Rational Method Irrationality with Rectification

Abstract

Rational method (RM) is the simplest approach for peak discharge calculation, but it has  many simplifying and unrealistic assumptions, which cause biased results in many applications.  Among the most important drawbacks are its applicability restriction to small areas, but it is  also used without much care even for large, flat and horizontal areas, even though drainage  basins might have significant slopes and rough topography. In the RM the rainfall intensity is  taken as constant during the storm rainfall duration and over the drainage area coverage. In this  paper, first the RM irrationalities are explained and then a modified formulation is proposed by  reconsidering geomorphologic and rainfall features. Nonlinear relationships, in the forms of  double-logarithmic functions, of peak discharge with drainage area and slope are incorporated  in the new formulation. Its application is achieved for a set of drainage sub-basins from the  Kingdom of Saudi Arabia.