22
Hydrodynamics and Sediment Transport Modelling Ramiro Neves [email protected]

Hydrodynamics and Sediment Transport Modelling Ramiro Neves [email protected]

Embed Size (px)

Citation preview

Page 1: Hydrodynamics and Sediment Transport Modelling Ramiro Neves ramiro.neves@ist.utl.pt

Hydrodynamicsand Sediment Transport Modelling

Ramiro [email protected]

Page 2: Hydrodynamics and Sediment Transport Modelling Ramiro Neves ramiro.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.

Page 3: Hydrodynamics and Sediment Transport Modelling Ramiro Neves ramiro.neves@ist.utl.pt

Instituto Superior Técnico

How do they look like

Page 4: Hydrodynamics and Sediment Transport Modelling Ramiro Neves ramiro.neves@ist.utl.pt

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.

Page 5: Hydrodynamics and Sediment Transport Modelling Ramiro Neves ramiro.neves@ist.utl.pt

Instituto Superior Técnico

Basic Processes

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

enhance re-suspension

Ws

Page 6: Hydrodynamics and Sediment Transport Modelling Ramiro Neves ramiro.neves@ist.utl.pt

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

Page 7: Hydrodynamics and Sediment Transport Modelling Ramiro Neves ramiro.neves@ist.utl.pt

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.

Page 8: Hydrodynamics and Sediment Transport Modelling Ramiro Neves ramiro.neves@ist.utl.pt

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)

Page 9: Hydrodynamics and Sediment Transport Modelling Ramiro Neves ramiro.neves@ist.utl.pt

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 !!!!

Page 10: Hydrodynamics and Sediment Transport Modelling Ramiro Neves ramiro.neves@ist.utl.pt

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

Page 11: Hydrodynamics and Sediment Transport Modelling Ramiro Neves ramiro.neves@ist.utl.pt

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)

Page 12: Hydrodynamics and Sediment Transport Modelling Ramiro Neves ramiro.neves@ist.utl.pt

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)

Page 13: Hydrodynamics and Sediment Transport Modelling Ramiro Neves ramiro.neves@ist.utl.pt

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

Page 14: Hydrodynamics and Sediment Transport Modelling Ramiro Neves ramiro.neves@ist.utl.pt

Instituto Superior Técnico

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.

Page 15: Hydrodynamics and Sediment Transport Modelling Ramiro Neves ramiro.neves@ist.utl.pt

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.

Page 16: Hydrodynamics and Sediment Transport Modelling Ramiro Neves ramiro.neves@ist.utl.pt

Instituto Superior Técnico

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.

Page 17: Hydrodynamics and Sediment Transport Modelling Ramiro Neves ramiro.neves@ist.utl.pt

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

Page 18: Hydrodynamics and Sediment Transport Modelling Ramiro Neves ramiro.neves@ist.utl.pt

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

Page 19: Hydrodynamics and Sediment Transport Modelling Ramiro Neves ramiro.neves@ist.utl.pt

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

Page 20: Hydrodynamics and Sediment Transport Modelling Ramiro Neves ramiro.neves@ist.utl.pt

Instituto Superior Técnico

Cohesive Sediment SimulationsCohesive Sediment Simulations

Tidal Cycle Spring-neap tide

Page 21: Hydrodynamics and Sediment Transport Modelling Ramiro Neves ramiro.neves@ist.utl.pt

Instituto Superior Técnico

Model Validation

Hydrodynamics Short term simulations:

Time series of concentrations Long term simulations:

Time series of concentrations, Rates of accumulation/erosion

Page 22: Hydrodynamics and Sediment Transport Modelling Ramiro Neves ramiro.neves@ist.utl.pt

Instituto Superior Técnico

7

30

25

30

5

27

Kg/s

11

1

7