the grid and river survey condition were imported, and the boundary condition was setted. The type of slover is chosen as standard one, the time step was 0.1sec, the calculation was failured. However, when i took 0.001sec as the time step, it worked. So how can i choose the right time step and what might be the reason when a calculation is failured? initial condition setting? the time step? or the topography is too complex?