I have a project in Alabama that is located on the south end, not very close to the coast, but close to a river connected to the coast. I have never used MSL, but I am working with an engineer that has a need to use MSL. As a designer, I would like to know what the cut-off distance is between use of an MSL tidal gauge and/or NAVD88. I believe some coastal areas still use MSL, but there must be a rule for knowing when it is no longer accurate or applicable. In my limited research, I have heard of 1-2 miles as rule of thumb (upland of gauge), but not found any reference to this in a white paper form or scientific type documentation, but there must be something?
Thanks in advance for your help with this, greatly appreciated.