Effect of Various Factors on the Estimation of T-Year Discharges for Water Management

The paper deals with the effect of various factors on the estimation of designed discharges. As input data, the series of daily discharges and annual peak discharges on the Topľa River at Hanušovce nad Topľou for the period of 1931-2015 were used. The first, maximum annual discharges (AM) approach was applied with the most widely used Log-Pearson III. probability distribution. The second, we analysed the effect of the time series length and the effect of seasonality (winter, summer) on the accuracy of T-year maximum discharges estimation. As an alternative to this approach (AM), the Peak Over Threshold (POT) method was used. We analysed the effect of the threshold level value selection and using of maximum daily discharges on the accuracy of T-year maximum discharges estimation. Results showed that not only the selection of the distribution function to estimate T-year discharges but also the type of used data series may affect the results of the estimation (length or selection of the period). Results also showed that the estimation of designed maximum discharges using by the POT method did not show significant differences at the selected various threshold levels, but for a relatively fast and large increase of discharges during floods, it would be necessary to have peak values for all waves included in the analysis. Determining the specific value of a 500- or 1000-year flood for engineering practice is extremely complex and in interpreting the results, it should be kept in mind that estimated values with very high return periods are extrapolated values. Each statistical method includes some uncertainty that may be caused by the method but also the data may be affected by certain measurement error, therefore, it is also necessary to specify confidence intervals in which the flow of a given 100-, 500-, or 1000-year flood may occur with probability, for example, 90%.