The concept of flushing time (retention or residence
time) is important in the consideration of the impact
of polluting substances because it is one of the
factors determining the exposure period to marine
organisms. In relation to estuaries, flushing time
is defined as the time required to replace the existing
freshwater accumulated in the estuary by the river
discharge. Flushing time is influenced primarily
by freshwater flow from the river. McLusky (1981)
states that the flushing time for the Forth estuary
under mean river flow conditions is 12 days, but
increases to 10 weeks in Summer with reduced river
flow and decreases to 6 days following severe rainfall.
Estuaries with shorter flushing times are better
able to accept effluents because mixing of fresh
and saltwater is greater and dilution of polluting
substances is better (McLusky 1981). Models which
may be used by the EA in predicting flushing time
are usually used to define the retention time of
an estuary, but the boundary conditions of the model
and the position of the estuary/seawater boundary
can make a large difference to the calculated retention
time. Models take into account the fact that water
is moved progressively out of the estuary over time
but may move up and down over shorter distances
with the ebb and flow of the tide. Retention times
can also be calculated for sea lochs and coastal
embayments where the complete turnover of the volume
of water within the waterbody can be estimated.
Flushing times can be influenced by stratification.
The importance of flushing time for non-toxic substances
is that shorter flushing times reduce the risk of
primary or secondary effects occurring. For example,
the risk of deoxygenation of the water column is
reduced if the oxygen depleting substances are better
mixed in the water column and are washed out of
an estuary quickly. Similarly, the risk of the secondary
effects of eutrophication are reduced if the phytoplankton
are rapidly removed from an estuary and do not die
off causing oxygen depletion. For toxic substances,
shorter flushing times result in reduced contamination
of the water column and sediments and reduced exposure
periods for organisms in the waterbody.
When setting consent conditions, it is important
that the longest estimated flushing time is used
(i.e. in an estuary, during minimum river flow and
neap tides) in calculations to maximise the potential
for protecting organisms in a European marine site.
In considering the effect of flushing time within
a European marine site, it is important that the
effect is considered over the entire site, e.g.
including its effect on residence time within offshore/coastal
parts of the site (where relevant) and not considering
flushing time of the estuary alone.