Long-Term Evolution of Morphology at Loc An Estuary, Vung Tau, Vietnam.

In the last decade, the downdrift coast of the Loc An river mouth, Vung Tau province, Vietnam was subject to severe erosion. This was mainly due to the downdrift migration of the river mouth which had reached in total several kilometers over the past 30 years. This study presents the migration of the Loc An river mouth and the related consequences using long-term satellite images and field data. According to wave data analysis, the waves were separated in two different monsoon seasons, northeast and southwest and the characteristics of these waves in the two seasons are noticeably different. This led to contrary dominant directions of the longshore sediment transport between the two monsoon seasons. The river mouth migrated in the southwest direction around 3000 m in the period from 1979 to 2017 (80 m/year). This migration caused the erosion and the collapsing of the geotextile jetties on the right side of the river mouth. The change rate of the surface area of the updrift sand spit is utilized to estimate the longshore sediment rate which equals the order of 2.0 × 105 m3/year. It was noted that a re-occurrence of the breaching would be very difficult in the future due to the strengthening of the back side of updrift sand spit and the operation of the Song Ray reservoir. The migration caused the two jetties to become obsolete, which were constructed to stabilize the river mouth and ensure a smooth passage for the fishing boats. The primary action would be to remove the jetties, however, if this action proves too difficult to be actualized, immediate action like dredging deposited sediment and closing the gap would need to be taken in order for the jetties to become functional again. Additionally, this study illustrates the usefulness of using satellite images to investigate the long-term evolution of the morphology which is still active and where there is still a large amount of change such as in the river mouth area. This is very useful for the research carried out at this specific place or in the country which has limitations in the field and observed data. [ABSTRACT FROM AUTHOR]