Sediment Transport

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As a user-specified option, CMS-M2D calculates sediment transport rates and resultant changes in water depth (bottom elevation) through gradients in thetransport rates. In CMS-M2D Version 3, three transport models are available:

  1. Equilibrium total load formulation
  2. Equilibrium Bed Load plus Non-equilibrium Suspended Load
  3. Non-equilibrium Total Load

CMS-Flow accounts for the two morphologic constraints of: (a) hard bottom(non-erodible substrate) and (b) bottom avalanching if the slope exceeds aspecified value. The approach taken to treat hard bottom allows for sand toaccumulate over the hard bottom if depositional conditions are present, and italso allows material overlying the hard substrate to erode while preservingsediment volume. Avalanching is invoked if the bottom slope between two cellsexceeds a specified critical slope. The avalanching algorithm applies an iterative approach to move material down slope until the critical slope is no longerexceeded, while conserving sediment volume. The transport rate formulas that can be applied in CMS-Flow are based on the shear stress concept, implying that bed roughness and resulting friction factors are key parameters to estimate when computing transport rates. A sediment transport formula is often developed (i.e., calibrated against data) based on specific equations for calculating bed roughness and associated shear stress. Thus, when calculating the transport rate in CMS-M2D with the threeimplemented formulas, slightly different approaches are taken to determine theshear stress. In the following sections, the methods are discussed for obtainingthe shear stress for each of the sediment transport formulas. Also, it is noted thatthe roughness and associated shear stresses within the sediment transportcalculations are different from those in the CMS-M2D hydrodynamiccalculations. The computed current field and properties of surface wavescalculated from a separate model are input to the roughness and shear stresscalculations for the sediment transport, but there is presently no feedback ofroughness or shear stress from the sediment transport component of the model tothe hydrodynamic or wave calculations.

CMS-Flow calculates the current velocity in two horizontal coordinatedirections (u, v). If there is negligible contribution from the waves to thetransporting velocity (e.g., sinusoidal waves), the net transport is in the direction of the resulting current vector. Thus, waves will contribute to the mobilization and stirring of sediment, but a mean current is needed to produce a net transport.

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