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

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Instituto Superior Técnico Contents of this talk n Relevance of suspended matter in estuaries and coastal lagoons, n Basic processes in sediment transport, n Coupling hydro and sediment transport models, n System modelling.

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Instituto Superior Técnico How do they look like

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Instituto Superior Técnico Relevance of sediment transport modelling n Light penetration, n Transport of chemicals, n Benthic habitat properties n Navigation channels fill-up: u dredging u deposition of dredged products.

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Instituto Superior Técnico Basic Processes n Advection-Diffusion, n Settling, Deposition/Erosion n waves, generate currents and enhance re-suspension WsWs

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Instituto Superior Técnico Settling n Sediments are denser than water and fall down. At what speed ? CdCd Re W= s gV D= C d w S (W s ) 2 Re= ( w D W s ) / WsWs (W s ) 2 =( s / w ) gD/C d

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Instituto Superior Técnico Flocculation n Formation of flocs gluing individual particles. n Increases the size of the falling particles, increasing Re and decreasing Cd. n Floc’s density depends on the properties of individual particles. A floc can include: terrigenous, detritus phyto, zoo, bacteria.

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Instituto Superior Técnico Flocculation Mechanism ( Particles must meet and glue ) n The probability of two particles to meet increases with: u number of particles (concentration) u random displacement (turbulence) n The gluing probability depends on: u number of free ions (salinity), u adhesive properties of particle surfaces (biology)

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Instituto Superior Técnico De-flocculation ( Destruction of flocs ) n 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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Instituto Superior Técnico W S =KC (salinity higher than 2‰) u 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). u For concentrations higher than the hindering settling concentration (C hs ). u Exponent m varies between 2 and 5. Calculation of settling velocity C hs WsWs C

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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”. bb 0 ( CD ) ( CE )

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Instituto Superior Técnico Erosion / deposition Rates n Erosion: n Deposition PARTHENIADES, (1965) KRONE (1962) STEPHENS et al. (1992) used A 1 =0.0012 m 2 s -2 and E=1.2 b =M sed /(total volume)

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Instituto Superior Técnico How to handle the bottom n Bottom sediment consolidate with time n Initial state must be known n what about consolidation rate ? u Is very slow (hopefully !) C hs Consolidation

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Instituto Superior Técnico Traditional ways of handling bottom n Defining a initial horizontal and vertical distribution of sediments density. u Running a consolidation model to update this distribution. n 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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Instituto Superior Técnico Short term simulations n Simulations during which a deposition zone doesn’t become an erosion zone. u Sediments entering in the domain will be alternatively deposited and re-suspended until they leave it or settle in a deposition area. n Why is the concept useful ? u Because erosion rates of consolidated areas are slow ! u Identifies location where vertical profiles are need.

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Instituto Superior Técnico How to identify deposition areas ? n Running the model ! u Assuming there are cohesive sediment whole over the estuary one can identify net deposition and erosion areas. n In “eroding areas” no sediments easily eroded are expected to exist.

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Instituto Superior Técnico Coupling hydro and sediment transport models Advection-diffusion module Hydrodynamic module Sediment module Settling velocity Bottom exchange Water fluxes, diffusivities, H 2 O: T,S, Shear stresses, Geometry. concentration Shear stresses WsWs Erosion/deposition rates

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Instituto Superior Técnico Sediment Module n Calculation: u Function to calculate settling velocity as a function of concentration u Subroutines to calculate erosion (explicitly) and deposition (implicitly) n Initialisation: u concentrations, parameters, u boundary conditions

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Instituto Superior Técnico The Sado Estuary n Located 40 km south of Lisbon, n about 20 km long and 4 km wide, n the average depth is 5m, and maximum depth is 50m

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Instituto Superior Técnico Tidal Cycle Spring-neap tide

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Instituto Superior Técnico Model Validation n Hydrodynamics Hydrodynamics n Short term simulations: u Time series of concentrations n Long term simulations: u Time series of concentrations, u Rates of accumulation/erosion

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Instituto Superior Técnico 7 30 25 30 5 2 7 Kg/s 11 1 7

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