Hydrodynamics
and Sediment Transport
Modelling
Ramiro Neves
ramiro.neves@ist.utl.pt
Contents of this talk
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Relevance of suspended matter in
estuaries and coastal lagoons,
Basic processes in sediment
transport,
Coupling hydro and sediment
transport models,
System modelling.
How do they look like
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Relevance of sediment
transport modelling
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Light penetration,
Transport of chemicals,
Benthic habitat properties
Navigation channels fill-up:
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dredging
deposition of dredged products.
Basic Processes
Ws
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Advection-Diffusion,
Settling, Deposition/Erosion
waves, generate currents and
enhance re-suspension
D= Cd wS (Ws
)2
Settling
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Sediments are denser than water
and fall down. At what speed ?
Ws
Cd
W=sgV
Re= (wD Ws) /
(Ws)2 =( s /w) gD/Cd
Re
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Flocculation
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A floc can include:
• terrigenous, detritus
• phyto, zoo, bacteria.
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Formation of flocs gluing individual
particles.
Increases the size of the falling
particles, increasing Re and
decreasing Cd.
Floc’s density depends on the
properties of individual particles.
Flocculation Mechanism
(Particles must meet and glue)
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The probability of two particles to meet
increases with:
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number of particles (concentration)
random displacement (turbulence)
The gluing probability depends on:
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number of free ions (salinity),
adhesive properties of particle surfaces
(biology)
De-flocculation
(Destruction of flocs)
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It’s a pleasure to
travel with you
Needs a force do separate the
particles. Shear (and thus
turbulence) is the main deflocculation mechanism.
!
Don’t leave me !!!!
Move faster !!
I can’t !!
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Calculation of settling
velocity
Ws
WS=KC (salinity higher than 2‰)
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Chs
C
K [few (mm s-1) / (kg m-3)] is a function of
individual particle properties and typical
turbulence properties of the system. Must be
estimated from experimental data (field or
laboratory).
Ws  kChs 1 k2 C  Chs 
m
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For concentrations higher than the hindering
settling concentration (Chs).
Exponent m varies between 2 and 5.
Erosion and Deposition
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(CD)
(CE)
b
Bottom erosion and deposition occurs
simultaneously. For experimental
convenience reasons
erosion/deposition are defined as “net
erosion” and “net deposition”.
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Erosion / deposition Rates
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Erosion:
 b
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 E  E 
 1
  CE
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CE  A1b 
E
b=Msed/(total
volume)
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PARTHENIADES, (1965)
STEPHENS et al. (1992) used
A1=0.0012 m2s-2 and E=1.2
Deposition

b
 D  CWs b 1 
  CD
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



KRONE (1962)
How to handle the bottom
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Bottom sediment consolidate with time
Initial state must be known
what about consolidation rate ?
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Consolidation
Chs
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Is very slow (hopefully !)
Traditional ways of
handling bottom
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Defining a initial horizontal and vertical
distribution of sediments density.
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Running a consolidation model to
update this distribution.
Settled sediments acquire properties
of the surface layer.
This method needs good data and the
consideration of a consolidation
model. Allows long term simulations.
Short term simulations
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Simulations during which a
deposition zone doesn’t become
an erosion zone.
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Why is the concept useful ?
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Sediments entering in the domain will be
alternatively deposited and re-suspended
until they leave it or settle in a deposition
area.
Because erosion rates of consolidated areas
are slow !
Identifies location where vertical profiles are
need.
How to identify
deposition areas ?
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Running the model !
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Assuming there are cohesive
sediment whole over the estuary
one can identify net deposition and
erosion areas.
In “eroding areas” no sediments
easily eroded are expected to
exist.
Coupling hydro and sediment
transport models
Sediment module
concentration
Hydrodynamic
module
Settling
velocity
Bottom
exchange
Shear stresses
Ws
Water fluxes,
diffusivities,
H2O: T,S,
Shear stresses,
Geometry.
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Advection-diffusion
module
Erosion/deposition
rates
Sediment Module
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Calculation:
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Initialisation:
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Function to calculate settling
velocity as a function of
concentration
Subroutines to calculate erosion
(explicitly) and deposition (implicitly)
concentrations, parameters,
boundary conditions
The Sado Estuary
Eulerian Transport Results
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Located 40 km south
of Lisbon,
about 20 km long
and 4 km wide,
the average depth is
5m, and maximum
depth is 50m
Cohesive Sediment Simulations
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Tidal
Cycle
Spring-neap tide
Model Validation
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Hydrodynamics
Short term simulations:
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Long term simulations:
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Time series of concentrations
Time series of concentrations,
Rates of accumulation/erosion
30
30
25
5
7
11
7
Kg/s
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