Stratification occurs as a result of a density
differential between two water layers and can arise
as a result of the differences in salinity, temperature
or a combination of both. Stratification is more
likely when the mixing forces of wind and wave action
are minimal and this occurs more often in Summer.
In estuaries, the prime reason for stratification
is restricted mixing of freshwater river flows and
saltwater tidal incursions. In summer, the effects
of temperature can reinforce the salinity stratification.
In some estuaries, this can divide the water into
two distinct layers which do not mix and are kept
separate by a sharp change in density.
In stratified estuaries, the lower, saline water
is more rapidly replaced, since less dense freshwater
inputs (at lower volumes during summer) do not mix
but float above the pycnocline. The saline water
is more strongly mixed with seawater by tidal flow
and so most exchange with the open coast occurs
through the bottom layer. In such estuaries, discharge
plumes usually rise to the surface and dilution/dispersion
occurs primarily in the surface waters which, during
stratification, have an increased retention time.
This provides an argument for using summer 'upper
retention times to assess environmental impact,
rather than whole estuary or annual mean retention
In sea lochs with sub-surface sills, the surface
layer of water is replaced at a rapid rate by tidal
action, while the lower water remains essentially
stagnant, physically constrained behind the sills,
often with a density barrier (pycnocline) forming
between the two layers.
In coastal waters around the UK, stratification
is a very transient phenomenon but can occur as
a result of high temperatures in periods of calm
weather or as a result of very high freshwater flow
following severe rainfall.
It is important to know the extent to which stratification
occurs in European marine sites comprising coastal
waters so that due consideration can be made in
the calculation of discharge consent conditions.