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The EFDC code is capable of simulating the transport and fate of multiple size classes of cohesive and noncohesive suspended sediment including bed deposition and resuspension. Water column transport is based the same high order advection -diffusion scheme used for salinity and temperature. A number of options are included for the specification of settling velocities. For the transport of multiple size classes of cohesive sediment, an optional flocculation model (Burban, et al., 1989&1990) can be activated. Sediment mass conservative deposited bed formulations are included for both cohesive and noncohesive sediment. The deposited bed may be represented by a single layer or multiple layers. The multiple bed layer option provides a time since deposition versus vertical position in the bed relationship to be established. Water column -sediment bed interface elevation changes can be optionally incorporated into the hydrodynamic continuity equation. An optional, one dimensional in the vertical, bed consolidation calculation can be performed for cohesive beds. step as the external, is implicit with respect to vertical diffusion. The internal solution of the momentum equations is in terms of the vertical profile of' shear stress and velocity ® shear, which results in the simplest and most accurate form of the baroclinic pressure gradients and eliminates the over determined character of alternate internal mode formulations. Time splitting inherent in the three time level scheme is controlled by periodic insertion of a second order accurate two time level trapezoidal step. The EFDC A model is also readily configured as a two-dimensional model in either the horizontal or vertical planes. The EFDC model implements a second order accuracy in space and time, mass conservation fractional step solution scheme for the Eulerian transport equations for salinity, te-nperature, suspended sediment, water quality constituents and toxic comtaninants. The transport equations are temporally integrated at the same time step or twice the time step of the momentum equation solution (Smolarkiewicz and Margolin, 1993). The advective step of the transport solution uses either the central difference scheme used in the POM or a hierarchy of positive definite upwind difference schemes. The highest accuracy upwind scheme, second order accurate in space and time, is based on a flux corrected transport version Smolarkiewicz's multidimensional positive definite advection transport algorithm (Smolarkiewicz & Clark, 1986, Smolarkiewicz & Grabowski, 1990) which is monotonic and minimizes numerical diffusion. The horizontal diffusion step, if required, is explicit in time, while the vertical diffusion step is implicit. Horizontal boundary conditions include time variable material inflow concentrations, upwinded outflow, and a damping relaxation specification of climatological boundary concentration. For the temperature transport equation, the NOAA Geophysical Fluid Dynamics Laboratory's atmospheric heat exchange model (Rosati & Miyakoda, 1988) is implemented. A.3 Cohesive Sediment Transport The EFDC code is capable of simulating the transport and fate of multiple size classes of cohesive and noncohesive suspended sediment including bed deposition and resuspension. Water column transport is based the same high order advection -diffusion scheme used for salinity and temperature. A number of options are included for the specification of settling velocities. For the transport of multiple size classes of cohesive sediment, an optional flocculation model (Burban, et al., 1989&1990) can be activated. Sediment mass conservative deposited bed formulations are included for both cohesive and noncohesive sediment. The deposited bed may be represented by a single layer or multiple layers. The multiple bed layer option provides a time since deposition versus vertical position in the bed relationship to be established. Water column -sediment bed interface elevation changes can be optionally incorporated into the hydrodynamic continuity equation. An optional, one dimensional in the vertical, bed consolidation calculation can be performed for cohesive beds.