The Rational Method - Frequently Used, Often Misused

The Rational Method has been in use in some form or another at least since 1889, where its use appeared in a paper by Kuichling titled “The Relation between Rainfall and the Discharge in Sewers in Populous Districts”. It is not so much that the method itself is judged as being rational, but more than the method relates to the ratio between rainfall and runoff (which is essentially the runoff coefficient, C).

The method enjoys a prominent place in flow estimation guidelines in New Zealand and internationally. Unfortunately it is frequently misapplied, possibly as a consequence of its apparent simplicity. There are (erroneous) perceptions that the method can be used to estimate discharge for a wide variation in rainfall duration, to estimate total runoff volume and hence for sizing of mitigation works, and for determination of flood hydrographs for unsteady analyses.

In this paper the limitations of the method are examined in detail, with the intention of clarifying its use and range of applicability. Detail into the derivation of runoff coefficients that are applied is given, and some lesswidely known applications of the method (including the “probabilistic approach”) are described. This paper is intended for a practitioner audience.