Tidal flushing time of marine fish culture zones in Hong Kong |
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| Authors: | Jing-qiao Mao Ken T M Wong Joseph H W Lee K W Choi |
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| Institution: | MAO Jing-qiao a,WONG Ken T.M.b,LEE Joseph H.W.c,1 and CHOI K.W.b a State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering,Hohai University,Nanjing 210098,China b Department of Civil Engineering,The University of Hong Kong,Hong Kong,China c Hong Kong University of Science and Technology,Hong Kong,China |
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| Abstract: | 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. |
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| Keywords: | flushing time fish culture zone hydrodynamic tracking carrying capacity tidal exchange physical self-purification |
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