Abstract
The article presents a comprehensive analysis of the long-term variability of mean annual runoff observed during the period 1967...2022 at the Malaya Arna–Ulanbel section of the Chu River in its lower reaches. The aim of the study is to determine the statistical characteristics of annual runoff and to assess the applicability of the derived design parameters for substantiating the reliability and operational safety of hydraulic structures. In the course of the study, long-term observation data were reconstructed and statistically processed using the method of moments and the maximum likelihood method. It was established that the coefficient of variation of annual runoff is relatively high (C_v = 1.1), while the coefficient of skewness is positive (C_s = 1.7), indicating pronounced variability and instability of river runoff. Analysis of the difference integral curve showed that low-flow periods vary in duration, whereas high-flow periods are relatively short but intensive. The design water discharges for different exceedance probabilities were estimated at 4.10...4.20 m³/s for a 50 % exceedance probability, 15.38...15.60 m³/s for a 10 % exceedance probability, and 0.47...0.48 m³/s for a 90 % exceedance probability. Statistical tests confirmed the homogeneity of the hydrological series, providing a basis for the further application of the derived design parameters. The results of the study can be used as scientifically justified design data for the planning, design, and operation of hydraulic structures in the investigated section of the lower reaches of the Shu River, thereby contributing to improved reliability and safety.