Coastal Erosion Caused by River Mouth Migration on a Cuspate Delta: An Example from Thanh Hoa, Vietnam

Abstract

Coastal erosion poses a significant threat to the infrastructure of the coastal community at the mouth of the Ma River in Thanh Hoa Province, Vietnam. In response, emergency solutions such as hard, protective structures are often implemented. However, this approach exacerbates the problem as the underlying mechanisms of coastal erosion are not adequately investigated and understood. In this study, the long-term configuration of the mouth of the Ma River in Thanh Hoa Province, Central Vietnam, is investigated using Landsat imagery spanning from 1987 to 2023. An analytical solution of a one-line model for shoreline change was also used to examine the sand discharge from the Ma River and the diffusion coefficient for the sand transported along the shore by breaking waves. The results showed an asymmetric configuration of the mouth of the Ma River over the past 37 years. The supply of sand from the Ma River is around 350,000 m3/year. The majority of sand (ranging from 55% to 75%) is mainly transported to the northern beach of the Ma River delta. This uneven distribution of sand from the Ma River has led to the asymmetrical morphology of the delta apex in which the northern part of the Ma River delta is experiencing northward movement while the southern part of the Ma River Delta is moving southward and landward. The asymmetrical morphology of the delta at the mouth of the Ma River has recently been identified as the cause of severe coastal erosion. The diffusion coefficient value determined for the transportation of longshore sand along the deltaic lobes of the Ma River delta corresponds to 90 m2/day. This study offers a practical method for investigating morphological changes in cuspate deltas, especially when measured field data are limited.