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.

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

Residual Flow in the Tagus

Water Trajectory

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.

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.,

Tidal analysis(M2)

Results validation

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.

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….

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)

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

___

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.

Temperature

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