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The next basic step is checking convergence of the numerical code to a certain asymptotic limit, presumably the actual solution of the equations solved. The grid steps $\Delta x$ and $\Delta y$ need to be halved, and the time step $\Delta t$ reduced appropriately to conform with the Courant-Friedrics-Levy (CFL) criterion. The optimal locations to check convergence are the extreme runup and rundown. A graph needs to be prepared presenting the variation of the calculated runup and rundown (ordinate) with the step size (abscissa). As the step size is reduced, the numerical predictions should be seen to converge to a certain value, and further reductions in step size should not change the results.