RIVERBANK EROSION ON THE OB AND THE IRTYSH AND ITS CONNECTION WITH THE RIVER CHANNEL MORPHOLOGY

This is the first study to provide generalized description of dangerous channel processes – riverbank erosion, conditions and patterns of its development for the Middle and Lower Ob (from the mouth of the Tom River to the city of Salekhard) and the Lower Irtysh (from the confluence with the Tobol River to the mouth). The geological and geomorphological conditions for the formation of the river channels and changes in the water content of the rivers downstream entail a change in the rate of riverbank erosion. On the Middle and Lower Ob, with a wide variety of channel types, the most active re-formations can be seen along the channel bends and the bends of the branches of the floodplain-channel branching and the bifurcated channel. On the Lower Irtysh, which has a predominantly meandering channel, riverbank erosion is confined to the free bends of the channel. The parameters of the channel forms, which determine the hydraulics of the flow, affect the characteristics of riverbank erosion (speed and length of the front). With an increase in the degree of bend development, the riverbank erosion rates increase with a simultaneous reduction in the length of the erosion front. On flat bends, there is no directional trend in the change in the riverbank erosion rates, but there is an increase in the length of the erosion front. The elongation of the riverbank erosion front is also traced for bends and forks of the channel with a larger step. The increase in the order of the river, which characterizes its water content, is accompanied by an increase in the rate of riverbank erosion on the Lower Irtysh. On the Ob, a significant contribution is made by the dispersal of the water runoff along the branches of the forks and forked channels, as well as the diversion of part of the runoff into floodplain channels (branches) and onto the inundated floodplain. The data obtained are of great importance for the development of water resources and the riverine areas of the Ob and Irtysh.