Biausque, M. and Senechal, N., 2020. Analysis of two contrasting seasonal recovery periods of an open sandy beach, using high frequency DGPS surveys. In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 377-381. Coconut Creek (Florida), ISSN 0749-0208.High frequency DGPS surveys were undertaken for 2.5 years (November 2015 until April 2018) on an open sandy double barred beach in the SW of France (Biscarrosse Beach) covering three winter and two summer periods. On those latitudes, summers are usually described as seasonal recovery periods. The morphological response of the beach during two consecutive summer periods (2016 and 2017) has been analyzed to identify processes involved in seasonal recovery. Analysis of alongshore mean profiles indicates that the summer 2016 is marked by a progressive berm reconstruction, manly driven by cross-shore sediment transport. The beach recovery in 2016 can be characterized as complete with a stable berm through the summer season associated with steady beach cusps, and a seaward dune foot migration. During this summer the variations in volume of each section of the beach (intertidal/supratidal beach and dune) show sediment transport firstly orientated from the intertidal beach toward the supratidal one, and, later, driven from the supratidal beach to the dune. This cycle is coherent with recent observations described by Philips (2017). In contrast, the summer 2017 is characterized by an incomplete recovery, an unstable berm and so, a much more complex dynamic: the berm, rapidly rebuild (by the end of June), started to erode in July. Six sequences of cross-shore sediment exchanges are measurable along the season between the intertidal and the supratidal beach, inducing successive erosion/ accretion of both sections of the beach. Thus, according to those results summers 2016 and 2017 present variable spatial and temporal dynamics. Moreover, analysis of hydrodynamic and environmental factors indicate that recovery periods not only depend on hydrodynamic conditions, such as wave energy and tide, but also on winter/summer morphological coupling, and sandbar/beach coupling.
A field experiment was conducted on a sandy beach with a low tide terrace (Nha Trang, Vietnam) to investigate the swash zone hydro-and morphodynamics throughout different tide and wave conditions.A 2D Lidar was used to measure runup properties and bed level changes on the swash zone.An energetic monsoon wave event provided energetic conditions during the initial stage of this experiment while mild wave conditions were observed during the remaining days.Swash dynamics were clearly modulated by wave and tide conditions.Preliminary results indicate that wave climate is linked with extreme runup and beach erosion and recovery processes while tide level seems to affect swash spectral signature (dominated by infragravity band during low tide and incident band during high tide) and linked with asymmetrical morphological response of the swash.
The characteristic wave period (or frequency) is a key parameter in most of hydrodynamic models and paradoxically its determination is not easy, particularly in presence of wind sea conditions or in the nearshore zone. In this paper, we propose to show why it is complex to clearly define this parameter and the implication that it can induce in the determination of wave energy decay parameterization. In particular we will focus on one example, which concerns the parameterization of the γ parameter (the ratio of significant wave height to local water depth) in the surf zone. We will show that the discrepancy previously observed between two parameterizations can be partially explained by the determination of the characteristic wave period (or frequency).
En zone de levée, l'énergie contenue dans la bande infragravitaire (fréquences typiquement inférieures à 0.05 Hz), est généralement négligeable par rapport à celle contenue dans les vagues (< 1%).Cette tendance s'inverse fortement en zone de surf où l'énergie infragravitaire peut devenir plus importante que celle contenue dans les vagues.Dans ce papier, nous montrerons, à partir de données in situ, que les processus physiques qui contrôlent l'énergie infragravitaire en zone de surf sont très différents de ceux des vagues et de ce fait que la structure spatio temporelle de cette bande se différencie de celle observée habituellement dans les vagues.
Rip currents are wave-driven intense seaward-flowing jets of water that are important to both beach morphodynamics and the overall ecosystem. Rip currents are also the leading deadly hazard to recreational beach users worldwide. More specifically, the African region is reported to have the highest rates of drowning in the world, yet both the occurrence and the type of rips developing along the African beaches are unknown. In February 2013, a 12-day field experiment was performed at the high-energy low-tide-terraced sandy beach of Grand Popo beach (Benin, West Africa). Human drifter data and video imagery are combined to address wave-driven circulation and rip current activity. Results show two prevailing rip current types. (1) Low-energy (~0.2-0.4 m/s) swash rips, with short life-spans of about 1 minute, extend about 5-10 m offshore and occur preferably at mid to high tide at fixed locations in the center of beach cusps. (2) Higher-energy (0.2 - 0.8 m/s) surfzone flash rips become active with the onset of intense wave breaking across the low-tide terrace. They tend to migrate downdrift with alonger time-span of about 2-5 minutes. The relatively weak longshore current (0.2 - 0.55 m/s) measured during the experiment suggests that flash rips were driven by vorticity generated by wave breaking rather than shear instabilities of the longshore current. Swash rips and flash rips are common at Grand Popo and often co-exist. We propose a conceptual model of both flash and swash rip activity on this stretch of the West African coast.
Despite a global context of shoreline retreat, coastal areas and in particular sandy coasts are increasingly attractive. To handle the problem of coastline retreat different management strategies are deployed and among them soft methods as windbreakers or hard ones as seawalls. But all those methods are known to interfere in the natural evolution of the beach/dune systems at different timescales. To underline potential influences of management strategies on erosion and recovery periods, high frequency DGPS surveys coupled with video images are recorded at a workshop-site exhibiting various management strategies, Biscarrosse beach (SW of France) from November 2015 until September 2016. Results for the winter 2016 highlight a global erosion of the beach associated to a dune foot retreat and an alongshore variability in the beach response to events. The same patterns can be observed during the seasonal recovery period (April to August), in particular a lag in the berm reconstruction in front of the seawall. The LVI (Longshore Variation Index) reflects possible sediment processes taking place between the different sections of the beach: while recovery seems to be dominated by cross-shore exchanges in the unmanaged section, longshore sediment processes seem to be the origin of the recovery in the managed section. This variability could be linked to a permanent rip current visible (98% of observation) in front of the seawall that could cause an offshore sediment export explaining both the lag in term of recovery timescale and the different sediment processes involved during the recovery period. During the erosion season, sediment exchanges between the beach and the dune are limited due to the presence of seawalls and beach erosion and dune retreat in the two ends on the wall accelerated.
