Tidal Flushing Time of Marine Fish Culture Zones in Hong Kong

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.

FLUSHING TIME OF THE YANGTZE ESTUARY BY DISCHARGE: A MODEL STUDY

Flushing time of the Yangtze estuary by discharge is one of the importantfactors responsible for the transport of pollutants from various sources located along the Yangtzeestuary: Therefore, an objective of the present stud-y, which analysis flushing time in the case ofdifferent discharge is very helpful to evaluate the water environmental of the Yangtze estuary.Using a dissolved conservative material as a tracer in the water, a three-dimensionadvection-diffusion water exchange numerical model was used to study the flushing time by dischargeand the discharge dominated region of the Yangtze estuary. The initial tracer concentration is setto 0.0 in the numerical domain of the Yangtze estuary, and the concentration value is set to 1. 0 onthe inflow boundary. The tracer flux normal to the solid boundary is set to 0. 0. The flushing timeand the out limit of discharge dominated region can be calculated in terms of the tracerconcentration. Estuarine, Coastal and Ocean Model (ECOM) is used as the hydro-dynamic model. Theresult shows that the flushing time is approximately in inverse proportion to the discharge at theupper stream. The out limit is farther from the upper inflow boundary as discharge increases. Theout limit at the north branch is different from that of the south branch because the discharge intothe north branch is much less than that into the south branch. The data is qualitative similar tothe observed data, which show the three-dimensional advection-diffusion equation can be used toestimate the flushing time and the discharge dominated region of the Yangtze estuary.