Comparative study of water exchange capacity evaluation methods for semi-enclosed water based on the outfitting zone of immersed tube tunnels

The strength of water exchange capacity serves as a proxy for the self-purification capacity of water bodies, thereby playing a pivotal role in enhancing the overall water environment. This study focuses on the secondary outfitting area of the immersed tube tunnel within the Shenzhen–Zhongshan Link, utilizing either the substances transport model or the particle tracking model. This framework is designed to delve into the similarities, differences, and appropriateness of the Eulerian and Lagrangian methods in appraising the water exchange capacity of semi-closed water areas. The following key insights have been derived: (1) The spatial distribution patterns of water exchange capacity computed using both methods align, indicating a congruency in their fundamental principles. However, in our case study, the average water transit time determined by the Lagrangian method is 1.6 times higher than the average water residence time calculated by the Eulerian method, suggesting a more conservative approach by the Lagrangian method. (2) The results obtained from varying particle release times or initial concentration release times within a tidal cycle in the case exhibit notable disparities, highlighting the sensitivity of the system to these factors. (3) The Lagrangian method exhibits a significant 12% difference in average water transit time, compared to the mere 0.9% difference observed in the average water residence time calculated by the Eulerian method. This underscores the Lagrangian method’s heightened sensitivity to the timing of particle release. Conversely, the Eulerian method offers more stable simulation outcomes in assessing the water exchange capacity of semi-closed water bodies, making it a preferred choice for such studies. However, if a comprehensive understanding of water exchange capacity, material transport trajectories, and final destinations is sought, the Lagrangian method would be the more appropriate approach. The research outcomes presented in this paper can serve as a valuable reference for selecting appropriate evaluation techniques for water exchange capacity in semi-closed water areas, encompassing not just immersed tube outfitting zones but also lagoons and harbor basins.