*2.3. Modeling Approach*

To account for the multi-scale physics of sediment delivery from rivers to the Gulf of Mexico, and subsequent mobilization by oceanic flows, the workflow was designed to operate as follows. Each model component was designed to deliver needed model inputs to the "downstream" models in a one-way coupling framework (Figure 1). Phasing of model development required coordination among the subject matter experts who developed various components of the workflow. The river discharge model (*WBMsed*) can provide values needed as input to the hydrodynamic ocean model. *ROMS* can use these discharges as point sources of freshwater and sediment, distributing the output from *WBMsed* for individual rivers onto the three-dimensional grid of the hydrodynamic model. For example, *WBMsed* provided Mississippi River discharges that were distributed to 39 Mississippi

River discharge grid cells that were spread around the bird-foot delta of the *ROMS* Grid-f. Then, using input winds and open boundary conditions from the lower resolution Gulf model (Grid-g), the local model (Grid-f, see Figure 4) was used in the *CSTMS* to estimate the dispersal and deposition of sediment delivered from the rivers. The bed stresses calculated by *ROMS* accounted for wave-current bed shear stress, and along with *WaveWatch III*® data, could be employed by the *HurriSlip* modules to identify times and locations of sediment mass-failure and density-flow ignition. These events detected by *HurriSlip*, could be used to trigger a turbidity current calculation via *RANS*/*TURBINS*; which would also be informed by topographic gradients, the sediment properties from *dbSEABED*, and near-bed current velocities and sediment depositions calculated by the *ROMS*/*CSTMS*.
