The Bolonia and Valdevaqueros coves are located near the town of Tarifa, on the SW Atlantic coast of Cádiz, 10 km from the Strait of Gibraltar, at the geographical coordinates 36°9’N; 5°78’W and 36°4’N; 5°41’W, respectively (Fig. 1). The shoreline has a NW-SE direction, and according to the classification of Davis and Hayes (1984), the coast has a microtidal environment (Fages et al. 2007, del Río et al. 2012), with a spring tidal range of 1.69 m and a neap tidal range of 0.5 m (IHM 2010). The relation between daily variations in sea level and meteorological forcing can be found in Aboitiz et al. (2008).

Both beaches have large transgressive dune systems that were part of an old bypass dunefield (Navarro et al. 2011) that until the mid-twentieth century extended from Valdevaqueros to Bolonia cove and from Bolonia cove to El Cañuelo Beach (Fig. 2). The Bolonia dunefield had an area of approximately 90 ha and was 2800 m long and 300-400 m wide, as shown in Fig. 2B. The original Valdevaqueros dunefield had an area of approximately 440 ha (Fig. 2C), with barchan dunes formed at the western end, where sand availability is more limited.

The Cádiz littoral zone, between the Guadalquivir River Mouth and the Strait of Gibraltar, has a step-like morphology conditioned by the existence of two groups of transverse faults whose tectonic directions are NNW-SSE and E-W (Gutiérrez-Mas et al. 1991, Gracia and Benavente 2000). This feature causes the deflection of river channels, which act as traps interfering with the littoral drift coming northwards from sand accumulations located south of the zone (Muñoz-Pérez et al. 2009). In particular, Valdevaqueros dune is bounded by the mouth of the Valle River to its east, which forms a small estuary whose position migrates seasonally along the beach as a function of the rainfall climate.

The areas surrounding these dunes are minimally inhabited areas, with the rural communities Punta Paloma (approx. 500 inhabitants) and El Lentiscal (approx. 200 inhabitants) in Valdevaqueros and Bolonia, respectively, including areas of great historical and cultural significance, particularly the Roman ruins of Baelo-Claudia (Fig. 3) close to Bolonia beach (Román-Sierra et al. 2004), which was presumably destroyed by an earthquake in the 4th century AD (Silva et al. 2005). The Bolonia and Valdevaqueros dune systems form part of the Natural Park of the Strait of Gibraltar, and in addition, the Bolonia dunefield was declared a Natural Monument in 2001 (BOJA 2001).

The wind regime in the Strait of Gibraltar is largely influenced by the surrounding orography, which is 400-800 m high. As a result of a pressure gradient between the Atlantic Ocean and the Mediterranean Sea, wind accelerates when going through the 20-km-wide gap (Dorman et al. 1995), but the wind dynamics in the Strait are complex and are not simply the result of Venturi effects. Maximum wind speed values are not located at the narrowest section of the gap but further to the west, in the area offshore of Bolonia (Capon 2006). For this reason, both dune ecosystems are strongly influenced by the local aeolian regime. Dominant winds come from the east with a lesser SE component, whose average is approximately 60 km h^–1 (Muñoz-Pérez et al. 2009), with wind gusts that reach 100 km h^–1 (Gómez-Pina et al. 2002). Westerly winds are also abundant, but with larger directional dispersion and lower speeds. Easterly and westerly winds include more than 90% of the wind speeds greater than 10 m s^–1 (Plomaritis et al. 2013). More specific wind information is available at Navarro et al. (2011, 2015).

These strong and frequent wind conditions are ideal conditions for sailing sports such as kiteboarding, which holds its world championship competition each year at Valdevaqueros beach (Fig. 4). Therefore, there is intense pressure from people on the beach and from the proliferation of paths and roads in the area where a foredune could be expected to develop (there is even a car park close to the river mouth). The dune sands have a mean grain size ranging from 0.28 to 0.30 mm, a similar value to that of all the beaches of the Gulf of Cádiz (Navarro 2011). A comprehensive list of the average grain sizes of these sands with their sorting, skewness and kurtosis is available in Román-Sierra et al. (2014), and the methodology used to obtain these data is in Román-Sierra et al. (2013).

Both dunefields have experienced very high activity levels during recent decades despite the implementation of several restoration techniques, such as sand fencing and plantations (Ley et al. 2011), and even extensive sand mining. Dune stabilization was intended to reduce the rate of dune front advance towards the pine groves and infrastructure. In the case of Valdevaqueros, the only road that connects to the rural settlement of Punta Paloma is being invaded constantly by dune sands. In contrast to the activities described in Gómez-Pina et al. (2007), which were never criticized, this process is causing a serious local problem that requires strategic coastal management. Similarly, in Bolonia, the advancing dune front is currently located approximately 300 m from the nearest road, so road invasion may occur in the medium term without mitigation.

To mitigate sand invasion towards anthropic infrastructure, the first interventions carried out during the 1930s consisted of installing several rows of 1-m-high concrete structures. These structures not only did not reduce the dune advance but strongly altered the natural dune profile, as the lack of porosity did not provide for natural aeolian sand accumulation and therefore prevented the development of vegetation. Indeed, subsequent studies demonstrated that the optimum porosity of sand fences should be approximately 50% (a detailed literature review on the efficiency of sand stabilization systems can be found in Willetts and Phillips 1978).

The Valdevaqueros and Bolonia dunefields were artificially fixed approximately in 1946 and 1953, respectively, by species such as Pinus pinea, Eucalyptus sp. and Retama monosperma (Gracia et al. 2011). Pine grove plantations near Bolonia covered the Camarinal headland, which is approximately 2.5 km long and 300 m wide, reaching the Baelo-Claudia Ruins (to the east), and similarly, the Valdevaqueros pine grove was planted from the eastern part of Bolonia beach to the Valdevaqueros cove (see Fig. 2). In addition, restoration activities were employed for the first time between 1961 and 1962 by means of wattle fences and, subsequently, wicker and wooden fencing. After every wind storm, the fences were renewed manually, as they were quickly covered by the sand.

From that point forward, dune growth was only limited by periodic sand extraction by individuals for construction use. Since the enactment of the Spanish Shore Act (BOE 1988), which banned dune sand mining for private profit, the Ministry of Environment decided that the best solution was to relocate large volumes of dune sand from Valdevaqueros to other beaches (Muñoz-Pérez et al. 2014) in addition to continuous sand fencing. Table 1 shows sand mining volumes removed by the Coastal Department, totalling more than 684000 m³ in 20 years of extraction activity (updated from Muñoz-Pérez et al. 2009). When the dune front reached the road in Valdevaqueros in 2004, sand extraction increased to prevent the road from being buried. Marram grass (Ammophila arenaria) has also been planted, especially during the last decade.

In spite of all of the human stabilization attempts, both dune systems have grown in an uncontrolled manner over the pine groves located on their leeward sides (Fig. 5), showing the difficulty of dealing with a highly active dune (Gracia et al. 2013).

Figure 6 shows the Bolonia dune’s evolution from 1974 to 2011, revealing an average total advance of 320 m in 37 years. At least two dune fronts have been identified since 1985. The most active front closely matches the surveyed western profile (B1). Since 1990, the dune area seems to have increased but narrowed at the same time, occupying the corridor bottom between Bolonia and Cañuelo. Despite the increasingly vertical climb of dune development, the fastest dune advance was experienced during the last six years of the research period.

Topographical observations reveal that the Bolonia dune profiles experienced a great accretion during the final three years of the research period (i.e. 2008-2011). As an example, the topographical evolution of the western profile of Bolonia (B1) can be observed in Figure 7. The dune elevation changes depending on the dune profile, with a maximum height above the datum (the lowest low water level, LLWL) of approximately 47 m in profiles B1 and B3 and 50 m in profile B2. The average elevation change was of the order of 1.10 m yr^–1 from 2008 to 2011.

The Bolonia dune has adopted irregular morphologies along the windward face, creating deflation basins, which are depressions that appear in the central area of the dunefield and have height differences of up to 10 m with barely 20 m of horizontal displacement (Fig. 8). The dune area and advance rates derived from the analysis of the Bolonia dune images are shown in Table 2, in which it is possible to identify the evolution of the western and eastern dune profiles (B1 and B3), with a total advance ranging from 284 to 356 m in almost 40 years.

The Valdevaqueros dunefield has been subjected to many more interventions than Bolonia to mitigate sand advance towards the adjacent road. Figure 9 shows the evolution of the Valdevaqueros dunefield from 1956 to 2011, with a total advance of nearly 300 m in 55 years (Table 3).

It is noteworthy that, in addition to its dune profile development, Valdevaqueros beach has also undergone changes in relatively short periods, such as from 1956 to 1974 (accretion) and from 1974 to 1979 (erosion). From 1985 to 1998, there was a major dune advance ranging from approximately 125 to 200 m, depending on the dune profile. Since 2004, the dune area has remained practically constant due to frequent sand mining (Table 1).

The evolution of the most active profile of the Valdevaqueros dune was monitored topographically from 1995 to 2009, as shown in Figure 10. This profile (V1) is located in the western part of the dunefield. The dune profile height increased from 24 to 42 m (an average elevation change of 1.29 m yr^–1), and horizontal displacement was 130 m in only 14 years (average advance rate of 9.28 m yr^–1). Therefore, the Valdevaqueros dune profile experienced substantial changes not only in area but also in shape.

The dune profile describes a shifting behaviour during three periods. The first period, from 1995 to 2000, which is considered as the “initial” profile, shows a uniform and progressive accretion of 480 m³ m^–1 yr^–1 (Muñoz-Pérez et al. 2009), in which orography plays a very important role. In this stage, the windward side of the dune had a more convex slope, becoming sharper as the dune profile was softened by mechanical action just before 2003. This action, in which the dune was reshaped by distributing the sand from the crest along the stoss slope (and the slipface), did not respect the natural slope angles of the dune. From the moment of this development, the windward face probably acted as a launching ramp from which sand grains were easily transported towards the crest. As a consequence, blown sands were kept away from the dune-beach system interaction. Dune elevation also obviously increased in proximity to the sand fencing, and after the sand reached the road in 2004, the dune experienced shifting morphologies (not only on the crest but also on the windward and leeward sides), resulting in a more complex profile. During the second period (2003 to 2008), the profile continued increasing in height and started to adopt sharp, irregular morphologies, possibly due to the wooden fencing at the dune crest. From May to December 2006, the profile underwent an annual accretion rate of 438 m³ m^–1 yr^–1. Finally, during the last phase of dune growth (from September 2008 to August 2009) the dune profile decreased in height, undergoing a net accretion (deducting sand extraction volumes) of approximately 280 m³ m^–1 yr^–1.

Based on observations of the dune behaviour in recent years, an average advance rate of profile V1 was calculated as 17.5 m yr^–1 (Muñoz-Pérez et al. 2009). In addition, after considering the “natural” transport rate experienced by profile V1 (taking into account the extracted sand volume and the remaining natural dune volume), it was concluded that only 2-3 weeks of moderate easterly winds (of approximately 40 km h^–1) would be required to move as much sand volume as during one of the prior years.

Current sand transport rates are one order of magnitude greater than reported in other studies (as Serra et al. 1997 or Cabrera and Alonso 2010). This finding could be explained by the singularity of the strong and frequent wind regime that occurs close to the Strait of Gibraltar. This information is crucial for dune management decision-making. In this case, the current strategy based on the physiographic profile reshaping by mobilization of dune sands from the dune front to the dune foot has proven to be no longer sustainable.

As a result, the Valdevaqueros dune showed very high mobility, especially during the final 14 years of the study period. The dune area accumulated a total net volume of approximately 2000 m³ m^–1 during these 14 years, i.e. approx. 142 m³ m^–1 yr^–1, comparable to the volume extracted per year, except from 2003 to May 2006, when the natural dune accumulation exceeded the sand mining volume.

As seen in Figure 11, the western end of the Valdevaqueros dune (V1) experienced the highest advance in this dune during the 50-year study period (1956-2006). This profile has followed a generally exponential advance, which can be split into three phases: The first phase, with a soft slope, is due to the existence of a single foredune near the backshore of the beach. Continuous and imprecise sand mining was carried out during this phase (1956-1985). During the second phase (1985-1998), a higher advance rate was identified, clearly accentuated by the enactment of the Spanish Shore Act (BOE 1988), according to which dune sand extraction for private profit was forbidden as a management tool for beach and dune system conservation. A critical dune profile advance phase was evident from 1998 to 2004, when profile V1 eventually reached the road and was urgently halted by sand extraction and profile reshaping by the Coastal Authority to address the social implications.

The eastern end of the Valdevaqueros dune (V5) also showed similar advance rates but its growth was not as fast and the advance was progressively linear (Fig. 11). Note that sand extraction was not carried out on this part of the dune, although other management tools, such as sand fencing and the planting of vegetation, were employed (Fig. 12) on the windward dune face. In addition, blowing sands are transported westward approximately 75% of the time during which easterly winds blow (Navarro et al. 2011), as in the profile V1 advance.

The western end of the Bolonia dune (B1) has undergone two slightly different phases (Fig. 12). During the first, from 1974 to 1998, the profile describes a linear advance, and from 1998 to 2011, it grows exponentially. This climbing dune was most likely influenced primarily by orographic features, as there was no sand mining and few restoration activities. Therefore, because of these orographic features and despite the aeolian sand transport following an E-W direction, the eastern end of the Bolonia dune (B3) underwent higher advance rates from 1985 to 1990 and from 2001 to 2011.

As shown in Table 4, the Bolonia and Valdevaqueros western profiles (B1 and V1) generally move faster than the eastern profiles (B3 and V5), undergoing higher average advance rates of 10.9 and 9 m yr^–1, respectively. This finding is reasonable considering that the resultant drift direction from wind in these areas is essentially E-W (Navarro et al. 2011).

The advance rates in the Bolonia and Valdevaqueros dunes were at first glance quite variable, both spatially and temporally. In the case of Valdevaqueros, the advance rates were highly influenced by sand mining, especially around the western profile (V1), which has quite variable values, depending on the date. Despite the anthropic interventions, the average advance rates ranged from 6.2 to 9 m yr^–1 for V1 and V5, respectively, although maximum values of 25 m yr^–1 were identified in V1. Similarly, the western profile of Bolonia (B1) showed a significant increase in advance rate during the last six years of the research period, reaching maximum values of 22.9 m yr^–1, with an average advance rate of 10.9 m yr^–1 from 1974 to 2011 (see Table 3), whereas the eastern part of the Bolonia dune system (B3) showed maximum values of 13.5 m yr^–1 and an average advance rate of 8.7 m yr^–1. Extrapolating these results, the Bolonia dune could reach the nearest road in approximately 12 years. Thus, monitoring these dunes is highly useful for predicting dune behaviour and estimating dune advance rates in order to adopt preventive measures.

According to Figure 12A, the Bolonia dune area increased from 3.1 10⁴ m² to 7.9 10⁴ m² between 1974 and 2011. Valdevaqueros has experienced an exponential area increase since 1956, when its delimited area (3.3 10⁴ m²) was similar to the initial Bolonia dune area (the final area of the Valdevaqueros dune in 2011 was 21.8 10⁴ m²). In terms of dune area increase (Fig. 12B), Bolonia has followed a single linear distribution with a slope of 0.117 10⁴ m² yr^–1, whereas Valdevaqueros has shown different phases with three linear distributions: from 1956 to 1974 (rate of 0.014 10⁴ m² yr^–1), from 1974 to 1998 (rate of 0.276 10⁴ m² yr^–1) and from 1998 to 2008 (rate of 1.12 10⁴ m² yr^–1, coefficients of determination (R2) ranging from 0.96 to 0.99).