Water temperature in estuaries is sensitive to thermal discharges and expansive tidal flats; as such, this parameter is essential in maintaining estuarine ecosystem. Semi-enclosed water bodies with poor water exchange easily accumulate heat. This is especially true for Xiangshan Bay, which contains two power plants and a large area of tidal flats. This bay was used as an example to study water temperature and heat dynamics, considering the thermal discharge and tidal flats. This study developed and validated a three-dimensional hydro-heat flux numerical model using field data on tidal elevation, currents, water temperature, and tidal flat temperature. The Finite Volume Community Ocean Model combines an accurate thermal discharge model with the air-water-tidal flat heat flux model. The findings showed that thermal discharge provides heat to the bay in summer and winter, and increases the water temperature of the entire bay by 0.7°C in summer, while maintaining water temperature at 0.52°C in winter. The atmosphere and open seas had greater impacts on heat in the bay in winter and summer. The atmosphere and tidal flats provided heat to the bay in summer and absorbed heat from the bay during winter; the opposite was true for the open sea. The effect of tidal flats in summer is less than that in winter, and provides 1.31 × 1013 J of heat to the bay in summer, while taking 8.63 × 1013 J of heat from the bay in winter. This study provides a comprehensive understanding of the effects of tidal flats and thermal discharge on water temperature and heat in macro-tidal bays and estuaries; its results are applicable to similar bays around the world.
Tidal flats provide a foundation for biological diversity and marine economy. Xiangshan Bay is a semi-enclosed bay that shelters large areas of tidal flats, and is known for its aquaculture. In this study, field trips were conducted in late autumn to measure the water level, current, water temperature, tidal flat temperature, and turbidity data of the tidal flat in the bay during Typhoon Lingling. The field data were well calibrated and used to investigate the hydrodynamics, temperature, and turbidity of the tidal flat. The results showed that the spring-neap tidal cycles at the sea surface level were well captured at both stations. The maximum tidal range was 5.5 m and 1.5 m during spring and neap tides, respectively. The tidal flat was occasionally exposed to air occasionally (30 min). The current velocity (<0.2 m/s) and waves (<0.15 m) at the field stations were weak, and the direction of flow was controlled by the geomorphology, even during Typhoon Lingling. Water was more turbid at station S2 (<0.8 kg/m3) than at station S1 (<0.2 kg/m3). The sea water temperature and tidal flat temperature were affected by tidal cycles, with larger variations occurring during spring tides than during neap tides. The maximum value of seawater temperature at S1 station was greater than that at station S2 during spring tides. The intrinsic mode functions (IMFs) of sea water temperature and surface tidal flat temperature were similar, as they are both subject to sea-air-tidal flat interactions. The IMFs of the middle and bottom layers in the tidal flat were less correlated. Temperature fluctuations in seawater and tidal flats were mainly affected by air temperature and tides. Small-scale features (>0.5 Hz) were important for water and tidal flat temperatures, particularly during typhoons. These findings provide field data for future studies on eco-hydrology and coastal engineering in tidal flats.
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