Ports), which coincided in all cases with the values in Table1. What follows is an analysis of the coastal dynamic, which serves to check whether any correspondence exists between storms and the beach dynamic. Although there are outside factors that may affect the dynamic of this beach, a great deal of dragging was done in the bay of Santander in the 1980s so that ships could reach the industrial port (inaugurated in 1985) and the ferry dock (inaugurated in 1989) [38,39].

3.3. Cartographical Study (1875–1985) For the cartographical analysis of the evolution of the beach of El Puntal, maps were available from the years of 1875, 1908, 1920, 1950, and 1985 from the Port Authority of Santander (Figure3). Remote Sens. 2018, 10, 1500 5 of 18

The different cartographies used (Figure3) were georeferenced in ETRS 89 so as to collect them all Remote Sens. 2018, 10, x FOR PEER REVIEW 5 of 17 in the same system of coordinates. Only in this way could comparisons be made among maps and other geomatic techniques be used (photogrammetry, topography, and terrestrial laser scanning (TLS)). Using the global navigation satellite system (GNSS), coordinates in ETRS 89 were given to 31 Using the global navigation satellite system (GNSS), coordinates in ETRS 89 were given to 31 elements elements present today (houses, churches, walls, etc.) distributed around the bay of Santander, which present today (houses, churches, walls, etc.) distributed around the bay of Santander, which are are representedrepresented onon severalseveral of of the the historical historical maps. maps.

(a) (b)

(c)

(d) (e)

Figure 3. Maps of Somo Beach for the years of (a) 1875, (b) 1908, (c) 1920, (d) 1950, and (e) 1985. Figure 3. Maps of Somo Beach for the years of (a) 1875, (b) 1908, (c) 1920, (d) 1950, and (e) 1985. Subsequent maps were georeferenced, and the 5-m-level curve was digitalized. Figure4 shows theSubsequent degree of maps ﬁt of the were georeferencing georeferenced, of the and different the 5- cartographies,m-level curve withwas maximumdigitalized. errors Figure of 40 4 m.shows the degreeThe ﬁt of thefit of georeferenced the georeferencing digitalizations of the ofdifferent the different cartographies, maps in the with area maximum of Puerto Chicoerrors were of 40 m. The fitchecked of the against georeferenced the orthophoto digitalizations in Figure4, atof 1the km different northeast ofmaps Punta in Rabiosa the area (Figure of Puerto1). Chico were checked againstThe curve the of orthophoto the 5-m level in was Figure then 4, digitalized at 1 km nor on eachtheast of theof Punta georeferenced Rabiosa maps, (Figure as this1). curve was present on all the maps analyzed (Figure5). It should be pointed out that the 5-m-level curve is normally situated between the upper part of the backshore and the base of the cliffed dune.

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Using the global navigation satellite system (GNSS), coordinates in ETRS 89 were given to 31 elements present today (houses, churches, walls, etc.) distributed around the bay of Santander, which are represented on several of the historical maps.

(a) (b)

(c)

(d) (e)

Subsequent maps were georeferenced, and the 5-m-level curve was digitalized. Figure 4 shows the degree of fit of the georeferencing of the different cartographies, with maximum errors of 40 m. RemoteThe fit Sens. of 2018the ,georeferenced10, 1500 digitalizations of the different maps in the area of Puerto Chico 6were of 18 checked against the orthophoto in Figure 4, at 1 km northeast of Punta Rabiosa (Figure 1).

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Figure 4. Differences in georeferencing in the digitalizations of the maps of 1875, 1908, 1920, 1950, and 1985. Orthophotography from 2010 (Spatial Data Infrastructure (SDI) Service of the Government of Cantabria, Spain).

FigureThe curve 4. Differences of the 5-m in level georeferencing was then in digitalized the digitalizations on each of of the the maps georeferenced of 1875, 1908, maps, 1920, 1950, as this and curve was 1985.present Orthophotography on all the maps fromanaly 2010zed (Spatial (Figure Data 5). It Infrastructure should be pointed (SDI) Service out that of thethe Government 5-m-level curve of is normallyCantabria, situated Spain). between the upper part of the backshore and the base of the cliffed dune.

FigureFigure 5. 5. TheThe 5 5-m-level-m-level curve curve corresponding corresponding to to the maps of 1875, 1908, 1920, 19501950,, and 1985. OrthophotographyOrthophotography fromfrom 20102010 (SDI(SDI ServiceService ofof thethe GovernmentGovernment ofof Cantabria,Cantabria, Spain).Spain).

3.4.3.4. Photogrammetric Analysis (1985–2014)(1985–2014) PhotogrammetricPhotogrammetric flightsflights were were made made over over the the city city of of Santander Santander since since 1946, 1946, but but those those made made prior prior to 1985to 1985 could could not benot taken be taken into consideration into consideration because because of their of low their quality. low quality.Therefore, Therefore, the flights the available flights fromavailable 1985 from to 2014 1985 were to 2014 as follows: were as follows: • FlightsFlights withoutwithout photogrammetric information information (1985, (1985, 1988 1988,, and and 2001): 2001): There There is isno no certification certification of ofcamera camera calibration. calibration. The The aerial aerial photography photography was was rectified rectified using using points points situated situated on the on beach the beach,, and andlater later,, the front the front of the of sandy the sandy slope slope waswas digitalized. digitalized. • PhotogrammetricPhotogrammetric flights flights (2005, (2005, 2007, 2007, 2010, 2010 and, and 2014): 2014 These): These are of are good of photogrammetric good photogrammetric quality andquality open and up the open possibility up the of orienting possibility pairs of of orienting photographs pairs and of generating photographs stereoscopes. and generating Camera calibrationsstereoscopes. were Camera certified calibrations and mapping were was certified performed and mapping in the digital was stereoplotter, performed in Digi the 3D. digital stereoplotter, Digi 3D. The upcoming section analyzes the two possible interpretations of the photogrammetric sessions (digitalizationThe upcoming and restitution). section analyzes the two possible interpretations of the photogrammetric sessions (digitalization and restitution). 3.4.1. Digitalization of Photogrammetric Flights (1985–2001) 3.4.1. Digitalization of Photogrammetric Flights (1985–2001) The flights in 1985, 1988, and 2001 were rectified using the program ArcMap of ArcGIS. To make the rectification,The flights in nearby 1985, 1988 control, and points 2001 onwere the rectified shore (level using zero) the program were used, ArcMap and the of upperArcGIS. part To ofmake the slopethe rectification of the sandy, nearby front was control later points digitalized on the (Figure shore6 (level). During zero) this were period used, (1985–2001), and the upper a retreat part inof thethe shorelineslope of the in thissandy area front was was detected. later digitalized (Figure 6). During this period (1985–2001), a retreat in the shoreline in this area was detected.

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The curve of the 5-m level was then digitalized on each of the georeferenced maps, as this curve was present on all the maps analyzed (Figure 5). It should be pointed out that the 5-m-level curve is normally situated between the upper part of the backshore and the base of the cliffed dune.

Figure 5. The 5-m-level curve corresponding to the maps of 1875, 1908, 1920, 1950, and 1985. Orthophotography from 2010 (SDI Service of the Government of Cantabria, Spain).

3.4. Photogrammetric Analysis (1985–2014) Photogrammetric flights were made over the city of Santander since 1946, but those made prior to 1985 could not be taken into consideration because of their low quality. Therefore, the flights available from 1985 to 2014 were as follows:  Flights without photogrammetric information (1985, 1988, and 2001): There is no certification of camera calibration. The aerial photography was rectified using points situated on the beach, and later, the front of the sandy slope was digitalized.  Photogrammetric flights (2005, 2007, 2010, and 2014): These are of good photogrammetric quality and open up the possibility of orienting pairs of photographs and generating stereoscopes. Camera calibrations were certified and mapping was performed in the digital stereoplotter, Digi 3D. The upcoming section analyzes the two possible interpretations of the photogrammetric sessions (digitalization and restitution).

3.4.1. Digitalization of Photogrammetric Flights (1985–2001) The flights in 1985, 1988, and 2001 were rectified using the program ArcMap of ArcGIS. To make the rectification, nearby control points on the shore (level zero) were used, and the upper part of the Remoteslope Sens.of the2018 sandy, 10, 1500 front was later digitalized (Figure 6). During this period (1985–2001), a retreat7 of 18in the shoreline in this area was detected.

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Figure 6. Digitalization of ﬂightsflights (1985, 1988,1988, and 2001) of Somo Beach. Orthophotography from 2010 (SDI(SDI ServiceService ofof thethe GovernmentGovernment ofof Cantabria,Cantabria, Spain).Spain).

The comparison between the digitalizations of maps from 1985 (Figure5 5)) andand thethe flightflight ofof 19851985 (Figure(Figure6 6)) show show planimetric planimetric differences differences of of over over 200 200 m betweenm between the the 5-m-level 5-m-level curve curve on theon mapsthe maps and and the upperthe upper part partof the of sandy the sandy slope from slope the from 1985 the flight 1985 (Figure flight7 (Figure). Figure 7).4 shows Figure that 4 shows it is not that a problem it is not of a georeferencing,problem of georeferencing and we can,, therefore,and we can interpret, therefore that, interpret the maps that from the 1985 maps fail from to reflect 1985 the fail true to reflect condition the oftrue Somo condition Beach. of The Somo reason Beach. for thisThe isreason that the forPort this Authorityis that the ofPort Santander Authority used of theSantander same map used from the 1950same to map 1985 from (see the1950 maps to 1985 of 1950 (see and the 1985maps in of Figure 19503 ).and Both 1985 maps in F representigure 3). theBoth mouth maps of represent the bay and the itsmouth bathymetry, of the bay information and its bathymetry, indispensable information to navigation indispens for accessable to to navigation the Port of for Santander; access to however, the Port theof Santander rest is identical; however, on both the maps.rest is Therefore,identical on the both map maps. of 1985 Therefore, was discarded the map and of the1985 digitalization was discarded of theand aerial the digitalization photography of of the the aerial same photography year was used. of the same year was used.

Figure 7. 7. ComparisonComparison of of digitalizations digitalizations of of the the cartography cartography 1985 1985 and and of of the the flight ﬂight of of 1985. Orthophotography fromfrom 20102010 (SDI(SDI ServiceService ofof thethe GovernmentGovernment ofof Cantabria,Cantabria, Spain).Spain).

3.4.2. Photogrammetric RestitutionsRestitutions (2005–2014)(2005–2014) The calibration of the cameras used on the photogrammetric flightsflights in 2005, 2007, 2010,2010, and 2014 by thethe SpatialSpatialData Data Infrastructure Infrastructure (SDI) (SDI Service) Service of of the the Government Government of Cantabriaof Cantabria was w certified.as certified. This This fact providesfact provides sufficient sufficient assurance assurance ofquality of quality to beto be able able to to draw draw up up the the cartography cartography on on a a 1/2500 1/2500 scale, inin whichwhich thethe maximummaximum errorerror inin thethe orientationorientation ofof flightsflights mustmust notnot bebe overover 0.50.5 mm (Table(Table2 ).2). The orientation orientation of of the the photographs photographs of these of these flights flights was determined was determined following following the General the General Process ofProcess Photogrammetry of Photogrammetry [40], which [40] consists, which of consists internal of and internal external and orientation. external orientation. For internal For orientation, internal theorientation certificate, the of certificate calibration of of calibration the camera of is the required camera (focal is required distance, (focal coordinates distance, of thecoordinates fiducial marks,of the andfiducial distortion marks, functionand distortion of the function lenses); forof the the lenses) external; for orientation, the external the orientation coordinates, the ofcoordinates the support of andthe support control points and control in ETRS points 89 are in needed. ETRS 89 Theare support needed. points The support are those points used are in thethose orientation used in the of theorientation various of cartographies the various cartographies for flights in for 2005, flights 2007, in 2010,2005, and2007, 2014, 2010, and and they2014, are and always they are the always same ones.the same The ones. control The points control are points the support are the support points not points used not in used the external in the external orientation. orientation. On each On of each the photogrammetricof the photogrammetric restitutions restitutions on a scale on a ofscale 1/2500, of 1/2500, the same the same control control points points were were measured measured and and the differencethe difference in the in coordinatesthe coordinates was analyzed.was analy Thesezed. These did not did surpass not surpass the maximum the maximum value ofvalue 0.5 mof in0.5 any m ofin theanycoordinates of the coordinates (x, y, and (x, zy), (Tableand z)2 (Table). 2).

Table 2. Difference in coordinates of the control points for the different flights (2005, 2007, 2010, and 2014). Coordinates are in ETRS 89 and the origin of altitudes is the mean sea level in Alicante (Spain).

Poin Coordinat Year 2005 Year 2007 Year 2010 Year 2014 Difference in x Difference in y Difference in z t e 2007–2005: 0.445 x 441,240.502 441,240.947 441,240.656 441,240.911 m 4,812,198.85 4,812,198.38 4,812,198.88 4,812,198.88 2014–2007: 0.499 1 y 8 4 1 3 m 2007–2005: 0.457 z 6.034 6.491 6.303 6.197 m 2010–2005: 0.133 x 440,874.853 440,874.947 440,874.986 440,874.947 m 4,811,762.36 4,811,762.65 4,811,762.26 4,811,762.76 2014–2010: 0.498 2 y 9 4 6 4 m 2007–2010: 0.496 z 33.641 34.002 33.506 33.584 m

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Table 2. Difference in coordinates of the control points for the different ﬂights (2005, 2007, 2010, and 2014). Coordinates are in ETRS 89 and the origin of altitudes is the mean sea level in Alicante (Spain).

Difference Difference Difference Point Coordinate Year 2005 Year 2007 Year 2010 Year 2014 in x in y in z 2007–2005: x 441,240.502 441,240.947 441,240.656 441,240.911 0.445 m 1 2014–2007: y 4,812,198.858 4,812,198.384 4,812,198.881 4,812,198.883 0.499 m 2007–2005: z 6.034 6.491 6.303 6.197 0.457 m 2010–2005: x 440,874.853 440,874.947 440,874.986 440,874.947 0.133 m 2 2014–2010: y 4,811,762.369 4,811,762.654 4,811,762.266 4,811,762.764 0.498 m 2007–2010: z 33.641 34.002 33.506 33.584 0.496 m 2007–2014: x 441,367.931 441,367.949 441,367.748 441,367.481 0.468 m 3 2010–2014: y 4,812,533.321 4,812,533.011 4,812,533.384 4,812,532.973 0.411 m 2007–2010: z 12.253 12.287 11.788 11.884 0.499 m 2005–2010: x 441,327.687 441,327.666 441,327.509 441,327.660 0.178 m 4 2010–2014: y 4,812,791.151 4,812,791.116 4,812,791.273 4,812,790.938 0.335 m 2007–2014: z 9.326 9.327 8.835 8.829 0.498 m 2007–2014: x 441,270.233 441,270.492 441,270.456 441,269.998 0.494 m 5 2007–2010: y 4,812,287.476 4,812,287.488 4,812,287.441 4,812,287.448 0.047 m 2005–2010: z 2.676 2.465 2.217 2.359 0.459 m 2005–2010: x 440,823.517 440,823.428 440,823.290 440,823.428 0.227 m 6 2010–2007: y 4,812,064.215 4,812,064.070 4,812,064.470 4,812,064.070 0.400 m 2005–2014: z 8.749 8.266 8.427 3.318 5.431 m Maximum difference of coordinates: x (point 5): 0.494 m; y (point 1): 0.499 m; z (point 3): 0.499 m.

Coordinate z in Table2 indicates the particular situation of point 6 owing to its disappearance following the winter storms in 2013–2014, the origin position (2010) of which is shown in Figure8a, and the situation in 2014 is shown in Figure8b. The flight of 2014 was, therefore, given the same planimetric coordinates as the flight of 2010 and its altimetric position (3.318 m) was determined. The loss of level at this point between 2005 and 2014 was 5.431 m. Between 2010 and 2014, loss was 5.109 m, and this value was later taken into account to evaluate the influence of the storms of 2013–2014. Once the process of photogrammetric orientation was complete, the upper part of the slope of the sandy front was restituted (Figure9). Both the digitalized cartography and photogrammetric flights prior to 2005 and the photogrammetric restitutions used the ETRS 89 system of coordinates. In order to check the fit of the georeferencing, common elements were used, such as the corners of houses, crossroads, or the corners of fields. The maximum differences between the digitalizations (cartography and photogrammetry) and the photogrammetric restitutions were no greater than 40 m. Remote Sens. 2018, 10, x FOR PEER REVIEW 8 of 17

2007–2014: 0.468 x 441,367.931 441,367.949 441,367.748 441,367.481 m 4,812,533.32 4,812,533.01 4,812,533.38 4,812,532.97 2010–2014: 0.411 3 y 1 1 4 3 m 2007–2010: 0.499 z 12.253 12.287 11.788 11.884 m 2005–2010: 0.178 x 441,327.687 441,327.666 441,327.509 441,327.660 m 4,812,791.15 4,812,791.11 4,812,791.27 4,812,790.93 2010–2014: 0.335 4 y 1 6 3 8 m 2007–2014: 0.498 z 9.326 9.327 8.835 8.829 m 2007–2014: 0.494 x 441,270.233 441,270.492 441,270.456 441,269.998 m 4,812,287.47 4,812,287.48 4,812,287.44 4,812,287.44 2007–2010: 0.047 5 y 6 8 1 8 m 2005–2010: 0.459 z 2.676 2.465 2.217 2.359 m 2005–2010: 0.227 x 440,823.517 440,823.428 440,823.290 440,823.428 m 4,812,064.21 4,812,064.07 4,812,064.47 4,812,064.07 2010–2007: 0.400 6 y 5 0 0 0 m 2005–2014: 5.431 z 8.749 8.266 8.427 3.318 m Maximum difference of coordinates: x (point 5): 0.494 m; y (point 1): 0.499 m; z (point 3): 0.499 m.

(a) (b)

Figure 8. (a) Position of control point 6 in the orthophotographs of 2010; (b) point 6 in the Figure 8. (a) Position of control point 6 in the orthophotographs of 2010; (b) point 6 in the Remoteorthophotographs Sens. 2018, 10, x FOR of PEER 2014. REVIEW SDI Service of the Government of Cantabria, Spain. 9 of 17 orthophotographs of 2014. SDI Service of the Government of Cantabria, Spain.

Once the process of photogrammetric orientation was complete, the upper part of the slope of the sandy front was restituted (Figure 9). Both the digitalized cartography and photogrammetric flights prior to 2005 and the photogrammetric restitutions used the ETRS 89 system of coordinates. In order to check the fit of the georeferencing, common elements were used, such as the corners of houses, crossroads, or the corners of fields. The maximum differences between the digitalizations (cartography and photogrammetry) and the photogrammetric restitutions were no greater than 40 m.

Figure 9. Restitutions of aerial flights ﬂights (2005, 2007, 2010 2010,, and 2014) of Somo Beach, inin which only the upper part ofof thethe slopeslope ofof thethe sandysandy front front is is represented. represented. Orthophotography Orthophotography from from 2010 2010 (SDI (SDI Service Service of theof the Government Government of Cantabria,of Cantabria Spain)., Spain).

3.5. Topographical Measurement (1988–1993)(1988–1993) For the topographical topographical bathymetric bathymetric monitoring monitoring of of the the beach beach of ofSomo Somo between between 1988 1988 and and 1993, 1993, 13 13profiles profiles (P1, (P1, P1’ P1’,, P2,P2, P3,P3, …, ...P8), were P8) were measured measured monthly monthly (Figure (Figure 10a) 10[10,4a) [1]10. ,41The]. bathymetry The bathymetry was wasobserved observed at high at high tide tideand and the thetopographical topographical land land measurement measurement at atlow low tide tide in in order order to to obtain obtain the intertidal overlap between the bathymetric measurements measurements (profile (profile with with one one sounding sounding on on the the vessel) vessel) and the terrestrial ones (topographical(topographical profileprofile onon thethe beach).beach). An analysis of the topographical profiles throughout of Somo Beach during the period 1988–1993 reveals few changes to the retreat of the shoreline in the area of Las Quebrantas. However, if we observe the photogrammetric restitution of 2005, the large retreat of the coast in Las Quebrantas between 1988 and 2005 is seen. Since 2011, research was, therefore, centered on the area of Las Quebrantas, where TLS data were collected [42,43].

(a)

(b)

Figure 10. (a) Planimetric representation of the 13 topographic profiles (1988–1993) on the photogrammetric cartography (2005, 2007, 2010, and 2014). The topographic, photogrammetric, and

Remote Sens. 2018, 10, x FOR PEER REVIEW 9 of 17

Figure 9. Restitutions of aerial flights (2005, 2007, 2010, and 2014) of Somo Beach, in which only the upper part of the slope of the sandy front is represented. Orthophotography from 2010 (SDI Service of the Government of Cantabria, Spain).

3.5. Topographical Measurement (1988–1993) For the topographical bathymetric monitoring of the beach of Somo between 1988 and 1993, 13 profiles (P1, P1’, P2, P3, …, P8) were measured monthly (Figure 10a) [10,41]. The bathymetry was observed at high tide and the topographical land measurement at low tide in order to obtain the intertidalRemote Sens. overlap2018, 10, 1500between the bathymetric measurements (profile with one sounding on the vessel)10 of 18 and the terrestrial ones (topographical profile on the beach).

(a)

(b)

FigureFigure 10. (a()a ) Planimetric Planimetric representation representation of of the the 13 13 topographic topographic profiles profiles (198 (1988–1993)8–1993) on on the the photogrammetricphotogrammetric cartography cartography (2005, (2005, 2007, 2007, 2010 2010,, and and 2014). 2014). The Thetopographic, topographic, photogrammetric photogrammetric,, and and terrestrial laser scanning (TLS) data collection area are also amplified ×10. (b) The seven profiles of measurement using photogrammetry and TLS over the cartography generated by TLS in 2011 are shown. Profile 4 (P4) coincides with the measurement of profile 7 (P7) in the period of 1988–1993. The position of the base in 1988 is also indicated.

3.6. Terrestrial Laser Scanner (2011–2017) The application of terrestrial laser scanning (TLS) for the monitoring and analysis of geomorphological dynamics on sandy coasts with millimetric precision is relatively recent and entirely efficient even at detecting annual changes [44], as well as complementing traditional analytical methods. TLS has functional sensitivity under adverse meteorological conditions [45] and a versatility that permits monitoring at any time. It has proven reliability on the coasts of the Cantabrian Sea in the study area, on confined beaches nearby, and in the ongoing observation of sand banks [42,43,46,47]. The use of TLS in the area of Las Quebrantas provided seven profiles, as shown in Figure 10b. The number of points measured with TLS in each survey was approximately 500 million with a precision at each point of 3 cm including the GNSS error. Two measurements were made per year to evaluate the evolution of the beach during the winter (October–March) and the summer (April–September). A shoreline of 400 m was selected to make the TLS measurements coincide with those from previous periods. In this way, profile 4 (P4) taken with TLS (2011–2017) is the same as profile 7 (P7), which was measured using topographical techniques (1988–1993), and this same profile 7 (P7) was collected using photogrammetry (2005–2014) (Figures 11 and 12). Remote Sens. 2018, 10, x FOR PEER REVIEW 11 of 17

425 m since 1926. During this period (1875–2017), our calculations determine a westward advance of 470 m.  This area of the El Puntal Beach retreated southward due to erosion, a process that is ongoing. There was an estimate of the shoreline retreat of 100 m between 1929 and 1960 [10], while Reference [48] suggests a retreat of 200 m between 1920 and 1985 (Table 4). Our calculations indicate a retreat of around 200 m between 1950 and 1985, and of approximately 375 m from 1875 to 1985 (Table 4).  Digitalizations of the flights in 1985, 1988, and 2001: The coastal retreat in this period was 25 m.  Digitalization of the flight in 2001 and the photogrammetric restitutions of the flights in 2005, 2007, 2010, and 2014 (Figure 11a): From 2005 to 2010, there was a mean retreat of the shoreline of 5 m, although there are sections with retreats of over 20 m and others that advanced in the period (Figure 11b). Between 2010 and 2014, there was an accentuated retreat of the shoreline, particularly in the winter of 2013–2014 as a result of large storms that hit the Cantabrian Sea Remote(Fi Sens.gure2018 11c)., 10, 1500The estimated retreat in this area during these storms was 10 m. Thus, in the11 200 of1 18– Remote2 014Sens. period2018, 10,, xthere FOR PEERwas aREVIEW retreat of around 15 m. 12 of 17

In total, the retreat in the area between Punta Rabiosa and Somo between 1875 and 2014 was 415 m with a margin of uncertainty of 40 m in the cartographic digitalizations (Table 4). It should be pointed out that these results are close to those of References [10,48].

Table 4. Summary of the historical evolution of El Puntal Beach.

Date Total Years Retreat (m) Retreat (m/yr) Authors 1929–1960 31 100 3.2 [10] 1920–1985 65 200 3.1 [48] 1875–1985 110 375 3.4 1950–1985 35 200 5.7 [41-43] 1875–2014 139 415 ± 40 2.7–3.3 (a) The study of the changes in the area between the village of Somo and Loredo Beach, known as Las Quebrantas Beach, for the period of 1875–2017 indicates the following:  The cartography of 1875, 1908, 1920, and 1950, and the flight of 1985: The values of coastal retreat are the same as those of the rest of the El Puntal Beach, that is, 375 m.  Digitalization of the flights in 1985, 1988, and 2001: The retreat of the shoreline during the period was 30 m. Within this period, the analysis of the topographical profile (P4) between 1988 and 1993 showed a retreat of less than 1 m (Figure 12). Between the topographical profile of 1993 and the photogrammetric restitution of 2005, the shoreline retreated by 25 m given that the upper

part of the slope of the sandy front between( b1988) and 1993 was found to be 25 m farther out to sea with respect to 2005.  Digitalization of the flight in 2001 and photogrammetric restitutions of the flights in 2005, 2007, 2010, and 2014: There was hardly any retreat in the foredune ridge between 2001–2014, although this was not the case at the base of the slope of the sandy front, which was affected by the winter storms of 2013–2014 (Figure 12). These variations were analyzed in greater detail using TLS.  Terrestrial laser scanning (TLS): Measurements taken using TLS in the period from August 2011 to November 2013 show stability with slight variations in the intertidal area. In April 2014, large retreats are seen with respect to the measurements made in November 2013 (Figure 13) and these coincide with the photogrammetric measurements (Figure 12). The retreat of the base of the slope of the sandy front in the position of pro(cfile) 4 between November 2013 and April 2014 was Figure8 m. The 11. fall(a) Restitutionsin the level of of the the slope entire of thebeach sandy by front3 m was in 2005, also 2007, significant, 2010, and even 2014. as (b )much Comparison as 5.431 m Figure 11. (a) Restitutions of the slope of the sandy front in 2005, 2007, 2010, and 2014. (b) Comparison ofin thesome evolution areas (point of the shorelines 6) (Table restituted 2). From in April 2005 and2014 2010. to April (c) Comparison 2017, the of upper the evolution part of ofthe the slope of the evolution of the shorelines restituted in 2005 and 2010. (c) Comparison of the evolution of the shorelinesretreated by restituted 2 m, stabilizing in 2010 and the 2014. gradient of the sandy slope (Figure 13). shorelines restituted in 2010 and 2014.

Figure 12. TopographicalTopographical measurements measurements of of profile proﬁle 4 (Figure 4 (Figure 10b) 10 betweenb) between 1988 1988 and and1993, 1993,and andphotogrammetric photogrammetric evolution evolution of profile of proﬁle 4 between 4 between 2005 2005 and and2014. 2014.

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4. Results and Discussion The geomatic techniques developed in the study of Somo Beach have maximum errors in the cartographic production for the calculation of the coastal retreat (Table3).

Table 3. Maximum errors obtained with each geomatic method.

Geomatic Method Maximum Error (m) Cartography (1875–1985) 40.00 Photogrammetry (1985–2014) 0.50 Topography (1988–1993) 0.07 Terrestrial Laser Scanner (2011–2017) 0.03

To determine the numerical values of the evolution of the shoreline, the beach of Somo was divided into two parts: the sandy strips between Punta Rabiosa and Somo (El Puntal Beach), and the cliffed dune between Somo and Loredo Beach (Las Quebrantas Beach) (Figure1). The results obtained between Punta Rabiosa and Somo between 1875 and 2014 were as follows:

• Cartography of 1875, 1908, 1920, and 1950, and digitalization of the flight in 1985: The curve of the 5-m level in Punta Rabiosa between 1875 and 1920 indicates a turn from west to northeast (Figure5). In the period of 1950–1985, it underwent a deviation in its trajectory toward the west (Figure5). This change in direction is still ongoing today and is possibly because of dredging at the mouth of the bay of Santander. There are estimates indicating that, from 1870 to the present, Punta Rabiosa grew by 1075 m toward the west [20], while [48] some estimate this growth to be 425 m since 1926. During this period (1875–2017), our calculations determine a westward advance of 470 m. • This area of the El Puntal Beach retreated southward due to erosion, a process that is ongoing. There was an estimate of the shoreline retreat of 100 m between 1929 and 1960 [10], while Reference [48] suggests a retreat of 200 m between 1920 and 1985 (Table4). Our calculations indicate a retreat of around 200 m between 1950 and 1985, and of approximately 375 m from 1875 to 1985 (Table4). • Digitalizations of the flights in 1985, 1988, and 2001: The coastal retreat in this period was 25 m. • Digitalization of the flight in 2001 and the photogrammetric restitutions of the flights in 2005, 2007, 2010, and 2014 (Figure 11a): From 2005 to 2010, there was a mean retreat of the shoreline of 5 m, although there are sections with retreats of over 20 m and others that advanced in the period (Figure 11b). Between 2010 and 2014, there was an accentuated retreat of the shoreline, particularly in the winter of 2013–2014 as a result of large storms that hit the Cantabrian Sea (Figure 11c). The estimated retreat in this area during these storms was 10 m. Thus, in the 2001–2014 period, there was a retreat of around 15 m.

Table 4. Summary of the historical evolution of El Puntal Beach.

Date Total Years Retreat (m) Retreat (m/yr) Authors 1929–1960 31 100 3.2 [10] 1920–1985 65 200 3.1 [48] 1875–1985 110 375 3.4 1950–1985 35 200 5.7 [41–43] 1875–2014 139 415 ± 40 2.7–3.3

In total, the retreat in the area between Punta Rabiosa and Somo between 1875 and 2014 was 415 m with a margin of uncertainty of 40 m in the cartographic digitalizations (Table4). It should be pointed out that these results are close to those of References [10,48]. Remote Sens. 2018, 10, 1500 13 of 18

The study of the changes in the area between the village of Somo and Loredo Beach, known as Las Quebrantas Beach, for the period of 1875–2017 indicates the following:

• The cartography of 1875, 1908, 1920, and 1950, and the flight of 1985: The values of coastal retreat are the same as those of the rest of the El Puntal Beach, that is, 375 m. • Digitalization of the flights in 1985, 1988, and 2001: The retreat of the shoreline during the period was 30 m. Within this period, the analysis of the topographical profile (P4) between 1988 and 1993 showed a retreat of less than 1 m (Figure 12). Between the topographical profile of 1993 and the photogrammetric restitution of 2005, the shoreline retreated by 25 m given that the upper part of the slope of the sandy front between 1988 and 1993 was found to be 25 m farther out to sea with respect to 2005. • Digitalization of the flight in 2001 and photogrammetric restitutions of the flights in 2005, 2007, 2010, and 2014: There was hardly any retreat in the foredune ridge between 2001–2014, although this was not the case at the base of the slope of the sandy front, which was affected by the winter storms of 2013–2014 (Figure 12). These variations were analyzed in greater detail using TLS. • Terrestrial laser scanning (TLS): Measurements taken using TLS in the period from August 2011 to November 2013 show stability with slight variations in the intertidal area. In April 2014, large retreats are seen with respect to the measurements made in November 2013 (Figure 13) and these coincide with the photogrammetric measurements (Figure 12). The retreat of the base of the slope of the sandy front in the position of profile 4 between November 2013 and April 2014 was 8 m. The fall in the level of the entire beach by 3 m was also significant, even as much as 5.431 m in some areas (point 6) (Table2). From April 2014 to April 2017, the upper part of the slope retreated by 2 m, stabilizing the gradient of the sandy slope (Figure 13). Remote Sens. 2018, 10, x FOR PEER REVIEW 13 of 17

FigureFigure 13. 13. EvolutionEvolution of of profile proﬁle 4 (Figure 10b)10b) with TLS between August 2011 and November 2017.

There werewere estimatesestimates indicating indicating a retreata retreat of of 210 210 m betweenm between 1870 1870 and 1926and 1926 in this in area thisof area the of beach, the beachand of, and 200 of m 200 between m between 1920 and 1920 1985 and [198548] (Table [48] (Ta5).ble Our 5). research Our research indicates indicates that, betweenthat, between 1875 1875 and 1985,and 1985 this, retreatthis retreat was 375was m, 375 and, m, betweenand, between 1985 and1985 2017, and the2017 retreat, the retreat was 32 was m (Table 32 m5 (Table). This 5). area This of areaLas Quebrantasof Las Quebrantas Beach, Beach thus, suffered, thus, suffered a retreat a retreat of 407 mof with407 m an with uncertainty an uncertainty of 40 m of from 40 m 1875 from to 1875 the topresent the present (2017). (2017).

TableTable 5. Summary of the historical evolution of the beach of Las Quebrantas.

DateDate YearsYears Retreat Retreat (m) (m) Retreat Retreat (m/yr) (m/yr) Authors Authors 1870–19261870–1926 5656 210 210 3.7 3.7 Badía,Bad 2003ía, 2003 1920–19851920–1985 6565 200 200 3.1 3.1 1875–19851875–1985 110110 375 375 ± ±4040 3.0– 3.0–3.83.8 SanjoséSanjos et éal.,et al.,in this in this work work 1985–20171985–2017 3232 32 32 1.0 1.0

Analyzing the results obtained between 1985 and 2017, large retreats of the shoreline are seen between 1993 and 2001 (Figure 12), as well as following the storms between December 2013 and March 2014 (Figure 13). There is a correspondence, therefore, with the data of Table 1, in which we see that the five-year periods of 1995–1999 and 2010–2014 were the most active since the series of swells became available (1985). To be specific, 1999 was outstanding in the series, as there were eight days with storms that year, and more recently, 2014, when there were six days of storms in February and the first week of March, of which four days had tidal range of over 100. The village affected by this coastal dynamic is Somo, located between El Puntal Beach and Las Quebrantas Beach. For the period of 1985–2017, the coast retreated by 32 m in this area (Table 5). Somo used to be a village with small individual houses; however, at the beginning of the 1980s, housing estates and attached houses began being built, some of them on the beach front. It is these buildings that this coastal retreat is now affecting, mainly when the swells are higher than 6 m, coinciding with periods of high tidal range of over 100 (Figure 14).

Figure 14. Damaged houses in the urban area of Somo due to the winter storms of 2013–2014.

Remote Sens. 2018, 10, x FOR PEER REVIEW 13 of 17

Figure 13. Evolution of profile 4 (Figure 10b) with TLS between August 2011 and November 2017.

There were estimates indicating a retreat of 210 m between 1870 and 1926 in this area of the beach, and of 200 m between 1920 and 1985 [48] (Table 5). Our research indicates that, between 1875 and 1985, this retreat was 375 m, and, between 1985 and 2017, the retreat was 32 m (Table 5). This area of Las Quebrantas Beach, thus, suffered a retreat of 407 m with an uncertainty of 40 m from 1875 to the present (2017).

Table 5. Summary of the historical evolution of the beach of Las Quebrantas.

Date Years Retreat (m) Retreat (m/yr) Authors 1870–1926 56 210 3.7 Badía, 2003 1920–1985 65 200 3.1 1875–1985 110 375 ± 40 3.0–3.8 Remote Sens. 2018, 10, 1500 Sanjosé et al., in this work 14 of 18 1985–2017 32 32 1.0

AnalyzingAnalyzing the results obtained between 1985 and 2017, large retreats of the shoreline are seen between 1993 1993 and and 2001 2001 (Figure (Figure 12 ), 12) as, wellas well as following as following the storms the storms between between December December 2013 and 2013 March and 2014March (Figure 2014 (Figure13). There 13). is There a correspondence, is a correspondence, therefore, therefore, with the datawith of the Table data1, inof whichTable we1, in see which that thewe five-yearsee that the periods five-year of 1995–1999 periods of and 199 2010–20145–1999 and were 201 the0–2 most014 were active the since most the active series since of swells the series became of availableswells became (1985). available To be specific, (1985). 1999 To be was specific, outstanding 1999 was in the outstanding series, as there in the were series, eight as daysthere with were storms eight thatdays year, with andstorms more that recently, year, and 2014, more when recently there, were 2014, six when days there of storms were six in Februarydays of storms and the in firstFebruary week ofand March, the first of whichweek of four March, days of had which tidal four range days of overhad tidal 100. range of over 100. The village affected by this coastal dynamic is Somo, located between El Puntal Beach and Las Quebrantas Beach.Beach. ForFor the the period period of of 1985–2017, 1985–2017 the, the coast coast retreated retreated by 32by m 32 in m this in areathis (Tablearea (Table5). Somo 5). usedSomo to used be a to village be a villagewith small with individual small individual houses; houses however,; however, at the beginning at the beginning of the 1980s, of the housing 1980s, estateshousing and estates attached and attached houses began house beings began built, being some built, of them some on of thethem beach on the front. beach It is front. these It buildings is these thatbuildings this coastal that this retreat coastal is now retreat affecting, is now mainly affecting, when mainly the swells when are the higher swells than are 6 m, higher coinciding than 6with m, periodscoinciding of highwith tidalperiods range of high of over tidal 100 range (Figure of over14). 100 (Figure 14).

Figure 14. Damaged houses in the urban area of Somo due to the winter storms of 2013–2014.2013–2014.

5. Conclusions The techniques and sources used to analyze the evolution of the coastal sandy system were useful in determining the retreat of the coast over the last 142 years. The geomatic techniques used were cartography, photogrammetry, topography, terrestrial laser scanning (TLS), and georeferencing with the global navigation satellite system (GNSS), which were effective for observing ongoing changes in the coastal system. The combination and complementarities of different techniques and sources, such as historical cartography, photogrammetric flight series, and TLS, were very useful to know the evolution and erosive rhythms on the foredune. The behavior of the entire beach was uniform over the long time scale between 1875 and 2017, with an overall retreat of around 415 m; however, differences could be established in short time scales. The retreat of the shoreline was 25 m between 1985 and 2001 in El Puntal Beach and between 1993 and 2005 in the area of Las Quebrantas Beach. A faster retreat took place during the period of 1995–1999 (Table1) characterized by the large number of strong storms [40]. Between 2010 and 2014, there were large storms, four of which came in the winter of 2013–2014, and the beach underwent a mean retreat of 8 m at the foredune base. These four storms coincided with tidal ranges of over 100, swells higher than 6 m, and periods of high tides. The storms that took place between December 2013 and March 2014 were exceptional because of their coincidence with large tidal coefficients, low atmospheric pressures, and successions of waves grouped together, which had a constant erosive effect on the beaches with the export of sediment and a significant retreat of the shoreline. The strong retreats of the shoreline in the coastal sandy systems were generated in extraordinary periods, as shown by what happened in the four months of the winter of 2013–2014, associated with extreme storms and erosive rates far higher than those estimated for the other periods analyzed. From April 2014 to April 2017, six TLS data collections were made, and the foredune talus slope stabilized with the beach recovered its original level. Remote Sens. 2018, 10, x FOR PEER REVIEW 14 of 17

5. Conclusions The techniques and sources used to analyze the evolution of the coastal sandy system were useful in determining the retreat of the coast over the last 142 years. The geomatic techniques used were cartography, photogrammetry, topography, terrestrial laser scanning (TLS), and georeferencing with the global navigation satellite system (GNSS), which were effective for observing ongoing changes in the coastal system. The combination and complementarities of different techniques and sources, such as historical cartography, photogrammetric flight series, and TLS, were very useful to know the evolution and erosive rhythms on the foredune. The behavior of the entire beach was uniform over the long time scale between 1875 and 2017, with an overall retreat of around 415 m; however, differences could be established in short time scales. The retreat of the shoreline was 25 m between 1985 and 2001 in El Puntal Beach and between 1993 and 2005 in the area of Las Quebrantas Beach. A faster retreat took place during the period of 1995–1999 (Table 1) characterized by the large number of strong storms [40]. Between 2010 and 2014, there were large storms, four of which came in the winter of 2013–2014, and the beach underwent a mean retreat of 8 m at the foredune base. These four storms coincided with tidal ranges of over 100, swells higher than 6 m, and periods of high tides. The storms that took place between December 2013 and March 2014 were exceptional because of their coincidence with large tidal coefficients, low atmospheric pressures, and successions of waves grouped together, which had a constant erosive effect on the beaches with the export of sediment and a significant retreat of the shoreline. The strong retreats of the shoreline in the coastal sandy systems were generated in extraordinary periods, as shown by what happened in the four months of the Remotewinter Sens. of 20120183,–102014,, 1500 associated with extreme storms and erosive rates far higher than those estimated15 of 18 for the other periods analyzed. From April 2014 to April 2017, six TLS data collections were made, and the foredune talus slope stabilized with the beach recovered its original level. The rock rock cliff cliff previously previously covered covered by by the the foredune foredune appeared appeared in 1999 in 1999,, and and in November in November 2016 2016,, 130 130m to m the to east the, east,a new a outcrop new outcrop rock arose rock on arose top onof the top dune of the (Figure dune 15). (Figure These 15 dune). These erosive dune processes erosive processescan generate can significant generate signiﬁcant changes, producing changes, producing a continuous a continuous cliff without cliff dunes without if the dunes erosive if the proces erosiveses processesremain. remain.

Figure 15. New outcrop of the rock cliff in the year 1999 and in November 2016.

The use of geomatic techniques can allow precision measurements to be made in the future complemented by other means (satellite photography, drone flights)ﬂights) to determinedetermine recent coastal evolution and its current dynamic with greater accuracy. The build build-up-up of a long storm data series will improveimprove knowledgeknowledge ofof thethe retreatretreat of of the the shoreline, shoreline, trends trends and and erosive erosive rates, rates and, and accumulation, accumulation as, wellas well as theas the response response of the of naturalthe natur systemal system of the of sea–land the sea– contactland contact strip in strip the contextin the context of global of change, global andchange as indicators, and as indicators of sea level of sea change. level Wechange. consider We consider that the behavior that the behavior analyzed analyzed in this article in this is similararticle tois similar that which to that occurs which in theoccur world’ss in the dune world’s fronts dun withe fronts similar with characteristics, similar characteristics and that their, and retreats that their are alsoretreats affected are also by largeaffected storms. by large storms.

Author Contributions: J.J.d.S.B. conceived, designed and carried out the project and the manuscript. M.G.-L., M.S.-F., E.S.-C. shared the project and helped record information in the field and to write the manuscript, and during the publication process. Acknowledgments: This research was made possible thanks to funding by the Junta de Extremadura and ERDF, through the references GR18053 to the Research Group NEXUS (University of Extremadura), and by the I+D+I project CGL2015-68144-R (MINECO, Spanish Government and ERDF), and the Research Group PANGEA (University of Valladolid). We thank the SDI Service of Government of Cantabria for providing the aerial photographic information. Conflicts of Interest: The authors declare no conflict of interest.

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