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

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Presentation on theme: "01/09/2014Dr. Joanna Waniek, IOW1 Definition and basic characteristics Secondary Circulation Partially Mixed Estuaries."— Presentation transcript:

1 01/09/2014Dr. Joanna Waniek, IOW1 Definition and basic characteristics Secondary Circulation Partially Mixed Estuaries

2 01/09/2014Dr. Joanna Waniek, IOW2 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)

3 01/09/2014Dr. Joanna Waniek, IOW3 Salinity of surface water considerably raised. River volume R is small compared to tidal volume V Max R=50m 3 /s, Max Tidal flow=7500m 3 /s Tidal Prism= 10 8 m 3 Tidal energy dissipated by bottom friction and “interfacial” friction Turbulent Eddies work against gravity and move salt upward and fresh water downward Partially Mixed Estuaries

4 01/09/2014Dr. Joanna Waniek, IOW4 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,

5 01/09/2014Dr. Joanna Waniek, IOW5 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. HeadMouth

6 01/09/2014Dr. Joanna Waniek, IOW6 (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.

7 01/09/2014Dr. Joanna Waniek, IOW7

8 01/09/2014Dr. Joanna Waniek, IOW8 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 m 3 s -1 is small compared to the tidal volume of more than 200 m 3 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 tideLow tide

9 01/09/2014Dr. Joanna Waniek, IOW9 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.

10 01/09/2014Dr. Joanna Waniek, IOW10 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 km 2 ; 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.

11 01/09/2014Dr. Joanna Waniek, IOW11 Mersey Estuary- Environmental implications 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

12 01/09/2014Dr. Joanna Waniek, IOW12 Mersey Estuary- Environmental implications 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.

13 01/09/2014Dr. Joanna Waniek, IOW13 Tidal Flow in Partially Mixed Estuary

14 01/09/2014Dr. Joanna Waniek, IOW14  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

15 01/09/2014Dr. Joanna Waniek, IOW15 Ribeiro et al., 2004 Transport of sediment/pollutants with ± 5 cms -1 in and out of the estuary SpringNeap IN OUT

16 01/09/2014Dr. Joanna Waniek, IOW16

17 01/09/2014Dr. Joanna Waniek, IOW17 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 mixing switched 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?

18 01/09/2014Dr. Joanna Waniek, IOW18 Transverse Circulation e.g. Southampton Water

19 01/09/2014Dr. Joanna Waniek, IOW19 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 2 nd Edition

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