To better understand the factors influencing the thermal structure of tributaries in the Three Gorges Reservoir （TGR）, a well validated three-dimensional hydrodynamic and water temperature model was proposed to simulate the water temperature distribution in the Xiangxi Bay, a representative tributary of TGR. The numerical results show that water temperature stratification seasonally occurred in the Xiangxi Bay, with stable vertical temperature profiles. It is found from the numerical experiments that three key factors are responsible for the formation of water temperature structure： （1） very often, the locations of thermocline are mainly determined by wind speeds, and the higher the wind speed is, the deeper the thermocline is located beneath the water surface, which could be expressed by a fitted exponential function, （2） the thermal structure is affected by static stability of water column, and the thermocline becomes closer to the water surface and its thickness increases with the increase of temperature, （3） due to the effect of the thermal density inflow, the water temperature of the hypolimnion tends to be uniform, however, even under the condition of larger inflow discharge, the influence of the inflow on the epilitnnion and the thermocline is not significant.
Water release operation is crucial for water quality in large reservoirs such as Three Gorges Reservoir(TGR),because it determines the hydrodynamics and hence the self-purification capability.As algal blooms were often observed in some tributary bays of TGR during the release periods,high frequency field observations were carried out in a typical eutrophic tributary bay(Xiangxi River) from February 9 to May 10,2009.In this paper we assess the hydrodynamic behaviour,density stratification,and trophic status in this bay,respectively using a series of observations for flow,physical,chemical and biological parameters.Then,we analyze the effects of reservoir release operations on algal blooms using correlation analysis method.An empirical prognosis is concluded for the likelihood of algal bloom occurrence as a function of daily fluctuation of water level(DFWL) and water temperature.Our results indicate that during the release period,the algal bloom occurrences are closely tied to the DFWL in that if the ratio of DFWL to total water depth ranges from -10×10-4 to 0,the possibility of algal blooms may reach up to 70%,and if the ratio is less than -10×10-4,then that risk can be significantly reduced to less than 10%.This paper finally suggests that a wave-type water release operation should be beneficial in reducing bloom frequencies in the tributary water bodies,which is helpful for TGR water quality management,especially for the water release operation optimization.
Accurate determination of flushing time is crucial for maintaining sustainable production in fish culture zones （FCZs）, as it represents the physical self-purification capability via tidal exchange with clean water in the outer sea. However, owing to the temporal and spatial complexity of the coastal flushing process, existing methods for determining flushing time may not be generally applicable. In this paper, a systematic method for determining the flushing time in FCZs is presented, in which bathymetry, runoff, tidal range and stratification are properly accounted for. We determine the flushing time via numerical tracer experiments, using robust 3D hydrodynamic and mass transport models. For FCZs located in sheltered and land-locked tidal inlets, the system boundary can be naturally defined at the connection with the open sea. For FCZs located in open＇waters, hydrodynamic tracking is first used to assess the extent of tidal excursion and thus delimit the initial boundary between clean water and polluted water. This general method is applied to all designated marine FCZs in Hong Kong for both the dry and wet seasons, including 20 sheltered FCZs （in semi-enclosed waters of Tolo Harbour, Mirs Bay, and Port Shelter） and 6 FCZs in open waters. Our results show that flushing time is the longest in inner Port Shelter （about 40 days in dry season）, and the shortest for the FCZs in open waters （less than one week in dry season）. In addition, the flushing time in dry season is commonly longer than that in wet season： 20%~40% for most well-sheltered FCZs; 2.6-4 times for the others. Our results indicate a positive correlation between the flushing time and distance to open boundary, supporting the view that the flushing time of a FCZ is closely related to its location. This study provides a solid basis for mariculture management such as the determination of carrying capacity of FCZs.
