Boulder movement monitored during "Christina" and "Nadja" 2014 storms in the Coxos boulder deposit, West coast of Portugal

2019-08-15T05:40:03Z (GMT) by Maria Alexandra Oliveira
A boulder deposit resulting from deposition of large limestone clasts by extreme marine inundations has been identified north and south of Coxos beach, in the western coast of Portugal (European southwest coast). The boulder accumulation was characterized as associated to an extreme marine inundation by Oliveira (2017). It comprises over 1500 limestone boulders showing evidence of transport against gravity, their source layers outcropping at lower altitudes. Boulders are parallelepiped in shape, they sit at 2-13m amsl, their mass is under 30Mg and their size decreases inland from the edge of the rocky platforms/cliffs Boulder movement by present-day storms was detected during January and February 2014. Boulders' initial position, prior to dislocation by storms, was considered as the position acquired during initial boulder mapping, undertaken from 2009 to 2010. Final position was obtained from monitoring field surveys during January and February 2014 focused on: (i) acquiring positions of particles displaced during that period and previously identified in earlier field campaigns; (ii) on mapping new boulders detached and emplaced by waves during this period and, whenever possible, of their original location/socket in the cliff/platform edges. This information was acquired using Real Time Kinematic Global Positioning System (RTK-GPS) equipment (accuracy better than 5cm). In addition, boulder thickness was also measured using a measuring tape. Rock samples were collected to measure rock mass density by using the volume of dislocated water in the laboratory. The corners of each boulder were transformed into a 3D surface, corresponding to the largest exposed boulder surface by using geographical information system software. Boulder mass was determined for each particle by combining area of boulder surface (automatically computed using geographical information system software), boulder thickness and rock mass density. As a result of this procedure, a database (excel spreadsheet) was created summarizing information regarding boulder mass, direction of movement and distance traveled associated with storms that impacted this coastal location in January and February 2014. In this archive, two shapefiles are also provided: (i) one containing mapped boulder paths (polyline shapefile); (ii) another containing mapped boulders' initial position (label field in the attribute table is "initial position"), position after movement (label field in the attribute table is "mooved"), boulder eroded (label field in the attribute table is "eroded") and new boulders (label field in the attribute table is "new") final position of the boulders for which movement was detected (polygon shapefile). There is also information regarding the date of movement and boulder mass in the attribute table.