Flood frequency analysis by peaks -over-a threshold method.

Flood frequency analysis (FFA) is useful in the determination of critical discharges for engineering design of hydraulic structures. Two approaches to FFA are the annual maximum (AM) series method and the peaks over a threshold (POT) series method. This study investigates the application of POT method to FFA at 15 gauging stations on two rivers in Tanzania. The distribution and parameter estimation (D/E) procedures applied consist of three probability distributions and two methods of parameter estimation. The probability distributions are the exponential, the generalized Pareto and the Weibull while the methods of parameter estimation include the methods of ordinary and probability weighted moments. The six D/E procedures investigated are as follows: exponential with method of moments (EXP/MOM), exponential with method of probability weighted moments (EXP/PWM), exponential with method of Hosking (1986), generalized Pareto with method of probability weighted moments (GPD/PWM), generalized Pareto with method of Hosking (1986), and Weibull distribution with method of probability weighted moments (WEB/PWM). The relative merits of the D/E procedures were gauged by two tests of descriptive and a test of predictive ability. Based on the results of these tests, the exponential distribution with its parameters estimated by the method of Hosking (1986) was found to be the most appropriate FFA procedure for the rivers being investigated.