Estuarine water circulation
Encyclopedia
Estuarine water circulation is controlled by the inflow of rivers, the tides, rainfall and evaporation
, the wind
, and other oceanic events such as an upwelling
, an eddy
, and storms. Water circulation patterns are influenced by vertical mixing and stratification
, and can affect residence time and exposure time.
is a key variable determining the health of an estuary
, particularly from human-induced stresses. Rapid flushing ensures that there is insufficient time for sediment
accumulation or dissolved oxygen depletion in the estuary; thus a well flushed estuary is intrinsically more robust than a poorly flushed estuary. Residence time also affects other parameters such as heavy metals
, dissolved
nutrients, suspended particulate matter, and algal blooms that may affect the health of estuaries.
A simple way to calculate residence time is using a simple classic estuary model, which can be useful for acquiring a conceptual understanding of an estuary, but is coarse in time and space. A classic estuary has the following components: 1) freshwater
inflow with a discharge Qf and a salinity
Sf (generally Sf = 0); 2) oceanic inflow with a discharge Qin and salinity So; and 3) outflow to the ocean with a discharge of Qout and a salinity S1. The inflow and outflow of water is equal because mass is conserved. Salt
is also a conserved, therefore, the inflow and outflow fluxes of salinity are also equal. If groundwater
inputs and evaporation are ignored, the continuity equation is:
The residence time T is the volume of water within the estuary (Vol) divided by the flow rate of the river:
may re-enter the system during a flood tide
. The amount of time a water particle spends in the estuary until it never returns back is called exposure time. The exposure time can be much larger than the residence time if the water particles are leaving with the ebb tides and returning with the rising tide. The ratio between the number of water particles returning to the estuary and the number of water particles leaving is known as the return coefficient, r.
In order to quantify exposure time, the water circulation outside of the estuary must be determined. However, the circulation near the mouth of the estuary is complex due to the tidal mixing processes that occur between the estuarine and ocean waters. If the coast is rugged with headlands, a mosaic of complex flow fields consisting of eddies, jets, and stagnation zones will occur, further complicating the circulation patterns outside of the estuary.
In cases involving delta
s or wetlands that drain into multiple tidal creeks, such as Missionary Bay, Australia
, water leaving one creek at the ebb tide may enter another estuary during the flood tide. When there is a series of estuaries involved, a large exposure time (larger than that of the individual estuaries), will occur if the tidal outflow from one estuary re-enters a different estuary during the flood tide. Along a rugged coastline with headlands, however, mixing of estuary and oceanic waters can be intense. When estuarine water leaves the estuary, it gets flushed out to coastal waters, so exposure time and residence time are close equal.
In some cases it is possible to measure the fluxes of volume, salt, and temperature across the mouth of an estuary through a tidal cycle. Using this data, (1-r) can be calculated (r is the return coefficient). (1-r) is equal to the fraction of the volume of water VTP (mean tidal prism volume) leaving the estuary during the ebb tide that is replaced with coastal waters prior to re-entering the system. When r = 1, the same water is re-entering the estuary, and if r = 0, the estuarine water that has left the estuary during the ebb tide has been replaced with coastal waters entering the estuary during the rising tide. The exposure time τ is estimated by:
Vestuary is defined as the mean estuarine volume and Ttide is the tidal period.
The total fluxes of brackish water
through the river mouth during tidal events is often much higher (often by a factor of 10 to 100) than the volume flux from riverine inflow. Therefore, if measurements are not precise, the estimate of the net flux will be unreliable. Direct measurements of the return coefficient are often complicated by unsteady oceanic events such an upwelling, the passage of an eddy, or storms, so the success of a correct direct measurement of the return coefficient is rare.
Vertical mixing determines how much the salinity
and temperature
will change from the top to the bottom, profoundly affecting water circulation. Vertical mixing occurs at three levels: from the surface downward by wind forces, the bottom upward by boundary generated turbulence (estuarine and oceanic boundary mixing), and internally by turbulent mixing caused by the water currents which are driven by the tides, wind, and river inflow.
Different types of estuarine circulation result from vertical mixing:
and the bottom layer of saline water
. River water dominates in this system, and tidal effects have a small role in the circulation patterns. The freshwater floats on top of the seawater and gradually thins as it moves seaward. The denser seawater moves along the bottom up the estuary forming a wedge shaped layer and becoming thinner as it moves landward. As a velocity
difference develops between the two layers, shear forces generate internal waves at the interface, mixing the seawater upward with the freshwater. An example is the Mississippi
estuary.
in both directions across the density boundary. Therefore, the interface separating the upper and lower water masses is replaced with a water column with a gradual increase in salinity from surface to bottom. A two layered flow still exists however, with the maximum salinity gradient at mid depth. Partially stratified estuaries are typically shallow and wide, with a greater width to depth ratio than salt wedge estuaries. An example is the Thames.
Evaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
, the wind
Wind
Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...
, and other oceanic events such as an upwelling
Upwelling
Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. The increased availability in upwelling regions results in high levels of primary...
, an eddy
Eddy (fluid dynamics)
In fluid dynamics, an eddy is the swirling of a fluid and the reverse current created when the fluid flows past an obstacle. The moving fluid creates a space devoid of downstream-flowing fluid on the downstream side of the object...
, and storms. Water circulation patterns are influenced by vertical mixing and stratification
Stratification (water)
Water stratification occurs when water masses with different properties - salinity , oxygenation , density , temperature - form layers that act as barriers to water mixing...
, and can affect residence time and exposure time.
Residence time
The residence time of waterWater
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
is a key variable determining the health of an estuary
Estuary
An estuary is a partly enclosed coastal body of water with one or more rivers or streams flowing into it, and with a free connection to the open sea....
, particularly from human-induced stresses. Rapid flushing ensures that there is insufficient time for sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
accumulation or dissolved oxygen depletion in the estuary; thus a well flushed estuary is intrinsically more robust than a poorly flushed estuary. Residence time also affects other parameters such as heavy metals
Heavy metals
A heavy metal is a member of a loosely-defined subset of elements that exhibit metallic properties. It mainly includes the transition metals, some metalloids, lanthanides, and actinides. Many different definitions have been proposed—some based on density, some on atomic number or atomic weight,...
, dissolved
Dissolution (chemistry)
Dissolution is the process by which a solid, liquid or gas forms a solution in a solvent. In solids this can be explained as the breakdown of the crystal lattice into individual ions, atoms or molecules and their transport into the solvent. For liquids and gases, the molecules must be compatible...
nutrients, suspended particulate matter, and algal blooms that may affect the health of estuaries.
A simple way to calculate residence time is using a simple classic estuary model, which can be useful for acquiring a conceptual understanding of an estuary, but is coarse in time and space. A classic estuary has the following components: 1) freshwater
Freshwater
Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...
inflow with a discharge Qf and a salinity
Salinity
Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...
Sf (generally Sf = 0); 2) oceanic inflow with a discharge Qin and salinity So; and 3) outflow to the ocean with a discharge of Qout and a salinity S1. The inflow and outflow of water is equal because mass is conserved. Salt
Salt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...
is also a conserved, therefore, the inflow and outflow fluxes of salinity are also equal. If groundwater
Groundwater
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...
inputs and evaporation are ignored, the continuity equation is:
- Qin So + Qf Sf = Qout S1
The residence time T is the volume of water within the estuary (Vol) divided by the flow rate of the river:
- T = (Vol/ Qf )(1 - S1/ So - Sf / So)
Exposure time
Residence time considers the time it takes for the water particles to leave the estuary, however, some water particles that leave the estuary during an ebb tideEbb Tide
"Ebb Tide" is a popular song, written in 1953 by the lyricist Carl Sigman and composer Robert Maxwell.-Recordings:* The best-known versions are by Frank Chacksfield & His Orchestra , Vic Damone , Roy Hamilton , Frank Sinatra , The Platters , Lenny Welch and the Righteous Brothers...
may re-enter the system during a flood tide
Flood Tide
Flood Tide is an adventure novel by Clive Cussler. This is the 14th book featuring the author’s primary protagonist, Dirk Pitt. He must rescue illegal immigrants from a Chinese tycoon and locate the bones of the Peking Man, the famous lost example of Homo erectus...
. The amount of time a water particle spends in the estuary until it never returns back is called exposure time. The exposure time can be much larger than the residence time if the water particles are leaving with the ebb tides and returning with the rising tide. The ratio between the number of water particles returning to the estuary and the number of water particles leaving is known as the return coefficient, r.
In order to quantify exposure time, the water circulation outside of the estuary must be determined. However, the circulation near the mouth of the estuary is complex due to the tidal mixing processes that occur between the estuarine and ocean waters. If the coast is rugged with headlands, a mosaic of complex flow fields consisting of eddies, jets, and stagnation zones will occur, further complicating the circulation patterns outside of the estuary.
In cases involving delta
River delta
A delta is a landform that is formed at the mouth of a river where that river flows into an ocean, sea, estuary, lake, reservoir, flat arid area, or another river. Deltas are formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river...
s or wetlands that drain into multiple tidal creeks, such as Missionary Bay, Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
, water leaving one creek at the ebb tide may enter another estuary during the flood tide. When there is a series of estuaries involved, a large exposure time (larger than that of the individual estuaries), will occur if the tidal outflow from one estuary re-enters a different estuary during the flood tide. Along a rugged coastline with headlands, however, mixing of estuary and oceanic waters can be intense. When estuarine water leaves the estuary, it gets flushed out to coastal waters, so exposure time and residence time are close equal.
In some cases it is possible to measure the fluxes of volume, salt, and temperature across the mouth of an estuary through a tidal cycle. Using this data, (1-r) can be calculated (r is the return coefficient). (1-r) is equal to the fraction of the volume of water VTP (mean tidal prism volume) leaving the estuary during the ebb tide that is replaced with coastal waters prior to re-entering the system. When r = 1, the same water is re-entering the estuary, and if r = 0, the estuarine water that has left the estuary during the ebb tide has been replaced with coastal waters entering the estuary during the rising tide. The exposure time τ is estimated by:
- τ = Vestuary Ttide / (1-r) VTP
Vestuary is defined as the mean estuarine volume and Ttide is the tidal period.
The total fluxes of brackish water
Brackish water
Brackish water is water that has more salinity than fresh water, but not as much as seawater. It may result from mixing of seawater with fresh water, as in estuaries, or it may occur in brackish fossil aquifers. The word comes from the Middle Dutch root "brak," meaning "salty"...
through the river mouth during tidal events is often much higher (often by a factor of 10 to 100) than the volume flux from riverine inflow. Therefore, if measurements are not precise, the estimate of the net flux will be unreliable. Direct measurements of the return coefficient are often complicated by unsteady oceanic events such an upwelling, the passage of an eddy, or storms, so the success of a correct direct measurement of the return coefficient is rare.
Vertical mixing and stratification
The residence time of water in an estuary is dependent on the circulation within the estuary that is driven by density differences due to changes in salinity and temperature. Less dense freshwater floats over saline water and warmer water floats above colder water (temperatures greater than 4°C). As a result, near-surface and near-bottom waters can have different trajectories, resulting in different residence times.Vertical mixing determines how much the salinity
Salinity
Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...
and temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
will change from the top to the bottom, profoundly affecting water circulation. Vertical mixing occurs at three levels: from the surface downward by wind forces, the bottom upward by boundary generated turbulence (estuarine and oceanic boundary mixing), and internally by turbulent mixing caused by the water currents which are driven by the tides, wind, and river inflow.
Different types of estuarine circulation result from vertical mixing:
Salt wedge estuary
These estuaries are characterized by a sharp density interface between the upper layer of freshwaterFreshwater
Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...
and the bottom layer of saline water
Saline water
Saline water is a general term for water that contains a significant concentration of dissolved salts . The concentration is usually expressed in parts per million of salt....
. River water dominates in this system, and tidal effects have a small role in the circulation patterns. The freshwater floats on top of the seawater and gradually thins as it moves seaward. The denser seawater moves along the bottom up the estuary forming a wedge shaped layer and becoming thinner as it moves landward. As a velocity
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...
difference develops between the two layers, shear forces generate internal waves at the interface, mixing the seawater upward with the freshwater. An example is the Mississippi
Mississippi
Mississippi is a U.S. state located in the Southern United States. Jackson is the state capital and largest city. The name of the state derives from the Mississippi River, which flows along its western boundary, whose name comes from the Ojibwe word misi-ziibi...
estuary.
Partially stratified estuary
As tidal forcing increases, the control of river flow on the pattern of circulation in the estuary becomes less dominating. Turbulent mixing induced by the current creates a moderately stratified condition. Turbulent eddies mix the water column, creating a mass transfer of freshwater and seawaterSeawater
Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% . This means that every kilogram of seawater has approximately of dissolved salts . The average density of seawater at the ocean surface is 1.025 g/ml...
in both directions across the density boundary. Therefore, the interface separating the upper and lower water masses is replaced with a water column with a gradual increase in salinity from surface to bottom. A two layered flow still exists however, with the maximum salinity gradient at mid depth. Partially stratified estuaries are typically shallow and wide, with a greater width to depth ratio than salt wedge estuaries. An example is the Thames.