agen 159 lec 6

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


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


v1 v

2

Qin Qout

A

1

A

2

Continuity EquationQ=vA

Q = flow ratev = velocity

A = area


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


Qt = Ad Q = flow ratet = timeA = area

D = depthd = Qt/A

Q = Ad/t

t = Ad/Q

A = Qt/d

Irrigator’s Equation


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


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


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


gy

xv

2

2

Orifice meter


22

21

12 2

aa

aghv

Float method


t

xv

Current meter


bNav

Pitot tube


gHv 2

Weirs


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


• 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


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


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


Parshall Flume

Parshall Flume


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


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

agen 159 lec 6

Education