3.23 Estuaries
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found underneath the low-salinity water water. Except for the sharp density interface
between the surface and bottom layers, there are little salinity differences in each of
the layers. Examples of salt-wedge estuaries include the world’s mightiest rivers, the
Conga, Amazon, and Mississippi Rivers, but this type of estuary can also develop
in much smaller rivers.
Highly stratified estuaries are classified by a slightly smaller R/V ratio of values >0.1. Here, substantial amounts of saltier water are entrained into the surface
layer outflow, such that both salinity and flow rate of this outflow increase along the
length of the estuary. The bottom layer consists of largely undiluted seawater, but
the loss of saline water to the surface layer implies a continuous bottom inflow of
seawater from the adjacent sea. The Hardanger Fjord in Norway, for example, turns
seasonally into a highly stratified estuary whereby the freshwater source is provided
by summertime melt of glaciers. Constrained by the existence of a sill, however,
the inflow of seawater is often not dense enough to replenish bottom waters of a
fjord. As a consequence of this, bottom waters can become depleted in dissolved
oxygen (anoxic) as dead organic matter decays in the bottom layer. In some fjords,
this situation can last for several years.
A further decrease of the R/V ratio to values >0.01 leads to establishment of
slightly stratified estuaries. The relative influence of tidal mixing is enhanced and,
in contrast to entrainment being a one-way process, this creates mixing between
both layers. Hence, the salinity increases in both layers along the length of the
estuary with a top-to-bottom salinity difference remaining approximately constant.
Examples of slightly stratified estuaries include Chesapeake Bay situated in the
north-western Atlantic Ocean. Maximum river flow in the spring supports a stronger
density interface preventing the fresh surface water and saltier bottom water from
mixing. Owing to severe shortage of dissolved oxygen, major kills of commercially
important bottom-dwelling animals occur during this time.
Positive R/V values <0.001 characterise a situation in which tidal mixing is
strong enough to suppress density stratification most of the time. This regime
characetrises vertically mixed estuaries. Salinity at any point of the estuary is almost
uniform from surface to bottom and salinity increases from head to mouth of the
estuary. The Bay of Fundy is an example of a vertically mixed estuary.
Finally, a negative R/V ratio corresponds to a net water loss of an estuary which
is characteristic of inverse estuaries. Examples of inverse estuaries are Spencer Gulf
and Gulf St. Vincent in South Australia.
3.23.9 Transport Timescales in Estuaries
Owing to high human population density, most estuaries are subject to various
kinds of pollution such as discharges of sewage and other wastewater, or oil spills.
Hydrodynamic models are frequently applied to study the dilution and dispersal of
pollutants in estuaries. Various timescales have been introduced by scientists for this
purpose. These timescales are briefly described in the following.
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found underneath the low-salinity water water. Except for the sharp density interface
between the surface and bottom layers, there are little salinity differences in each of
the layers. Examples of salt-wedge estuaries include the world’s mightiest rivers, the
Conga, Amazon, and Mississippi Rivers, but this type of estuary can also develop
in much smaller rivers.
Highly stratified estuaries are classified by a slightly smaller R/V ratio of values >0.1. Here, substantial amounts of saltier water are entrained into the surface
layer outflow, such that both salinity and flow rate of this outflow increase along the
length of the estuary. The bottom layer consists of largely undiluted seawater, but
the loss of saline water to the surface layer implies a continuous bottom inflow of
seawater from the adjacent sea. The Hardanger Fjord in Norway, for example, turns
seasonally into a highly stratified estuary whereby the freshwater source is provided
by summertime melt of glaciers. Constrained by the existence of a sill, however,
the inflow of seawater is often not dense enough to replenish bottom waters of a
fjord. As a consequence of this, bottom waters can become depleted in dissolved
oxygen (anoxic) as dead organic matter decays in the bottom layer. In some fjords,
this situation can last for several years.
A further decrease of the R/V ratio to values >0.01 leads to establishment of
slightly stratified estuaries. The relative influence of tidal mixing is enhanced and,
in contrast to entrainment being a one-way process, this creates mixing between
both layers. Hence, the salinity increases in both layers along the length of the
estuary with a top-to-bottom salinity difference remaining approximately constant.
Examples of slightly stratified estuaries include Chesapeake Bay situated in the
north-western Atlantic Ocean. Maximum river flow in the spring supports a stronger
density interface preventing the fresh surface water and saltier bottom water from
mixing. Owing to severe shortage of dissolved oxygen, major kills of commercially
important bottom-dwelling animals occur during this time.
Positive R/V values <0.001 characterise a situation in which tidal mixing is
strong enough to suppress density stratification most of the time. This regime
characetrises vertically mixed estuaries. Salinity at any point of the estuary is almost
uniform from surface to bottom and salinity increases from head to mouth of the
estuary. The Bay of Fundy is an example of a vertically mixed estuary.
Finally, a negative R/V ratio corresponds to a net water loss of an estuary which
is characteristic of inverse estuaries. Examples of inverse estuaries are Spencer Gulf
and Gulf St. Vincent in South Australia.
3.23.9 Transport Timescales in Estuaries
Owing to high human population density, most estuaries are subject to various
kinds of pollution such as discharges of sewage and other wastewater, or oil spills.
Hydrodynamic models are frequently applied to study the dilution and dispersal of
pollutants in estuaries. Various timescales have been introduced by scientists for this
purpose. These timescales are briefly described in the following.