Fluvial Hydraulics

the branch of hydraulics that studies the flow of water in river channels. In rivers, the motion of a liquid exhibits its most complex form—unsteady motion in nonprismatic deforming channels. The kinematics and dynamics of river flow vary significantly with time, from high water resulting from a freshet to low water, and the structure of the flow becomes particularly complex when it emerges from the main channel into a floodplain. When there is an active interaction between the flow and the channel being eroded, the flow becomes two-phase, that is, it consists of water and solid particles (sediment).

The following basic problems are studied in fluvial hydraulics: (1) the kinematics of river flow, that is, the distribution of velocities and the relation between characteristic velocities in a cross section and the resistance to the flow of water in channels with complex shapes; (2) the shapes of the free surface of a flow resulting from a head of water caused by the construction of a hydraulic engineering installation; and (3) the unsteady, or wave, motion in river channels, observed during high water resulting from a freshet or during discharges from reservoirs, and the calculation of such motion. Other basic problems include the flow of water below an ice cover, which significantly affects the velocity field in a cross section, increasing resistance to the flow of water because of the increase in the wetted perimeter, and the effect of flow control and hydraulic engineering installations on the flow of liquid and sediment. In the latter case, the primary object studied is the reservoir, in which the flow velocities of water are greatly reduced in comparison with the natural flow, and the ability of the flow to transport sediment is reduced; gradual silting of the reservoirs therefore occurs. In tail waters, that is, below dams, the rate of deformation of the river bottom is increased, both because of the increase in flow velocity and because a flow free of sediment has greater erosion capacity than does a turbid flow.

The formation of river channels, flow turbulence, sediment transport, and deformations (erosion and aggradation) of the bottom and banks of rivers are studied in a branch of fluvial hydraulics—stream dynamics. Soviet scientists who have made major contributions to the development of fluvial hydraulics include N. M. Bernadskii and S. A. Khristianovich.