The Courant number is defined as follows:
(6.5)
where c is the celerity, Dt the time step and Dx the grid spacing. For a tidal wave the celerity is
(6.6) 
where g is gravity and h is the water depth.
As the information (about water levels and fluxes) in the computational grid travels at a speed corresponding to the celerity, the Courant number actually expresses how many grid points the information moves in one time step.
