01/09/2014Dr. Joanna Waniek, IOW1 Definition and basic characteristics Secondary Circulation Partially Mixed Estuaries

11/04/23 Dr. Joanna Waniek, IOW 1

Definition and basic characteristicsSecondary Circulation

Partially Mixed Estuaries



– The salinity driven stratification is sufficiently intense to limit the penetration of bottom generated turbulence into the body of flow

– The halocline moves slightly over a tidal cycle

– Horizontal density gradient exists because of the steady increase in salinity towards mouth.

– River Flow low compared with tidal prism (difference in volume between HT & LT)


•Salinity of surface water considerably raised.

•River volume R is small compared to tidal volume V

•Max R=50m3/s, Max Tidal flow=7500m3/s

•Tidal Prism= 108m3

•Tidal energy dissipated by bottom friction and “interfacial” friction

•Turbulent Eddies work against gravity and move salt upward and fresh water downward



•To discharge a given volume of FRESH water outflow has to be enhanced

•To maintain continuity of volume need increased flow of SALINE water

•There is some increase in surface velocity and upper layer transport towards the sea

•This type of estuary is widespread in temperate and subtropical climates; many examples are found around the world. E.g. Southampton Water UK, James River USA,


Surface and bottom salinity increase from station 1 to station 4, but surface salinity is always lower than bottom salinity.

Top: Salinity as a function of depth and distance along the estuary, numbers indicate station locations, mixing between upper and lower layer is indicated by the faint arrows.

Bottom: in vertical salinity profiles for stations 1-4.

Salinity in a partially mixed estuary.

Head Mouth


(b) Salinity-depth profile along the dashed vertical line in (a) showing the poorly defined halocline. (c) Salinity difference between surface and bottom values along the estuary from river to sea. (d) Velocity-depth profile showing marked upstream residual flow of seawater at the bed.

Longitudinal section of water circulation and salinity gradient. The dashed sub-horizontal line is the depth at which is no horizontal residual flow either seawards or landwards.



Mersey River, UK

•The river widens near the city of Liverpool but connects with the Irish Sea through a narrow passage of 8 km length and 1 - 2 km width known as the Mersey Narrows.

•The Irish Sea has significant tides, and the tidal range in the 20 m deep Mersey Narrows regularly exceeds 5 m and reaches 9.4 m at spring tide.

•The average river discharge rate of 25 m3 s-1 is small compared to the tidal volume of more than 200 m3 s-1. As a result the estuarine water body moves in and out with the tide over several km, and there is no overlap between the salinity range observed at high tide and the salinity range at low tide. Isohalines are slanted from surface to bottom, as is typical for a partially

mixed estuary.

Salinity sections along the Mersey Narrows. Adapted from Hughes (1958).

High tide Low tide


Mersey Estuary

The situation: 20 rivers drain into the Mersey over an area of 2,000 square miles, encompassing densely populated parts of Greater Manchester and Merseyside. In the past Victorian sewers emptied directly into the rivers. Recently extensive industrial development along the Mersey’s banks and busy docklands contributed another cocktail of pollutants. The clean-up started in the 1970s.


Mersey Estuary- Environmental implications

• The length of the River Mersey is 110 km, of which the Estuary is 26 km long;

• The total area drained by the Mersey and its tributaries is 4,680 km2;

• Over 5 million people live and work within the catchment;

• The name ‘Mersey’ originates from Old English ‘Maere’ meaning boundary; • The Mersey was the ancient boundary between the Saxon kingdoms of Mercia and Northumbria; • The River Mersey forms at the confluence of the River Tame with the River Goyt in Stockport.



• The Mersey Estuary continues through the ‘Narrows’ a straight narrow channel with depths of up to 30 m driven by a change in geology. • It forms the Outer Estuary, a large area of inter-tidal sand and mud banks as it flows into Liverpool Bay on the Irish Sea. • The Mersey is a tidal river with the second highest tidal range in the UK of about 10 m. These strong tides have created deep channels and sandbanks throughout the Mersey Estuary, which can make navigation difficult.

PAST• Low oxygen• High ammonia• DDT & PCBs

PRESENT poor

good



•A target to achieve 10% dissolved oxygen levels along the length of the estuary was set in 1976. •Low oxygen levels were still being recorded during the 1990s. •In 2002 oxygen levels could support fish along the entire length of the estuary•In 2002 sea trout and 26 salmon were seen - exceeding all expectations.


Tidal Flow in Partially Mixed Estuary


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•

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Cadland buoy: mooring site

Dockhead ABP’s tidal gauge

SOC/Metoffice meteorological station

Calshot tidal gauge

- Mooring at Cadland buoy - operated over a total of 280 days- depth ranged from 7.5 to 12.5 m

- 1200 kHz ADCP- 2 Conductivity-Temperature loggers

11/04/23 Dr. Joanna Waniek, IOW 15Ribeiro et al., 2004

Transport of sediment/pollutants with ± 5 cms-1 in and out of the estuary

SpringNeap

IN

OUT



Estuarine circulation is a 2-layer circulation driven by the longitudinal density gradient and resulting in a bi-directional net transport.

estuarine circulation established by a longitudinal density gradient

turbulent mixing shuts down estuarine circulation and stratification

Linden & Simpson (1986) lab experiment:

air bubbles as source of turbulent mixingswitched on and off periodically.

Does the spring-neap fluctuation in the tidal mixing modulate the estuarine circulation and consequently the net transport in the estuary?


Transverse Circulation e.g. Southampton Water


Summary

• Partially mixed estuaries have a strong longitudinal circulation which flushes estuary quickly

• Mersey Estuary (30 years ago one of most polluted estuaries in UK, today habitat of salmon), Rotterdam Waterway, James River

• Transverse circulation can be important e.g. Southampton Water

• In estuaries where tidal flows are significant, the tidal motion of water in an estuary will generate turbulence on the bed and banks of the estuary. The turbulence acts to mix the fresh and saline waters and reduce saline stratification. Vertical mixing is important

• The inflow of salt water is favoured on the flood tide and outflow of fresh water on the ebb tide. This can greatly enhance the flushing of such systems beyond that produced by fresh water flow alone.

• See also K. Dyer Estuaries 2nd Edition

Documents

01/09/2014Dr. Joanna Waniek, IOW1 Definition and basic characteristics Secondary Circulation Partially Mixed Estuaries