of Department Oceanography SOES2009 Estuaries
Lecture 20
Lecture 20: Physical Processes in Partially Mixed Estuaries By Dr. N.C. Wells
Physical Processes Partially mixed estuaries are those in which the tidal flow generates turbulence which promotes vertical mixing leading to a two-layered structure, but which is not sufficiently vigorous to break down the difference in density between the layers. The layered structure enables differential flow to occur in the upper and lower layers, and thus a secondary (longitudinal) circulation can develop. It is the presence of the secondary circulation, leading to an up-estuary transport of salt partially by advection, which characterises the partially mixed estuary, and defines its location on the Hansen/Rattray Classification Diagram.
R
Becomes saltier downstream
Isohalines are mainly vertical. Salinity patterns move up and down with the tide
Becomes fresher upstream A B
0 Salinity profile at A
0 Salinity profile at B
R
Salinity distribution
Within each layer, turbulent mixing keeps the salinity and velocity structure uniform. At the interface, mixing is suppressed by the density gradient, but if a current shear develops across the layer, shear instability occurs leading to Kelvin-Helmholtz billows. This enables diffusive mixing of salt from the lower to the upper layer, and a mixing down of "freshness".
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Department of Oceanography SOES2009 Estuaries
Lecture 20
Freshwater and the salt mixed up from the layer are advected out of the upper layer by the river flow. Salt penetrates up the lower layer by tidally driven diffusion. But this is not all; salt is also advected up the estuary in the lower layer by a mean upstream flow. Typical flows through the tidal cycle
Flood Lower layer
Time
Ebb
Upper layer
Secondary Circulation Without the upstream advective transport, the lower layer would progressively lose its salt, because the upstream diffusion would not keep pace with the turbulent exchange across the interface. What causes the secondary circulation to develop? The redistribution of salt by diffusive processes, particularly the vertical transport, is sufficient to drive the secondary circulation. In particular, the turbulent energy which lifts salt up from the lower to the upper layer (doing work on it) provides the energy to drive the secondary circulation, analogous to a pump. The forces experienced by the water are pressure forces. In the upper layer, the surface slopes down towards the mouth, and the resulting pressure force toward the mouth ensures that the upper layer is driven towards the mouth, accelerating towards the mouth. In the lower layer, the differential weight of salty water along the estuary causes a reverse pressure gradient, up the estuary, which drives water into the estuary. To preserve continuity, there has to be entrainment from the lower layer into the upper layer, by billow turbulence across the interface. Thus there is vertical advection of salt as well as vertical diffusion:
Pressure Force
No Horizontal Pressure Force Pressure Force
P P
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Department of Oceanography SOES2009 Estuaries
Lecture 20
Without the tidally driven diffusion, the circulation would not be maintained, but it is the advection by the secondary flow which helps to flush out the estuary most effectively. In some estuaries, the flux of the recirculating flow may be ten or twenty times the river flow. e.g.:
R 6R
5R
At the head of the estuary, the tidal flow may be weaker, in which case turbulent mixing is reduced and the lower layer becomes a (diluted) salt wedge, with no downward mixing of fresh water:
R
Salinity distribution
Examples: James River (USA), Southampton Water. Lateral Effects in a Partially mixed estuary. The cross-section of the estuary is normally not uniformly rectangular. In the deep part of the cross-section the stratification is established and the secondary circulation is set up. In the shallow parts, only the upper layer occurs, there is no deeper, saltier layer, no secondary circulation, and so the water is slightly less salty than in mid channel: B A S
Z
The secondary flow occurs in the deep channel not on the shallower banks. The deep channel is very saline.
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Department of Oceanography SOES2009 Estuaries
Lecture 20
Salt Wedge Estuary In the full salt wedge estuary, the seawater penetrates right up the floor of the estuary.
River Flow
Entrainment along interface Salt Wedge
The river water flows out over the top of the sea water which penetrates up the estuary floor because it is denser. As the estuary widens, the river water spreads out laterally and therefore the freshwater layer becomes thinner towards the mouth of the estuary. At the interface, from time to time, shear instability is likely to occur. This results in entrainment of the denser seawater into the upper freshwater layer, which gradually becomes more saline down-estuary. The entrained salty water is rapidly mixed into the upper layer by the turbulence of the flowing river water. The freshwater does not mix down into the lower layer because is stationary and there is no turbulence. The salt wedge therefore occurs where the tidal flow is weak, i.e. the tidal prism small is compared with the river inflow per tidal cycle. The transport process is entirely by advection.The resulting density distribution is:
6 R 9 12 15 18 21 24 isohaline
Salinity distribution
University of Southampton
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Department of Oceanography SOES2009 Estuaries
Lecture 20
The profiles of salinity and water flow near the head (A) and the mouth (B) of the estuary are:
A Salinity Profiles S S A Tidal mean velocity profile 0 0 0 S B B
0
0
0
Where does the Salt wedge fit on the Hansen-Rattray Classification Diagram?
10 No mixing ?S/So 1
Salt wedge ?
1
1.5 Us/Uf
10
Self-stabilisation of the Salt Wedge The salt wedge is a stable structure because (as the river water inflow rate changes) it moves up or down the estuary until it reaches an equilibrium position:
River flow increases ? Velocity shear increases ? ? Richardson Number, Ri, decreases ? Shear instability - entrainment University of Southampton 5
Department of Oceanography SOES2009 Estuaries ? See lecture 5 p.4 for definition ? ? ?
Lecture 20
Entrainment erodes wedge Upper layer deepens Salinity structure moves down estuary
? ? ? ? ? ?
River discharge is now distributed through a thicker layer Velocity reduces Ri increases Interface stabilises Entrainment stops Salinity pattern stabilises in new down estuary position
River Flow decreases ? Ri increases further ? Interface very stable ? Pressure due to dense lower layer forces the salt wedge up the estuary
? ? ? ? ?
Upper layer thins velocity decreases Ri decreases - becomes less stable steady state reached when intermittent entrainment starts to occur
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Department of Oceanography SOES2009 Estuaries Physical Processes in Fjords
Lecture 20
Fjords are long, deep, parallel-sided estuaries formed by glacial action. They often have a shallow region, a sill, at the mouth which is characteristic of glacial formation. To describe an estuary as a fjord is therefore to classify it in terms of geomorphology. However, because of their typical and unique shape, estuaries have a characteristic circulation and stratification pattern and so have a specific location on the Hansen-Rattray classification diagram.
River flow Freshwater layer, gradually getting saltier towards mouth (by entrainment)
Entrainment Weak return flow to feed entrainment
Sill
Outside sea
Deep saline layer Section view Steep sides
Deep, uniform cross section
Plan view
Typical physical characteristics of fjords: ? low tidal range; tidal amplitude is reduced by constricted entrance ? weak tidal currents (a consequence of the great depth of the fjord even if the tidal range is large) ? low turbulence ? highly stratified - the river inflow runs over the stationary deep salty water ? weak entrainment of saline water into the upper layer ? no downward mixing of freshwater into the static lower layer There is a weak circulation produced by the entrainment, giving rise to a weak flow into the fjord in the deep layer. This may be sufficient to maintain reasonable flushing of the estuary. The dominant transport mechanism for salt is advection. However, the shallow depth of the water over the sill can restrict the...