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Runoff Period

Runoff Period

Runoff Period

Why, when modelling a relatively small catchment (2Ha) with an equally short time of concentration (12 mins) does the runoff hydrograph indicate that outflow continues to the 24 hour mark i.e. approx 12 hours after the peak. In fact the runoff volume is also reduced if the time span is set to less than 24 hours. Intuitively the storm event creating maximum peak discharge for such a small catchment would not be responsible for runoff 12 hours later. Is this just an anomaly of the modelling method or is my intuition failing me ?

RE: Runoff Period

The SCS/NRCS runoff procedure is normally used with a storm of 24-hour duration.  The same storm duration is used for (virtually) any subcatchment, regardless of the time of concentration.  Since the rainfall continues until 24 hours, there will always be some runoff until 24 hours.  A larger Tc will extend the runoff beyond 24-hours, but it will never be shorter.

Note that the calculation time span is different - it sets the "viewing window" for calculations, without changing the timing of the rainfall, which normally remains fixed at 24-hours.  In general, you should use the default time span of 5-20 hours unless you get a related warning message, or have a specific reason to use another time span.

Since volumes are calculated over the calculation time span, a shorter time span will reduce the apparent volume.  You will see a warning message if this occurs.  Just click the message for full details.

For further information please click the Help button on the applicable screens or see www.hydrocad.net/rainfall.htm  You may also want to review the "Hydrology" section of the stormwater training slides, which is included on the HydroCAD 7.1 CD.  You will also find an on-line version of the slides at www.hydrocad.net/selftraining.htm

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