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Modeling the Effect of Stratification on Sediment Deposition

Modeling the Effect of Stratification on Sediment Deposition. Keisuke Nakayama National Institute for Land and Infrastructure Management. Visiting Associate Professor, Tokyo Institute of Technology Currently visiting Centre for Water Research on Gledden Senior Fellowship.

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Modeling the Effect of Stratification on Sediment Deposition

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  1. Modeling the Effect of Stratification on Sediment Deposition Keisuke Nakayama National Institute for Land and Infrastructure Management Visiting Associate Professor, Tokyo Institute of Technology Currently visiting Centre for Water Research on Gledden Senior Fellowship

  2. Background of the study Tokyo Bay

  3. Background of the study COD>5 5>COD>4 60km 20km 1973 1989 1994

  4. Red tide in Tokyo Bay Number of occurrence days 1979 1990 1999

  5. Red tide and POM in summer Nutrient Flux from rivers Red tide Nutrient Flux (Nakayama and Imberger, JPO, submitted, 2005) (Wallace and Wilkinson, JFM, 1988) (Gloor et al., Hydrobiologia, 1994) (Steinman et al., WASP, 1997) (Sutherland, JPO, 1996)

  6. Internal wave effects Horizontal circulation due to wind (Nakayama, HP, submitted, 2005)

  7. Time series of vorticity of wind in August 2001 Wind Vorticity of wind

  8. Internal wave effects Vertical circulation due to stratification Turner J.S., Buoyancy effects in fluids, 1973 Imberger, J. and Ivey G.N., Boundary mixing in stratified reservoirs, JFM, 248, 477-491, 1993 Benthic boundary layer (BBL)

  9. POM in Tokyo Bay

  10. MEL3D • Finite difference method • Immersed boundary • CIP method • Non-hydrostatic model • First order LES for horizontal plane • Second order closure for vertical coordinate • ALE method • Residual cutting method for solving the Poisson’s equation (Nakayama, Okada and Nomura, ECSS, 2005) (Nakayama, IJNMF, 2005) (Arthur, Imberger and Nakayama, JGR, in preparation, 2005)

  11. Target and Initial condition Target: Clarification of the mechanisms of vertical circulation due to stratification • Modified Tokyo Bay: 60km x 6km x 100m (60 x 6 x 40) case 1: stratified field case 2: no stratification

  12. Residual current and density With stratification Without stratification

  13. POM transport 180 days computation Falling speed is taken as 0.3 m day-1

  14. Theoretical analysis The first layer: The third layer: The second layer:

  15. Assumption used in the theoretical analysis • Water depths for each layer are taken as a function of space not a function of time (Steady state assumption). • Well mixed in the third layer. • No residual current in the first layer.

  16. Comparisons with the numerical computation Bay mouth Bay head

  17. Conclusion • On-slope residual current due to stratification effect is confirmed to be reproduced by using non-hydrostatic model, MEL3D. • The spatially distributed POM concentration was found to be influenced by BBL. • The applicability of theoretical solution was verified through good agreements with the numerical computations. Thank you very much.

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