Kritulių parametrų, lietaus nuotakynui ir valykloms projektuoti, analizė

Abstract

During the hot season of 1967-1996 in Lithuania occurred on average 32 rainfall events with precipitation bigger than 2.5 mm. In Fig. 2.1 are presented oscillations of the hot season of precipitation (a) and of the rainy days (b) and curve-line trends for Biržai MS. Correlation between the number of rainy days and the amount of precipitation was observed. The correlation coefficients vary from 0.77 to 0.94. The analysis of differences in amounts of precipitation measured by pluviographs and ordinary rain gauges was carried out. In the third section of the dissertation rainfall intensity-duration-frequency relationships are determined. Data from fourteen rainfall recording gauges in Lithuania, collected during the period 1967-1996 (treated periods of record range from 17 to 30 years), with the average value of 23 years were analyzed. Before processing, the rainfall series were divided into statistical independent rainfall events. Two events were separated from each other by rain-free periods or periods with the rain intensity below 0.02 mm/min during 30 min or longer. For each rain event, maximum average intensity in the intervals from 10 to 300 min was calculated. Since we are interested only in extreme values, the material was statistically treated with intensity values over a 1/3 year threshold. During the data treatment 14 728 rainfall events were selected. Fig. 3.1 shows some characteristics of the employed rainfall event definition. Rainfalls up to 2 hours duration on average makes up about 80% and events smaller than 6 mm about 75% of all rainfall events. It is determined that a normal distribution function fits a rainfall intensity series. Fig. 3.2 shows a plot of the average 20 min intensity per rainfall event as a function of return period. The geographical variability is illustrated as fractiles in the population of rain gauge. Fractiles are calculated as corresponding to the mean value ± one and two times the standard deviation. [...]