Tidal Boundary Setup: When Are Harmonic Functions Necessary Alongside Downloaded Water Level Data?

1

Dear everyone,

I am currently working on hydrodynamic modeling of an estuary using EFDC 11.6. When setting up the open boundary conditions for ocean tides, I noticed that I can use downloaded water level data and also include harmonic function data. My question is: If I am using water level data downloaded directly from EFDC, do I still need to set up harmonic functions? Under what circumstances should harmonic functions be configured?

2

Hello User,
Thank you for your questions.
You can use tidal data downloaded from the EFDC tool, and you do not need to set up harmonic functions.
However, there are a few points to note:

  • If you have observed field data, that would be the best option. Data from the EFDC tool should only be used as examples.
  • If you can accurately determine the tidal constituents for your region (such as Q1, O1, etc.), you can use harmonic functions.
    Thank you!
3

Thank you for your response. Regarding the tidal data in the EFDC model, I have observed that the downloaded boundary condition data applies identical values across all open boundary cells. However, given the considerable spatial separation between these nodes (potentially spanning several kilometers), tidal wave propagation theory suggests that variations in phase and amplitude should exist among them. Could this uniform treatment be a standard configuration? If spatial variability in the tidal field needs to be accounted for, how should the boundary conditions be properly adjusted?

4

Hello, User

Thank you for raising this issue. In the EFDC model, assigning identical tidal values to all open boundary cells is often a simplified configuration, especially when tidal data lacks spatial detail or when the boundary area is relatively small. However, if the open boundary cells are spaced several kilometers apart, tidal wave propagation theory suggests there should be differences in phase and amplitude between those locations.

Applying uniform values under such conditions may introduce errors in simulating flow dynamics and sediment distribution, particularly in areas that are sensitive to tidal forcing.

However, the use of high spatial resolution tidal data is extremely rare and, in many cases, practically unfeasible. In reality, tide gauge stations may be spaced tens to even hundreds of kilometers apart along the coastline, making it challenging to construct a spatially detailed tidal boundary field.

Therefore, if the model requires high accuracy in representing the tidal field, it is important to carefully weigh the trade-off between technical complexity and the availability of input data in order to determine the most appropriate boundary condition setup. In our typical applications, we usually apply a single uniform boundary band.