Frequency Factors
Typical application of Rational Method is within in the context of a peak flow or a design run simulation. To aid in the pairing of larger storm events with the Rational Method or Modified Rational Method, one may option to utilize the calculation options available for employing Frequency Factors.
With the use of a Frequency Factor term the Rational Method effectively becomes:
Q=C'iA
C'=CfC
- Cf = Frequency Factor corresponding to the storm's Return Period (T)
- C = Runoff Coefficient (Unadjusted)
For larger Return Events the adjusted Runoff Coefficient may become larger than 1. Additional Frequency Factor calculation options include ability to declare this as an invalid condition. When the calculation option Allow Runoff Coefficient to Exceed 1.0? is set to a state of False, a user notification will appear and simulation upon Compute will not be successful when this condition is detected.
Within calculation options, user may specify a Frequency Factor to be paired with each potential Return Period (T). By launching the Frequency Factors dialog from the Frequency Factors calculation option, a modeler may declare a Multiplier (Cf) for each Average Recurrence Interval (T).
The table contains the following columns: