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Instituto Superior Técnico
Contents of this talk
Relevance of suspended matter in estuaries and coastal lagoons,
Basic processes in sediment transport,
Coupling hydro and sediment transport models,
System modelling.
Instituto Superior Técnico
How do they look like
Instituto Superior Técnico
Relevance of sediment transport modelling Light penetration, Transport of chemicals, Benthic habitat properties Navigation channels fill-up:
dredging deposition of dredged products.
Instituto Superior Técnico
Basic Processes
Advection-Diffusion, Settling, Deposition/Erosion waves, generate currents and
enhance re-suspension
Ws
Instituto Superior Técnico
Settling
Sediments are denser than water and fall down. At what speed ?
Cd
Re
W=sgV
D= Cd wS (Ws)2
Re= (wD Ws) /
Ws
(Ws)2 =( s /w) gD/Cd
Instituto Superior Técnico
Flocculation
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.
A floc can include:• terrigenous, detritus
• phyto, zoo, bacteria.
Instituto Superior Técnico
Flocculation Mechanism(Particles must meet and glue) The probability of two particles to meet
increases with: number of particles (concentration) random displacement (turbulence)
The gluing probability depends on: number of free ions (salinity), adhesive properties of particle surfaces
(biology)
Instituto Superior Técnico
De-flocculation (Destruction of flocs) Needs a force do separate the
particles. Shear (and thus turbulence) is the main de-flocculation mechanism.It’s a pleasure to
travel with you !
Move faster !!
I can’t !!
Don’t leave me !!!!
Instituto Superior Técnico
WS=KC (salinity higher than 2‰)
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).
For concentrations higher than the hindering
settling concentration (Chs).
Exponent m varies between 2 and 5.
Calculation of settling velocity
mhshss CCkkCW 21
Chs
Ws
C
Instituto Superior Técnico
Erosion and Deposition
Bottom erosion and deposition occurs simultaneously. For experimental convenience reasons erosion/deposition are defined as “net erosion” and “net deposition”.
b
0 (CD) (CE)
Instituto Superior Técnico
Erosion / deposition Rates Erosion:
Deposition
1
CE
bE E
CD
bbsD CW
1
PARTHENIADES, (1965)
KRONE (1962)
EbCE A 1STEPHENS et al. (1992) used
A1=0.0012 m2s-2 and E=1.2
b=Msed/(total volume)
Instituto Superior Técnico
How to handle the bottom
Bottom sediment consolidate with time Initial state must be known what about consolidation rate ?
Is very slow (hopefully !)
Chs
Consolidation
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Traditional ways of handling bottom Defining a initial horizontal and vertical
distribution of sediments density. 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.
Instituto Superior Técnico
Short term simulations Simulations during which a
deposition zone doesn’t become an erosion zone. Sediments entering in the domain will be
alternatively deposited and re-suspended until they leave it or settle in a deposition area.
Why is the concept useful ? Because erosion rates of consolidated areas
are slow ! Identifies location where vertical profiles are
need.
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How to identify deposition areas ? Running the model !
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.
Instituto Superior Técnico
Coupling hydro and sediment transport models
Advection-diffusion module
Hydrodynamic
module
Sediment module
Settling velocity
Bottom exchange
Water fluxes,diffusivities,H2O: T,S,
Shear stresses,Geometry.
concentration
Shear stresses
Ws
Erosion/depositionrates
Instituto Superior Técnico
Sediment Module
Calculation: Function to calculate settling
velocity as a function of concentration
Subroutines to calculate erosion (explicitly) and deposition (implicitly)
Initialisation: concentrations, parameters, boundary conditions
Instituto Superior Técnico
The Sado Estuary
Located 40 km south of Lisbon,
about 20 km long and 4 km wide,
the average depth is 5m, and maximum depth is 50m
EulerianEulerian Transport Results Transport Results
Instituto Superior Técnico
Cohesive Sediment SimulationsCohesive Sediment Simulations
Tidal Cycle Spring-neap tide
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Model Validation
Hydrodynamics Short term simulations:
Time series of concentrations Long term simulations:
Time series of concentrations, Rates of accumulation/erosion
Instituto Superior Técnico
7
30
25
30
5
27
Kg/s
11
1
7