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Hans Burchard Leibniz Institute for Baltic Sea Research Warnemünde

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Periodic straining, a process which lakes (due to seiches ) and estuaries (due to tides) have in common. . Hans Burchard Leibniz Institute for Baltic Sea Research Warnemünde [email protected]. Principle of estuarine circulation. MacCready and Geyer (2010). - PowerPoint PPT Presentation
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Hans Burchard Leibniz Institute for Baltic Sea Research Warnemünde [email protected] Periodic straining, a process which lakes (due to seiches) and estuaries (due to tides) have in common.
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Page 1: Hans  Burchard Leibniz Institute for  Baltic Sea Research  Warnemünde

Hans Burchard

Leibniz Institute for Baltic Sea Research Warnemünde

[email protected]

Periodic straining, a process which lakes (due to seiches) and estuaries (due to tides) have in common.

Page 2: Hans  Burchard Leibniz Institute for  Baltic Sea Research  Warnemünde

Principle of estuarine circulation

MacCready and Geyer (2010)

Page 3: Hans  Burchard Leibniz Institute for  Baltic Sea Research  Warnemünde

The principle of tidal straining

MacCready and Geyer (2010), after Simpson et al., 1990

Page 4: Hans  Burchard Leibniz Institute for  Baltic Sea Research  Warnemünde

Observations of tidal straining in Liverpool Bay

MacCready and Geyer (2010), after Simpson et al., 1990

Stratifying during ebb, destratitifying during flood

Bott

om-t

op s

alin

ity

diff

eren

ce

Page 5: Hans  Burchard Leibniz Institute for  Baltic Sea Research  Warnemünde

GOTM application: Tidal straining in Liverpool Bay

Simpson et al., 2002

Page 6: Hans  Burchard Leibniz Institute for  Baltic Sea Research  Warnemünde

GOTM application: Tidal straining in Liverpool Bay Observations

Salinity

Longitudinal vel.

Transverse vel.

Temperature

Simulation Observed dissipation rate Simulated dissipation rate

Simpson et al., 2002

Page 7: Hans  Burchard Leibniz Institute for  Baltic Sea Research  Warnemünde

MacCready & Geyer (2010) after Jay & Musiak (1994)

Tidal straining as driver of estuarine circulation

75% level

75% level

Page 8: Hans  Burchard Leibniz Institute for  Baltic Sea Research  Warnemünde

Result:Tidal straining makes about 2/3 of estuarine circulation.

With full-scale 1Dmodel (GOTM):Gravitational circulation and tidal straining profiles

Burchard and Hetland (JPO 2010)

Estuarine circ.Straining

Gravitational

Tidally-averaged currents fortypical Wadden Sea conditions

Page 9: Hans  Burchard Leibniz Institute for  Baltic Sea Research  Warnemünde

Non-dimensional parameters of this problem

1. Non-dimensional bed roughness:

2. Strouhal number:

3. Inverse Ekman number:  

4. Simpson number:

Hzzb

b 00~

*UHSt

*UfHEi

2*

2

UHbSi x

Page 10: Hans  Burchard Leibniz Institute for  Baltic Sea Research  Warnemünde

Estuarine circulation and SPM* transport

Analytical solution for constant eddy viscosity/diffusivity:

* SPM = Suspended Particulate MatterAnalytical solution for parabolic viscosity/diffusivity, see Burchard et al. (JPO, 2013)

Page 11: Hans  Burchard Leibniz Institute for  Baltic Sea Research  Warnemünde

Enhancement of estuarine circulation in channelised tidal flow

(2D slice modelling with GETM)

Burchard et al. (JPO 2011)

www.getm

.eu

Page 12: Hans  Burchard Leibniz Institute for  Baltic Sea Research  Warnemünde

Circulation in transverse estuary

Page 13: Hans  Burchard Leibniz Institute for  Baltic Sea Research  Warnemünde

Transverse structure of estuarine circulation

Burchard et al. (JPO 2011)

Tidal straining circulation Gravitational circulation

Advective circulation Barotropic circulation

Page 14: Hans  Burchard Leibniz Institute for  Baltic Sea Research  Warnemünde

Does this all happen in nature?Ok, let’s go out to the Wadden Sea and measure:

Page 15: Hans  Burchard Leibniz Institute for  Baltic Sea Research  Warnemünde

Campaign in Lister Deep (April 2008)

Becherer et al. (GRL 2011)

shoals

Page 16: Hans  Burchard Leibniz Institute for  Baltic Sea Research  Warnemünde

Becherer et al. (GRL 2011)

Page 17: Hans  Burchard Leibniz Institute for  Baltic Sea Research  Warnemünde

Puzzling however:W

ater

col

umn

stab

ility

Tidal phase

Near lateral shoals, stratification kicks in already during flood ...This is topic of another study ...

Becherer et al. (GRL 2011)

Page 18: Hans  Burchard Leibniz Institute for  Baltic Sea Research  Warnemünde

Do similar things also happen in this lake (Constance) …

… or in this lake (Alpnach)?

Page 19: Hans  Burchard Leibniz Institute for  Baltic Sea Research  Warnemünde

Simulation Lake Alpnach (Switzerland)

Becherer & Umlauf (2011)

Page 20: Hans  Burchard Leibniz Institute for  Baltic Sea Research  Warnemünde

Upslope currents: Destabilisation

Downslope currents: Stabilisation

stable mixingunstable mixing

GOTM

1D

simul

atio

n

Periodic straining in lakes and non-tidal basins?

down-slope up-slope

Umlauf and Burchard (2011)

Page 21: Hans  Burchard Leibniz Institute for  Baltic Sea Research  Warnemünde

Observations in Lake Constance

Lorke et al. (2008)

Page 22: Hans  Burchard Leibniz Institute for  Baltic Sea Research  Warnemünde

Take home:Tidal straining is the interaction between tidal oscillations and lateral buoyancy gradients as they typically occur in coastal areas.

Tidal straining drives estuarine residual circulation in a comparablemagnitude as gravitational circulation.

Cross-sectional straining due to lateral buoyancy gradients may bemuch more effective than longitudinal straining.

In lakes, the same mechanisms work, with seiches providing theoscillating forcing, and vertical stratification along sloping bedsproviding the along-flow buoyancy gradients.

Question: do these seiches also cause near-bottom up-sloperesidual circulation? If so, this may have significant consequences forlake ecosystems.


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