Estuary Processes and Managment - River Dynamics and Integrated River Management - page 487

Time scales are often used to quantify the exchange and transport processes and to characterize the
physical self-purification ability in an aquatic system. Tidal flushing time is a commonly used time scale,
whose use can be traced back to the early 1950s. It is also known as residence time, total exchange time,
turnover time, or detention time, but there is no unique agreed definition or method of determination. Very
often, the flushing time is taken as the time required for a tracer mass to reduce to a certain level (such as
e -1 , 50% or 10%) of the initial mass, so under certain assumptions it could be equivalent to such parameters
as turnover time—the time taken for the mass level to fall to e -1 of the initial level (Prandle, 1984).
Fig. 8.23 Schematic illustration of tidal flushing mechanism for a semi-enclosed bay
In nature, the flushing time refers to the time needed for the entire volume of a specific water body to
be removed through its open boundaries, but more often interest is focused on the removal of the dissolved
substance in the water. For environmental management purposes, since many water quality processes
occur at time scales much larger than that of tidal variation, hydrodynamic and water quality models can
be effectively coupled by lumping all the tidal exchanges for a given system into a flushing rate. The
main attractiveness of such a simplified approach is the immediate practical value of linking pollution
input and ecosystem response in a tractable manner. For example, as a lumped measure of the effectiveness
of hydrodynamic processes in removing any substance from the water body, the flushing time concept
(or its inverse, flushing rate) is extremely important for ecosystem-scale nutrient budget assessment, or
can be used to determine how much of a potentially harmful substance an embayment can tolerate.
A semi-enclosed embayment (e.g., fish farms or tidal inlets, or typhoon shelters) can be seen as a
separate system within a larger water body connected to the outer sea. In general, the flushing time is
governed by tidal exchanges between the system and the outer sea—which is a complicated function of
the freshwater runoff, tidal range, topography and bathymetry, density stratification, and wind. Therefore,
the exact value of flushing time is difficult to measure, but could be estimated from long term freshwater
and salinity data when available, or alternatively be determined by hydrodynamic models.
The tidal flushing of a system can be studied with respect to a hypothetical tracer experiment. For a
semi-enclosed system (Fig. 8.24) with a mean volume, V , and mean net flow, Q e , a mass of tracer is
instantaneously introduced into the system, and then the tracer mass in the system will decrease with
time as tidal advection and dispersion act to remove the tracer through the open boundaries. If the system
of water is initially labeled with a conservative tracer at initial mass, M 0 , then assuming no mass return to