Study on simulation and improvement measures of water exchange capacity in semi-enclosed lakes: A case study of Longjiang Pond in Haitang Bay

Abstract

[Objective] Water exchange in semi-enclosed lakes mainly relies on river inflows, making it prone to seasonal and tidal water shortages. Under the combined influence of climate change and human activities, the issue of water scarcity in these lakes has become increasingly prominent, leading to complex problems such as water quality deterioration and ecological degradation, urgently requiring effective solutions. [Methods] Longjiang Pond was selected as the study area, and a two-dimensional hydrodynamic-water quality mathematical model of Longjiang Pond was established. Relative pollutant concentration and half time of water exchange were taken as evaluation indicators to compare the water exchange capacity of Longjiang Pond before and after the completion of the water system connectivity project. [Results] The result showed that under current conditions, both relative pollutant concentration and half time of water exchange in Longjiang Pond exhibited an increasing trend from south to north. In the northern lake area, the pollutant concentration decreased the slowest, with the half time of water exchange generally exceeding 2 weeks. In contrast, the southern lake area experienced the fastest decline in pollutant concentration, with the half time of water exchange generally within one week. After the implementation of the water system connectivity project, both the relative pollutant concentration and the half time of water exchange became higher in the western region than in the eastern region. The pollutant concentration in the northern lake area declined more rapidly, especially in the eastern region near Water System Connectivity Project No. 1, where the relative concentration decreased to 0.17 on the second day of simulation. In the southern lake area, the decline in pollutant concentration slowed during the first 10 days, especially in the western area, where the half time of water exchange increased from 6 to 14 days. After 20 days, the relative pollutant concentration decreased to below 0.2. [Conclusion] The findings demonstrate that the water system connectivity project helps accelerate the reduction in relative pollutant concentration and effectively shorten the half time of water exchange in semi-enclosed lakes. However, it weakens the hydrodynamic conditions near the original connecting channels. These findings can provide a practical reference for improving the water environment of similar semi-enclosed lakes.

Keywords

semi-enclosed lake / water exchange capacity / water system connectivity project / numerical simulation / relative pollutant concentration / half time of water exchange