Determination Of The Coefficient Of Discharge Of A Rectangular Weir 4e19i

College of Engineering Civil Engineering Department

LABORATORY MANUAL Subject: CE 47 – Hydraulics Experiment #: 3 Experiment Title: Determination of the Coefficient of Discharge of a Rectangular Weir Objectives: To determine the coefficient of discharge of a rectangular weir. Apparatus: Stopwatch, meter stick, water tank (with rectangular weir Procedure: 1. Take note of the initial level of water at the side of the tank. 2. Set the stopwatch to zero. 3. Open the main gate valve. At the start of the opening of the valve, the water level is slowly increasing until such time that the water discharging over the orifice is equal to the incoming water from the main supply line. When the rate of water flowing in the channel is equal to the rate of water outflowing in the weir, then the level of water will not increase anymore. The head over the weir will be constant. 4. Measure the head over the weir. Take note that the head over the crest of the weir is not the head over the weir. Measure this head from the crest of the weir to water surface at a point upstream from the weir. This horizontal distance is 12 dc. The critical depth (dc) is approximately 2/3 of the head over the weir. 5. Simultaneously note the level of water in the tank and time. 6. Take observations at every quarter point of the tank. 7. Undertake two trials for every quarter point. 8. After the observations have been made, close the main gate valve. Calculations: The discharge over the rectangular weir is given by the following Manning’s formula: Q = Cw L [(H+hv)3/2 – hv3/2] Where: Q = actual discharge Cw = the weir factor L = the length of the weir with the consideration of the effective length given by the formula: L’ = L(1-0.10nH) n = the number of contracted portion of the weir H = head over the weir hv = velocity head just at the upstream side of the weir Observations: Weir Head Time Level Theoretical Actual Qa Dimension (m) (sec) Reading (m) Qt (cu. m/s) (cu. m/s)

Results: The average computed discharge, Qt = The average actual discharge, Qa =

Drafted by: SJ, 12/07/12

The average coefficient of discharge, C =

College of Engineering Civil Engineering Department

Precautions: 1. Start the data gathering only when the level of water is almost constant. 2. The head just above the crest of the weir is the true head. The slope of the water line here is big. The true head is far back at the upstream side of the weir. 3. If the side of the weir is rounded then this side is suppressed. In case of the contracted weir, the effective length is computed as given in the above mentioned formula. Discussion: 1. Does the coefficient of discharge, obtained from the experiment, conform with the standard C for rectangular weir? If not, then cite the possible reasons. 2. Enumerate possible ways to increase the efficiency of the weir. 3. Cite two practical uses of a rectangular weir.

Drafted by: SJ, 12/07/12