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Development of a Sediment Transport Model for the Chesapeake Bay: Supporting Physical Data

Development of a Sediment Transport Model for the Chesapeake Bay: Supporting Physical Data. Co-PIs: Lawrence P. Sanford 1 Carl T. Friedrichs 2 Jerome P.-Y. Maa 2 1 University of Maryland Center for Environmental Science, Horn Point Laboratory, Cambridge MD

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Development of a Sediment Transport Model for the Chesapeake Bay: Supporting Physical Data

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  1. Development of a Sediment Transport Model for the Chesapeake Bay: Supporting Physical Data Co-PIs: Lawrence P. Sanford1 Carl T. Friedrichs2 Jerome P.-Y. Maa2 1University of Maryland Center for Environmental Science, Horn Point Laboratory, Cambridge MD 2College of William and Mary, Virginia Institute of Marine Science, Gloucester Point, VA

  2. Overall Project Description • Year 1, 2004 • Deliver physical data from previous studies • 4 seasonal axial surveys of upper half of Potomac • 5 d process study (3 vessels) near Gunston Cove • All surveys complete, data return good (not perfect), data mostly processed • Year 2, 2005 • Deliver Year 1 data • 4 seasonal axial surveys of upper half of Potomac • 5 d process study (3 vessels) of Potomac ETM region • All surveys complete, data return very good, data partially processed • Year 3, 2006 • Complete data processing and analysis • Deliver Year 2 data

  3. Conclusions, part 1

  4. 2004 Intensive Erosion Testing Sites

  5. 2005 Intensive Erosion Testing Sites

  6. Stress Limited, Locally Linear Erosion

  7. 2005 Site 1 in the ETM channel, compared toall 2002 CB ETM cores is remarkably similar

  8. 2005 Site 1 in the ETM channel, compared to 2005 Site 2 in the channel downstream of the ETM. Site 2 is less erodible.

  9. 2004 Site 2 opposite Gunston Cove, compared to 2005 Site 3 at same location. 2005 is similar, slightly less erodible

  10. 2004 Site 1 inside Gunston Cove, compared to 2004 Site 3 on the inside channel edge. Site 1 is similar to Site 2, not very erodible. Site 3 is similar to 2005 Site 1, in the ETM channel.

  11. Conclusions, part 2

  12. Modeling resuspension and deposition with a dynamically varying mixed sediment bed

  13. Consolidation causes tc to increase rapidly with depth into the bed and with time after deposition.Example critical stress (shear strength) profile from laboratory tests by Parchure and Mehta (1985)

  14. Erodibility can change significantly in response to disturbance.Passage of a tropical storm, upper Chesapeake Bay, Sept 1992. Dredged sediment disposal site, 5 m depth.

  15. Interactions between physical and biological disturbance of the sea bed can lead to distinct layering or near homogeneity ( x-radiographs courtesy of Linda Schaffner) Increasing physicaldisturbance of sea bed 10 cm Bioturbation dominates fine sediment fabric at Chesapeake Bay site Physical disturbance overwhelms bioturbation at upper York River site

  16. Present approach: • Use a layered bed model with continuous profiles of tc, layer-averaged erosion constant M and sand fraction fs • Use sediment bed mass m as independent variable instead of depth (better for consolidation) • 2-component mixture of sand and mud • Separate erosion parameters for sand and mud (interaction effects not yet incorporated) • Erosion rates proportional to fractions at interface • Mud erosion follows Sanford and Maa (2001) • Sand erosion follows Harris and Wiberg (2001) • Assume constant tc,sand • Assume that tc,mud approaches an equilibrium profile at a first order rate g • Allow for sediment mixing (bioturbation, bedload transport) • Subtract or add active bed layers to follow the interface during erosion or deposition • Only mix mass between layers when a threshold is exceeded (minimizes numerical dispersion) • Model evolution of tc, mud and fs as a function of m and time

  17. Critical Stress and Erosion Rate Constant, all upper Chesapeake Bay UMCES microcosm data from 2001-02

  18. Example: Erosion, deposition, and consolidation of a pure mud and a sand-mud mixture • Bed consists of 20 layers 0.075 kg m-2 thick • Critical stress profile initiated with average of upper Chesapeake Bay microcosm profiles, also assumed to be equilibrium profile • Assume M=constant=0.001 kg m-2 s-1 Pa-1 • Spring-Neap cycle of tidal shear stress, max varies between 0.15-0.25 Pa • A 2-day event from day 20-22 doubles the max stress • A 2-day event from day 34-36 triples the max stress • Very low sediment mixing of 0.01 cm2 yr-1 • wsm = 36 m d-1, h = 2 m • Consolidation rate = 1.0 d-1, swelling rate = 0.05 d-1 • wss = 363 m d-1, tcsand = 0.125 Pa (fine sand)

  19. All mud, very low sediment mixing

  20. 50/50 sand-mud, very low sediment mixing

  21. 50/50 sand-mud, sediment mixing 10 cm2 yr-1

  22. Conclusions • Layered bed model for critical stress profile in terms of bed mass simplifies formulation. • Specification of equilibrium conditions based on observed erosion behavior promising, but may need tweaking. • “Consolidation” formulation predicts reasonable behavior with little computational effort, but needs more validation • Mud-sand mixture and diffusive mixing schemes lead to realistic complex bed structures and directly affect resuspension

  23. Conclusions, continued • Need to incorporate sand-mud interaction effects on erodibility • Need to simplify code, translate into Fortran subroutine(s) for incorporation into sediment transport models • Community modeling/open source code approach favored

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