Extended rational method (ERM)

A hydrological model that produces hydrographs, such as the Horton (ILSAX), RORB, RAFTS and WBNM models, must be applied to analyse the storage routing effects of detention basins. The rational method is not suitable because it only calculates peak flows.

It is possible to calibrate other models to rational method peak flowrates by altering their parameters, but the results are seldom satisfactory due to fundamental differences in their loss and routing procedures. A hydrograph model might be adjusted to make flow peaks agree for one average recurrence interval (ARI) and one storm duration, but peak flows for other ARIs and storm durations are unlikely to agree.

As an alternative, the extended rational method (ERM) has been made available in DRAINS. This applies the same time-area routing procedures as the Horton (ILSAX) model, but substitutes a simpler rational method loss model for the Horton infiltration loss model. The ERM operates in the same way as the Horton (ILSAX) model, applying rainfall patterns such as those from Australian Rainfall and Runoff, 1987, synthetic rainfall data or hydrological data. When applied with a set of rainfall blocks based on an intensity-frequency-duration (I-F-D) curve, the ERM will produce hydrographs with the same peaks as the rational method. When applied with a set of Australian Rainfall and Runoff patterns, it will calculate higher or lower peaks, due to the different rainfall inputs to these models.

To apply the method, first set up an ERM model from the Hydrological Models dialog box:

After the Add Extended Rational Model button is pressed, the following property sheet appears:

This is the same as the property sheet for the rational method, except that only the Australian Rainfall and Runoff 1987 rational method procedure can be applied. You must enter a pervious area runoff coefficient based on the 10 year ARI, 1 hour rainfall intensity for the site. (Refer to Equation 14.12 on p. 307 of Volume 1 of Australian Rainfall and Runoff, 1987).

The ERM applies a proportional continuing loss, as in the Modified Rational Method used in the United States. The proportion of loss at any time is taken to be (1 - C) times the rainfall intensity, where C is the runoff coefficient applied in the pipe design procedure in Australian Rainfall and Runoff.

In the property sheet above, there is the option of performing time-area calculations separately for the pervious and impervious areas, or of lumping them and using a C value scaled according to the proportions of the pervious and impervious areas, and the longer of the times of concentration for the two areas. This latter choice will give the same answers as the rational method when applied with synthetic rainfall patterns.