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HydroCAD Stormwater Modeling - Since 1986

Runoff Calculations
Comparing SCS Runoff Rational Method

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.

Click for complete self-study materialsHow 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.  The widely-used SCS UH has a peak factor of 484, while the Delmava UH has a PF of 284. 

To change the peak factor you must select the appropriate UH.  HydroCAD includes a complete selection of UH tables to suit most applications, or you can create a custom UH definition. To explore the UH definitions installed on your computer, start HydroCAD and select View|Unit Hydrograph.

For further details read about unit hydrographs.

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?

Click for complete self-training materialsThe 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?

With the SCS/NRCS method, the most common reason for getting no runoff is because the precipitation depth is less than the initial abstraction, resulting in no runoff.  This frequently occurs with a low rainfall depth and/or curve number.  As a cross-check you can evaluate the SCS runoff equation by hand.  Details here.

Using an incorrect time span can also result in no runoff.  If the span ends before the actual runoff begins, no flow will be shown.   For details read about the calculation time span.

Click for complete self-training materialsWhat is the calculation "Time Span"?

The time span specifies the timer period over which runoff and routing calculations will be performed.  Using an appropriate time span is essential to getting meaningful results from your model.  Details here.

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.

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:

bulletTo model the melting of snow (no rainfall effects), use a link to introduce the appropriate hydrograph into the model.  This may be a constant of varying flow.   (Sorry, but we can't help you with this calculation.)  In the case of a constant flow, it may be easier to add this as a base flow within a reach or pond.
bulletTo model rainfall runoff from a snow-pack, set the antecedent moisture condition (AMC) to 3 or 4.  This will increase the curve number to approximate wet or saturated ground respectively.  (For programs without a built-in AMC adjustment, you will need to adjust the CN value manually.)  Then use the rainfall distribution and depth prescribed for your study.
bulletSome studies may utilize the "1-day/10-day" rainfall distribution provided with HydroCAD.  This is a rainfall of 10 days duration with a peak at 5 days, where the middle 24 hours contains 40% of the total 10 day precipitation.
bulletThe "Constant Intensity" rainfall distribution may be used to simulate a steady rate of snowmelt "precipitation".

Keep in mind that these are simply examples of some of the snowmelt calculations we have encountered.  You should consult your reviewer or regulatory agency to determine what is appropriate in each situation.

Also read about rainfall, subcatchments, curve numbers and Time-of-Concentration.

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