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The Littoral Sedimentation and Optics Model (LSOM)

The Littoral Sedimentation and Optics Model (LSOM). Timothy Keen Naval Research Laboratory Code 7322 Stennis Space Center, Mississippi 10 May 2002. Motivation.

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The Littoral Sedimentation and Optics Model (LSOM)

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  1. The Littoral Sedimentation and Optics Model (LSOM) Timothy Keen Naval Research Laboratory Code 7322 Stennis Space Center, Mississippi 10 May 2002

  2. Motivation The Navy needs a capability to predict nearshore bottom properties and water column optical characteristics at forecast intervals up to 72 hours.

  3. OUTLINE • Overview of LSOM • Resuspension • Active Layer • Transport Algorithms • Mass (bed) Conservation • Bed Definitions

  4. LSOM: OVERVIEW • 2D finite-difference computation grid • 1D BBLM computes profiles • Mass-conservation equations for spectrum of silt/sand sizes • Mud algorithms • Bed load, advection, and diffusion terms • Spectral and total erosion and deposition • Algorithms for optical scattering and diver visibility

  5. LSOM: FLOW INPUT: 2D current, wave, and sediment parameter fields 2D GRID LOOP BBLM u*, C0, hA, etc. SIZE LOOP Sediment profiles Resuspension depth HR Recompute profiles YES Reduce reference concentration. HR > hA Bed, suspended, and diffusion fluxes OUTPUT: 2D beds; sediment profiles; scattering coefficients; diver visibility; Solve mass conservation equation Bed parameters Integrated bed parameters

  6. BBLM: FLOW INPUT: CURRENT, WAVE, AND SEDIMENT PARAMETERS Guess reference current UA and physical roughness kb Find skin friction Calculate kb and reference concentrations Find total shear stress using kb and UA OUTPUT: SHEAR STRESSES; CURRENT PROFILE; SEDIMENT REFERENCE CONC., C0;RIPPLE PARAMETERS Find mean and maximum shear stresses u*c and u*cw, apparent bottom roughness kbc, and wave boundary layer height Find current profile Calculated and specified reference currents match? NO YES

  7. SILT/SAND REFERENCE CONCENTRATION • Instantaneous reference concentration cn(z0) • Average over wave period to find mean concentration cmn(z0) normalized excess skin friction bed concentration size class instantaneous resuspension coefficient

  8. COHESIVE SEDIMENT DYNAMICS: ENTRAINMENT • Resuspended concentration*: Empirical Coefficients (kg/m2) Time since deposition • Need to examine the empirical coefficients a, n, and m using mineralogical and chemical data. • Replace power-law formulation with physical models for clay diagenesis and entrainment.

  9. SUSPENDED SEDIMENT PROFILES Dimensionless fall velocity Mean concentration within wave boundary layer: Mean concentration above wave boundary layer: Diffusivity parameter Wave boundary layer height

  10. ACTIVE LAYER CALCULATION • The BBLM computes a resuspension depth for each size class. • depth is limited by: • near-bed transport: • ripple height: Relative density Critical Shields parameter Max Shields parameter diameter Break-off range Equilibrium range Sediment parameter Wave orbital diameter

  11. COUPLING BBLM TO TRANSPORT MODEL • The BBLM is applied independently at each grid point on a 2D horizontal grid. • The corrected suspended sediment profiles are coupled to 2D transport equations for all size classes. • The active layer is found from near-bed transport and ripple height.

  12. MASS-CONSERVATION EQUATIONS • The suspended sediment ADVECTION flux in x direction for size n:

  13. BED LOAD • Modified Bagnold formulation: Thrust height on grain Critical shear velocity Critical shear stress Bed concentration Dynamic friction coefficient

  14. SEDIMENT DIFFUSION EQUATIONS • The suspended sediment DIFFUSION flux in x direction for size n: Horizontal diffusivity • AH is Smagorinsky formulation

  15. MASS-CONSERVATION EQUATIONS • Total derivative in x direction for size n: Suspended sediment diffusion flux Suspended sediment advection flux Elevation change Bed porosity Sediment density Bedload transport rate Y dimension

  16. BED DEFINITIONS • Storm Bed: thickness of reworked sediment • Resuspension Bed: equivalent thickness of resuspended sediment • Transported Bed: sediment deposited after transport by steady currents from another location

  17. INSTANTANEOUS TOTAL BEDS Erosion Resuspension Initial Deposition Reference Elevation ZR Bed Elevation ZB Apparent Erosion Depth ZS Resuspended Bed Storm Bed Transported Bed These definitions apply to a single time step in the model. BT BC BR

  18. CUMULATIVE TOTAL BEDS Transport Bed BT Resuspended Bed BR Total bed BC ZB ZR ZS BT = ZB - ZR BC = ZB - ZS BR = ZR - ZS These beds are present after last model integration.

  19. CONCLUSIONS • LSOM is a modular, scalable, multipurpose model for Navy needs. • It is being enhanced with cohesive sediment algorithms. • A PC-based windows version is currently under development.

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