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Classification Of Estuaries

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					Classification of estuaries by circulation and salinity structure

• • • • •

Why Classification ? Basis of Classification Using the Hansen-Rattray Diagram Limitations Other Classifications
Dr. Joanna Waniek, IOW 1

03/03/2009

Why Classification ?
• Although every estuary is unique, it is possible to some extent to categorise estuaries into distinct types. • Once a newly studied estuary is classified, its characteristics can be inferred from knowledge of a well studied estuary of a similar type.

03/03/2009

Dr. Joanna Waniek, IOW

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Basis of Classification
• We expect  to vary according to both the stratification in the estuary and the mean circulation patterns. • Stratification and flow can be measured, allowing a method of categorising an estuary. The question is: what combinations of stratification and flow give rise to the same salt transport parameter  ? • The answer was provided by modelling studies of Hansen and Rattray (J. Marine Research, 23, 104-122; 1965).

03/03/2009

Dr. Joanna Waniek, IOW

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Basis of Classification
• Relative contribution of advection and diffusion to the transport of salt in the estuary.

• An estuary where salt transport is achieved by advection is much better flushed than one which relies only on diffusive processes. • We define a parameter,  = fraction of horizontal salt transport caused by diffusion. If  = 1, diffusion dominates, if  = 0, advection dominates

03/03/2009

Dr. Joanna Waniek, IOW

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Basis of Classification
They simulated different estuarine types, and defined each in terms of: • A stratification parameter,
S
So  difference in salinity from seabed to surface (tidal averaged) Depth - averaged salinity (tidal averaged)

• A circulation parameter,
us surface velocity (tidally averaged)  u f depth - averaged velocity (tidally averaged)

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Dr. Joanna Waniek, IOW

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They then evaluated from the model how  varied with the stratification and circulation parameters :

1

 is the fraction of the total horizontal salt transport tendency caused by diffusion =1 all diffusion - no advection =0 all advection - no diffusion

S/So 10-1 =0.99 =1 10-2 1 =0.9

=0.5 =0.1 =0.01

1.5

10
Us/Uf

102

103

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Dr. Joanna Waniek, IOW

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Hansen & Rattray diagram
1A 1B 2A,B 3A

3B 4

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Dr. Joanna Waniek, IOW

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Note…
• Different positions along the estuary may plot at different places on the diagram
• The same place may classify differently in different seasons or tidal conditions

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Dr. Joanna Waniek, IOW

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Limitations of H-R diagram
• Method not applicable when lateral circulation occurs. The model on which the classification is based did not allow for transverse flows or variations. Hence only applies to narrow and deep estuaries.
• Need a more sophisticated model to include transverse circulations…see later lecture !

03/03/2009

Dr. Joanna Waniek, IOW

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Hansen-Rattray Diagram
• Advantages: (i) Simple to use (ii) Gives a general classification for estuary (iii) Robust Methodology

03/03/2009

Dr. Joanna Waniek, IOW

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Hansen-Rattray Diagram
– Disadvantages:
• Gives a qualitative description of an estuary • Not sufficient for many purposes ( water quality, pollution incidents). • Does not give information during tidal cycles. • Requires a large field data set over all conditions of the estuary e.g Low/high runoff Spring/Neap tides, Strong/weak winds

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Dr. Joanna Waniek, IOW

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Other Classifications: Definition of estuaries & empirical estuary classification

An estuary is a semi-enclosed coastal body of water which has a free connection with the open sea and within which sea water is measurably diluted with fresh water derived from land drainage (Cameron and Pritchard, 1963).

Estuaries described by this definition are known as positive estuaries.

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Dr. Joanna Waniek, IOW

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All other types of estuaries are derived by applying three changes to the definition:

1) Intermittent closure of the estuary to the sea.
This situation is common with gently sloping, sandy coasts in regions with strong seasonal or irregular fluctuations of rainfall; reduced river runoff during the dry periods allows the establishment of a sand bar, turning the estuary temporarily into a brackish lake.

2) Evaporation exceeds the fresh water supply from rivers and from local rain, and where as a consequence the water in the
estuary is more saline than in the open ocean; such estuaries are known as negative or inverse estuaries.

03/03/2009

Dr. Joanna Waniek, IOW

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3) Estuaries are narrow, is required to separate estuaries from Mediterranean seas. modified definition An estuary is a narrow, semi-enclosed coastal body of water which has a free connection with the open sea at least intermittently and within which the salinity of the water is measurably different from the salinity in the open ocean.

03/03/2009

Dr. Joanna Waniek, IOW

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Classification using topography Pritchard (1952)
Pritchard (1952) distinguishes

1. coastal plain estuaries (drowned river valleys), 2. fjords 3. bar-built estuaries 1) Coastal plain estuaries (Chesapeake Bay, USA; Thames, Mersey, Southampton Water, UK) - were formed by the flooding of river valleys following a rise in sea level over geological time. - show little sedimentation, so that the ancient river valley still determines the estuarine topography. - shallow, with depths of ca. 30 m or so, and mostly located in the temperate climate zones.
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2) Fjords (Loch Etive, Scotland; Sogne Fjord, Norway)
- river valleys deepened by glaciers during the last ice age

- the scouring of the valley floor results in very deep estuaries, sometimes exceeding 800 m
- the characteristic feature of fjords is the existence of a shallow sill at the mouth formed by accumulated rock at the glacier front - these sills can be as shallow as 4 m, but sill depths of between 40 m and 150 m are more common

- true fjords are found only in the temperate zones
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3) Bar-built estuaries (Vellar, India)
-are drowned river valleys with a high sedimentation rate

- they are mostly very shallow, with depths of a few metres, and often branch out towards the mouth into shallow waterways
- the sediment accumulates near the mouth of the estuary and forms a bar and the water depth decreases even further - common in the subtropics and tropics but can occur wherever the coastal zone is characterised by deposition of sediment
03/03/2009 Dr. Joanna Waniek, IOW 17

Classification by Tides Davies (1964)
Microtidal Mesotidal Macrotidal Hypertidal <2m <4m, >2m <6m, >4m > 6m

•In high tidal range estuaries the volume of water between HW and LW, the tidal prism, is large compared with the low tide volume.
•The interaction between tidal wave propagating in the estuary and the morphology leads to variations in the range of the tide and the strength of the currents. •Convergence at the estuary sides compresses the wave laterally, and the tide increases if the friction is low, whereas friction in shallow water diminishes the tide.
03/03/2009 Dr. Joanna Waniek, IOW 18

Classification by Tides (cont.)

Head

Mouth

Head

Mouth

Head

Mouth

Hypersynchronous Convergence>Friction

Synchronous Convergence=Friction

Hypersynchronous Convergence<Friction

The tidal range & currents increase towards the head of the estuary until the convergence diminishes & friction reduces the tide
03/03/2009

The friction and convergence have equal and opposite effects on the tide
Dr. Joanna Waniek, IOW

Friction is stronger and the tide diminishes along the estuary

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