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There are three Types of Open BC that the user may select from: Elevation Specified, Radiation Separation - Zero Tangential, and Radiation Separation - Free Tangential. Elevation Specified is the standard approach for water surface elevations, using a combination of harmonic constituents and time series. With the Radiation Separation boundary condition the incoming wave at an open boundary is separated from the outgoing wave and the incident wave is assumed to be twice the surface elevation required (Zhou, 2014). By default, the outgoing characteristic is left undefined, allowing waves generated interior to the model domain to pass outward across the boundary with no reflection. When using the Elevation Specified option the user should deactivate one cell in the corner of the grid where two open boundary directions meet. When using a Radiation Separation boundary then the user must have 2 cells deactivated in the corner as shown in Open Boundary Condition Editor. This is because an open boundary always sets a mask in E-W direction for a S boundary, likewise an open boundary sets a mask in N-S direction for a W boundary. Users should also be aware that if they have rapidly changing bathymetry at the open boundary then it is possible to get mass balance errors.
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