Modes of onshore sandbar migration at a single-barred and swell-dominated beach

Abstract

Eroded sediment can return to the subaerial beach through onshore sandbar migration, thus understanding this process is key for rebuilding coastal morphology. While offshore sandbar migrations are fairly predictable based on wave conditions alone, landward migrations are less predictable and more dependent on sandbar morphometrics. In this study, monthly collected topographic and bathymetric data from August 2014 to September 2018 were analyzed and related to the incoming wave conditions to investigate the control of sandbar location and morphometry on the landward migration process. Four distinct onshore sandbar migration modes were identified, primarily governed by the sandbar crest location and size (volume and height), and the preceding cumulative wave energy. A cross-shore distance of 150 m equivalent to ~3 m depth was defined as a boundary for the fate of the sandbar during the landward migration process. Sandbar welding (SW; Mode I) occurred during low-energy conditions after a moderately energetic winter (cum Pt ≤ 50 kWm−1) when the sandbar (of up to 1 m height and ~ 100 m3m−1) was located shoreward from the cross-shore boundary. A large sandbar (> 100 m3m−1; > 1 m height) that formed offshore from the boundary during a highly-energetic winter (cum Pt >50 kWm−1) turned into a terrace-bar (STT; Mode II) in a subsequent very low-energy period. A terrace-bar of ~100 m3m−1 located near the boundary (~ 3 m depth) became a sandbar (TST; Mode III) during moderately energetic conditions at the beginning of the winter. Sandbar splitting (SS; mode IV) occurred during low-energy conditions when a sandbar of ~100 m3m−1 situated offshore from the boundary; the outer section flattened and followed a net offshore migration cycle while the inner section migrated shoreward. This study highlights the need for considering sandbar morphometric properties in addition to the preceding wave conditions to adequately predict the shoreward migration process of the sandbar.