The Rational Method (and its variants) bases its calculations on Rainfall Data in the form of Intensity, Duration and Frequency (IDF) data.



IDF files are created on the principle that for a given Return Period (or Frequency) and a set duration, a storm will have a known average intensity. For example a storm near Seattle in the USA with return period of 5 years (frequency), and a duration of 20 minutes has an average intensity of 17.5 mm/hr.

The IDF file represents a single frequency and is a sequential list of average intensities for different durations of storm. In the Rational Method, each pipe or channel is designed for the average intensity of storm of a duration equal to its time of concentration. It could be said, therefore, that each pipe or channel in a network is designed for a different storm, as each pipe has a unique time of concentration.

The Rational Method is therefore not suitable for backwater analysis, or any analysis that involves the effect of one continuous storm, as no two pipes are designed using the same storm. A full analysis of the system should be used to determine the real hydraulic grade lines, which will vary minute by minute throughout a storm.

IDF data can be stored in the rainfall manager database for a single return period (Single RP IDF) or for multiple return periods (Table IDF). The rainfall manager shows these under distinct sub branches of the IDF branch.


Single RP IDF

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New

Allows a new IDF record to be created.

Load

Allows an IDF record to be loaded from file. These include:

    • IDF File (.xpidfx) - The xpdrainage file format for IDF records.

    • Text File (.csv,.txt) - Allows IDF data to be loaded from comma seperate files via the Universal Importer.

    • Excel File (.xls,.xlsx) - Allows IDF data to be loaded from Microsoft Excel format.

Save

Save the current rainfall event to a .xpidfx file so it can be reused elsewhere.

Label

This is the label given to the IDF data that is then shown on the Design Criteria form. It can be used to give the IDF data a suitable label. i.e. 5 year return period.

Spreadsheet

Duration

The duration that (average) intensity applies to.

Intensity

The average intensity for the duration at the given return period (ARI).

Buttons

1, 2 & 5 mins

Creates a set of 100 entries at the specified interval ready for data to be entered.

Repeat

Repeats the currently selected Intensity value down the spreadsheet.

Clear All

Clears ALL of the information in the spreadsheet


Table IDF

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Open

Allows an IDF table to be loaded from file. These include:

    • Table IDF File (.xpidtx) - The xpdrainage file format for IDF Tables.

    • NOAA Atlas 14 Point Precipitation Frequency Estimates (.csv).

      (These files can be created at NOAA's National Weather Service website)

Save

Save the current rainfall event to a .xpidtx file so it can be reused elsewhere.

Table Layout

Select a predefined layout for the spreadsheet. Currently supported layouts are NOAA Atlas 14, KOSTRA 2000 and KOSTRA 2010.

Delete Row

Delete the currently selected row in the spreadsheet.

Clear All

Clears ALL of the information in the spreadsheet.

Add Return Period

Add a new return period column to the spreadsheet.

Delete Return Period

Remove the current return period column from the spreadsheet.

Edit Return Period

Modify the return period of the currently selected column.

Label

This is the label given to the IDF data that is then shown on the Design Criteria form. It can be used to give the IDF data a suitable label. i.e. 5 year return period.

Show As

The data in the spreadsheet can be shown as rainfall depth or intensity.

Spreadsheet

Duration

The duration that (average) intensity applies to.

Intensity/Depth

All other columns are the average intensity or depth for the duration at the given return period (ARI). The return period is shown in the column title.