Flow Measurement (Weirs

1. Flow Measurement (Weirs)
Prof Ir Dr Ab Halim Abu Bakar

2. Weirs
• Discharges can be measured with the use of discharge
structures such as weirs
• Weirs are sharped-crested overflow structures that are
built across open canals
• They are easy to construct and can measure the discharge
accurately when correctly installed
• The water level upstream of the structure is measured
using a measuring gauge as shown in Figure 31 where the
difference between the water and the crest of the weir is
marked H.
• The discharge corresponding to that water level is then
read from a table which is specific for the size and type of
weir being used.

3. Types of Weirs
• Three well-known weir types are : the rectangular
weir, Trapezoidal weir and the 90 degree V-notch
• The rectangular weir has a rectangular has a
rectangular opening
• The trapezoidal weir is a improved rectangular
weir and has a slightly higher capacity for the
same crest length.
• Its opening is inclined at a slope of 4 to 1
• The 90 deg V-notch weir has a rectangular
opening and is well suited to measure small flows
with high accuracy

7. Discharge measurement
• Reading the water level on the gauge in figure
31 is o.12, so H = 0.12
• Assume the structure in figure 31 is a
rectangular weir with a crest length of 1.25 m
• Look at table A-2.1,find the column for being
used is 1.25
• The value at the point where the column and
the row cross is 94 and that is the discharge in
litres per second is Q = 94 l/s

8. Table A-2.1

9. Discharge Measurement
• For a trapezoidal weir ,crest length is 1.0m and
the head reading is 0.17
• Table A- 2.2 the nearest value is used,0.16 and
0.18m
• For H = 0.16 and L = 1.0 Q = 119l/s
• For H = 0.18 and L= 1.0 Q = 142 l/s
• Therefore Q = (119+142)/2 = 131 l/s

10. Table A2.2