Model Construction:Time-step
The time-step (Dt) is the duration of the computational time-step, in seconds. The time-step is a critical variable in determining the wall clock execution time for a simulation. Typical time-steps for GSSHA simulations range from 20 sec up to 5 min. For particularly hard problems, the time-step may need to be very short, 10 sec, 5 sec, or even less. One second (1 sec) is the smallest permissible time-step.
Note that the overall model time-step must be less than and divisible into the smallest increment of time in the rainfall file. For example, for 1-min rainfall data the maximum time-step is also 1 min. Other permissible values would be 30, 20, 10, 5, 2, and 1 sec. If saturated groundwater flow is being simulated, then the overall model time-step must also be equal to or less than, and integer divisible into, the groundwater time-step.
The general rule for overland routing is that shorter time-steps must be used for higher intensity storms, finer horizontal grid resolution (grid spacing, Dx), steeper watershed slopes, larger watershed areas, and smoother surfaces. Stability of the explicit routing routines depends in part upon the Courant number. The Courant number is a dimensionless number that expresses the wave celerity, water velocity for steady flows, over the model celerity, Dt/Dx. For model stability, the Courant number must be less than 1.0; that is, water should not move more than one grid cell during a single time-step.
Shorter time-steps must be used when backwater effects are significant, which occurs mainly in flat areas. If the time-step is too long, the surface water depth in flat areas may show a checkerboard pattern, i.e., oscillations are observed in the water surface level. In such cases, the time-step should be decreased and the simulation repeated. Alternately the flat areas can be removed by editing the elevations.
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