Video-Based Nearshore Bathymetric Inversion on a Geologically Constrained Mesotidal Beach during Storm Events

Abstract

Although geologically constrained sandy beaches are ubiquitous along wave-exposed coasts, there is still a limited understanding of their morphological response, particularly under storm conditions, which is mainly due to a critical lack of nearshore bathymetry observations. This paper examines the potential to derive bathymetries from video imagery under challenging wave conditions in order to investigate headland control on morphological beach response. For this purpose, a video-based linear depth inversion algorithm is applied to three consecutive weeks of frames collected during daylight hours from a single fixed camera located at La Petite Chambre d’Amour beach (Anglet, SW France). Video-derived bathymetries are compared against in situ topo-bathymetric surveys carried out at the beginning and end of the field experiment in order to assess the performance of the bathymetric estimates. The results show that the rates of accretion/erosion within the surf zone are strongly influenced by the headland, whereas the beach morphological response can be classified into three main regimes depending on the angle of wave incidence θp: (1) under deflection configuration (θp>0°), the alongshore sediment transport was trapped at the updrift side of the headland, promoting sand accretion. (2) Under shadowed configuration (θp<0°), the interruption of the longshore current drove a deficit of sand supply at the downdrift side of the headland, leading to an overall erosion in the surf zone. (3) Under shore-normal configuration (θp=0°), rip channels developed, and up-state beach transition was observed. A comparison between video-derived bathymetries and surveys shows an overall root mean square error (RMSE) around 0.49 to 0.57 m with a bias ranging between −0.36 and −0.29 m. The results show that video-derived bathymetries can provide new insight into the morphological change driven by storm events. The combination of such inferred bathymetry with video-derived surface current data is discussed, showing great potential to address the coupled morphodynamics system under time-varying wave conditions.

Holocene Erosional Processes in a Highly Exposed Intertidal Sandstone Reef Inferred from Remote Sensing Data

“…According to Gallop et al [16], the existence of rock controls, rather than being infrequent, is the rule on sandy beaches on coasts worldwide. Several studies have pointed out the determinant influence of hard rock structures, such as reefs, headlands or underlying outcrops, on the morphodynamics of sandy beaches [16][17][18][19][20]. They act as protectors against coastal erosion, generating strong energy gradients between exposed and protected sectors [18,21].…”

“…A complete understanding of the morphodynamic behavior of rock-controlled coastal systems requires a simultaneous examination of subaerial and subaqueous geomorphic transformations. Topo-bathymetric studies have become frequent, especially in the monitoring of rapid response sedimentary systems, such as beaches [20]. The monitoring of the submerged surfaces presents a greater technical challenge than the monitoring of land surfaces.…”

“…Traditional techniques have used boat-borne echosounders [23,24], but high costs often do not allow for high-recurrence surveys and they are not operational in shallow intertidal areas. The need for more versatile and less expensive alternative techniques has led to the development of bathymetric procedures, such as video monitoring [20,25] or hyperspectral sensing [26,27]. The results of these new techniques have generally proved to be reliable and represent new tools for research.…”