Problem

My reviewer or locale requires an adjustment to be made to the C factor, which is different for each return frequency. How can I apply a different adjustment factor to the rational C for each return event? 

Solution

Applying a C adjustment factor to catchment area for the PondPack

To apply a different adjustment to the C factor for each return event, you can use the "C Adjustment" field in the properties of the modified rational catchment. Here are the complete steps:

1) Under Analysis > Alternatives, create a new Hydrology alternative for each return event. It is recommended that you label them according to the return frequency. If you only see the "Base Hydrology" parent alternative, you can use that to represent one of your return events and make child alternatives for the others.

2) Assign each of the Hydrology alternatives to the scenario representing the respective return event.

3) For each return event, make the respective scenario active, open the modified rational catchment properties and make sure "User defined" is selected for the "Outflow Criteria".

4) Enter the Target peak flow (allowable predevelopment outflow) for that return event and enter your adjustment factor in the "C Adjustment" field. The adjustment will be applied to the postdevelopment rational C coefficient(s).

5) Repeat step 4 for each return event.

If you do not wish to enter the predevelopment target flow values but would rather enter them by way of the predevelopment C, area and Tc, choose "Predevelopment" as the "Outflow Criteria", then manually adjust the predevelopment and postdevelopment Rational C coefficients based on your factor. In either case, this information is stored in the Hydrology alternative, so the setting you choose will be kept separate when the scenarios are using different hydrology alternatives.

Applying a C adjustment factor to the GVF Rational (StormCAD) solver

Starting with V8i SELECTseries 5, a calculation option is available to set Rational Method Frequency Factors. This enables you to apply a multiplier to the Rational C for all catchments, and have that multiplier be different for each return event.

 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'=CfC

Cf = Frequency Factor corresponding to the storm's Return Period (T)

C = Runoff Coefficient (Unadjusted)  

You can enable use of frequency option through the calculation options. 

See Also

Climate Change Adjustment vs. Rational Method Frequency Factor

Unable to enter the same Rational Method Frequency Factor multiplier more than once