Hans Burchard

Leibniz Institute for Baltic Sea Research Warnemünde

hans.burchard@io-warnemuende.de

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

Principle of estuarine circulation

MacCready and Geyer (2010)

The principle of tidal straining

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

Observations of tidal straining in Liverpool Bay

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

Stratifying during ebb, destratitifying during flood

Bott

om

-top

salin

ity d

iffere

nce

GOTM application: Tidal straining in Liverpool Bay

Simpson et al., 2002

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

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

Tidal straining as driver of estuarine circulation

75% level

75% level

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

Non-dimensional parameters of this problem

1. Non-dimensional bed roughness:

2. Strouhal number:

3. Inverse Ekman number:  

4. Simpson number:

H

zz

bb 00~

*U

HSt

*U

fHEi

2*

2

U

HbSi x

Estuarine circulation and SPM* transport

Analytical solution for constant eddy viscosity/diffusivity:

* SPM = Suspended Particulate Matter

Analytical solution for parabolic viscosity/diffusivity, see Burchard et al. (JPO, 2013)

Enhancement of estuarine circulation in channelised tidal flow

(2D slice modelling with GETM)

Burchard et al. (JPO 2011)

www.g

etm

.eu

Circulation in transverse estuary

Transverse structure of estuarine circulation

Burchard et al. (JPO 2011)

Tidal straining circulation Gravitational circulation

Advective circulation Barotropic circulation

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

Campaign in Lister Deep (April 2008)

Becherer et al. (GRL 2011)

shoals

Becherer et al. (GRL 2011)

Puzzling however:

Wate

r colu

mn

sta

bilit

y

Tidal phase

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

Becherer et al. (GRL 2011)

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

… or in this lake (Alpnach)?

Simulation Lake Alpnach (Switzerland)

Becherer & Umlauf (2011)

Upslope currents: Destabilisation

Downslope currents: Stabilisation

stable mixing

unstable mixing

GO

TM

1D

si

mula

tion

Periodic straining in lakes and non-tidal basins?

down-slope up-slope

Umlauf and Burchard (2011)

Observations in Lake Constance

Lorke et al. (2008)

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.