Spatial resolution is extremely important for accurate solution of the Saint-Venant equations. In general higher Froude numbers and shallow flows require finer spatial resolution.
Open channel systems are modelled by taking the channel parameters and computing levels and discharges at a set of discrete cross-sections. Each cross-section is separated from the next by a distance Dx. Likewise the solution is carried forward in time by a series of discrete timesteps Dt.
The rules of thumb for estimating values for Dx and Dt are different for Tidal and Non-Tidal reaches.