88c40e_Suppressed Weir Lecture.pptx

gis

1. Determination of Discharge and Co-efficient of Discharge through a Suppressed Weir Hydraulics and Irrigation Engineering Lab Batch-03 Semester-08 Experiment # 05: By Engr. Waleed Ahmad

2. Sequence Of Presentation Introduction Objective Theoretical Background Experimental Procedure Observations and Calculations Interpretation of Results

3. Introduction A weir is basically an obstruction in an open channel flow path . Weirs are typically classified as being either sharp-crested or broad-crested . This lecture is devoted to the more widely used Suppressed sharp-crested weir. Sharp-crested weirs are typically constructed by placing a thin, rust resistant metal plate, with a notch in the top of it, perpendicular to the flow of water. Water will flow through the notch and the depth of water that flows through it, will correlate to the discharge in the channel . Common Weir Terminology: Notch – the opening through which water flows Crest – the edge which water flows over Nappe – the sheet of water that flows over the weir Length – the “width” of the weir notch

4. Objective Weirs are commonly used for measurement of open channel flow rate . “The objective of this experiment is the determination of discharge co-efficient ‘Cd’ for the Suppressed Rectangular Weir model installed in the hydraulic tilting flume.”

5. Theoretical Background A weir functions by causing water to rise above the obstruction in order to flow over it. The height of water above the obstruction correlates with the flow rate, so that measurement of the height of the flowing water above the top of the weir can be used to determine the flow rate by the use of an equation, graph or table. The weir goes across the entire channel width for a suppressed rectangular, sharp-crested weir, as shown in the diagram. The end contractions are suppressed (not present) in this configuration; hence the name suppressed rectangular weir.

6. Continued… The Bureau of Reclamation, in their Water Measurement Manual , gives following equation, as an equation suitable for use with suppressed rectangular , sharp-crested weirs if the conditions noted below are met: (U.S. units: Q in cfs , B & H in ft ): To be used only if : For H/P > 0.33 or H/B > 0.33, T he more comprehensive Kindsvater -Carter equation should be used: (Note : Above Equation is a dimensional equation. Q is in cfs and H, P, & L are in ft.)  

7. Experimental Procedure Check the reservoir water level and the drain in the tank basin . Make sure the drain is closed during the performance of the experiment. Measure the upstream channel cross-section width. Measure the length ‘L’ of the weir plate. Install the suppressed rectangular weir in the channel. Position the hook gage next to the weir plate and zero the scale at the weir crest elevation. For the rectangular weir, measure the crest height from the channel bottom. Determine a maximum flow rate by opening the flow control valve, and turning on the pump. The maximum flow rate will fill the flume tank and will not over flow the channel. For more accurate measurements, the water surface upstream of the weir should not be too turbulent. Calculate the actual flow rate by the digital flow meter or the float method which ever is convenient. For the first measurement, use the maximum flowrate found in step 5. Obtain a steady flow through the channel and use the hook gage to determine the water surface elevation above the weir crest. Close the drain. As the water fills the tank the volumetric flow meter will start to rise. Use the stopwatch to record the time it takes to fill the tank to a given volume. Compute and record the flowrate. Use the data collected in step 7. Decrease the flow rate and repeat steps 5 through 8 for five different flow rates.

8. Observations and Calculations S/No Volume (lit) Time (sec) Q act H 1 (cm) H 2 (cm) H (m) Q th C d =Q act / Q th Q(lit/s) Q(m 3 /s) 1.                   2.                   3.                   4.                   5.                   Crest Length ( L) =______________

9. Interpretation of Results Plot on a Excel graph (at log-log scale ) Q vs. H and comment on the relationship. (use similar units such as cfs and feet). Is the average Cd value within the recommended range? If not, specify why?

10. Assignment Write the objective, procedure, observations and Comments of the experiment in your Lab Notebooks. Take a clear picture of the experiment from the said notebook and paste it in a MS word document and upload it to the LMS portal. Save the MS Word Document on your Name and Roll #. Deadline: One week after uploading of the lecture . Note: Leave the Observations and Comments section Empty since experiment will be performed once regular classes are resumed.