open TELEMAC-MASCARET The mathematically superior suite of solvers

hello
I want to ask about something.i'm trying to simulate effect of tide on a lagoon.
the water depth at boundaries equal to zero and the bottom elevation is 12m at first. after that there is a wave because of tide which has an increase of 12.3m water elevation and then decrease of 11.7m water elevation.
when the water elevation goes below 12m ( for example 11.7m) and we know that the bottom elevation equals to 12m. so the water depth will be negative and telemac deals with it as zero water depth. how can i manage this?