# Best practice of using harmonic boundary condition in EFDC Explorer

I have some confusion about the harmonic boundary conditions in EFDC Explorer. I would appreciate if anyone could help me with these boundary condition issues:

• In EFDC model there is an option to use the harmonic constituents as the boundary. When we are using the harmonic constituents, then I think we also need to provide the offset of the average of the series from the reference datum (most of the times MLLW). Sometimes, while using the harmonic constituents, we might be losing a large chunk of residual difference which might affect our results. Would you please suggest me what are the best practices on using the harmonic constituents as the boundary conditions?
• I used the observed water surface elevation as the model calibration boundary. However, for the scenario studies associated with the model I want to use the harmonic boundary condition. While using the harmonic constituent how can I account for the residual error especially when harmonic constituents and the observed water surface have difference?
• Would you please help me explain the three options that EFDC model has for specifying the harmonic constituents. They are constant, linear and quadratic. Under what conditions we need to use these options ? I believe the original EFDC model only has one option and also has an option to add mean sea-level which EFDC doesn’t have. I believe we need to combine the water surface elevation and the harmonic constituents also which defining the harmonic constituent boundary condition.

Thank you so much

John

Hi John,

When specifying an open boundary, you can specify both a harmonic series and a pressure series. The harmonic series is added to the pressure series for the final pressure to be used by EFDC in the solution. That is how you would adjust for a datum shift (i.e. the pressure series would just be a constant offset).

The difference between constant, linear and quadratic are a bit more uncertain. We implemented these methods when updating EFDC_DSI to the latest EFDC_GVC a couple of years ago. I believe these were added to EFDC by TT for the Florida Bay model. We validated the EE/EFDC_DSI computations against the EFDC_GVC version to ensure that they were being computed properly. However, we have never seen any documentation on these options. You can look at the EFDC code and see how they are computed, but basically, when using the linear and quadratic options, you specify an additional one or two amplitude and phase for each tidal constituent, respectively. EFDC varies the final (i.e. applied) phase and amplitude for each tidal constituent using the specified method and values. If you come on better information and/or documentation we would love to also have a copy.

Regards,

Paul