25 Revista de la Sociedad Geológica de España 34 (1) ISSN (versión impresa): 0214-2708 ISSN (Internet): 2255-1379 OSTENDE AND ARRIGUNAGA BEACHES, TWO DIFFERENT INTERVENTIONS AND SUBSEQUENT GEOMORPHOLOGICAL CHANGES IN NORTHERN SPAIN: AN APPROPRIATE POLICY TO FOLLOW? Las playas de Ostende y Arrigunaga, dos intervenciones diferentes y cambios geomorfológicos sufridos en el norte de España:¿una apropiada política a seguir? Javier Elorza Departamento de Mineralogía y Petrología. Universidad del País Vasco UPV/EHU, Apdo. 644, 48080 Bilbao. [email protected] Abstract: The small towns of the Basque-Cantabrian coast (Spain) suffer from strong demographic pressure in summer time that has made it necessary to develop new beach areas suitable for bathing. In 1991 in Castro Urdiales (Cantabria), gravel was dumped in Urdiales cove (350,000 m3) to create the Ostende artificial beach. In Getxo (Bizkaia), the regeneration of Arrigunaga urban beach involved a complex intervention between 1995 and 1999, with the construction of groynes, the removal of sands and conglomerates of cemented cast iron slag (beachrock), the subsequent deposition of natural bioclastic sand (213,000 m3) and the complete urbanization of the beach. Several decades having now elapsed, the present work analyses how the two interventions have evolved, whether the migration of sand-gravel, with losses and accumulations in different areas, poses a risk for the proper use of the beaches, and whether the groynes meet their original objectives and maintain themselves despite the numerous sea storms to which they are subject. As a new feature, the rapid formation of new geomorphological aquafact structures on the rocky protrusions generated by the mechanical abrasion of water/sand-gravel. Finally, the question is raised whether this type of intervention is appropriate in the current context of environmental conservation. Keywords: coastal dynamics, beach regeneration, blast furnace slag, beachrock, aquafacts, Basque-Cantabrian coast. Resumen: El bienestar económico producido en el último tercio del siglo XX lleva consigo el disfrute, en la época estival, de una segunda vivienda situada en la costa. Las pequeñas localidades costeras sufren una fuerte presión demográfica que obliga a acondicionar nuevos espacios de esparcimiento playeros. En Castro Urdiales (Cantabria) en 1991 se realizó un vertido de gravilla (350.000 m3) en la Ensenada de Urdiales, para pasar a convertirse en la playa artificial de Ostende. En Getxo (Bizkaia), la regeneración de la playa urbana de Arrigunaga conllevó una intervención compleja entre los años 1995 y 1999, con la construcción de espigones, retirada de arenas y conglomerados de escorias de fundición cementados (beachrock), el vertido de arena bioclástica natural (213.000 m3) y la completa urbanización de la playa. Una vez que han pasado varias décadas, en este trabajo se analiza cómo han evolucionado ambas intervenciones, si la migración de la arena- gravilla, con pérdidas y acumulaciones en diferentes zonas, supone un riesgo para el pleno uso de las playas, y si los espigones cumplen los objetivos iniciales y se mantienen a pesar Revista de la Sociedad Geológica de España, 34 (1), 2021 26 OSTENDE AND ARRIGUNAGA BEACHES, TWO DIFFERENT INTERVENTIONS IN NORTHERN SPAIN de los numerosos temporales marítimos sufridos. Como novedad, se constata la rápida formación de acuafactos como nuevas estructuras geomorfológicas sobre las protusiones rocosas por la abrasión mecánica de agua/arena-gravilla. Por último, surge la pregunta sobre si este tipo de intervención resulta adecuado en el actual contexto de conservación ambiental. Palabras clave: dinámica costera, regeneración de playas, escorias alto horno, beachrock, acuafactos, costa Vasco-Cantábrica. Elorza, J., 2021. Ostende and Arrigunaga beaches, two different interventions and subsequent geomorphological changes in northern Spain: An appropriate policy to follow? Revista de la Sociedad Geológica de España, 34 (1): 25-35. Introduction The last third of the 20th cen- tury saw a certain economic boom in Basque society, which allowed the most fortunate to acquire a se- cond residence in small coastal towns along the Basque-Cantabrian coast (Bizkaia, Cantabria) (Fig. 1A). This led to a major increase in po- pulation in these small towns during the summer period, with the conse- quent demand for public assistance services and recreational spaces. In this sense, according to 2019 data, the municipality of Castro Urdiales (Cantabria) went from a stable cen- sus population of 32,069 inhabitants to more than 70,000 inhabitants du- ring the summer season, whereas in the municipality of Getxo (Bizkaia) the population surveyed in 2019 rea- ched 79,946 inhabitants. One of the most frequently re- quested services during the summer season was for new beaches to be developed as recreation areas, for- cing the responsible administra- tions (municipal, provincial, and the Coastal Demarcation Office) to coordinate and undertake specific projects to satisfy the growing de- mand. In 1991, the municipality of Castro Urdiales undertake a work on the Urdiales cove, initially wi- thout sand and with a small marsh area, that was to become the so-ca- Fig. 1.- A: Geographical location of the Ostende artificial beach (Castro Urdiales, Cantabria) lled artificial beach of Ostende and the Arrigunaga beach (Getxo, Bizkaia) along the Basque-Cantabrian coast. *(b) Location (around 750 m in length, Fig. 1B). of the Abra-Ciervana buoy. B: View from the northern part of the current Ostende artificial beach (750 m in length), showing the tracks from the mechanical levelling that is carried out In the municipality of Getxo, by daily. C: General view of the current Arrigunaga beach from its western part (630 m in length). contrast, the work was undertaken on the Arrigunaga urban beach (around 630 m in length, the area, with the construction of three groynes and a lar- Fig. 1C) that due to its deep anthropic footprint was prac- ge breakwater (associated with Punta San Ignacio). It was tically in disuse and was not recommended for bathing. completed in 1999 with the addition of bioclastic natural This situation was mended by a complex intervention in sand to the beach. Revista de la Sociedad Geológica de España, 34 (1), 2021 J. Elorza 27 More than two decades after these interventions (30 rocky protrusions and in the breakwater blocks that stand and 21 years, respectively), it seems appropriate to review out in the foreshore. how the processes of coastal dynamics have acted upon them. In this sense, this study aims to inquire whether the Geological context and characteristics of the behaviour has been as theoretically predicted or whether interventions there have been substantial changes in the profiles of each beach. The loss and/or migration of the sands concentra- Ostende beach (Urdiales Cove) ted in certain areas may have modified the expected beach half-life. As regards the groynes and the breakwater built The Ostende artificial beach (43º 23’ 21” N and 3º on Arrigunaga beach, the question is whether they have 13’ 33” W, Fig. 1A) belongs to the municipality of Cas- been effective or been overcome by the coastal dynamics. tro Urdiales (Cantabria). From a geological point of view, Additionally, this study also deals with how the interaction the beach substrate is composed of limestones from the of tides and sediments has promoted an abrasive process so-called “Planar-parallel limestone of the Urdiales Cove producing new geomorphological structures in the natural Formation” (Rosales, 1995, 1999). Dated to the lower Al- bian, it is composed of 200 m-thick rhythmic alternations of marls, marly limestones and limestones in decimetre-thick beds. Rosales (1995) suggested that this formation belongs to a hemipelagic platform related to the distal part of a carbo- nate ramp-type shelf. Westwards the unit grades laterally to shallow shelf limestones (Oriñon Limestone For- mation). Refurbishment works were ca- rried out in a short period of time (from 29/12/1988 to 02/01/1991) and consisted of the modification of Urdiales cove by the deposition and levelling of crushed gravel (6-8 mm in diameter) along a coastal strip around 750 m in length, using a to- tal volume of 330,580 m3 to this end (data provided by the Cantabrian Coastal Demarcation Office) (Figs. 1B, 2A-C). Currently, the sediments found on the artificial beach corres- pond to two types: a) a minor part of fine-medium bioclastic natural sands (0.2-0.3 mm) typical of the beaches of the Cantabrian coast; and b) a majority of limestone gravels (6-8 mm) that corresponds to the artificial crushed deposit, extracted from the Santullán quarry, current- ly showing a clear increase in the roundness index (Figs. 2D-F). Arrigunaga beach Three sectors bounded by faults can be distinguished in the cliffs Fig. 2.- Ostende beach, Castro Urdiales. A: Aerial view of Urdiales cove, dating from 1965. bordering the Arrigunaga beach B: Aerial image similar to the previous one, taken in 2015, with evidence of the sand-gravel (43º 21’ 22.0” N and 3º 01’ 10.00” spill creating the recreation area (750 m in length). C: Current picture of the artificial beach from the southeast, where the protrusions of Urgonian limestone can be observed. D: Detail W, Fig. 1A), Getxo (Bizkaia) (Ro- of the fine bioclastic sands that make up a small part of the sandbank. E-F: Appearance of dríguez-Lázaro et al., 1989). The the artificial angular gravels (E), now notably rounded (F) by the abrasive effect exerted over northeastern sector comprises tur- three decades. Marker and coin are 2 cm and 2.2 cm in diameter, respectively. bidite sediments that belong to the Revista de la Sociedad Geológica de España, 34 (1), 2021 28 OSTENDE AND ARRIGUNAGA BEACHES, TWO DIFFERENT INTERVENTIONS IN NORTHERN SPAIN middle Eocene and consist of an alternation of calcarenites a family beach with limited use, little recommended for and coarse calcisiltites with marls.