Beach adaptation to intraseasonal sea level changes

Abstract

Coastal areas such as beaches with steep upper slope and flat low-tide terrace, are expected to be increasingly affected by sea level changes. Related impacts due to the paramount rise in sea level have been intensively investigated, but there is still little evidence of the impact of shorter timescales variations on the coast, particularly those induced by trapped coastal waves. Using the latest advances in video bathymetric estimation, daily observations over 3.5 years (February 2013 to June 2016) on Grand Popo Beach (West Africa) reveal that intraseasonal sea level variations impact the beach profile. The intraseasonal sea level variations are dominated by the propagation of wind forced coastal trapped waves with periods ranging 15-95 days. It is shown that the beach goes through a transient state with a deformation of the profile: an intraseasonal sea level rise leads to a 2 m erosion of the upper beach and a widening of the flat terrace at the lower beach. Although the underlying mechanism must be tested through beach profile modelling, this study highlights the active adaptation of the beach profile to variations in sea level.

Modelling Cross-Shore Shoreline Change on Multiple Timescales and Their Interactions

“…A source of shoreline data are shore-based video cameras that generally collect data during daylight on a 30 min basis [31]. In comparison to shoreline-walking GPS surveys, these video-based shorelines provide significantly better temporal resolution, which makes video-derived shorelines particularly suitable to study the importance of different, multiple, response timescales [2,32].…”

Modelling cross-shore shoreline change on multiple timescales and their interactions

“…These spectra can be considered before model application and can be used to determine if the SF-MT model is more favorable to use with respect to SF-ST (considering model while there are multiple dominant forcing timescales present. However, it should also be mentioned that in terms of forcing, this site is very unique (other than multiple dominant forcing components): see for example Ondoa et al (2020), in which they reveal that intra-seasonal sea level variations impact the beach profile, which is a process that is not accounted for in the model. Moreover, at Narrabeen, Nha Trang and Tairua, the ∆AIC score is larger than 1 for the calibration phase, indicating a considerable model improvement is acquired when accounting for model skill and complexity.…”

“…A source of shoreline data are shore-based video cameras that generally collect data during daylight on a 30-min basis (Holman & Stanley, 2007). In comparison to shoreline-walking GPS surveys, these video-based shorelines provide significantly better temporal resolution, which makes video-derived shorelines particularly suitable to study the importance of different, multiple, response timescales (Ondoa et al, 2020;Pianca et al, 2015).…”

“…It is an open-ocean, sandy stretch of coast which is situated far from any anthropological influences. The nearest one (20 km updrift) is a field of groynes, constructed in the last five years (Ondoa et al, 2020;Anthony et al, 2019). Grand Popo beach is characterised by sand with a D50 of 0.6 mm.…”

Coastal Zone Changes in West Africa: Challenges and Opportunities for Satellite Earth Observations

“…The West African coast is generally considered a storm-free environment, dominated by North Atlantic distant swells (Sadio et al 2017;Almar et al 2019;Ndour et al 2020) with their interannual fluctuation described by the North Atlantic Oscillation (Almar et al 2019). The wave regime in the Gulf of Guinea area is mainly driven by extratropical storms, and their variability is modulated by the South Atlantic Annular Mode (SAM) (Almar et al 2015;Melet et al 2016;Abessolo et al 2020). The evolution of the sandy coast is controlled by strong longshore sediment drift resulting from oblique waves.…”