Hello,
This keyword is no longer used and is kept only for the compatibility with already existing models. You can remove it from your steering files. The old option consisted in using the Shallow Water Equations in the algorithm but a finer theory showed that there is a mistake in doing this, which is that discretising and then depth-averaging is different from depth-averaging and then discretising. Now the 3D continuity equations are first discretised then summed on the vertical to give a 2D continuity equation that provides the new depth at every time step.
With best regards,
Jean-Michel Hervouet