Common Runoff Issues
How is the runoff calculated?
HydroCAD provides a choice of runoff techniques, including the SCS/NRCS Unit
Hydrograph procedure (aka TR-20), the Santa Barbara Urban Hydrograph (SBUH) and
the Rational method. The runoff method is
selected on the Settings|Calculation screen.
How do I set the rainfall?
The rainfall parameters are specified on the Rainfall
tab of the Settings|Calculation screen.
Please read about rainfall issues.
How do I change the "Peak Factor"?
The Peak Factor is a characteristic of the Unit Hydrograph
curve, which is selected on the Unit Hydrograph tab of the
Settings|Calculation screen. HydroCAD includes the SCS UH (with a PF of 484) and the Delmarva UH
(with a PF of 284), which cover the vast majority of applications. To use any other PF you must provide the appropriate
UH definition file.
To explore the UH definitions installed on your computer, start HydroCAD and
select View|Unit Hydrograph.
How do I model the rainfall on a pond?
Include the pond's area within one of your subcatchments, or set up a separate subcat
with a Tc of zero and a CN of 98-100. This will produce instantaneous runoff equal to
the precipitation. Starting with HydroCAD 8.5,
you can use one of the new entries in the CN lookup table that allow the water
surface to be reported as pervious or impervious.
Why does the runoff always peak near 12 hours?
The most common rainfall distributions are synthetic storms, with the
peak rainfall intensity intentionally placed at the mid-point in the storm.
The runoff will always peak at some time after the peak rainfall.
The exact timing depends on the time-of-concentration, but it can never be
earlier than the rainfall peak.
Why don't I get any runoff until about 10 hours?
According to the SCS runoff equation, there will be no runoff until the
rainfall exceeds the initial abstraction, which may not happen until 8-10 hours
into the storm. Until that point, the runoff will be zero. This
effect is most pronounced for low CN values and low rainfall depths, such as
water quality events. For example, a CN value of 80 has an initial
abstraction of 1/2 inch, which isn't satisfied until 11.2 hours for a 2-inch
24-hour Type II storm, and significant runoff does not occur until about 11.7
hours.
Why don't I get any runoff at any time?
You may be using an incorrect time span for the selected runoff method (see
below). If the span ends before the actual runoff begins, no flow will be shown.
An examination of the SCS runoff equation will also reveal that zero runoff will
occur when using a low rainfall and/or low curve number.
Details here.
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 "viewing window" allows you to focus the computation 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 you are using and the specifics of your
subcatchments. For a 24-hour SCS storm, 8 or 10 hours is a common starting time.
** The Time Span should end after all the important events have occurred. This
should at least include the hydrograph peak, and will often extend further.
HydroCAD 4.5 (and later) will issue a warning if a hydrograph fails to meet these two
criteria. Even with this self-check, it is recommended that you review the plot of each
hydrograph to be sure your results are meaningful!
What time increment (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.
(Smaller values can be achieved by adjusting the number of decimal places,
although this is not generally recommended.)
Since the time increment can have an effect on peak flow calculations, it's
generally recommended that you use the same increment for when comparing
different scenarios, such as existing and proposed conditions.
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. For additional assistance
with comparing runoff results, please see the step-by-step comparison guide.
Why is the peak runoff greater than TR-20 or TR-55?
For small Tc values, the peak runoff may be slightly greater than predicted
by TR-20 or TR-55. This happens because HydroCAD uses a polynomial-based
rainfall distribution which provides better "resolution" than the tabular
rainfall data used in TR-20. If you want to match the TR-20 results (or
just verify the reason for the difference) you can
download a tabular version of the rainfall tables for use with HydroCAD.
But keep in mind that the tabular rainfalls were not intended for use with small
Tc values, and will become less accurate as the Tc value decreases.
If you need further assistance with resolving a runoff difference, please see
the step-by-step comparison guide.
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.
How do I model a snowmelt event?
There is no single approach to modeling snowmelt or rainfall on a snow-pack, nor are
there specific rainfall distributions for these studies. You need to determine
exactly what calculations are appropriate before you attempt to model the situation in
HydroCAD. Here are some of the techniques we commonly see:
