Estimation of Design Flood

A new class of methods such as Smemax (Bethlahmy, 1977), Power Transformation (Chander et al., 1978), and Modified Smemax (Rasheed et al., 1982), has recently been developed for estimation of design flood using the approach of transforming the skewed distribution of observed data into normal distribution. It has been demonstrated that by these methods skewness practically in all cases reduces to a value very near to zero, but in many cases Kurtosis does not reduce to 3. To account for the effect of this residual Kurtosis,Chander et al. (1978) have recommended the use of a correction factor. These methods thus do not truly transform the skewed data into normal distribution. The complete transformation can be achieved by the proposed transformation called Two Step Power Transformation (TSPT), described in detail in the paper and it is demonstrated through its application in large number of cases that it is capable of converting flood data of any degrees, of skewness and kurtosis to a normal distribution. A comparison of the results of TSPT with those obtained by other methods has been made.