In EFDC there is an option to use the harmonic constituents or tides are the boundary conditions. It is very important to understand all the parameters in the EFDC model. The model allows you to enter the Harmonic constituents symbols and the period and amplitude and phase for each of the tide constituents.
To compute the tidal constituent itself is a difficult thing. However, there are certain codes / softwares available which would allow us to convert the water surface elevations into the harmonic constituents. It should however be noted that while using the harmonic constituents the constant elevation of the average depth should be used combined with the harmonic constituents in order to account for the offset from the MLLW or whatever datum you use. Most of the times we use the MLLW dataum.