Earth Sciences 1: Weather and Water
Week 5: The Water Cycle
Lab 2b: Observing a hydraulic jump
You will be a small flume to experiment with a phenomenon called a hydraulic jump.
Hydraulic jump is a natural phenomenon that occurs whenever flow changes from
supercritical to subcritical conditions. Supercritical flow is shallow, high velocity flow
which cannot be maintained for long because of high energy losses. Subcritical flow is
deeper, lower velocity flow, which is the natural state of water flow. The froude number,
Fr, distinguishes one from the other. Fr > 1 is supercritical, Fr < 1 is subcritical and Fr =
1 is critical flow
In a hydraulic jump, the water surface rises abruptly, surface rollers are formed, intense
mixing occurs, air is entrained, and often a large amount of energy is dissipated. There
are common hydraulic jumps that occur in everyday situations such as during the use of a
household sink. There are also man-made hydraulic jumps created by devices like weirs
or sluice gates. In general, a hydraulic jump may be used to dissipate energy, to mix
chemicals, or to act as an aeration device.
y1 = depth of upstream, supercritical (high velocity) flow
y2 = depth of downstream, subcritical (low velocity) flow.
Q = flow rate
Instructions
1. Turn on flow to the Bench Flume and allow the water to pass under the gate at the
flume entrance
2. Manipulate the downstream overshot weir to induce a steady hydraulic jump
3. Measure flow depths, y1 and y2.
4. Measure flowrate, Q.
5. Measure the flow areas A1 and A2 = flow depth (y) times channel width (w)
Vary the flow and repeat the measurements two more times.
Calculate the Froude number upstream and downstream of the hydraulic jump:
Fr 
v
gy
where v = velocity = Q/A;
Q = flow rate
A = flow area = y*w
y = flow depth
w = channel width
g = gravitational constant.
Make sure your units are consistent. Fr is a dimensionless number.
Do the Fr values confirm supercritical upstream and subcritical downstream of the jump?
Calculate the energy loss through the hydraulic jump, h, as
h
 y 2  y1 3
4 y1 y 2