Modelling of embayed beach equilibrium planform and rotation signal

Bruno Castelle (1) , Arthur Robinet (2) , Déborah Idier (2) , Maurizio d'Anna (2)

1 EPOC - Environnements et Paléoenvironnements OCéaniques
2 BRGM - Bureau de Recherches Géologiques et Minières (BRGM)

9 Embayed beaches are highly attractive sandy beaches bounded laterally by rigid boundaries, which 10 deeply affect equilibrium beach planform and shoreline dynamics. We use LX-Shore, a state-of-the-art 11 shoreline change model coupled with a spectral wave model to address embayed beach shoreline 12 dynamics driven by longshore sediment transport processes. The model is applied to different 13 idealized embayed beach configurations including variations in headland lengths. The model simulates 14 a large range of equilibrium embayed beach planforms and associated spatial and temporal modes of 15 shoreline variability. For short headlands enabling occasional headland sand bypassing, both embayed 16 beach curvature and maximum erosion at the upwave side of the embayment increases with increasing 17 headland length. Beach curvature also increases with increasing headland length for headlands long 18 enough to prevent any headland sand bypassing. In contrast, at the same time, embayed beach 19 becomes increasingly curved and symmetric, with maximum localised erosion within the embayment 20 decreasing in intensity. When there is no headland sand bypassing, rotation signal decreases in 21 amplitude and becomes increasingly symmetric with increasing headland length. The modal (time-22 invariant) directional spreading of incident waves is critical to embayed beach behaviour, with the 23 envelope and variance of cross-shore shoreline change and time-averaged shoreline curvature all 24 increasing with decreasing modal directional spreading. Embayed beach rotation characteristic 25 timescale increases with increasing embayed beach length, while the narrower the embayment the 26 smaller the cross-shore amplitude of shoreline variability. Our simulations provide new insight into the 27 influence of embayment characteristics and incident wave conditions on equilibrium planform and 28 shoreline dynamics of embayed beaches. This work also implies that the degree of potential headland 29 sand bypassing should be taken into account for modelling of beach rotational dynamics and embayed 30 beach dynamic planform configuration. 31 Highlights 32 • Embayed beach shoreline response is simulated with a hybrid shoreline model 33 • Headland length and headland sediment bypassing control shoreline response 34 • Wave directional spreading is critical to both mean shoreline and rotation signal 35 • Embayment beach length controls rotation characteristic timescale 36 37 Keywords: embayed beach ; hybrid shoreline model ; headland length ; rotation ; equilibrium 38 beach planform; headland sand bypassing 39 40

Domaines: Océanographie

Bruno Castelle, Arthur Robinet, Déborah Idier, Maurizio d'Anna. Modelling of embayed beach equilibrium planform and rotation signal. Geomorphology, 2020, 369, pp.107367. ⟨10.1016/j.geomorph.2020.107367⟩. ⟨hal-02982620⟩