Date post: | 30-Dec-2015 |
Category: |
Documents |
Upload: | nicholas-zoie |
View: | 19 times |
Download: | 0 times |
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.
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
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
Transverse structure of estuarine circulation
Burchard et al. (JPO 2011)
Tidal straining circulation Gravitational circulation
Advective circulation Barotropic circulation
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)
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)
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.