Hans Burchard
Leibniz Institute for Baltic Sea Research Warnemünde
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-t
op s
alin
ity
diff
eren
ce
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:
Hzzb
b 00~
*UHSt
*UfHEi
2*
2
UHbSi x
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)
Enhancement of estuarine circulation in channelised tidal flow
(2D slice modelling with GETM)
Burchard et al. (JPO 2011)
www.getm
.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: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)
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 mixingunstable mixing
GOTM
1D
simul
atio
n
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.