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Motivation Non-cohesive formulation Biodiffusive mixing Cohesive sediment Future work

Motivation Non-cohesive formulation Biodiffusive mixing Cohesive sediment Future work. Cohesive Sediment Algorithms in ROMS and Sediment Test Cases Chris Sherwood 1 , Larry Sanford 2 , John Warner 1 Bénédicte Ferré 1 , Courtney Harris 3 , Rich Signell 1 , and Alan Blumberg 4.

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Motivation Non-cohesive formulation Biodiffusive mixing Cohesive sediment Future work

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  1. Motivation Non-cohesive formulation Biodiffusive mixing Cohesive sediment Future work Cohesive Sediment Algorithms in ROMSand Sediment Test CasesChris Sherwood1, Larry Sanford2, John Warner1Bénédicte Ferré1, Courtney Harris3, Rich Signell1, and Alan Blumberg4 1US Geological Survey 2Univ. of Maryland3Univ. of Virginia 4Stevens Institute ROMS/TOMS Workshop Alcalá de Henares, Nov. 6, 2006 Funded by US EPA and USGS

  2. Palos Verdes Shelf near Los Angeles • Spatial distribution of DDE reflects dominant transport pathway • 9 million m3 of effluent affected sediment within 40 km2 • DDTmax250 ppm PCBmax  20 ppm • MassDDT = 120 MTMassPCB = 12 MT • 70% of DDE is on the shelf ( 100 m) Contoured region:1993 Inventory >500 mg/cm2p,p’-DDE Lee et al., 2002

  3. Palos Verdes Deposit- Two cohesive mud layers- DDE in lower layer- No sediment supply- Erosion of SE edge?

  4. Porosity (from resistivity and water content)at six sites in Feb 04 Higher porosity =easier to erode (?) Stevens, Lewis, and Wheatcroft, 2004

  5. Will the cohesive mud erode? Sediment in ROMS • Non-cohesive sediment (sand and silt) • Bed response determined by particle characteristics • Armoring caused by differential erosion • Cohesive sediment (mud) • Bed response determined by bulk characteristics • Armoring caused by compaction (and biogeochemistry)

  6. Sediment Variables • Sediment class variables dimension(NST) • Median size, particle density, settling velocity, critical shear stress • Bottom variables dimension(NX,NY,MBOTP) • Average grain size, critical shear stress, ripple geometry, hydraulic roughness, parameters to specify “reference” critical shear stress and biodiffusion profiles, cohesive time scale • Bed variables dimension(NX,NY,Nbed,MBEDP) • Thickness, volume solids fraction of each class, porosity, age, critical shear stress, biodiffusivity • Bed mass dimension (NX,NY,NBed,NST) • Bed fraction dimension (NX,NY,NBed,NST) NST = # non-cohesive + # cohesive sediment types MBOTP = # of bottom parameters MBEDP = # of bed parameters Nbed = # of bed layers

  7. Bed Model Active layer thickness (Harris and Wiberg, 1997) Sediment Transport Components Suspended sediment transport Erosion formulation when tb > tce Deposition formulation Bed load transport: Meyer-Peter Muller non-dimensional shear stress non-dimensional sediment flux bed load transport rate, kg m-1s-1

  8. Sand – Armoring

  9. Massachusetts BaySorting of Initially Uniform Sediments Seafloor sediment distribution Observed Modeled Warner, J.C., Butman, B., and Dalyander, P.S. (submitted) "Storm-driven sediment transport in Massachusetts Bay"

  10. Biodiffusive Mixing • Implicit solution of diffusion equation • Mixing profile Db(z) defined by five parameters • Typical values in top ~5-8 cm of the bed are 10 cm2/y (O 10-7 m2 s-1) Constant (in surfacelayer (< 5 cm) Exponential decrease Zero below some depth (~30 cm)

  11. Sand - Biodiffusion

  12. Cohesive Sediment Algorithm • Key bed property is critical shear stress τcr • τcr = F(depth, porosity, grain size, biology…) • Assume τcr = F(mass depth) only • So bulk density ρb is important • Assume ρb = F(depth) only • Assumes bed properties tend toward reference profiles • Determine reference profiles empirically • When system is perturbed (erosion or deposition), nudge back toward reference profiles with appropriate time scale

  13. Erosion Chamber Data 100 g/m3 ln( ME ) = -0.34 + 2.00 ln( τ ) Photos and data: P. Wiberg, UVa

  14. Erosion • Initial τcref curve (red) • Application of bed stress τb = 2 Pa • Material with τcr < 2 Pa erodes • Remaining material has higher τcr (black) • τcr gradually relaxes to new, deeper τcr_ref

  15. Deposition ρb kg m-3

  16. Sequence of Bed Operations • Erode / deposit to top layer • New layer? Add to top; combine bottom • [ Mix w/ mass conservation ] • Determine active layer thickness • Ensure top layer >= active layer • Split / combine bottom layers • Calc. bulk layer properties • [ Relax bulk density toward reference profile] • [ Relax τc profile toward reference profile ]

  17. Geostatistical Simulations of Erodibility • Monte Carlo estimates of the slope term in the critical erosion profile • How does spatial variability affect sediment-transport calculations? Chris Murray, Pacific Northwest National Lab

  18. Next Steps • Get the bugs out • Combine cohesive and non-cohesive calculations • Investigate sensitivity to time scale • Apply to Palos Verdes • Long term: try to characterize reference curves from bed properties

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