HYDRAULHYDRAULICS LABORATORY

Experiment No.7-ADetermination of the Coefficient of Discharge

over a Rectangular Weir

Weir is defined as a barrier over which the water flows in an open channel. A weir is a notch on a larger scale – usually found in rivers. It is used as both a flow measuring device and a device to raise water levels. The flow over the weir can be determined by a single measurement: the upstream submergence or weir head. This is the distance between the weir crest (sharp edged) and the still water surface (the water surface before the drop-down curve).

This type of weir is used in practice to measure small free-surface flows. From application of the Bernoulli equation we get the equation below relating the height of fluid above the weir crest and the weir width to the flow over the weir.

For a suppressed rectangular weir the width does not change with depth so there is no relationship between L and depth H.Substituting this with the general weir equation gives:

Qt=23L√2 g H

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If the sides of the weir are said to be contracted, the effective length of the weir is given by the expression:

L’ = L – 0.1H (singly contracted), L’ = L – 0.2H (doubly contracted)

Where:L’ – effective lengthL – length of the weirH – head over the weir

To calculate the actual discharge we introduce a coefficient of discharge, Cd, which accounts for losses at the edges of the weir and contractions in the area of flow, giving:

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Qa=Cd23L√2g H

32= volume

time

where Cd = coefficient of discharge, L = width of notch-weir and H = height of water (head) above the weir crest

I. Experiment Objectives

- To observed the characteristics of open-channel flow over a rectangular weir.- To determine value of the discharge coefficient for rectangular weir.

II. Materials and Apparatuses Required

The apparatuses necessary to perform this experiment are the following:

- SHEM Apparatus - Calibrated Bucket- Stopwatch- Metric Scale- Rectangular Weir

III. Procedures

a) Attach the rectangular weir to the channel (SHEM).b) Measure the width (L) of the rectangular weir.c) Turn on the pump and open the control valve until water discharges over the

weir.d) Regulate the flow to maintain a water level in the flow channel so that the weir is

filled to the highest possible level without overflowing the channel.e) Allow a steady flow to develop throughout the entire circuit.f) Measure the height of the water level above the top of the weir as an H to start

the analysis.g) Measure the flow rate with a stopwatch and calibrated bucket.h) For each set of measurements, measure the flow rate at least three times and

take the average.i) Reduce the approach, repeat step e to step h by using different flow rate of

water, which can be done by adjusting the water supply. Measure and record the H and the time to fill a certain volume of water in the bucket.

j) Compute for the theoretical discharge using the equation;

Qt=23L√2 g H

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k) Calculate the actual discharge by volumetric method using the formula;

Qa=volumetime

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l) Determine the coefficient of discharge of a rectangular weir:

Cd=QaQt

IV. Laboratory Data

TRIAL

ACTUAL DISCHARGE THEORETICAL DISCHARGE COEFFICIENT OF DISCHARGE

VOLUME (m3)

TIME (sec)

Qa L (m) H (m) Qt Cd Cd (AVERAGE)

1

             

              

             

2

             

              

             

3

             

              

             

COEFFICIENT OF DISCHARGE, Cd  

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V. Observations

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VI. Conclusions

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