Understanding the Time Span
What is the calculation "Time Span"?
The calculation settings screen allows you to set the time span to be used for runoff calculations. This is the time period over which the hydrograph will actually be generated. This "calculation window" allows you to focus the computations on just the period of time that is relevant to your particular needs, thereby allowing faster computations and more concise reports. However, any runoff occurring outside the specified time span will be excluded from all calculations!
How do I choose the
time span? (SCS Method)
The time span should generally begin at or before the earliest runoff. This will vary depending on the rainfall distribution and the specifics of your subcatchments. For a typical 24-hour storm, 5 to 10 hours is a common starting time, although an earlier start is often required to include runoff from impervious surfaces.
The time span should end after all the important events have occurred. This should at least include the hydrograph peak(s), and will often extend further as required to see the delayed outflow from the site. By default, the time span will end at 20 hours, which is suitable for most applications.
HydroCAD will issue a warning if the time span fails to include early runoff, or the peak appears to fall outside the time span. However, it is recommended that you review each hydrograph plot to be sure the time span is appropriate for your situation and purposes.
Why is the reported runoff depth less than predicted by TR-55?
To match the runoff depth given by the SCS runoff equation, the time span must include all times at which runoff will occur. To "see" the entire runoff volume (and the equivalent runoff depth) the time span must end sometime after the end of the storm. For a 24-hour storm, a span of 0-30 hours will generally include all the runoff.
When I use a long time span, the peaks are hard to view!
This can be resolved by setting a separate reporting time span. This will reduce the time span that is shown in the hydrographs, while still using a longer time span for the actual calculations. This option is available on the "Reports" tab of the Settings|Calculation screen. Unless you specifically need to use a reduced reporting span, this option should not be selected, thus allowing HydroCAD to automatically use the full calculation time span for all reports.
What
time span should I use for the Rational Method?
Since the Rational Method usually involves much shorter periods of study, the time span must be reduced accordingly. A span of 0-1 hours or 0-2 hours may be appropriate for many studies. For Rational Method the span should start at zero, and should extend past the Tc and/or duration being studied.
Also read about time step, rainfall, subcatchments, curve numbers and Time-of-Concentration.