A Tombolo Alternating Between a Double Tombolo and a Salient on the West Coast of Honghai Bay, Guangdong, China, Driven by Dynamic Fluvial and Coastal Interactions

A small tombolo on the west coast of Guangdong’s Honghai Bay was investigated using over a decade of satellite imagery. Occasionally, this stream forms a lagoon behind the island, giving the appearance of a double tombolo. However, analysis of satellite imagery reveals that the double tombolo was not consistently formed and that the tombolo tip was not always attached to the leeward side of the island. This suggests that the tombolo was in a transitional state between the formation of a tombolo and a salient. The beaches on both sides of the tombolo are headland-bay beaches. Therefore, MEPBAY and XBeach, coupled with grain size analysis, were utilized to investigate the dynamic geomorphological processes of the tombolo. This study shows that the headlands at both ends of the beaches, along with waves approaching perpendicular to the shore, inhibit longshore drift on either side of the tombolo. The sediment sustaining the tombolo originates from the stream sands and offshore sands transported onshore by waves. When wave-driven sediment transport exceeds stream sediment supply, a tombolo forms. Conversely, only a salient develops. This specific case study reveals previously undocumented phenomena, thereby offering valuable insights into the mechanisms of double tombolo formation.