Implications of Distinct Methodological Interpretations and Runoff Coefficient Usage for Rational Method Predictions

The Rational Method is one of the most widely used methods for estimating peak discharge in small catchments. There are at least three forms of the Rational Method in use: deterministic, stochastic, and hybrid Rational Methods. These different forms are associated with distinct definitions of the runoff coefficient and produce distinct design flows, each of which has different risks associated with their exceedence. In this study, we firstly differentiate these forms of the Rational Method and show that a key point of difference between the forms lies in their interpretations of the runoff coefficient parameter. We then focus on the widely used hybrid Rational Method and demonstrate that the runoff coefficient is not only challenging to interpret, but is also dependent on land cover types, storm duration, and infiltration losses. With the understanding that most design manuals treat the runoff coefficient as a constant dependent on land cover and independent of storm properties, we explore the magnitude of error in design flows resulting from these assumptions. They suggest that the magnitudes of error associated with the conventional application of the Rational can be >500%. These issues with interpretation and internal consistency in the treatment of terms in the Rational Method suggest that it may not be possible to achieve reliable or consistent peak flow estimates using the Rational Method, motivating the use of more complex design tools.