Toward A New Rational Method

A revision is proposed in the formulation of the rational method and in the procedure of its application. Two misleading concepts are eliminated: that a watershed has a unique time of concentration equal to the time of flow along the watershed; and that a rainfall event of a given occurrence probability results in a runoff event of the same probability. Based upon laboratory results and field data, a method is proposed that links the peak discharge to the intensity and duration of the causative rainfall, with no explicit reference to a time of concentration. The relationship uses an envelope curve for variables derived from measurable data. Upon application of the relationship to a complete series of rainfalls, subtracting abstractions, a complete series of runoff peaks would be generated. This series is readily suitable for a statistical analysis, similar to that applied to a measured series, from which the design discharge is determined.