The IDF Polynomial Log Equation definition type calculates the storm data using the following equation:

Where

i = rainfall intensity

D= rainfall duration

a, b, c, d = rainfall equation coefficients

When editing a definition of the IDF Polynomial Log Equation type, the tabbed area of the dialog contains the following controls:

Storm Data Input Tab

Equation Duration Unit: Specify the unit to be used for duration (D) value in the equation.

Equation Intensity Unit: Specify the unit to be used for intensity (i) value in the equation.

Add Return Event Button: Adds another Return Period row to the table. When you click this button an Add Return Period dialog appears, allowing you to specify the return period for the new row.

Remove Return Event Button: Removes the currently highlighted Return Period row from the table.

a (log): Specify the value to be used for the a coefficient in the equation for the associated return period.

b (log): Specify the value to be used for the b coefficient in the equation for the associated return period.

c (log): Specify the value to be used for the c coefficient in the equation for the associated return period.

d (log): Specify the value to be used for the d coefficient in the equation for the associated return period.

Results Table: This table displays the calculated rainfall intensity values for the each of the return periods in the table at durations of 5, 10, 15, 20, 30, 40, 50, 60, 80, 100, and 120 minutes.

Notes Tab

This tab contains a text field that allows you to enter descriptive notes that will be associated with the currently highlighted storm data definition.

Library Tab

This tab displays information about the storm data definition that is currently highlighted in the list pane. If the storm data definition is derived from an engineering library, the synchronization details can be found here. If the storm data definition was created manually for this model, the synchronization details will display the message Orphan (local), indicating that the storm data definition was not derived from a library entry.

To Import a comma or space delimited ASCII text file:

1. Click Import in the Storm Data dialog, and select the location and name of the file containing the rainfall table in ASCII format to import. You may see a prompt warning you that any existing storm data will be overwritten - if you do not want this, click Cancel.
2. Select Open to import the ASCII text file, or Cancel to exit without saving the changes. When importing an ASCII text file, the following format is assumed: The first line of the imported text file contains the return periods. The first entry in each succeeding line of the file contains the storm duration. All other entries represent rainfall intensities, which are assumed to be in the current display unit (i.e. in/hr, mm/hr, etc.).

An example in comma separated format is given below for return periods of 1, 2, 5, 10, 20, 50 and 100 years, and durations of 5, 15, 30 and 60 minutes):

1 , 2 , 5 , 10 , 20 , 50 , 100 5 , 47.0 , 63 , 87 , 103 , 125 , 157 , 183 15 , 29.7 , 39.5 , 54 , 63 , 76 , 95 , 110 30 , 20.8 , 27.5 , 37.1 , 43.5 , 52 , 64 , 75 60 , 14.0 , 18.4 , 24.5 , 28.6 , 34.0 , 41.8 , 48.1