Environmental Engineering Reference
In-Depth Information
Table 1.4
Comparative study of tide- and wave-dominated estuaries
Point
Tide-dominated estuary
Wave-dominated estuary
Catchment
'
s
input
Freshwater entering from the catchments is relatively lower. In negative
estuaries, the net inflow of marine water exceeds the outflow of catchment-
derived (fresh) water. In such cases, the hypersaline water is usually
exported to the ocean
Freshwater enters from the catchments. Although the volume of freshwater
input varies spatially and temporally (depending on local catchments and
climate conditions), it is often relatively high in positive estuaries
Freshwater
input
The volume of freshwater entering the estuary is too low to cause signi
cant
level of strati
cation. High tidal ranges may tend to accelerate mixing of any
freshwater inputs and marine water
Water circulation in wave-dominated estuaries generally ranges from well
mixed to salinity strati
ed, depending on the degree of wave mixing,
volume of freshwater input and climate.
wave-dominated estuaries
have lower salinity water towards their head. The volume of freshwater
causes strati
cation (or layering) in the water column, which varies with
seasonal flow. Buoyant low-salinity fresh water floats above the denser,
high-salinity ocean water
'
Positive
'
Salinity
High rates of evaporation cause increases in salinity within the estuary. The
resulting high-density hypersaline water sinks beneath the buoyant marine
water which penetrates through the estuary mouth and
A
or intrusion of denser saline marine water penetrates through
the entrance along the bed of the estuary. Some mixing occurs at the
interface between the freshwater and marine water. The distance that the
salt-wedge penetrates is dependant on tidal range and the amount of
'
salt-wedge
'
ows out of the
estuarine entrance into the coastal ocean through a process known as reverse
stratication. A large degree of mixing occurs between the two layers
fl
fl
uvial
fl
ow received by the estuary. During high
fl
uvial
fl
ow events (which may be
seasonal), fresh
oodwater may push the salt water beyond the mouth.
However, the large volume of central basins typical of wave-dominated
estuaries tends to reduce this effect
fl
Evaporation
Evaporation is the dominant process in negative tide-dominated estuaries
due to arid climatic conditions and the extensive area of shallow intertidal
environments. Aridity and the degree of evaporation may vary seasonally;
however, by de
nition, evaporation in
'
negative
'
estuaries is much larger
than freshwater input. Consequently, negative estuaries tend to have longer
residence times than positive estuaries
While signi
cant evaporation can occur in wave-dominated estuaries
characterized by positive circulation, evaporation (by de
nition) does not
exceed the amount of freshwater input
Water
exchange
Exchange of sea water and estuarine water occurs through the wide entrance
of the estuary. Flood and ebb tides may follow different routes into and out
of the estuary, and the tidal prism tends to be large. In negative estuaries, the
net in
Exchange of ocean water and estuarine water occurs through the entrance of
the estuary, although the magnitude of exchange depends on the size and
length of the entrance channel. In positive wave-dominated estuaries, the
out
ow of freshwater derived from
the catchments. In such cases, the hypersaline water is usually exported to
the ocean
fl
ow of marine water exceeds the out
fl
ow of marine water. During dry
conditions, the entrance of the estuary may be intermittently closed
fl
ow of freshwater exceeds the in
fl
