FLOOD FREQUENCY-ANALYSIS WITH SYSTEMATIC AND HISTORICAL OR PALEOFLOODDATA-BASED ON THE 2-PARAMETER GENERAL EXTREME-VALUE MODELS

Historical and paleoflood data have become an important source of info rmation for flood frequency analysis. A number of studies have been pr oposed in the literature regarding the value of historical and paleofl ood information for estimating flood quantiles. These studies have bee n generally based on computer simulation experiments. In this paper th e value of using systematic and historical/paleoflood data relative to using systematic records alone is examined analytically by comparing the asymptotic variances of flood quantiles assuming a two-parameter g eneral extreme value marginal distribution, type 1 and type 2 censored data, and maximum likelihood estimation method. The results of this s tudy indicate that the value of historical and paleoflood data for est imating flood quantiles can be small or large depending on only three factors: the relative magnitudes of the length of the systematic recor d (N) and the length of the historical period (M); the return period ( 7) of the flood quantile of interest; and the return period (H) of the threshold level of perception. For instance, for N = 50, M = 50 and T = 500, the statistical gain for type 2 censoring becomes significantl y larger than for type 1 censoring as H becomes greater than 100 years . In addition, computer experiments have shown that the results regard ing the statistical gain based on asymptotic considerations are valid for the usual sample sizes.