Analysing decadal-scale crescentic bar dynamics using satellite imagery: A case study at Anmok beach, South Korea

Abstract

Understanding long-term sandbar dynamics can be crucial for informed coastal zone management, but is often hampered by data availability. To increase the number of sandbar observations available from bathymetric surveys, this study proposes and evaluates a method to manually extract the sandbar location using freely available satellite imagery for the case study of Anmok beach in South Korea. Validation of the satellite extracted sandbar locations against 9 in-situ measurements shows good agreement with errors well within the pixel resolution of the satellite imagery (i.e. 30 m for Landsat missions). The applicability of the method is constrained to locations where (1) the cross-shore crescentic length scales are larger than the image resolution, (2) frequent wave breaking and clouds are absent and (3) the water clarity is sufficient to enable the manual extraction of the sandbar crest line. Using the additional sandbar observations from the satellite imagery significantly increases the temporal extent and resolution of the dataset for Anmok beach. This allows the study of sandbar characteristics, dynamics and impacts of human interventions to an extent that would not have been possible without the satellite imagery. Within the study period 1990-2017 it is found that the sandbar maintains a persistent crescentic pattern that is only altered during prolonged and very intense storm conditions. The cumulative alongshore migration of the sandbars is investigated and found to be in the order of hundreds of meters over the 27 years study period. Comparing the sandbar characteristics prior and after the construction of Gangneung port shows that both the amplitudes and wavelengths of the sandbar crescents near the port have decreased after its construction.

Observations on decadal sandbar behaviour along a large‐scale curved shoreline

“…In this way, an approach as described by Athanasiou et al . (2018) was followed. Profiles with maximum horizontal interval ∆ x of 20 m in the middle shoreface part of the profile were included (average of 19.6 measurements per transect).…”

“…The degree of phase coupling between sandbar and shoreline is, according to the beach state model of Wright and Short (1984), dependent on their separation distance. This suggestion was confirmed with field observations (Van de Lageweg et al ., 2013; Athanasiou et al ., 2018; amongst others). An out‐of‐phase coupled sandbar–shoreline was typically explained by the dominant effect of the cross‐shore wave‐breaking location induced by the water depth variability along the sandbar (Coco and Murray, 2007; Castelle et al ., 2010a; Coco et al ., 2020).…”

“…shoreward‐directed shoals face offshore shoreline perturbation) and in‐phase (i.e. shoreward‐directed shoals face onshore shoreline perturbations) on timescales up to years (Ruessink et al ., 2007; Quartel et al ., 2008; Van de Lageweg et al ., 2013; Athanasiou et al ., 2018).…”

Morphodynamics Assessment by Means of Mesoforms and Video-Monitoring in a Dissipative Beach

“…Beach morphodynamics is closely linked with the presence of bars, which can trigger wave breaking and associated sediment transport [5]. They can also influence beach width [6], change the shoreline shape with their pattern [7], interact with drift currents [8], or induce rip currents caused by normal waves resulting in rip channels over the bar [9]. Furthermore, as wave energy-driven features, they may vary their position with respect to shoreline according to wave energy conditions; after storms, they tend to be located far away from the coast (outer bars) and they move shoreward under low energy conditions (inner bars) [6].…”

Dynamicity of multi-channel rip currents induced by rhythmic sandbars

“…Alternate straightforward study of rip currents is the observation by either optical images or field equipment. Satellite images are convenient source to directly identify rip currents as dissected narrow dark water between sandbars or discontinuous white-cap breaking waves for a broad range of coastlines, but with limited acquisition times (Lippmann and Holman, 1989;Athanasiou et al, 2018). Aerial or fixed optical cameras are able to capture rip currents through consecutive observations covering long enough time and large scope of view (Stockdon and Holman, 2000;Yoon et al, 2014;Liu and Wu, 2019).…”