Fuzzy risk analysis of dam overtopping from snowmelt floods in the nonstationarity case of the Manas River catchment, China

With the significant climate change that has occurred in the Manas River catchment, the temporal and spatial patterns of the natural changes in the regional water cycle have changed dramatically in the past 30 years, and the frequency of extreme hydrological events has increased, which has changed the overall stability of the hydrological system of the catchment. According to the annual maximum peak flow data for the period 1956–2014 in the Manas River catchment, we used variable fuzzy set theory and the Mann–Kendall test to conduct trend and change point tests, respectively, and decomposition synthesis theory was used to handle consistency correction. Combined with the flood routing results for the Kensiwate reservoir using Monte Carlo simulation, a reservoir overtopping risk model based on right-angle trapezoidal fuzzy numbers was established, and the fuzzy risk index intervals and the corresponding fuzzy risk rate intervals for the Kensiwate reservoir were considered for past and present conditions. The results show that the local tendency and jumping of the maximum flood peak series showed significant changes; the annual runoff had an obvious growth trend in the period 1985–2006 and a gradually varying qualitative change from 1975 to 2000. In particular, the characteristic parameter reached an extreme qualitative change level of − 0.0016 in 1993. Then, we used decomposition synthesis theory to process the sequence with jumping points and obtained the past and present hydrological time series. Under the present conditions, the average flood peak sequence is 58.10% higher than in the past, and the flood design value decreases with increasing design frequency; however, when the present sequence increases, the change rates of both flood design values increase. The reservoir overtopping risk increased under the current conditions of snowmelt floods, which was affected by a significant increase in catchment temperature. Moreover, the use of right-angle trapezoidal fuzzy numbers to describe the reservoir overtopping risk was more in line with the objective reality than a traditional triangular fuzzy number. The results of this study can be used for the efficient utilization of water resources in the Manas River catchment and provide a new reference for the scientific management of reservoirs.