Effects of storms on a composite beach, San Felipe, Gran Canaria

Abstract

San Felipe is a pebble beach placed in the northern coast of the island of Gran Canaria. The morphodynamics of beaches located along this area undergoes variations due to waves generated by Trade winds, from NNE, prevailing since April to October. However, the greatest changes occur during the stormy season. During this period, from November to March, wave direction changes from NW to NE, being waves from NNW the most frequent and which show higher maximum wave heights. Therefore, owing to the magnitude of changes during storms, the aim of this paper is to study pebbles and cobbles dynamics in that period, as well as beach morphological changes associated with the wave regime. The study area is approximately 200 m long and 40 m average width during winter. San Felipe is a composite beach, according to Jennings and Shulmeister (2002) classification, consisting mainly of phonolitic and basaltic pebbles and cobbles. In the nearshore a submerged sand bar is present, being most of these sediments of basaltic origin. Pebbles and cobbles tracking is performed using a RFID system, based on that described by Allan et al. (2006). This technology allows the identification of pebbles with a unique number for each of the 198 selected particles. The entire beach was tracked 5 times from 19 December 2013 to 21 February 2014 with an antenna to locate the tagged particles. Temporal spacing between those dates depended on the occurrence of stormy events and tide conditions, but never exceeding a month. Once each individual tagged particle was identified, it was located with a Leica total station TCR307. From the measured coordinates the movement of each individual cobble has been calculated. Topographic data for the whole beach were also collected with the total station. According to hourly wave data recorded at the deep water wave buoy “Gran Canaria”, owned by Puertos del Estado, dominant waves during this study came from N with an average of Hs 2.33 m and Tp 13.56 s. During the 2 months study period, 10 stormy events took place, all them with average Hs higher than 2.70 m. Only 3 of them should be pointed out: First one was a swell storm that took place between 6-7 January, with average Tp 20.29 s and 350ºN approaching direction. Second event took place between 23-26 January. In this case waves show average Tp 9.92 s and Hs 3.55 m from 36ºN. Third storm was the lasting one. It took place between 15-18 February and average waves were Hs 3.74 m, from 1ºN. During winter, the sand bar is completely submerged, and it is not until the beginning of the Trade wind season, when it moves onshore covering the cobbles located on the foreshore. For this reason, during the stormy season pebbles and cobbles in the lower foreshore are completely exposed to incident waves, which is the forcing agent for their movement. By contrast, particles located above the berm will be affected only in situations where high waves and high spring tides are coincident, and this type of situations took place in 4 of the considered stormy events. Sedimentary balance showed two different periods, the first of them from 19 December to 14 January, with the removal of sand from the foreshore and the formation of a shingle berm all along the eastern sector of the beach. The net sedimentary balance was -5160 m3, mostly due to the offshore movement of the sand. During the second period both erosion and accretion events took place, with an average value of 350 m3. It was at this stage where morphological changes due to pebbles movements occur. In eastern and central parts of the beach, a pebbles berm was formed with sediment gains of 1080 m3. Subsequently this berm moved onshore because maximum Hs coexist with high spring tides. However, the western area had erosive processes until 21 January, when it began the accumulation of 900 m3 of pebbles. The analysis of tagged pebbles with movement greater than 1m indicates that both time intervals between 19 December and 14 January and 21 to 28 January showed small particle displacements, but the greatest distances travelled took place during periods between 14 to 21 January and 28 January to 21 February. Movement directions changed for each of these periods. At the beginning 36.4% of pebbles and cobbles moved southwards, which confirms the shingle berm formation showed from the topography. During second interval (14-21 January) there was a net eastward transport, since 33.3% and 22.2% of particles moved towards E (38 m average distance) and W (11 m) respectively. During third period (21-28 January), directions E and W had the same percentage of representation, 28.6%, but in this case westward movement had higher average magnitude, 20.7 m. Finally, during fourth interval (28 Jan-21 Feb), the largest and strongest of the different stormy events took place, since Hs greater than 4 m were recorded during 35 consecutive hours. For this reason distances were the largest, with a mean value of 67 m, but with maximum distances up to 160 m. These movements occurred towards W. Main factor responsible for the movement of shingles are waves. During this study 10 storms have taken place, but two of them have generated the greatest movements and topographic variations. These two storms show maximum wave heights and spring high tides at the same time. The largest sediment transport is longshore, so that when the eastern sector erodes, the western area accretes and vice-versa. However, due to the prevailing northern waves, the cross-shore movements produce significant changes in beach morphology, in particular changes related with the formation, onshore migration and disappearance of the berm.