Probability assessment of the Kyiv reservoir overflow

Abstract

Reservoirs are an integral part of the world’s hydraulic infrastructure and form the basis of modern water management in most countries including Ukraine. However, reservoirs are also sources of an essential danger to the environment, infrastructure, and population. The potential danger and risks to the population living near reservoirs especially downstream may be no less than to people living near nuclear facilities or chemical plants, with which experts and the public usually associate problems of technogenic safety. Moreover, statistics show that about a third of all accidents on dams and levees occurred due to overflow of reservoirs when upstream water levels exceeded allowable values.There are 1103 reservoirs in Ukraine with a total water volume of about 55,500 million m3. The Kyiv reservoir is the third one by volume and water surface area in the country. In addition, the reservoir is created by one of the longest dams in the world; the total dam length of the reservoir reaches 70 km.Admittedly, the overflow of a reservoir can be caused by an extreme flood with inflow parameters exceeding the capacity of hydraulic structures. The challenge is that the capacity of water passage structures may be insufficient both due to the inaccuracy of the hydrological forecast and because of faults, poor functioning, or failures of the hydraulic structures during a design flood. In particular, long-term forecasts of floodwater discharges maxima of the inflow into the Kyiv reservoir based on using various probability distribution functions show the essential divergence of the obtained results. As well, as practice shows, the unavailability of some water passage tracts of the reservoir can reach several months in a year. Sometimes repair works were performed even during floods.The aim of the study consisted of probabilistic forecasting the emergency situation on the Kyiv reservoir as a result of its uncontrolled overflow through the possible inaccuracy of the hydrological forecast concerning an actual water inflow into the reservoir and due to failures of water passage hydraulic structures during floods. To achieve the study aim the following tasks were solved: (1) there was proposed a method of hydrological forecasting, which allows taking into account results of long-term forecasts of floodwater discharges maxima based on using various probability distribution functions and fuzzy modelling; (2) there was performed hydrological forecasting of floodwater discharges maxima of the Dnieper affecting the condition of the Kyiv reservoir based on the actual data collected the Vyshgorod water level gauge; (3) there was assessed the probability of the Kyiv reservoir overflow taking into account the occurrence possibility of a shortage of the capacity of water passage structures with using the failure and fault tree method. Totally, six incompatible hypothetical emergency situations at the Kyiv reservoir were considered. The calculations showed the total probability of the Kyiv reservoir overflow equal to 3.84*10–4 (year–1), which is acceptable to guarantee the hydrological safety of infrastructure and the population.