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Modeling Earth Surface Dynamics from Source to Sink Matthew Wolinsky NCED Videoconference April 25, 2006

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Introduction NCED studies the diverse channel systems that serve as the arterial network of Earth's “Critical Zone” –mountain streams, alluvial fans, river floodplains and deltas, submarine canyons and fans, … Over geologic time channel systems sculpt erosional landscapes and deposit sedimentary records of the past On continental scales linked channel systems transport sediment from high mountain source areas to deep marine sink areas

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Processes, Environments, and Boundaries

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Bedrock-Alluvial Transition Shoreline Shelf Edge

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Sedimentary Processes and Boundary Coupling Boundary coupling is as important as sedimentary processes in determining surface dynamics and stratigraphy!

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Long Profile Model A unified framework to explore … Large-scale evolution of source to sink system –Coupled landscape, seascape, and stratigraphic evolution –signatures of paleo-environments and processes Medium-scale system evolution of sub-systems –migration of boundaries between sedimentary environments –propagation of “signals” upstream/downstream within environments Large-scale consequences of alternative process models and hypotheses developed by NCED research

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Outline of Talk 1) Overview of modeling framework –Conservation of mass and momentum between flow and sediment –Generalized morphodynamic evolution equation 2) Prototype model –Simplified bedrock-alluvial-marine model –Response to sea level cycles 3) Gravel-sand transitions –Sharp vs diffuse transitions –Physics of equilibrium transitions –Transient modeling –MATLAB Dynamic Stratigraphy Toolbox

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Conservation of Sediment Mass (Exner Equation) surface elevation, q sediment flux subsidence surface elevation = sediment thickness + basement elevation surface change = deposition/erosion + uplift/subsidence

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Sediment Flux Laws Non-Equilibrium Flux Laws bedrock incision passive settling “Fast” equilibration between sediment flux and bed/flow conditions “Slow” equilibration between sediment flux and bed/flow conditions Equilibrium Flux Laws bedload diffusion

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Exner + Flux Law Advection-Diffusion-Reaction Equation Nonlinear coefficients (Velocity, Diffusivity, Source) Conservation of (grain-size specific) sediment flux Morphodynamic Evolution Equation

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Simplified Bedrock-Alluvial-Marine Model Single grain size Lump marine processes into “diffusion” Linear coefficients + moving boundaries Nonlinear system ( Humphrey and Heller,1995; Jordan and Flemings, 1991 ) EnvironmentV Threshold Bedrock V 0 x h , q q a Alluvial aa > z SL Marine mm z SL

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Simplified Bedrock-Alluvial-Marine Model

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Over long timescales source area corresponds to region of uplift, sink area corresponds to region of subsidence “Equilibrium” bedrock-alluvial transition = tectonic transition Sea level forcing causes shoreline migration, which forces migration of bedrock-alluvial transition Boundary migration triggers upstream waves of deposition and erosion, as seen by …

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Depositional History and Moving Boundaries

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Summary of Bedrock-Alluvial-Marine Model Large change (discontinuity) in deposition across shoreline, with strong localization of deposition (erosion) near shoreline Transgression triggers upstream waves of deposition in coastal plain (“upward tilt” to deposition contours) Sea level changes cause perturbations in relative uplift, preventing equilibrium bedrock channel profiles Perturbations in bedrock erosion rates “passively” advected upstream without decay (steady tectonics)

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Gravel-Sand Transitions Typically downstream fining is relatively continuous within gravel-bed rivers and within sand-bed rivers However there is typically a rapid downstream transition in bed grain size and slope between these two river types Modeling formation and dynamics of gravel-sand transitions an essential component of the long profile model Two classes of models: explicit interface vs self-organized –Explicit interface models assume a sharp transition –Self-organized models allow for diffuse transitions

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Explicit-Interface Gravel-Sand Models (Marr et al., 2000) Two grain sizes: gravel + sand Aggregated diffusive flux law EnvironmentThreshold Deposition Gravel q g gg ∂ x q g Sand q g ss ∂ x q s

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Self-Organized Gravel-Sand Models Conservation of grain-size fractions (Hirano, 1971): P s sand fraction in transport F s sand fraction in active layer H a active layer thickness

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Grain-size specific (diffusive) flux laws Quasi-static hydrodynamic momentum balance Effective diffusivity averaged over bed composition Mixed-Grain Bedload Flux

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Preferential Transport of Sand (Ferguson, 2003) Sand more mobile than gravel preferential sand transport

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Steady State Gravel-Sand Transition Steady aggrading profile (e.g. downstream dam) Steady bed composition, constant aggradation

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Steady State Gravel-Sand Transition Steady Hirano + Flux laws system of equations for unknown sand fractions and bed shear stress Degenerate 3 rd order ODE system (index-1 DAE) Integrate downstream to solve

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Transport coefficients a function of relative shear stress … Grain-size specific reference shear stress Transport “maxes out” at large relative stress Transport Coefficients: Similarity Collapse (Wilcock and Crowe, 2003)

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Reference stress depends on mean grain size … Reference Shear Stress: Hiding Effects (Parker and Klingeman, 1982; Wilcock and Crowe, 2003) But also on constituent grain sizes … (hiding effects) –Larger grains harder to move, but protrude higher into flow –Smaller grains easier to move, but “hidden” among larger grains

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Steady Gravel-Sand Transition: Bed Composition Sharp transition at low sediment influx Diffuse transition at higher sediment influx

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Steady Gravel-Sand Transition: Bed Profile Sharp transition has abrupt slope break Diffuse transition has smooth slope

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Sharpness of transition depends on preferential transport … … So need large contrast in transport coefficients W i Depends on ratio of constituent grain sizes (potential contrast) But also on stress level (i.e. sediment influx) Scaling and Dimensional Analysis Large D * + Small Q * Abrupt transition

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Transient Gravel-Sand Modeling Changes in external forcing transient response Must solve transient Hirano with possibility of erosion –Must keep track of bed composition (i.e. stratigraphy) –Rarely able to solve transient Hirano analytically Stratigraphy is typically very dynamic –columns grow/shrink due to deposition/erosion –A difficult computational problem Use discrete data structure to efficiently store, access, and update stratigraphy … while hiding details from user

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% SedLayer data structure SedLayer = { dz,phi,tdep,F[Ngrain] } % Matlab interface routines StratPtr = InitStrat(eta0,Hactive,Ngrain) SrfLyr = GetSrfLyr(StratPtr,col) EroLyr = ColUpdate(StratPtr,col,deta) Strat = GetStrat(StratPtr) FreeStrat(StratPtr) % Matlab display routines Zcont = ContourTime(Strat,Tcont) ShadeStrat(Strat,fname) MATLAB Dynamic Stratigraphy Toolbox

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Transient Gravel-Sand Transition: Steady Forcing Time to reach equilibrium appears to depend on two factors: 1) gravel diffusion time and 2) basin filling time

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Transient Gravel-Sand Transition: Cyclic Forcing (P s0 ) Sharp transition simulation has small fluctuations in interface position, but large fluctuations in slope (unconformities)

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Example MATLAB Routine

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Future Work Complete alluvial component of long-profile model –gravel-sand model, floodplain evolution (mud) Refine and implement marine process models –turbidity currents, sand-mud dynamics Input from NCED community –What problems/questions are of interest? –What capabilities are needed to address these? –What sedimentary processes/models are appropriate? –MATLAB Dynamic Stratigraphy Toolbox (anybody interested?)

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Acknowledgements Vaughan Voller Chris Paola Participants in the 2 nd NCED Workshop on Geomorphic Interfaces

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