The current study presents the effects of seasonal variations in the depth-of-closure ($D_c$) on shoreline evolution using a numerical, one-line shoreline model. Beach erosion of the southern beach of the Nha Trang Coast, which is located in south central of Vietnam, is selected as the study area. This study area is immensely influenced by the tropical monsoon climate that has a clear pattern of large waves in the northeast monsoon season and calm waves in the non-monsoon season. The analysis of the long-term measured shoreline variations from a video-camera system has found a strong correlation of these variations to the monsoon-dominated wave characteristics in the Nha Trang Bay. Therefore, a new approach for determining the depth-of-closure with consideration of the seasonal wave climate changes is purposed in the current study. By implementing this new approach into a numerical, one-line shoreline model, it is found that the seasonal variations of $D_c$ appear to better describe the periodical shoreline evolution due to the monsoon-dominated wave characteristics for the Nha Trang Coast. Such important findings are considered to commonly apply for monsoon-dominated coastal regions in general. These findings are useful information not only for scientific readers but also for the coastal authorities and managers in order to make better countermeasure plans against this kind of erosion mechanism in the future.

当前的研究提出了封闭深度的季节性变化的影响（$d_C$），使用数值一线海岸线模型进行海岸线演化。位于越南中南部的芽庄海岸南部海滩的海滩侵蚀被选为研究区域。该研究区域受到热带季风气候的极大影响，热带季风气候在东北季风季节具有明显的大浪模式，而在非季风季节具有平静的海浪模式。通过对摄像机系统长期测量的海岸线变化的分析发现，这些变化与芽庄湾以季风为主的波浪特征密切相关。因此，本研究旨在提出一种考虑季节波动的气候变化来确定封闭深度的新方法。通过将这一新方法应用于数字式单线海岸线模型，$d_C$由于芽庄海岸以季风为主的波浪特征，似乎可以更好地描述周期性海岸线的演变。一般认为，这种重要发现通常适用于以季风为主的沿海地区。这些发现不仅对科学读者有用，对于沿海当局和管理者也是有用的信息，以便将来针对这种侵蚀机制制定更好的对策计划。