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DEFINITION
OF AN ESTUARY |
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"What
is an estuary? The most
common definition for an estuary is that of Cameron and Pritchard
(1963) that states, "An estuary is a semi-enclosed coastal body
of water which has a free connection with the open sea and within
which sea water is measurably diluted with fresh water derived from
land drainage” .
According
to Ref.1 (DWAF 2004), “A
partially or fully enclosed body of water which is open to the sea
permanently or periodically, and within which the sea water can be
diluted, to an extent that is measurable, with freshwater drained from
land. The upstream boundary of an estuary is the extent of tidal influence”.
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CLASSIFICATION |
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Estuaries
can be classified according to its topography, mouth conditions and
salinity structure. Ratios of flow parameters, stratification, and
circulation provide some measures to define the salinity/hydrodynamic
characteristics of an estuary. |
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SALINE
CHARACTERISTICS OF ESTUARIES |
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Typically
three types of categories describe the saline characteristics of an
estuary:
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Stratified
estuaries
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Stratified
estuaries are salt wedge type estuaries, in which the inflowing fresh
water overrides a saline (dense) tidal inflow. The saline wedge
advances along the bottom until the freshwater flow forces can no
longer be overcome. At this point, the tip of the salt wedge will be
blunt during inflowing tide and tapered during ebb tide. Mixing occurs
at the saltwater/freshwater interface by entrainment, a process caused
by shear forces between the two moving layers. As small amounts of
dense water are mixed in the upper layers, more fluid enters the
estuary near the bottom to compensate for the loss, and more fluid
leaves the estuary in the upper layers to attain equilibrium of
forces. Normally the isohaline is ‘deep’ and the freshwater
surface layer is almost homogeneous. Only during low fresh water
inflows, the maximum salinity gradient will reach the surface. At the
mouth, the salinity structure will differ from the upstream areas,
because of larger tidal velocities, resulting in weaker stratification
and usually a better-mixed water column.
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Partially
mixed estuaries
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These
are estuaries (normally relative shallow) in which tidal energy is
dissipated by bottom friction, resulting in turbulent upward mixing of
salt water and downward mixing of fresh water. As the salinity at the
surface increases, the surface outflow is increasing to maintain fresh
water inflow as well as the upward-mixed saline water, resulting in a
compensating inflow at the bottom. The salinity structure is different
from a stratified estuary because of the efficient mixing of salt and
fresh water. Normally, undiluted fresh water now occurring only near
the head of the estuary.
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Well-mixed
estuaries
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Where
tidal flow is much larger than the fresh water inflow and bottom
friction large enough to mix the entire water column, a vertically
homogeneous density structure exists. If the estuary is wide,
horizontal flow separation can result in a vertically homogeneous, but
laterally non-homogeneous conditions.
Vertically and laterally homogeneous conditions occur in
‘narrow’ estuaries, typically like the Milnerton estuary, in which
salinity increases evenly towards the mouth.
However,
because of continuous changing processes (river and tidal flows), an
estuary cannot be exactly categorized, but will continuously move
through different stages of categories between the mouth and the head
of the estuary. At the head of the estuary where tidal range is
reduced and fresh water inflow dominates, a stratified condition may
exist. Farther downstream as tidal amplitudes increase with subsequent
turbulent mixing, partially mixed conditions occur and closer to the
mouth where the tidal flow becomes larger than fresh water inflow,
vertically homogeneous (well mixed) conditions will occur. These
conditions will vary diurnally and seasonally.
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Useful
links |
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References |
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Ref.1.
RSA DWAF (2004) Operational Policy For The Disposal Of Land-Derived
Water Containing Waste To The Marine Environment Of South Africa.
Department of Water Affairs & Forestry. |
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Ref.2. Cameron
WM, Pritchard DW, 1963, Estuaries,
In: The Sea (Ed. MN Hill), Vol. 2, Wiley, New York, 306-324. |
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Ref.3. Botes, W.A.M. &
Le Roux, M. Milnerton
Estuary: An assessment of the effects for reduced flows from the
Potsdam WWTW. Reference:
PW56/CMC/Milnerton_01.
Submitted
to: CMC/BVI.
July
2004. |
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