# 3.5 Hydraulic Jump

When the flow is Supercritical in an upstream section of a channel and is then forced to become subcritical in a downstream section, a rather abrupt change in depth usually occurs and considerable energy loss accompanies the process. This flow phenomenon, known as hydraulic jump, is described in the following figure.

If a hydraulic jump occurs in a rectangular channel, the depth *y*2 (known as conjugate depth) is expressed as a function of *y*1 and the Froude number (Fr) as follows:

where *y*2 = water depth in the section 2 ( m, ft)

*y*1 = water depth in the section 1 (m, ft)

Fr1 = Froude number at the section 1, defined as

where *V*1 = upstream velocity [*V*_{1} = *Q*/(*y _{1}B*)] (m/s, ft/s)

*B* = channel bottom width (m, ft)

*Q* = flow (m3/s, ft3/s)

# Hydraulic Jump

The hydraulic jump dialog box is shown below.

## · Input for hydraulic jump:

· **Flow Unit** – Select the desired flow unit.

· **Solving Target** – Discharge, upstream depth or downstream depth.

· **Upstream Depth** – Supercritical flow depth at upstream of the jump.

· **Downstream Depth** – Subcritical flow depth at downstream of the jump.

· **Discharge** – Channel flow rate.

· **Channel Bottom Width** – Bed width of the channel.

## · Output for hydraulic jump:

· **Area (Upstream/Downstream)** – Flow area at upstream section and downstream section of the jump, respectively.

· **Velocity (Upstream/Downstream)** – Flow velocity at upstream section and downstream section of the jump, respectively.

· **Froude Number (Upstream/Downstream)** – Froude number at upstream section and downstream section of the jump, respectively.

· **Critical Depth** – Flow depth corresponding to the minimum specific energy for the flow rate.

· **Head Loss in the Jump** – Difference in specific energy at the upstream end and the downstream end of the jump.