Will Taiwan lose its lagoon? Effects of sandbar migration and sediment transport on lagoon siltation in Southwest Taiwan

Cigu lagoon in Southwest Taiwan is susceptible to disappear due to sandbar migration and sedimentation issues over the past few decades. According to the Water Resources Agency in Taiwan, the sandbar has retreated more than 800 meters to the landward from 1975 to 2005. As a result, the Cigu lagoon has been shrinking from 10,000 to 1,350 hectares nowadays. In addition, various river and coastal management decisions in the last century have also contributed to the disruption of the natural sediment balance along the Cigu coast. A compounding consequence of these processes continuously occurs, changing the hydrodynamic characteristics and accelerating the siltation process of the lagoon.A deeper understanding of the erosive and deposition processes at play in the Cigu lagoon is essential to inform current coastal management practices in the area. This study combined remote sensing and a 2-D hydrodynamic model to examine the sediment transport and siltation process in the Cigu lagoon. A remote sensing technique is conducted to complement the initial suspended sediment concentration (SSC) on the model. The results of this study indicate that tidal currents and wave forcings primarily influence the sediment transport and siltation process of the Cigu lagoon. The annual siltation of the Cigu lagoon is 0.82 cm, and this process mainly occurs during the winter season. The strong winter wave induces sediment mixing and suspension in the water column, which causes the SSC and sedimentation of Cigu higher than those in the summer.In this particular case, lagoon siltation was also exacerbated by sediment from the sandbar. The overwash phenomenon swept a large amount of sediment from the sandbar to the lagoon. Moreover, based on our sediment budget analysis, about 0.1 million m3 of sediment in the Cigu sandbar is eroded every year, and some sediment is transported to the lagoon by tidal currents and wave actions. The study implies that many elements have contributed to the siltation of Cigu lagoon, and the whole coastal system management needs to be taken into consideration for managing lagoon siltation.