Open Boundary Condition

Owned by Kester Scandrett
Last updated: 23 Apr, 2018
3 min read

The form for the open boundary condition is shown in Figure 1. In the Current Boundary Group Information frame the user may assign the Open Face Direction and Type of Open BC with drop down menus.

There are four Types of Open BC that the user may select from: 

  • Elevation Specified
  • Radiation Separation – Zero Tangential
  • Radiation Separation – Free Tangential
  • Elevation Specified – 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 for each open boundaries when they meet. When using a Radiation Separation boundary then the user must have 2 cells deactivated in the corner for each boundary. This is shown in Figure 2 where four cells are deactivated where the south and east boundaries meet. 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.



Figure 1   Boundary Condition Settings – Open Boundary.

Figure 2   Open Boundary Condition Settings – delete corner cells for Radiation Separation option.


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. Rapidly changing bottom elevation at the open boundary can lead to EFDC becoming unstable. To easily resolve this issue it is recommended that the first two cells going out in the direction of the open boundary i.e. in southward direction for the southern open boundary, be at the same elevation. To facilitate this a new tool has been added to the Open Boundary Condition form as shown in Figure 1. Selecting the Flatten Two Adjacent Interior Cells button will set all the first two cells the same bottom.  

This feature works on a group basis and uses the type ie the direction N, S, E, W, to know which cells need to be smoothed.

In EE8.4 a BC option has been which allows the use of an elevation specified flow but allows for a “free tangential”.  Normally an open BC requires two cells at the border. In some models where there are tight boundaries this can be restrictive. The Elevation Specified-Free Tangential option allows EFDC to only use one cell at the border.


Group Settings Frame

For open BC's, unlike flow BC's, the concentrations specified in the Constant Concentrations frame are always used for boundary assignments. The constant concentrations will be added to any time series concentrations defined. For example, if the user has a tidal time series of salinity that varies from 5 to 10 ppt and specifies a concentration for salinity of 10 ppt in the Constant Concentrations frame, EFDC will apply time variable salinity concentrations ranging from 15 to 20 ppt.

Interpolation Sub-frame

The Interpolation sub-frame provides access to two functions:

  • Assign generates new pressure series for the unassigned cells interpolated from the cells along the boundary that were assigned prior to pressing Assign. Interpolated series always have an ID that begins with "I:" to identify the series as an interpolated series. Each unassigned cell has its own interpolated series generated, then that series is assigned to that cell. Two or more prior assigned cells to existing series is required. The cells at either end of the open boundary must be defined for this process to work properly.
  • Reset scans the current boundary group and finds any cells whose pressure series ID begins with "I:". If it finds cells with an interpolated series, it deletes the series and sets the pressure series index equal to zero (i.e. resets the assignment).