Dinámica sedimentaria y evolución geomorfológica de las playas, plataforma sumergida y dunas de Maspalomas (Gran Canaria)

Abstract

ABSTRACT The Maspalomas Sedimentary System (S of Gran Canaria Island) is composed of the Maspalomas Dune Field, Maspalomas and El Inglés beaches, connected by the sandy strip of La Bajeta Cape, and the surrounding submerged area, with multiple sedimentary interactions among them. The main objective of this study is to describe the current mophodynamics of this sedimentary system. According to it, the specific objectives have been: i) the characterization of the wind and wave regimes, ii) the quantification of the sedimentary fluxes, iii) the quantification of the sedimentary thickness and availability of unconsolidated sediments in the dunes, beaches and proximal submerged area, iv) the determination of the geomorphological evolution and its influence in the present morphodynamics, and v) the analysis of the influence of the climate change in the future evolution of the system. Fieldwork studies have been carried out during 2005-2010, including accurate topo-bathymetric surveys by Differential GPS (Global Positioning System) and multibeam/monobeam echosounder, combined with geophysical techniques (seismic profiles, electrical tomographies and Side-Scan Sonar (SSS) images). For the topographic monitoring, twenty-one Differential GPS topographic surveys of the supratidal, intertidal and upper subtidal zones of Maspalomas and El Inglés (including La Bajeta Cape) were carried out by crosshore zigzag and alongshore trajectories during low spring tides, obtaining more than 30,000 data for each survey. Initial topography was measured in September 2005. Monthly surveys were performed from March 2006 to February 2007, and after that, seasonally surveys were carrying out until June 2009. The bathymetric survey on the subtidal beaches and the inner shelf of the Maspalomas sedimentary system was initially carried out in February 2007, using a GeoAcoustic Geoswath 250kHz multibeam echosounder. Later, seasonal short-term topographic changes at the submerged area were monitored via three additional bathymetric surveys (in August 2007, November 2007 and February 2008) taken by a Valeport Midas Surveyor 210kHz monobeam echosounder. A total of 28 seismic profiles (each one of 115 m long, with seismic sensors every 5 m), and 4 electric tomography profiles (each one of 235 m long, with electrodes every 5 m) were performed over the dune field in March and September 2006 respectively. High-resolution 3.5 kHz seismic-reflection profiles were acquired along a marine geophysical survey in October 2010 using a Chirp Knudsen 3200 system. Thus, a total length of 89,649 m parallel and 49,850 m perpendicular to the isobathic lines were recorded. The facies cartography on the same area (about 6km2) of the inner shelf seafloor was obtained from SSS images recorded using an Edgetech 4125-P. Data analysis began with the local wind and wave characterization by wind profiles and wave record measurements during 2006-2007 fieldwork, as well as the design of wind and wave roses from data bases of a local AEMET meteorological station and offshore wind and wave regimes obtained from HIRLAM and WAM models applied by Puertos del Estado. Local wave and littoral currents were simulated by the SMC propagation model, which was applied over the bathymetry and beach topography measured in this study. Digital Elevation Models (DEM) for the entire area of the Maspalomas sedimentary system has been carried out. All the maps were referred to the World Geodetic System 84 (WGS84) and the REGCAN (Red Geodésica de Canarias) geodesic systems. Thus, the obtained DEM integrate dunes (subaerial), beaches (supratidal to subtidal) and shelf (submerged) reliefs. Shoreline variations maps relatives to mean sea level were obtained too from topo-bathymetric maps. Topo-bathymetric data has allowed quantify erosion and accretion periods for Maspalomas and El Inglés beaches, which present similar or opposite trends after the dominance of the crosshore or longshore sediment transport, respectively. Maspalomas and El Inglés beaches present a W-E and N-S shoreline orientation respectively, and both beaches supply sediments to La Bajeta Cape, which show the highest shoreline variations, until 100 m at its eastern margin. This sedimentary pattern is in agreement with the analysis of the wind and wave regimes, as well as the results of the numerical wave propagations studies. Therefore, according to its morphodynamical behavior, this complex sedimentary system has been defined as a cuspate foreland. Storm erosion has been identified as the main factor inducing erosion in the Maspalomas Sedimentary System, mainly due to the winter SW storm waves with Hso higher than 2,5 m and return period of 5 years. Sediment budget has shown a shortage of sediment for the entire area in short (months) to medium-term (months to years) scales. Therefore, it has been deduced that theses beaches are not controlled by a classical annual erosion-accretion cycle, but almost a decade (7,8 years) are necessaries to recover the Maspalomas beaches after intense SW storm erosion events. The analysis of the accurate topo-bathymetric DEM combined with geophysical profiles for the emerged and submerged sectors have allowed deduce the geomorphological evolution and to determine the sediment thickness of the system. Thus, seismic profiles allowed identify the Pleistocene alluvial terraces above the actual Holocene deposits for the whole system. The new identification of a submarine alluvial terrace has allowed explain the wide morphology of the proximal submerged plain southward of Maspalomas Beach, in contrast with the narrow shelf eastward of El Inglés and La Bajeta Cape, where this alluvial terrace was not developed. The top of these alluvial deposits is more superficial in the northern and western areas of the Maspalomas Sedimentary System, for both the emerged and submerged sectors. The geophysical profiles and DEM have also allowed design the map of the wall for the modern unconsolidated sediment layer, and calculate the isopach map of this layer. Thus, an intense easterward progradation has been deduced from the isopach map. Consequently, the origin and present erosive tendency of the Maspalomas dune field and Maspalomas Beach can be explained by the sediment scarce on the west and northern areas, which explain the frequent outcroup of the alluvial substrate. In contrast, El Ingles Beach and La Bajeta Cape present a high availability of sand and the alluvial substrate does not outcroup. Sediment thickness has been determined in both the emerged and the submerged area by geophysical methods (seismic refraction and electrical tomography for the subaerial sector; seismic reflection respectively). Thus, a total volume of 97.76×106 m3 has been calculated for the entire system, including the dune field, beaches and inner shelf of Maspalomas, with most of the available sediments (64.55%) in the inner shelf. The analysis of the DEM and the SSS images allowed obtain the sedimentary cartography of the submerged area, which shows the presence of several facies: rocky substrate, volcanic boulders, sandstone cobbles and gravel, sand covered with vegetation, and finally mobile sand, including ripples and megaripple fields. The presence of ripples and megarriples fields close to the talus has been related to the main sinking area of the Maspalomas sedimentary system, where sediments output mainly due to SW storm-wave events. The effect of the climate change in the Canary Islands region has been described in the literature, by numerical models prediction, as a reduction of the trade winds intensity and an increase of wave storms. Consequently, the Maspalomas Sedimentary System, and particularly Maspalomas Beach, could be affected in the near future by a higher erosion trend.The data gathered in this thesis could be very helpful to improve the knowledge about the development of a sustainable and durable natural environment as sensitive to climate change and anthropogenic activities. It is meant to be used as a reference to help in decisions of future actions aimed to preserve this outstanding natural space.