Hazardous and adverse hydrological phenomena for the Russian part of the Ural River basin

Hazardous and adverse hydrological phenomena in rivers of the Russian part of the Ural River basin have been analyzed and assessed based on data of long-term observations of the level regime at hydrological gages with established critical water levels. A procedure developed in the State Hydrological Institute was used, and a generalized criterion was proposed, which enabled identifying gages with the largest flood hazard. The analysis has shown that the recurrence of hazardous hydrological phenomena at gages located at Ashcherbutak, Sergievka, Maiskii, and Adamovka settlements is >10%, with most hazardous floods observed at the Ashchebutak and Adamovka settlements. The recurrence of unfavorable hydrological phenomena at gages at the towns of Orsk and Kuvandyk and at Novoorsk Settlement is >50%. At the same gages, the mark was exceeded many times (up to four times per year) in different phases of water regime (spring flood and freshets). This indicates the need to take measures to prevent the adverse effect of water in areas subject to a high risk of inundation. Critical marks of low water level were recorded at two sites on the Ural River – the City of Orenburg and the Town of Verkhneuralsk. The lowest water level at Orenburg gage has never dropped below the level of a hazardous hydrological phenomenon. At the gage of Verkhneuralsk Town, against the background of considerable changes in the lowest water levels after 1975, its level dropped 4 times below the mark of unfavorable hydrological phenomenon.