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

Q = Rational Flow
C' = Runoff Coefficient (Adjusted)
i = Intensity
A = Drainage Area

C'=C_fC

C_f = 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 (C_f) for each Average Recurrence Interval (T).

The Frequency Factors dialog box contains buttons and a return periods vs. frequency factors table. The dialog box contains the following controls:


New	This button creates a new row in the table.
Delete	This button deletes the currently highlighted row from the table.
Report	Opens a print preview window containing a report that details the input data for this dialog box.

The table contains the following columns:

Column	Description
Average Recurrence Interval	Enter the return period for the corresponding frequency factor.
Multiplier	Enter the frequency factor for the corresponding return period.