INTRODUCTIONTop

Coastal dune research has become especially important in recent decades because of the multiplicity of roles played by dunes (shoreline protection, sedimentary balance, ecological functions, landscape singularity, etc.) and the vulnerability arising from their location in the coastal environment (Martínez et al. 2006Martínez M.L., Gallego J.B., García-Franco J.G., et al. 2006. Assessment of coastal dune vulnerability to natural and anthropogenic disturbances along the Gulf of México. Environ. Conserv. 33 (2): 109-117., Williams and Davies 2001Williams A.T., Davies P. 2001. Coastal dunes of Wales; vulnerability and protection. J. Coastal Conserv. 7: 145-154). Furthermore, dune systems are the most efficient and least expensive defence against shoreline erosion (Gómez-Pina et al. 2002Gómez-Pina G., Muñoz-Pérez J.J., Ramírez J.L. et al. 2002. Sand dune management problems and techniques. J. Coastal Res. SI: 325-332.) and the consequent risks (del Río 2007del Río L. 2007. Riesgos de erosión costera en el litoral atlántico gaditano. PhD Thesis, Univ. Cádiz, 496 pp.).

Many lines of research into these ecosystems are currently focused on the lack of dune mobility. Dune stabilization takes place when the original conditions of dune development are modified because of changes in the vegetation cover or sediment supply or a decrease in migration rates (Jackson and Cooper 2011Jackson D.W.T., Cooper J.A.G. 2011. Coastal dune fields in Ireland: rapid regional response to climatic change. J. Coastal Res. 64: 293-297.). However, coastal dunes can be stabilized or remobilized in response to changes in wind power (Levin et al. 2007Levin N., Kidron G.J., Ben-dor E. 2007. A field quantification of coastal dune perennial plants as indicators of surface stability, erosion or deposition. Sedimentology 55(4): 751-772.). Indeed, Yizhaq et al. (2007)Yizhaq H., Ashkenazy Y., Tsoar H. 2007. Why do active and stabilized dunes coexist under the same climatic conditions? Phys. Rev. Lett. 98(18): 188001. and Tsoar et al. (2009)Tsoar H., Levin N., Porat N., et al. 2009. The effect of climate change on the mobility and stability of coastal sand dunes in Ceará State (NE Brazil). Quaternary Res. 71 (2): 217-226. observed that in rainy areas with an environment capable of supporting vegetation cover and a prevalence of intermediate wind speeds, mobile and stabilized dunes can coexist, whereas under lower wind speeds dunes are always fixed. Nevertheless, it should be noted that mature soils do not necessarily have to be geomorphologically inactivity either in coastal or inland areas (Paskoff 2003Paskoff R. 2003. La conservation des dunes littorales implique-t-elle leur stabilisation? L’exemple de la côte atlantique. Natures sciences sociétés 11(3): 288-294.).

Conversely, in other dune systems the problem lies in the loss of stability and a subsequent increase in mobility. It is assumed that under extremely high wind stress, vegetation is destroyed and dunes are re-activated. Furthermore, it has been shown that long droughts or mega-droughts lead to a reduction in vegetation cover and dune reactivation (Schmeisser et al. 2009Schmeisser R.L., Loope D.B., Wedin D.A. 2009. Clues to the medieval destabilization of the Nebraska Sand Hills, USA, from Ancient Pocket Gopher Burrows. Palaios 24 (11-12): 809-817., Hanson et al. 2010Hanson P.R., Arbogast A.F., Johnson W.C., et al. 2010. Megadroughts and late Holocene dune activation at the eastern margin of the Great Plains, north-central Kansas, USA. Aeolian Res. 1: 101–110.).

Del Valle et al. (2008)del Valle H.F., Rostagno C.M., Coronato F.R., et al. 2008. Sand dune activity in north-eastern Patagonia. J. Arid Environ. 72(4): 411-422. found high migration rates (over 9 m yr^–1) in some coastal dunes in Argentine Patagonia. Girardi and Davis (2010)Girardi J.D., Davis D.M. 2010. Parabolic dune reactivation and migration at Napeague, NY, USA: Insights from aerial and GPR imagery. Geomorphology 114(4): 530-541. recognized a recent reactivation phase of parabolic dunes in the Walking Dune Field in Napeague (New York) from GPR images. Maia et al. (2005)Maia L.P., Freire G.S.S., Lacerda L.D. 2005. Accelerated dune migration and aeolian transport during El Nino events along the NE Brazilian coast. J. Coastal Res. 21(6): 1121-1126. found that migration rates observed in the dunes of Ceará (Brazil) depended inversely on the size of the dunes, so migration decreased as the size of the dunes increased. Jackson et al. (2013)Jackson D.W.T., Cruz-Avero N., Smyth T., et al. 2013. 3D airflow modelling and dune migration patterns in an arid coastal dune field. J. Coastal Res. SI 65: 1301-1306. also determined migration patterns in Gran Canaria by using 1-m resolution LiDAR surveys. However, there are noteworthy cases in Europe, such as the Rubjerg Knude dune in Denmark, where the accumulation of aeolian sand began quite recently (approximately 300 BP) and the modern high dune only began to form after 1885, apparently in association with an acceleration in the rate of coastal cliff retreat (Saye et al. 2006Saye S.E., Pye K., Clemmensen L.B. 2006. Development of a cliff-top dune indicated by particle size and geochemical characteristics: Rubjerg Knude, Denmark. Sedimentology 53 (1): 1-21.). In the Pilat littoral dune (France), different phases of dune activity have been identified; the most recent one was believed to have occurred after the planting of pine forests along the French coast in the late 19th century (Tastet and Pontee 1998Tastet J.P., Pontee N.I. 1998. Morpho-chronology of coastal dunes in Medoc. A new interpretation of Holocene dunes in Southwestern France. Geomorphology 25: 93-109.). These transverse dunes of increasing height that grow landward are mainly formed when there is sparse vegetation cover, abundant sand supply and moderate wind power. Another example of a transverse dunefield has recently been identified and studied by Jackson et al. (2014)Jackson D.W.T., Cooper J.A.G., Green A.N. 2014. A preliminary classification of coastal sand dunes of KwaZulu-Natal. J. Coastal Res. SI 70: 718-722. in northern KwaZulu-Natal (South Africa). Within this transverse dune family, transgressive dune systems are large aeolian sand deposits formed by sand sheets deposited over vegetated or semi-vegetated dunefields. These dunes have two different slopes: moderate on the windward side and steeper on the leeward side (Tsoar et al. 2004Tsoar H., Blumberg D.G., Stoler Y. 2004. Elongation and migration of sand dunes. Geomorphology 57: 293-302.). The windward side ends in a practically horizontal dune crest and a leeward slope generally close to the critical angle of repose of sand (34° to 40° depending on the humidity).

These dunes often act as major sand sinks and tend to move sand systematically inland, away from the beach and the littoral active zone (Rust and Illenberger 1996Rust I.C., Illenberger W.K. 1996. Coastal dunes: sensitive or not? Landscape and Urban Planning 34, 165-169.). This process may cause the invasion of other adjacent ecosystems, changing the ecological conditions of the invaded site, including shrub species, pine and broom groves, salt marshes (Cooper and McLaughlin 1998Cooper J.A.G., McLaughlin S. 1998. Contemporary multidisciplinary approaches to coastal classification and environmental risk analysis. J. Coastal Res. 14(2): 512-524.), cropland, grassland, and even human settlements (Avis 1989Avis A.M., 1989. A review of coastal dune stabilization in the Cape Province of South Africa. Landscape Urban Plan. 18: 55-68., Bate and Ferguson 1996Bate G., Ferguson M. 1996. Blowouts in coastal foredunes. Landscape Urban Plan. 34 (3): 215-224., Hertling and Lubke 1999Hertling U.M., Lubke R.A. 1999. Use of Ammophila arenaria for Dune Stabilization in South Africa and its Current Distribution-Perceptions and Problems. Environ. Manag. 24: 467-482., Yao et al. 2007Yao Z.Y., Wang T., Han Z.W., et al. 2007. Migration of sand dunes on the northern Alxa Plateau, Inner Mongolia, China. J. Arid Environ. 70: 80-93.).

Generally, degraded dune ecosystems can be recovered by means of ecological restoration, providing long-term stability. The main goal of restoration is to help the ecosystem to recover its dynamic equilibrium by developing methods and soft technologies such as protective systems (walkways, fencing, signs and communications systems, etc.) and regeneration systems (sand fencing, vegetation plantations, and removal of invasive species), as described by Ley et al. (2011)Ley C., Navarro-Pons M., Muñoz-Pérez J.J. 2011. Métodos generales de restauración de sistemas dunares. In: Sanjaume Saumell E., Gracia F.J. (eds). Las dunas en España. Pub. Spanish Geomorphology Society.. Therefore, rehabilitation programmes of these systems are currently one of the priorities of coastal managers. One of the latest studies, assessing the efficiency of the recuperation techniques of a foredune system, was carried out by Portz et al. (2015)Portz L., Manzolli R.P., Hermanns L., et al. 2015. Evaluation of the efficiency of dune reconstruction techniques in Xangri-lá (Rio Grande do Sul, Brazil). Ocean Coast. Manage. 104: 78-89. in Shangri-la (Brazil). Digital terrain models and the sediment volume variation were used in order to discuss the unanticipated alterations due to the interventions carried out. Despite these management tools, some types of invasive dunes make monitoring much more difficult because they are not stabilized systems (Román-Sierra et al. 2004Román-Sierra J., Navarro-Pons M., Muñoz-Pérez J.J., et al. 2004. Ecosistemas dunares en la provincia de Cádiz: Estabilizaciones en Bolonia y Valdevaqueros (Tarifa). Revista de Obras Públicas 3450: 65-76.).

Along the Cádiz coast (SW Spain), two of the biggest coastal dunes in southern Europe, the Bolonia and Valdevaqueros dunes, are moving systematically inland. To reduce sand invasion towards rural settlements and other infrastructures, several stabilization and restoration activities have been implemented over recent decades. The persistent growth of the dunes has led to great social pressure, especially in the case of Valdevaqueros. For this reason, before elaborating suitable management strategies, it is necessary to understand more about the development of these dunes. The aim of this paper is to describe and compare the morphodynamic evolution of these transgressive dune systems, taking into account human intervention over the last half century, analysing the different measures performed in an attempt to stabilize them, and extracting conclusions to improve dune management strategies in similar scenarios.

STUDY AREATop

Features of the dune environment

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)Davis R.A. Jr., Hayes M.O. 1984. What is a wave-dominated coast? Mar. Geol. 60: 313-329., the coast has a microtidal environment (Fages et al. 2007Fages L., Gómez-Pina G., Navarro M., et al. 2007. Integrated study of alternatives for Valdevaqueros dune area (Tarifa, Spain). Proceed. Int. Conf. Manag. Restoration Coast. Dunes. Santander, Spain, 168-171., del Río et al. 2012del Río L., Plomaritis T.A., Benavente J., et al. 2012. Establishing storm thresholds for the Spanish Gulf of Cádiz coast. Geomorphology 143: 13-23.), with a spring tidal range of 1.69 m and a neap tidal range of 0.5 m (IHM 2010IHM (Instituto Hidrográfico de la Marina). 2010. Anuario de Mareas. San Fernando, Ministerio de Defensa.). The relation between daily variations in sea level and meteorological forcing can be found in Aboitiz et al. (2008)Aboitiz A., Tejedor B., Muñoz-Pérez J.J., et al. 2008. Relation between daily variations in sea level and meteorological forcing in Sancti Petri Channel (SW Spain). Cienc. Mar. 34(4): 491-501..

Both beaches have large transgressive dune systems that were part of an old bypass dunefield (Navarro et al. 2011Navarro M., Muñoz-Pérez J.J., Román-Sierra J., et al. 2011. Assessment of highly active dune mobility in the medium, short and very short term. Geomorphology 129(1-2): 14-28.) 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. 1991Gutiérrez-Mas J.M., Martín A., Domínguez S., et al. 1991. Introducción a la geologia de la provincia de Cádiz. Serv. Publ. Univ. Cádiz, 315 pp., Gracia and Benavente 2000Gracia F.J., Benavente J. 2000. Geomorfología de la costa atlántica gaditana. In: de Andrés J.R., Gracia F.J. (eds), Geomorfología Litoral. Procesos activos. Spanish Society of Geomorphology, Monograph 7, Instituto Tecnológico y Geominero de España, Univ. Cádiz, pp. 235-255.). 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. 2009Muñoz-Pérez J.J., Navarro M., Román-Sierra J., et al. 2009. Long-term evolution of a transgressive migrating dune using reconstruction of the EOF method. Geomorphology 112: 167–177.). 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. 2004Román-Sierra J., Navarro-Pons M., Muñoz-Pérez J.J., et al. 2004. Ecosistemas dunares en la provincia de Cádiz: Estabilizaciones en Bolonia y Valdevaqueros (Tarifa). Revista de Obras Públicas 3450: 65-76.), which was presumably destroyed by an earthquake in the 4th century AD (Silva et al. 2005Silva P.G., Borja F., Zazo C., et al. 2005. Archaeoseismic record at the ancient Román City of Baelo Claudia (Cádiz, south Spain). Tectonophysics 408: 129-146.). 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 2001Boletín Oficial de la Junta de Andalucía (BOJA) nº 135/2001. Declaration of Natural Monuments in Andalucia, 22/11/2001, 18834.).

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. 1995Dorman C.E., Beardsley R.C., Limeburner R. 1995. Winds in the Strait of Gibraltar. Q. J. R. Meteorol. Soc. 121: 1903-1921), 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 2006Capon R.A. 2006. High resolution studies of the Gibraltar Levanter validated using sun-glint anemometry. Meteorol. Appl. 13(3): 257-265.). 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. 2009Muñoz-Pérez J.J., Navarro M., Román-Sierra J., et al. 2009. Long-term evolution of a transgressive migrating dune using reconstruction of the EOF method. Geomorphology 112: 167–177.), with wind gusts that reach 100 km h^–1 (Gómez-Pina et al. 2002Gómez-Pina G., Muñoz-Pérez J.J., Ramírez J.L. et al. 2002. Sand dune management problems and techniques. J. Coastal Res. SI: 325-332.). 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. 2013Plomaritis T.A., Puyana I., Benavente J., et al. 2013. Can grain size trend analysis be used to identify Aeolian sediment transport pathways? The case of Bolonia dune (SW Spain). J. Coastal Res. SI 65: 1325-1330.). More specific wind information is available at Navarro et al. (2011Navarro M., Muñoz-Pérez J.J., Román-Sierra J., et al. 2011. Assessment of highly active dune mobility in the medium, short and very short term. Geomorphology 129(1-2): 14-28., 2015)Navarro M., Muñoz-Pérez J.J., Román-Sierra J., et al. 2015. Characterization of wind-blown sediment transport with height in a highly mobile dune (SW Spain). Geol. Acta 13(2): 155-166..

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 2011Navarro M. 2011. Modelización de la evolución morfodinámica de la duna de Valdevaqueros (T.M. Tarifa) mediante la aplicación de Funciones Empiricas Ortogonales a corto, medio y largo plazo. PhD Thesis. Univ. Cádiz.). 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)Román-Sierra J., Muñoz-Pérez J.J., Navarro-Pons M. 2014. Beach nourishment effects on sand porosity variability, Coast. Eng. 83: 221-232., and the methodology used to obtain these data is in Román-Sierra et al. (2013)Román-Sierra J., Muñoz-Pérez J.J., Navarro-Pons M. 2013. Influence of sieving time on the efficiency and accuracy of grain-size analysis of beach and dune sands. Sedimentology 60(6): 1484-1497..

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. 2011Ley C., Navarro-Pons M., Muñoz-Pérez J.J. 2011. Métodos generales de restauración de sistemas dunares. In: Sanjaume Saumell E., Gracia F.J. (eds). Las dunas en España. Pub. Spanish Geomorphology Society.), 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)Gómez-Pina G., Fages L., Román-Sierra J., et al. 2007. An example of Integrated Coastal Management in Punta Candor (Co. Rota, Spain). In: 1st Int. Conf. on Restoration and Manag. Coastal Dunes, Santander: 1-6., 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.

Dune stabilization attempts

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 1978Willetts B.B., Phillips C.J. 1978. Using Fences to Create and Stabilise Sand Dunes. Coastal Engin. Proceed. 1(16): 2040-2050.).

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. 2011Gracia F.J., Benavente J., Alonso C., et al. 2011. Las dunas del litoral gaditano. In: Sanjaume E., Gracia F.J. (eds.): Las dunas en España. Spanish Society of Geomorphology, pp. 359-384.). 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 1988Boletín Oficial del Estado (BOE). 1988. Spanish Shore Act (in Spanish) 29/07/1988: 18762.), 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. 2014Muñoz-Pérez J.J., Román-Sierra J., Navarro-Pons M., et al. 2014. Comments on “Confirmation of beach accretion by grain-size trend analysis: Camposoto beach, Cádiz, SW Spain” by Poizot et al. (2013) Geo-Marine Letters 33(4), Geo-Mar. Lett. 34 (1): 75-78) 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. 2009Muñoz-Pérez J.J., Navarro M., Román-Sierra J., et al. 2009. Long-term evolution of a transgressive migrating dune using reconstruction of the EOF method. Geomorphology 112: 167–177.). 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. 2013Gracia F.J., Nuño T., del Río L., et al. 2013. Valdevaqueros, Cádiz: La difícil gestión de una gran duna móvil. Enseñanza de las Ciencias de la Tierra 21(3): 332-335.).

METHODSTop

To investigate the morphological evolution and advance of the Bolonia and Valdevaqueros dunefields, several topographic surveys were performed using differential global positioning system (DGPS) techniques and total station theodolites. The DGPS can provide accuracy to the centimetre for carrier phase tracking (usually less than 10 cm), consistent with the methodology followed by Rodríguez-Santalla et al. (2009)Rodríguez-Santalla I., Sanchez M.J., Montoya I., et al. 2009. Internal structure of the aeolian sand dunes of El Fangar spit, Ebro Delta (Tarragona, Spain). Geomorphology 104 (3-4): 238-252..

On the other hand, the methodology related to the contribution of the principal statistical parameters (mean grain size, sorting, skewness and kurtosis) presented by other authors (e.g. Alcántara-Carrió and Alonso 2001Alcántara-Carrió J., Alonso I. 2001. Aeolian sediment availability in coastal areas defined from sedimentary parameters. Application to a case study in Fuerteventura. Sci. Mar. 65(S1): 7-20., Alcántara-Carrió et al. 2010Alcántara-Carrió J., Fernández S., Alonso I. 2010. Source area determination of aeolian sediments at Jandia Isthmus (Fuerteventura, Canary Islands). J. Mar. Syst. 80: 219-234.) has not been followed in this paper because the sand parameters are almost identical at Bolonia and Valdevaqueros. Interested readers may turn to Navarro et al. (2015)Navarro M., Muñoz-Pérez J.J., Román-Sierra J., et al. 2015. Characterization of wind-blown sediment transport with height in a highly mobile dune (SW Spain). Geol. Acta 13(2): 155-166. for data obtained in these dunes.

At the Bolonia dunefield, two topographical campaigns were carried out, in 2008 and 2011 (Fig. 5). Three longitudinal profiles in the direction of dune movement were surveyed in each campaign. At the Valdevaqueros dunefield, 11 campaigns were performed from 1995 to 2009, each one comprising 5 dune profiles.

In addition, aerial photos from 1956, 1974, 1979, 1985, 1990, 1998, 2002, 2005, 2008 and 2011 for Bolonia and photos from 1956, 1974, 1979, 1985, 1998, 2001, 2004, 2006, 2008 and 2011 for Valdevaqueros were used for dune monitoring. Most of the aerial photographs are vertical (e.g. the photos of 1956, scale 1: 9100 and of 1990, scale 1: 5000, available from the Army Geographical Service and the Directorate General of Coasts, respectively). Other photographs are orthorectified projections and properly georeferenced (e.g. year 1998 and scale 1:60000) and can be downloaded from the Junta de Andalucia webpage (http://www.juntadeandalucia.es/medioambiente/site/rediam/).

Dune front activity was determined by the use of the OGC Generic Viewer of the Environmental Network of Andalusia (south Spain) and GRAPHER software for mapping. A percentage of minimum error was assumed during the interpretation of the available cartography (del Río and Gracia 2013del Río L., Gracia J. 2013. Error determination in the photogrammetric assessment of shoreline changes. Nat. Hazards 65(3): 2385-2397.). The methodology can be found at Domínguez et al. (2004)Domínguez L., Gracia F.J., Anfuso G. 2004. Tasas de avance/retroceso de la línea de costa mediante morfometría fotogrametrica en el sector Sanlucar de Barrameda–Rota (Cádiz). Rev. Soc. Geol. España 17(1-2): 71-86.. Finally, a very interesting study of the historical evolution of the Valdevaqueros dune, including ancient topographical maps (1917 and 1955), can be consulted (Bello-Millán et al. 2015Bello-Millán F.J., Somoano M., Clavero M., et al. 2015. Evolución histórica del sistema dunar de Valdevaqueros. XIII Jornadas Españolas de Ingeniería de Costas y Puertos. Avilés.).

RESULTSTop

Due to their particular geographical location and the strong and frequent easterly winds that produce mass aeolian transport from the dry beach to the dune crest, the Bolonia and Valdevaqueros dune systems have experienced substantial growth in the last 50 years.

Bolonia dunefield evolution

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.

Valdevaqueros dune evolution

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. 2009Muñoz-Pérez J.J., Navarro M., Román-Sierra J., et al. 2009. Long-term evolution of a transgressive migrating dune using reconstruction of the EOF method. Geomorphology 112: 167–177.). 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. 1997Serra J., Riera G., Argullos J., et al. 1997. El transporte eólico en el Delta del Ebro. Evaluación y contribución al modelado litoral. Bol. Geol. Minero 108(4): 477-485. or Cabrera and Alonso 2010Cabrera L., Alonso I. 2010. Correlation of aeolian sediment transport measured by sand traps and fluorescent tracers. J. Mar. Syst. 80(3): 235-242.). 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.

Comparison of Bolonia and Valdevaqueros dune evolution

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 1988Boletín Oficial del Estado (BOE). 1988. Spanish Shore Act (in Spanish) 29/07/1988: 18762.), 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. 2011Navarro M., Muñoz-Pérez J.J., Román-Sierra J., et al. 2011. Assessment of highly active dune mobility in the medium, short and very short term. Geomorphology 129(1-2): 14-28.), 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. 2011Navarro M., Muñoz-Pérez J.J., Román-Sierra J., et al. 2011. Assessment of highly active dune mobility in the medium, short and very short term. Geomorphology 129(1-2): 14-28.).

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).

DISCUSSIONTop

The advance rate values of both dunes, previously presented, are similar to those identified at the barchans of Jericoacoara (NE Brazil), with values ranging from 14.6 to 21 m yr^–1 (Jiménez et al. 1999Jiménez J.A., Maia L.P., Serra J., et al. 1999. Aeolian dune migration along the Ceará coast, north-eastern Brazil. Sedimentology 46(4): 689-701.). However, at Ceará, the migration of dune fields was calculated as approximately 6 m yr^–1 despite being caused by the effects of the climatic phenomenon of El Niño (Maia et al. 2005Maia L.P., Freire G.S.S., Lacerda L.D. 2005. Accelerated dune migration and aeolian transport during El Nino events along the NE Brazilian coast. J. Coastal Res. 21(6): 1121-1126.). In general, advance rates of several metres per year are characteristic of transgressive dune systems. According to Rust and Illenberger (1996)Rust I.C., Illenberger W.K. 1996. Coastal dunes: sensitive or not? Landscape and Urban Planning 34, 165-169., these individual dunefields are so dynamic that they often show shifting behaviours not only in their shape but also in their location, depending on the wind regime.

During the initial phase, the artificial dune stabilization at Bolonia and Valdevaqueros had a twofold effect: the reduction of sand supply inland was achieved, but a slow shoreline advance started as a result of the increasing dune elevation and the steeper windward slope, producing higher sand sedimentation at the dune foot and the backshore.

Sand extraction was performed as the dune grew, to mitigate the sand invading the road, which occurred more frequently during the most recently researched years. Wooden and wicker fencing was the tool most widely used for dune stabilization. However, at Bolonia, fencing was almost experimental in nature, only working in isolated cases. Wooden fences were mainly located on the dune crest, whereas wicker and wattle fencing was employed on the windward side. As a result, the dune morphology developed accordingly. Whereas wicker and wattle fencing provide a more natural and stable profile (due to a higher fence porosity), wooden fencing increases dune elevation, creating a more artificial profile. Moreover, marram grass (Ammophila arenaria) was planted manually during a few experimental episodes, always on the windward foot. This intervention usually produced unsuccessful results, except for the plantations located on the eastern part of the dune, where aeolian sedimentation was not so intensive.

In the case of the Valdevaqueros dune, it is obvious that human activities on the beach-dune area have caused a great alteration of the ecosystem, especially during recent decades. However, changes in the Valle River mouth and its seasonal lagoon, the frequency of river flooding, and the construction of the car park prior to the enactment of the Spanish Shore Act (BOE 1988Boletín Oficial del Estado (BOE). 1988. Spanish Shore Act (in Spanish) 29/07/1988: 18762.) might also have strongly influenced dune system development and growth in terms of sediment balance. Thus, the location of the shoreline, the water table and other features proximal to the research area must have caused important changes in sediment availability since 1956.

As the Bolonia dunefield was considered a Natural Monument in 2001 and anthropic interventions have been completely stopped since then, it is considered to have developed more naturally than the Valdevaqueros dune. However, in both cases, unlimited sand availability still seems to represent a serious risk for the uncontrolled advance and growth of the dune.

In brief, the Bolonia and Valdevaqueros dunefields have undergone great transformation since 1956, showing average advance rates of 10.9 and 9.0 m yr^–1 and maximum values of 23 and 25 m yr^–1 for the most active profiles, respectively. Though the two dunes have similar features and wind velocity regimes due to their proximity to the Strait of Gibraltar, the Valdevaqueros dune has experienced more change during the overall research period than the Bolonia dune, mainly caused by sand mining activities. Whereas the Bolonia dune area has always increased linearly (0.117 104 m² yr^–1), Valdevaqueros has had a more pronounced tendency to increase over time, at a rate twice this value from 1974 to 1998, and ten times this value from 1998 to 2008. Though the Bolonia dune has not undergone so much human intervention as Valdevaqueros (only marram grass plantations and sand fencing in isolated cases), both dune profiles have grown not only in width (two advance fronts have been identified) but also in height, with average elevation change rates of 1.10 m yr^–1 for Bolonia and 1.29 m yr^–1 for Valdevaqueros. These results show that in addition to sand transport generated by wind-blown sand availability, which is similar for both systems, sand distribution is also strongly linked to orography, creating specific morphologies of different heights. It might be argued that these results could also be due to the closer proximity of Valdevaqueros to the Strait of Gibraltar. Unfortunately, although it would provide very interesting outcomes, simultaneous data in both dunes have never been surveyed.

After analysing attempts to mitigate road invasion by blowing sands, some practical and useful conclusions can be established:

- Pine grove plantations did not reduce aeolian sand transport at all and therefore should not be considered an effective method for dune stabilization.

- Sand fencing and marram grass plantations (later management activities), have produced only occasionally successful results due to the massive aeolian sand transport westward.

- Wooden fencing has strongly modified the profile morphology, especially on the Valdevaqueros dune crest, by increasing its height and producing steeper slopes, therefore reducing its stability and giving it a more complex profile. The relationship between this artificial profile and the increase of the migration rate of the dune has not been demonstrated but this possibility should not be excluded.

Thus, as sand supply has never been a limiting factor (Bello-Millán et al. 2015Bello-Millán F.J., Somoano M., Clavero M., et al. 2015. Evolución histórica del sistema dunar de Valdevaqueros. XIII Jornadas Españolas de Ingeniería de Costas y Puertos. Avilés.), taking into account that the Valdevaqueros dunefield has developed despite sand extraction in recent decades, given the current profile of the dune, it seems that only 2-3 weeks of moderate easterly winds can produce a sand transport, as happened over a full year when no human intervention took place. As a conclusion, current management measures based on sand removal (which do not respect the natural dune profile) are no longer sustainable.

Moreover, some of the results obtained from this research (such as the fact that the Bolonia dune could reach the nearest road in approximately 12 years) are very useful for conceiving or developing solutions in advance (a summary of the different solutions studied until now is available in Gracia et al. 2013Gracia F.J., Nuño T., del Río L., et al. 2013. Valdevaqueros, Cádiz: La difícil gestión de una gran duna móvil. Enseñanza de las Ciencias de la Tierra 21(3): 332-335.). Therefore, it is extremely important to understand the dynamics of dune development based on historical anthropic intervention in addition to accounting for the special environmental characteristics within the entire physiographic unit when designing future management strategies.

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