modeling the effect of stratification on sediment deposition
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Modeling the Effect of
Stratification on Sediment
DepositionKeisuke NakayamaKeisuke Nakayama
National Institute for Land National Institute for Land and Infrastructure and Infrastructure
ManagementManagementVisiting Associate Professor, Tokyo Institute of Technology
Currently visiting Centre for Water Research on Gledden Senior Fellowship
Background of the studyBackground of the studyTokyo Bay
Background of the studyBackground of the study
1973 1989 1994
COD>5 5>COD>4
20km
60km
Red tide in Tokyo BayRed tide in Tokyo Bayda
ys
Num
ber
of o
ccur
renc
e
1979 1990 1999
Red tide and POM in Red tide and POM in summersummer Nutrient Flux from rivers
Nutrient Flux
Red tide
(Nakayama and Imberger, JPO, submitted, 2005)(Wallace and Wilkinson, JFM, 1988)(Gloor et al., Hydrobiologia, 1994)(Steinman et al., WASP, 1997)(Sutherland, JPO, 1996)
Internal wave effectsInternal wave effectsHorizontal circulation due to wind
(Nakayama, HP, submitted, 2005)
139.7 139.8 139.9 140
35.2
35.25
35.3
35.35
35.4
35.45
35.5
35.55
35.6
35.65
20m/s
図-5 2001年8月10日の風の分布●( は風の観測地点)
1
139.7 139.8 139.9 140
35.2
35.25
35.3
35.35
35.4
35.45
35.5
35.55
35.6
35.65
-50
-40
-30
-20
-10
0
10
20
30
40
x10-4[1/s]
0 5 10 15 20 25 30-3
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5
x10-4[1/s]
August 2001 [day]
WindVorticity of wind
Time series of vorticity of wind in August 2001
Internal wave effectsInternal wave effects
Vertical circulation due to stratificationTurner J.S., Buoyancy effects in fluids, 1973Imberger, J. and Ivey G.N., Boundary mixing in stratified reservoirs, JFM, 248, 477-491, 1993
Benthic boundary layer (BBL)
POM in Tokyo BayPOM in Tokyo Bay
MEL3DMEL3D Finite difference methodFinite difference method Immersed boundaryImmersed boundary CIP methodCIP method Non-hydrostatic modelNon-hydrostatic model First order LES for horizontal planeFirst order LES for horizontal plane Second order closure for vertical coordinateSecond order closure for vertical coordinate ALE methodALE method Residual cutting method for solving the Residual cutting method for solving the
Poisson’s equationPoisson’s equation
(Nakayama, Okada and Nomura, ECSS, 2005)(Nakayama, IJNMF, 2005)(Arthur, Imberger and Nakayama, JGR, in preparation, 2005)
Target and Initial Target and Initial conditioncondition
Target: Clarification of the mechanisms of vertical circulation due to stratification
case 1: stratified fieldcase 2: no stratification
Modified Tokyo Bay: 60km x 6km x 100m (60 x 6 x 40)Modified Tokyo Bay: 60km x 6km x 100m (60 x 6 x 40)
Residual current and Residual current and densitydensityWith stratification Without stratification
POM transportPOM transport180 days computationFalling speed is taken as 0.3 m day-1
Theoretical analysisTheoretical analysis
The first layer:
111
1
111
1
2F
xhg
xH
gxu
utu
1111 Ehu
xth
The second layer:
xH
gxH
gxu
utu
2
2
121
2
122
2
2221
12 2
Fx
hx
hg
2222 Ehu
xth
The third layer:
xH
gxH
gxH
gxu
utu
3
3
232
3
121
3
133
3
3332
21
13 2
Fx
hx
hx
hg
3333 Ehu
xth
Assumption used in the Assumption used in the theoretical analysistheoretical analysis
Water depths for each layer are taken as a Water depths for each layer are taken as a function of space not a function of time function of space not a function of time (Steady state assumption).(Steady state assumption).
Well mixed in the third layer.Well mixed in the third layer. No residual current in the first layer.No residual current in the first layer.
Comparisons with the Comparisons with the numerical computationnumerical computation
Bay headBay mouth
ConclusionConclusion
On-slope residual current due to On-slope residual current due to stratification effect is confirmed to be stratification effect is confirmed to be reproduced by using non-hydrostatic reproduced by using non-hydrostatic model, MEL3D.model, MEL3D.
The spatially distributed POM The spatially distributed POM concentration was found to be influenced concentration was found to be influenced by BBL.by BBL.
The applicability of theoretical solution The applicability of theoretical solution was verified through good agreements was verified through good agreements with the numerical computations.with the numerical computations.
Thank you very much.
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