Sediment Model
The ROMS model represents sediment using separate cohesive and non-cohesive categories, each with an unlimited number of user-defined size classes. Each class has fixed attributes of grain diameter, density, settling velocity, critical shear stress for erosion, and erodibility constant. These properties are used to help determine the bulk properties of each bed layer. In the present version, although the framework exists, cohesive sediment dynamics (i.e. consolidation, flocculation) have not yet been implemented.
Sediment bed The sediment bed is represented by three-dimensional arrays with a fixed number of layers beneath each horizontal model cell. Each cell of each layer in the bed is initialized with a thickness, sediment-class distribution, porosity, and age. The mass of each sediment class in each cell can be determined from these values and the grain density. The bed framework also includes two-dimensional arrays that describe the evolving properties of the seabed, including bulk properties of the surface layer (active layer thickness, mean grain diameter, mean density, mean settling velocity, mean critical stress for erosion) and descriptions of the subgrid scale morphology (ripple height and wavelength). These properties are used to estimate bed roughness in the BBL formulations and feed into the bottom stress calculations. The bottom stresses are then used by the sediment routines to determine resuspension and transport, providing a feedback from the sediment dynamics to the hydrodynamics.
The bed layers are modified at each time step to account for erosion and deposition and track stratigraphy. At the beginning of each time step, an active layer thickness za is calculated based on the relation of Harris and Wiberg (1997). The thickness of the top bed layer has a minimum thickness equivalent to za. If the top layer is thicker than za, no action is required. If the top layer is less than za thick, then the top layer thickness is increased by entraining sediment mass from deeper layers until the top layer thickness equals za. If sediment from deeper than the second layer is mixed into the top layer, the bottom layer is split to enforce a constant number of layers and conservation of sediment mass.
Each sediment class can be transported by suspended-load and/or bedload (described below). Suspended-load mass is exchanged vertically between the water column and the top bed layer. Mass of each sediment class available for transport is limited to the mass available in the active layer. Bedload mass is exchanged horizontally between the top layers of the bed. Mass of each sediment class available for transport is limited to the mass available in the top layer.
Suspended-sediment that is deposited, or bedload that is transported into a computational cell, is added to the top bed layer. If continuous deposition results in a top layer thicker than a user-defined threshold, a new layer is provided to begin accumulation of depositing mass. The bottom two layers are then combined to conserve the number of layers. After erosion and deposition have been calculated, the active-layer thickness is recalculated and bed layers readjusted to accommodate it. This step mixes away any very thin layer (less than the active layer thickness) of newly deposited material. Finally the surficial sediment characteristics, such as D50, ripple geometry, etc., are updated and made available to the bottom stress calculations.
Suspended-sediment transport Sediment suspended in the water column is transported, like other conservative tracers (e.g., temperature and salinity) by solving the advection–diffusion equation (5) with a source/sink term for vertical settling and erosion. The vertical advection algorithm includes a piece-wise parabolic method (Colella and Woodward, 1984) and a weighted essentially non-oscillatory (WENO) scheme (Liu et al., 1994). This method allows the integration bounds of depositional flux to use multiple grid boxes in the vertical direction, so it is not constrained by the CFL criterion. Zero-flux boundary conditions are imposed at the surface and bottom in the vertical diffusion equation. The source or sink term in the advection equation represents the net of upward flux of eroded material and downward settling.
Bedload transport This version of ROMS implements two methods for computing bedload transport: 1) the Meyer-Peter Müeller (1948) formulation for unidirectional flow and 2) the formulae of Soulsby and Damgaard (2005) that accounts for combined effects of currents and waves. The formulae depend on the characteristics of individual sediment classes, including size D, density , specific density in water , and critical shear stress . Non-dimensional transport rates Φ are calculated for each sediment class and converted to dimensional bedload transport rates.
A more detailed description can be found in :
"Development of a three-dimensional, regional, coupled wave, current, and sediment-transport model" by
Warner, Sherwood, Signell, Harris, and Arango
submitted to Computers and Geosciences.
