"Water Depth" is the depth of the water at your location. If you need to consider tidal changes, then you can use WAV1, WAV2, etc where you vary the depth.
"Free Surface Elevation" does work in connection with the global coordinates you set in the piping input. For tidal considerations, it would be best if the first node of the model was at the top, with the model coded down to the bottom. Then the free surface would always be zero and all you would have to do is change the water depth. If the first node is at the bottom, and the model was coded upward, then you would need to adjust the free surface to consider the "other" tidal condition.
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Regards,
Richard Ay - Consultant