3.6.21D Domains
For the 1D channels the Courant criterion is expressed in the form:
1 D Scheme (2)
The time step selected should not be greater than the minimum value for any channel (except non-inertial channels such as bridges, culverts, etc). Accuracy of the results is also influenced by time step. The limiting value adopted is usually a compromise between accuracy, stability and simulation time, and sensitivity checks are recommended. The occurrence of mass errors may indicate the use of too high a timestep (see Section 6.4.4).
Typical timestep values are 60 or 120 seconds for a model with a minimum channel length of 500 metres, down to 1 second for 1D domains with small pipes. Where a few channels must be much shorter than the rest, it may be economical to specify them as non-inertial channels. The timestep can then be chosen on the requirements of the shortest remaining channel. Care should be exercised when specifying non-inertial channels to ensure that errors are not introduced by the non-inertial representation, particularly if these channels are in a region of particular interest. Any approximations can usually be assessed by a few selected runs without the non-inertial approximation and with the necessary shorter time step.
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