California State University Sacramento



California State University Sacramento -- Department of Civil Engineering

CE 135: Hydraulics Laboratory

Open Channel Flow -- PROCEDURES

Recommended Reading:

Crowe, C. T., D.F. Elger and J.A. Roberson, Engineering Fluid Mechanics, 8th ed., John Wiley and Sons, 2001, pp. 620 - 629

Crowe, C. T., D.F. Elger and J.A. Roberson, Engineering Fluid Mechanics, 7th ed., John Wiley and Sons, 2001, pp. 675 - 689

Objectives:

To examine the characteristics of subcritical and supercritical open channel flow

To measure the alternate depths and specific energies for various flows

Apparatus:

▪ Rectangular, horizontal flume (0.5 m wide) with sluice gates at the upstream and downstream ends

▪ Water surface gage

Experimental Procedure:

Set-up

1 Open both sluice gates as high as they will go.

2 Measure the depth of the bottom of the flume with the point gage located approximately in the middle of the flume lengthwise. (This value will be the datum against which other measurements are compared.)

3 Close the drain valve; start the large pump; and open the inlet valve.

Measure a range of subcritical depths at a single flow.

1 Establish a steady flow in the flume of about 400 gpm. (The exact value doesn’t matter but record it.)

2 Drop the downstream sluice gate until it barely impinges on the flow. Use a tape measure to measure the height of the gate above the wall of the flume.

3 As the gate impinges on the flow, the water will back up in the flume on the upstream side of the gate. The flow out of the gate opening will be supercritical; the flow behind (upstream side) the gate will be subcritical. Slowly lower the gate until the flume is nearly full. Wait for the flow to come to steady state between movements of the gate. If you lower the gate too far, the water will overtop the flume and your feet will get wet!

4 Measure and record the height of the water surface with the point gage located as in Step 1b (i.e., upstream of the gate). Use a tape measure to measure the height of the gate above the wall of the flume.

5 Raise the gate a short distance; wait for the flow to come to steady state; and measure the water surface again. Repeat this step until you have at least 6 measurements of depth. The distance to raise the gate each time can be judged by the height of the gate above the weir.

Measure a range of supercritical depths at the same flow.

1 Open the downstream gate until it is clear of the water.

2 Lower the upstream gate (also called a sluice gate) until it barely impinges on the flow. Use a tape measure to measure the height of the gate above the wall of the flume.

3 Slowly lower the gate until the head tank is nearly full. Wait for the flow to come to steady state between movements of the gate. As before, if you lower the gate too far, the water will overtop the flume. You’ll have less time to react at this end of the flume than you did at the downstream end.

4 Measure and record the height of the water surface with the point gage located as in Step 1b (i.e., downstream of the gate). Use a tape measure to measure the height of the gate above the wall of the flume.

5 Raise the gate a short distance; wait for the flow to come to steady state; and measure the water surface again. Repeat this step until you have at least 6 measurements of depth downstream of the gate.

Change the flow to a high value and repeat steps 2 and 3.

Water surface profile

1 Open both gates until they are clear of the water.

2 Using the point gage, measure the water surface starting at the downstream drop-off and working upstream to just downstream the sluice gate (or wherever the water starts being uniform flow).

Shut down: Turn off the pump; close the inlet valve; and open the drain valve on the head tank.

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