ESTUARINE FLOW, TURBIDITY AND PHYTOPLANKTON, DATA AND MODEL EXPERIMENTS

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ESTUARINE FLOW, TURBIDITY AND PHYTOPLANKTON,
DATA AND MODEL EXPERIMENTS

• Supervisors
• Huib E. de Swart
• HenkE M. Schuttelaars
• Dulce Perez
• Elena Quevedo
• Ana Mendonça
• Antonino Viviano
• Youen Kervella

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Outline

• Introduction
• Field observations
• Numerical model
• Circulation module
• Suspended sediment module
• Phytoplankton module
• Model results (sensitivity analysis)
• Conclusions

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Introduction

• Objectives and Approach
• Study of a well mixed estuary, to understand
  formation of phytoplankton blooms and compare
  results qualitatively with EMS estuary.
• The growth rate is a function of
• - Nutrient availability N
• - Ligth intensity I

Hydrodynamic module u, w (x-dir,z-dir)
Sediment transport module (concentration -
turbidity maxima, settling velocity)
Phytoplankton module (nutrients for phyto growth)
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Introduction
Initial growth of phytoplankton using a linear
stability analysis Initial state no
phytoplankton Predict in which conditions
phytoplankton will grow
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Introduction

• Analysed Processes
• Hydrodynamics
• Tidal averaged model (effects of tide in mixing
  parameters)
• Fresh water discharge (q) ? horizontal salinity
  gradient ?density-driven circulation
• Sediment transport

Sediment suspension (transported and eroded)
Currents
Large bottom shear stresses
Convergence/divergence of sediment transport
Assumption Considering no net sediment transport
(no deposit/erosion of bottom sediments) and
sediment conservation on the upper surface
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Real Case The Ems Estuary
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ETM (350-400)
River
Sea
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Fluorescence Measurements
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Circulation Module

• Domain
• two-dimensional (x,z)
• depth and width are constant
• x increase toward the river
• z is positive upward

z
x
river q lt 0
sea
h

• Assumptions
• steady-state
• neglected nonlinear terms
• hydrostatic pressure
• laterally homogeneous estuary
• Boussinesq approximation
• mixing coefficient Av is constant (related with
  tide)
• linear equation of state r ro (1 b S )

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Circulation Module

• Equations

• in which
• - u, w are horizontal and vertical velocity
• - p is the pressure (hydrostatic)

Salinity distribution is a data of the problem

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Circulation Module

• If river flow q ? 0, this is the velocity
  pattern

sea
river
There will be a point near the bottom in which
u 0 namely STAGNATION POINT
Near this point Turbidity Maxima region is
expected (ETM)
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Suspended Sediment Module

• HYPOTHESIS
• Sediment does not affect water density
• Sediment does not influence hydrodynamics
• Ws is constant ? Constant grain size

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Sediment Module

• Processes that affect sediment transport
• Settling velocity
• Advection
• Diffusion
• Sediment conservation equation

advection
diffusion
temporal variation
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Sediment Module

• Steady motion, at the first order of magnitude

diffusion
advection
temporal variation
Vertical balance settling velocity vertical
diffusion
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Sediment Module

• Boundary conditions
• No flux through the free surface nor through the
  bottom

Solution Vertical concentration profile
Now we need to solve Cb(x)
Cb
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Sediment Module

• Horizontal problem Integrated equation
• The flux of sediment across each section must
  vanish
• Integral condition

Y
Concentration
River side
Sea side
X-direction
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Linear Stability Analysis

• Basic state P0 0 (no growth)
• Small perturbation P P exp(?t)
• if ?gt0 eigenfunction increase so the
  phytoplankton will grow

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Model Results
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Sensitivity analysis

• Allows salt to intrude farther up the estuary.
• Greater landward migration of turbidity maxima.

small
discharge (q)

• Increase in stratification and strength the
  estuary circulation.
• Increase in the magnitude of the vertical
  velocity.
• Seaward displacement of the turbidity maxima.

large

• Decrease in the estuary circulation.
• Seaward displacement of the turbidity maxima.

small
water depth (h)

• Strength the estuary circulation.
• Landward displacement of the turbidity maxima.

large

• More suspended sediment in the water column.

small
settling velocity (ws)
large

• More deposition.

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Tide influence
Av Eddy viscosity coefficient m2/s kz
Vertical eddy diffusion coefficient m2/s
Spring tide
Av 0.002 m2/s kz 0.002 m2/s
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Tide influence
Av Eddy viscosity coefficient m2/s kz
Vertical eddy diffusion coefficient m2/s
Neap tide
Av 0.0005 m2/s kz 0.0005 m2/s
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Maximum bottom concentration in domain five
times more sediment in the water
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Conclusions

• Knowledge of ETM and distribution of nutrients
  can predict the spatial distribution of
  phytoplankton blooms
• Maximum growth at the seaside of the ETM since
  the river side is light limited and the seaside
  nutrient limited
• Parameters sensitivity can be easily explored and
  understood using these simplified models