Estuary Processes and Managment - River Dynamics and Integrated River Management - page 490

three flushing coefficients (i.e., Ȗ , k 1 and k 2 ), and the local flushing time is solely dependent on the
characteristics of the tracer mass removal curve. A similar relation is also found between the system-wide
flushing time and its tracer mass removal process (Choi and Lee, 2004). With the advance of numerical
hydrodynamic and mass transport models, the three flushing coefficients may be determined numerically
from results of 3D numerical simulation, and then the flushing time can be determined subsequently. For
estimating the local flushing time, the basic idea is that a mass of hypothetical conservative tracer is
instantaneously introduced into a region of interest. A unit tracer concentration is prescribed initially
inside that region, and the subsequent advective dispersion of this mass is then obtained by solving the
mass transport equation numerically and the time-variation of the tracer mass inside the region is tracked.
For estimating the system-wide flushing time, however, mass is released from everywhere inside the
semi-enclosed water system.
As an example, the study for local flushing time and the system-wide flushing time in Yung Shue Au marine
fish culture zone (FCZ), Three Fathoms Cove in Tolo Harbor is presented (Fig. 8.25). Figure 8.26(a)
shows the tracer concentration distribution computed by a Lagrangian random walk method after 12 hours.
It is found from Fig. 8.26(b) that the changes in tracer mass for both local flushing and system-wide
flushing follow double-exponential curves (Choi and Lee, 2004).
Table 8.2 lists the flushing time and flushing rate of six representative fish culture zones (FCZs) in
Hong Kong for the dry season and the wet season, respectively, including four FCZs in Tolo Harbor:
Yung Shue Au, Lo Fu Wat, Yim Tin Tsai, and Yim Tin Tsai East (Fig. 8.25), and Sok Kwu Wan and Ma
Wan. In general, assisted by the density-driven estuarine circulation, the flushing rate in the wet season is
substantially greater than in the dry season.
Generally speaking, unlike the traditional approach of determining the flushing time using salinity or
freshwater concentration, the numerical approach discussed herein is applicable even for cases when
salt-balance methods cease to apply: negligible freshwater runoff and weak longitudinal salinity gradients,
or when detailed salinity data are lacking (e.g., for new projects).
Fig. 8.25 Location of four marine fish culture zones in Tolo Harbor, Hong Kong: Yung Shue Au, Yim Tin Tsai,
Yim Tin Tsai East and Lo Fu Wat