Hydrodynamics and Sediment Transport Modelling Ramiro Neves ramiro.neves@ist.utl.pt.

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Transcript of Hydrodynamics and Sediment Transport Modelling Ramiro Neves ramiro.neves@ist.utl.pt.

Hydrodynamicsand Sediment Transport Modelling

Ramiro Nevesramiro.neves@ist.utl.pt

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.

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How do they look like

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Relevance of sediment transport modelling Light penetration, Transport of chemicals, Benthic habitat properties Navigation channels fill-up:

dredging deposition of dredged products.

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Basic Processes

Advection-Diffusion, Settling, Deposition/Erosion waves, generate currents and

enhance re-suspension

Ws

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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

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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.

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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)

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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 !!!!

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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

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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)

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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)

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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.

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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.

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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

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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

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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

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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

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7

30

25

30

5

27

Kg/s

11

1

7