1. 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 Motivation Non-cohesive formulation Biodiffusive mixing Cohesive sediment Future work 1 US Geological Survey 2 Univ. of Maryland 3 Univ. of Virginia 4 Stevens Institute ROMS/TOMS Workshop Alcalá de Henares, Nov. 6, 2006 Funded by US EPA and USGS

2. Palos Verdes Shelf near Los Angeles Contoured region: 1993 Inventory >500  g/cm 2 p,p’-DDE Spatial distribution of DDE reflects dominant transport pathway Spatial distribution of DDE reflects dominant transport pathway 9 million m 3 of effluent affected sediment within 40 km 2 9 million m 3 of effluent affected sediment within 40 km 2 DDT max  250 ppm PCB max  20 ppm DDT max  250 ppm PCB max  20 ppm Mass DDT = 120 MT Mass PCB = 12 MT Mass DDT = 120 MT Mass PCB = 12 MT 70% of DDE is on the shelf (  100 m) 70% of DDE is on the shelf (  100 m) 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 Stevens, Lewis, and Wheatcroft, 2004 Higher porosity = easier to erode (?)

5. Will the cohesive mud erode? Sediment in ROMS Non-cohesive sediment (sand and silt)Non-cohesive sediment (sand and silt) –Bed response determined by particle characteristics –Armoring caused by differential erosion Cohesive sediment (mud)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. Sediment Transport Components Suspended sediment transport when t b > t ce Erosion formulation Deposition formulation non-dimensional shear stress non-dimensional sediment flux bed load transport rate, kg m -1 s -1 Bed load transport: Meyer-Peter Muller Bed Model Active layer thickness (Harris and Wiberg, 1997)

8. Sand – Armoring

9. Massachusetts Bay Sorting of Initially Uniform Sediments Seafloor sediment distribution Modeled Observed 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 D b (z) defined by five parameters Typical values in top ~5-8 cm of the bed are 10 cm 2 /y (O 10 -7 m 2 s -1 ) Zero below some depth (~30 cm) Exponential decrease Constant (in surface layer (< 5 cm)

11. Sand - Biodiffusion

12. Cohesive Sediment Algorithm Key bed property is critical shear stress τ cr τ cr = F(depth, porosity, grain size, biology…) Assume τ cr = F(mass depth) only So bulk density ρ b is important Assume ρ b = 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 Photos and data: P. Wiberg, UVa ln( ME ) = -0.34 + 2.00 ln( τ ) 100 g/m 3

14. Erosion Initial τ cref curve (red) Application of bed stress τ b = 2 Pa Material with τ cr < 2 Pa erodes Remaining material has higher τ cr (black) τ cr gradually relaxes to new, deeper τ cr_ref

15. Deposition ρ b kg m -3

16. Sequence of Bed Operations 1.Erode / deposit to top layer 2.New layer? Add to top; combine bottom 3.[ Mix w/ mass conservation ] 4.Determine active layer thickness 5.Ensure top layer >= active layer 6.Split / combine bottom layers 7.Calc. bulk layer properties 8.[ Relax bulk density toward reference profile] 9.[ Relax τ c 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