Hydraulic Jump | Irrigation Engineering - Civil Engineering (CE) PDF Download

Introduction

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In an open channel when rapidly flowing stream suddenly changes to slowly flowing stream a rise or jump in the elevation of liquid surface takes place. This phenomenon is known as hydraulic jump or standing wave. In this process, kinetic energy of the rapidly flowing stream is converted to potential energy.
Due to this there is a loss of kinetic energy. Hydraulic jump occurs at the toe of spillways or below a sluice gate where velocity is very high.
For a hydraulic jump to occur the depth of flow should be less than the critical depth or the flow should be a shooting flow or rapid flow.

Referring to the figures above, corresponding to specific energy head OG, depth of flow before the jump is d₁ = GH and after the jump is d₂ = GI.

Depth after Hydraulic Jump

Depth as a Function of Froude Number

∴

Loss of Specific Energy Due to Hydraulic Jump

ΔE_loss = E₁ - E₂

But
∴

i.e.,
Height of standing wave of hydraulic jump = h₂ - h₁.

Length of hydraulic jump: Exact points of commencement and end of the jump are not well defined. For purposes of analysis, it may be assumed that length of hydraulic jump is 5 to 7 times the height of the jump.

Relation between pre-jump and post-jump Froude numbers: It can be shown that,
For a hydraulic jump to occur Fr₁ should be greater than 1 and Fr₂ should be less than 1.