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Water Controlling and Measuring Structure
Md. Nurul KadirLecturerDepartment of Agricultural Engineering
Sher-e- Bangla Agricultural University, Dhaka
04/18/2023
Overview
Why is water measurement important to IWM?Explain some of the mathematics of water measurementDiscuss some of the common measuring devices Discuss other opportunities for measurementWork some example problems
04/18/2023Md. Nurul Kadir Lecturer, Dept. of Agril. Engg. SAU
Dhaka
04/18/2023Md. Nurul Kadir Lecturer, Dept. of Agril. Engg. SAU Dhaka
Permits intellegent use
Reduces excessive loss
Allows distribution according to needs and rights
Necessary for monitoring and evaluation of irrigation projects
Efficient water use depends on measurement
Reduced cost due to leached nutrients
Reduced environmental impact from over-irrigation
Purpose of measuring irrigation water
– Regulatory – Billing
Units of water measurement
04/18/2023Md. Nurul Kadir Lecturer, Dept. of Agril. Engg. SAU Dhaka
Water measured under two conditions:i.At rest (reservoir, ponds, soil) in units of volumeii.In motion (pipe, canal, river) in rates
Units of volume: litre, m3, ha-cmUnits of rate: l/s, cumec, cusec, ha-cm/h
• Volume: length3
• Flow Rate (Q): volume/time• Velocity: length/time• Area: length2
Water Measurement Mathematics
04/18/2023Md. Nurul Kadir Lecturer, Dept. of Agril. Engg. SAU Dhaka
v1 v
2
Qin Qout
A
1
A
2
Continuity EquationQ=vA
Q = flow ratev = velocity
A = area
04/18/2023Md. Nurul Kadir Lecturer, Dept. of Agril. Engg. SAU Dhaka
Continuity Equation
Given: d=12 inchesv=2.5 ft/s
Find: Q in cfs
Qv=2.5 ft/s
12 in.
Solution: Q = vA4
2dA
4
1 2)ft(A
A = 0.785 ft2
Q = 2.5 ft/s x 0.785 ft2 = 1.96 ft3/s
Irrigator’s Equation
04/18/2023Md. Nurul Kadir Lecturer, Dept. of Agril. Engg. SAU Dhaka
Qt = Ad Q = flow ratet = timeA = area
D = depthd = Qt/A
Q = Ad/t
t = Ad/Q
A = Qt/d
Irrigator’s Equation
04/18/2023Md. Nurul Kadir Lecturer, Dept. of Agril. Engg. SAU Dhaka
1. Given: d = 3 inchesA = 50 acresQ = 2 cfs
Find: Time required to apply d
2. Given: t = 36 hoursA = 20 acresQ = 2 cfs
Find: Depth of applied water, d
Solution
04/18/2023Md. Nurul Kadir Lecturer, Dept. of Agril. Engg. SAU Dhaka
1. t = dA/Q 1 cfs ≈ 1 ac-in/hr
t = 75 hourshr
inac)ac)(in(
t
2
503
2. d = Qt/A
d = 3.6 inchesac
)hr)(hrinac(
d20
362
Techniques of measuring pump discharge and water flow in distribution system
04/18/2023Md. Nurul Kadir Lecturer, Dept. of Agril. Engg. SAU Dhaka
Measuring pump discharge
Coordinate methodOrifice meterPiezometer
Measuring channel flowi.Float method (avg velocity = 0.85float velocity)ii.Current meter (mean velocity = avg velo of 0.2d & 0.8d)iii.Pitot tubeiv.Portable weirs/notches:
Notch is an opening on the side of a tankWeir is a structure over which water flows
v.Flumesi. Parshall flumesii.Cut-throat flume
Coordinate method
04/18/2023Md. Nurul Kadir Lecturer, Dept. of Agril. Engg. SAU Dhaka
gy
xv
2
2
Orifice meter
04/18/2023Md. Nurul Kadir Lecturer, Dept. of Agril. Engg. SAU Dhaka
22
21
12 2
aa
aghv
Float method
04/18/2023Md. Nurul Kadir Lecturer, Dept. of Agril. Engg. SAU Dhaka
t
xv
Current meter
04/18/2023Md. Nurul Kadir Lecturer, Dept. of Agril. Engg. SAU Dhaka
bNav
Pitot tube
04/18/2023Md. Nurul Kadir Lecturer, Dept. of Agril. Engg. SAU Dhaka
gHv 2
Weirs
04/18/2023Md. Nurul Kadir Lecturer, Dept. of Agril. Engg. SAU Dhaka
A weir is an overflow structure installed perpendicular to open channel flow• Has a unique depth of water at an upstream measuring point for each
discharge• If the water springs clear of downstream face, acts as sharp-crested weir• A long, raised channel control crest is a broad-crested weir
Weirs
04/18/2023Md. Nurul Kadir Lecturer, Dept. of Agril. Engg. SAU Dhaka
• Usually named for the shape of the overflow opening– Rectangular– Triangular– Cipolletti
• Lowest elevation on overflow is zero reference elevation for measuring h
Rectangular weirs can be either contracted or suppressed• Suppressed weirs use side of flow
channel for weir ends– No side contraction occurs– Often used in divide boxes
Side slope: 1 horizontal 4 vertical
Weirs
04/18/2023Md. Nurul Kadir Lecturer, Dept. of Agril. Engg. SAU Dhaka
General equation
For contracted weir
For suppressed weir
2/332 )2.0(2 HHLgCQ d
2/332 2 LHgCQ d
)(2 2/32/332
ad hHLgCQ
whereq = flow rate (m3/s)H = head on the weir (m)L = width of the weir (m)g = 9.81 (m/s2) - gravity cd= discharge constant for the weir - must
be determinedcd must be determined by analysis and
calibration tests. For standard weirs - cd - is
well defined or constant for measuring within specified head ranges
V-notch
04/18/2023Md. Nurul Kadir Lecturer, Dept. of Agril. Engg. SAU Dhaka
2/5158 2 HgCQ d
Limitations in using weirs
• Not accurate unless proper conditions are met
• Requires considerable head loss• Difficult to combine with turnout
structures• Not suitable for water carrying silt
Q = 8/15 cd (2 g)1/2 tan(θ/2) H5/2
04/18/2023Md. Nurul Kadir Lecturer, Dept. of Agril. Engg. SAU Dhaka
Parshall Flume
Parshall Flume
04/18/2023Md. Nurul Kadir Lecturer, Dept. of Agril. Engg. SAU Dhaka
Open channel type measuring deviceSelf cleaning deviceMay be free flow or submerged flowReasonably accurate even under
submerged condition
If ha =up stream head and hb = down stream head
Degree of submergence
Free flow depends on flume size and degree of submergence
a
b
h
h
Cut-throat flume
04/18/2023Md. Nurul Kadir Lecturer, Dept. of Agril. Engg. SAU Dhaka
Improved version of Parshall flumeFlat bottom, vertical wall and zero throatNo throat, so cut-throat (Skogerboe, Hyatt, Anderson & Eggleston, 1967)For free flow: critical depth at throatAt critical flow: u/s depth ha unaffected by d/s depth hb
Free flow depends on flume size and submergence ratio
For free flow:
Q = flow rateC = free flow coefficientn = exponent depends on flume length
For submerged flow both ha and hb required for Q
n
aChQ
04/18/2023
Md. Nurul Kadir Lecturer, Dept. of Agril. Engg. SAU
Dhaka