The rational formula from the runhydrograph

  • M Parak
  • GGS Pegram


The rational formula is possibly the simplest flood estimation technique available using rainfall-runoff relationships. In spite of the many criticisms regarding its over-simplification of the processes of rainfall conversion into runoff, it remains possibly the most widely used method for estimating peak flood flows for urban drainage systems and small (<100 km2) rural catchments. However, as a result of the criticisms, the formula carries with it many cautions. One such caution regards the determination of the formula\'s runoff coefficient c, which is seen as the main difficulty in the design application of the formula. Mindful of this, it was decided to investigate the calibration of this coefficient, on past flood peak and flood volume pairs for a number of catchments in South Africa. To this end the “data set” of runhydrographs, which describe the characteristic peak and volume discharges of a catchment for a given recurrence interval, was used to calibrate the coefficients for selected catchments and to explore the assumptions underpinning this simple model. This article describes the methods employed in achieving this as well as a discussion of the results.

Water SA Vol.32 (2) 2006: pp.163-180

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eISSN: 0378-4738