Unbiased plotting positions for historical flood information

Abstract

It is explained why the existing plotting position formulae for historical floods introduce large bias. It is proposed that the exceedance probability of historical floods in the exceedance based formula can be estimated initially by an analytical method using information about magnitudes of both the systematic and historical floods and assuming an appropriate parent distribution. It is shown that, for the Gumbel distribution, such a modification to the exceedance based formula reduces the bias to an almost negligible amount. © 1991.