Rapidly Varied Flow | Civil Engineering Optional Notes for UPSC PDF Download

Rapidly varied flow refers to the flow of water in open channels where the depth of the water changes rapidly over a relatively short distance.  

This type of flow occurs when there is a sudden change in channel slope, a constriction or expansion in the channel cross-section, or the presence of hydraulic structures such as weirs or hydraulic jumps. Rapidly varied flow is an important concept in hydraulic engineering and is commonly encountered in various natural and engineered hydraulic systems.

There are two main types of rapidly varied flow:

• Subcritical flow transitions: In this type of flow, the flow velocity is initially subcritical (i.e., slower than the wave speed of small disturbances in the flow), and it transitions to supercritical flow (i.e., faster than the wave speed) or vice versa. This transition leads to abrupt changes in the water surface profile, known as hydraulic jumps or standing waves. Subcritical flow transitions typically occur at abrupt changes in channel slope or when the flow encounters a sudden constriction or expansion in the channel. 
• Hydraulic jumps: Hydraulic jumps occur when there is a sudden increase in the flow velocity or a decrease in the water depth, causing the flow to transition from supercritical to subcritical flow. Hydraulic jumps are characterized by a rapid increase in water depth, turbulent mixing, and energy dissipation. They are commonly observed downstream of weirs, spillways, or other hydraulic structures where there is a sudden change in flow conditions.