Unidirectional Aqueous Flow

Unidirectional water flows are vital agents of erosion, transportation, and deposition in many Earth surface environments and can occur in a wide variety of depositional settings from continental rivers to flows in the deep sea. Unidirectional flows move in one principal direction, with no time-averaged reverse flows within the depth-averaged fluid and, apart from any local deviations caused by bed topography, experience no reverse or oscillatory motion, such as may be produced by waves and tides. Unidirectional flows can be either uniform, where the flow does not vary in velocity or cross-sectional area along its path, or nonuniform, where the fluid velocity and cross-sectional area do change spatially. Non-uniform flows show convective acceleration, where the cross-sectional area decreases and velocity increases, or convective deceleration, where the cross-sectional area expands and the flow slows. In addition to this spatial change in flow properties, unidirectional flows may vary temporally in their behaviour. Flows that show no temporal change in their behaviour are termed steady, whereas those whose velocity changes over time are termed unsteady. Unsteady flows show temporal increases and decreases in velocity, which are often related to the passage of a discrete event such as a flood. Water flowing over a boundary, whether solid or mobile, develops a flow structure that depends on the velocity and depth of the fluid together with its density and viscosity. The surface over which the fluid moves exerts a frictional drag on the flow, and the region of flow near the bed that is retarded....