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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. . Hans Burchard Leibniz Institute for Baltic Sea Research Warnemünde hans.burchard@io-warnemuende.de. Principle of estuarine circulation. MacCready and Geyer (2010).

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Hans Burchard Leibniz Institute for Baltic Sea Research Warnemünde

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  1. 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 hans.burchard@io-warnemuende.de

  2. Principleofestuarinecirculation MacCreadyand Geyer (2010)

  3. The principleoftidalstraining MacCreadyand Geyer (2010), after Simpson et al., 1990

  4. Observationsoftidalstraining in Liverpool Bay Bottom-top salinitydifference Stratifyingduringebb, destratitifyingduringflood MacCreadyand Geyer (2010), after Simpson et al., 1990

  5. GOTM application: Tidalstraining in Liverpool Bay Simpson et al., 2002

  6. GOTM application: Tidalstraining in Liverpool Bay Observations Simulation Simulateddissipation rate Observeddissipation rate Temperature Longitudinal vel. Salinity Transverse vel. Simpson et al., 2002

  7. Tidalstrainingasdriverofestuarinecirculation 75% level 75% level MacCready & Geyer (2010) after Jay & Musiak (1994)

  8. Tidally-averagedcurrentsfor typicalWaddenSeaconditions Estuarinecirc. Withfull-scale1D model (GOTM): Gravitationalcirculation andtidalstrainingprofiles Straining Gravitational Result: Tidal straining makes about 2/3 of estuarine circulation. Burchard andHetland (JPO 2010)

  9. Non-dimensional parameters of this problem • Non-dimensional bed roughness: • Strouhal number: • Inverse Ekman number:   • Simpson number:

  10. Estuarine circulation and SPM* transportAnalytical solution for constant eddy viscosity/diffusivity: * SPM = Suspended Particulate Matter Analytical solution for parabolic viscosity/diffusivity, see Burchard et al. (JPO, 2013)

  11. Enhancement of estuarine circulation in channelised tidal flow (2D slice modelling with GETM) www.getm.eu Burchard et al. (JPO 2011)

  12. Circulation in transverse estuary

  13. Transverse structure ofestuarinecirculation Gravitationalcirculation Tidalstrainingcirculation Advectivecirculation Barotropiccirculation Burchard et al. (JPO 2011)

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

  15. Campaign in Lister Deep (April 2008) shoals Becherer et al. (GRL 2011)

  16. Becherer et al. (GRL 2011)

  17. Puzzling however: Water column stability Tidal phase Near lateral shoals, stratification kicks in already during flood ... This is topic of another study ... Becherer et al. (GRL 2011)

  18. Do similar things also happen in this lake (Constance) … … or in this lake (Alpnach)?

  19. Simulation Lake Alpnach(Switzerland) Becherer & Umlauf (2011)

  20. Periodic straining in lakes and non-tidal basins? down-slopeup-slope Upslope currents: Destabilisation GOTM 1D simulation stable mixing Downslope currents: Stabilisation unstable mixing Umlauf and Burchard (2011)

  21. Observations in Lake Constance Lorke et al. (2008)

  22. Take home: • Tidalstrainingistheinteractionbetweentidaloscillationsand • lateral buoyancygradientsastheytypicallyoccur in coastalareas. • Tidalstrainingdrivesestuarine residual circulation in a comparable • magnitude asgravitationalcirculation. • Cross-sectionalstraining due to lateral buoyancygradientsmaybe • muchmoreeffectivethan longitudinal straining. • In lakes, the same mechanismswork, withseichesprovidingthe • oscillatingforcing, andverticalstratificationalongslopingbeds • providingthealong-flowbuoyancygradients. • Question: do theseseiches also causenear-bottomup-slope • residual circulation? If so, thismayhavesignificantconsequencesfor • lakeecosystems.

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