HydroCAD Stormwater Modeling - Since 1986

Understanding the Time Step

What is the calculation "Time Step"?

The time step or "dt" is the time increment at which hydrographs are generated and routed.  This is the same as the "main time increment" used in TR-20.  A smaller time step will generally provide greater accuracy and stability, while a longer time step will allow faster calculations.

HydroCAD uses a default time step of 0.05 hours, which provides a good balance for most applications. However, this value can be adjusted to address special situations.

What time step (dt) should I use?

Start with the default value of 0.05 hours.  If you get a warning message suggesting a smaller value, you can reduce to 0.01 hours as required.

bulletTo change the time step, open the Settings|Calculation screen and select the Time Span tab.

Since the time increment can have an effect on peak flow calculations, it's generally recommended that you use the same increment when comparing different scenarios, such as existing and proposed conditions.

Using a time step larger than the default of 0.05 rarely has any advantages and is not recommended, unless you are attempting to reproduce other results that were calculated with a larger time step.

What if I need an even smaller time step?

In certain situations (such as oscillations) you may need an even smaller time step.  This can occur with very small storage volumes, coupled ponds, or ponds with sharp inflections in the rating curves, such as an underground storage system that is being filled beyond capacity.

bulletTo reduce the time step below 0.01 hours you will first need to adjust the minimum allowed value:

1) Open the Settings | Units screen
2) Click the Custom button
3) Scroll down and select the "Time Inc" parameter
4) Increase the number of Decimal Places from 2 to 3
5) Click OK twice.

Now you can enter a value as small as 0.001 hours on the Settings|Calculation screen.

Why do the HydroCAD reports use a larger time step?

In order to produce compact reports, HydroCAD automatically uses a "Shrink" feature.  When Shrink is enabled, a larger time increment is used for tabular hydrograph reports.  The calculations are still being performed at the specified time step, but the report uses every n-th value in order to condense the table so it will fit on a single page or screen.

If necessary, the exact time step used by the Shrink feature can be set on the Reports tab of the Settings|Calculation screen.  Note that the reporting time step must be an exact multiple of the calculation time span, otherwise the closest multiple will be used.

When the Shrink feature is disabled, HydroCAD will attempt to display all time values, generally resulting in much larger reports.

What about the rainfall time step?

Most rainfall distributions use their own time step to define the internal rainfall data.  This value is independent of the time step used for hydrograph generation and routing.  A rainfall time step of 0.1 hours is fairly common because it provides adequate precision to represent rainfall data collected at 5 or 6 minutes intervals.

What about the "Finer Routing"?

The Finer Routing parameter can be used to perform routing calculations at a smaller time interval, without having to reduce the overall time step.  Details here.

Also read about time span, rainfall, subcatchments, peak flow, and Time-of-Concentration.

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