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About Estuarine Dynamics Transient Flow, Residual Flow, Residence time, Salinity, Temperature Evolution, Free Surface Exchange

About Estuarine Dynamics Transient Flow, Residual Flow, Residence time, Salinity, Temperature Evolution, Free Surface Exchange

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Page 1: About Estuarine Dynamics Transient Flow, Residual Flow, Residence time, Salinity, Temperature Evolution, Free Surface Exchange

About Estuarine Dynamics

Transient Flow, Residual Flow, Residence time,

Salinity, Temperature Evolution, Free Surface Exchange

Page 2: About Estuarine Dynamics Transient Flow, Residual Flow, Residence time, Salinity, Temperature Evolution, Free Surface Exchange

Transient Flow• Is the instantaneous flow generated by the

tide, by the wind and by density gradients.

Page 3: About Estuarine Dynamics Transient Flow, Residual Flow, Residence time, Salinity, Temperature Evolution, Free Surface Exchange

Residual Flow

• Is the flow solution integrated in time.

• Gives the tendency of the transport.

Tt

t

ii dtuT

u0

0

1

Tt

t

HdtT

H0

0

1

Tt

t

ii dtuT

Hu0

0

1

H

Huu i

iT

Page 4: About Estuarine Dynamics Transient Flow, Residual Flow, Residence time, Salinity, Temperature Evolution, Free Surface Exchange

Residual Flow in the Tagus

Page 5: About Estuarine Dynamics Transient Flow, Residual Flow, Residence time, Salinity, Temperature Evolution, Free Surface Exchange

Water Trajectory

Page 6: About Estuarine Dynamics Transient Flow, Residual Flow, Residence time, Salinity, Temperature Evolution, Free Surface Exchange

Boundary conditions

• Must include the variability we are looking for: Tide, Wind, Heat Fluxes, River Discharge, Vertical Temperature and Salinity Profiles at the Open Boundary.

• In a 2D model (or in a 3D barotropic model) Density gradients are not considered and thus Temperature and Salinity are not required.

• River discharge can change levels and is necessary in both models.

Page 7: About Estuarine Dynamics Transient Flow, Residual Flow, Residence time, Salinity, Temperature Evolution, Free Surface Exchange

Tide

k

kkk twazt cos)( 0

In every boundary point the tidal level is imposed as:

Where z0 is the average sea level, ak is the harmonic amplitude, w its phase velocity and “fi” is the phase lag. These values are imposed in the file “tide.dat”

Imposing the tide at the boundary the model computes the tidal flow. Performing the tidal analysis of the results one obtains the amplitudes and phase lags inside the estuary. Comparing these values with measurements one can validate the model results.,

Page 8: About Estuarine Dynamics Transient Flow, Residual Flow, Residence time, Salinity, Temperature Evolution, Free Surface Exchange

Tidal analysis(M2)

Page 9: About Estuarine Dynamics Transient Flow, Residual Flow, Residence time, Salinity, Temperature Evolution, Free Surface Exchange

Results validation

Page 10: About Estuarine Dynamics Transient Flow, Residual Flow, Residence time, Salinity, Temperature Evolution, Free Surface Exchange

Wind Forcing

• Is time and space variable. This variability description requires a meteorological model.

• Meteorological stations are few (Aeroporto, Caparica, Cacém…). They characterize time variability in detail, but hardly describe spatial variability.

• Wind generates momentum fluxes (shear stress), Heat fluxes (sensible and latent), gas fluxes (O2, CO2, …). Evaporation and Raain can be important in some systems.

Page 11: About Estuarine Dynamics Transient Flow, Residual Flow, Residence time, Salinity, Temperature Evolution, Free Surface Exchange

Residence time or Flushing time?

• Residence time is the time required by the river water to cross the estuary and reach the Sea.

• Flushing time is the time necessary to renew the estuarine water.

• They are synimous….

Page 12: About Estuarine Dynamics Transient Flow, Residual Flow, Residence time, Salinity, Temperature Evolution, Free Surface Exchange
Page 13: About Estuarine Dynamics Transient Flow, Residual Flow, Residence time, Salinity, Temperature Evolution, Free Surface Exchange

2006-10-04 Introduction to MOHID

Residence Time

Volume Tracers in Estuary / Volume Box(Box 5)

0.0

0.2

0.4

0.6

0.8

1.0

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0

Simulation Time (days)

Page 14: About Estuarine Dynamics Transient Flow, Residual Flow, Residence time, Salinity, Temperature Evolution, Free Surface Exchange

Residence time from Salinity data

f

VolEstuary SW

SW

f

VolEstuary SW

R Q

dvolS

SS

Q

dvolSS

T

1

This equation fraction in the integral gives the part of fresh water in each point of the estuary. Integrated over the estuary gives the volume of fresh water. Divided by the river discharge gives the time necessary to discharge that volume inside the estuary.

Hypotheses: Salinity does not change in time, The river discharge does not change during the residence time.

FreshWaterOfFractionLocalS

S

SeaWaterOfFractionLocalS

S

SW

SW

___1

___

Page 15: About Estuarine Dynamics Transient Flow, Residual Flow, Residence time, Salinity, Temperature Evolution, Free Surface Exchange

Salinity Distribution

• Salinity is a conservative property. It is used to validate the model results when the river discharge is well known.

• Inside the estuary can change only due to advection and diffusion.

• Other properties have sources and sinks. Their correlation with salinity shows the importance of transport on their evolution. No correlation with salinity means that sources and sinks are more important than transport.

Page 16: About Estuarine Dynamics Transient Flow, Residual Flow, Residence time, Salinity, Temperature Evolution, Free Surface Exchange

Temperature

• Depends on mixing and on heat exchange across the free surface. The deviation to temperature shows the importance of those fluxes.