Canal Outlets Department of Civil Engg UET Lahore, Pakistan
Introduction • An outlet is a hydraulic structure conveying irrigation water from a state owned distributary to privately owned water course. compared to other • The outlets are large in number as compared irrigation structures in an irrigation system and hence their design and type has maximum bearing on the equitable distribution of water. Therefore e proper proper design of outlet is of utmost • Therefor importance.
Introduction • An outlet is a hydraulic structure conveying irrigation water from a state owned distributary to privately owned water course. compared to other • The outlets are large in number as compared irrigation structures in an irrigation system and hence their design and type has maximum bearing on the equitable distribution of water. Therefore e proper proper design of outlet is of utmost • Therefor importance.
Canal Outlet
Tail cluster
Paa naa !Turnouts"
Essential Reuirements of an outlet •
An outlet should be strong and be without movable parts to minimi#e tempering
•
Tempering by cultivators should be readily detectable
•
The outlet must carry its fair share of silt from parent channel
•
$t should be able to wor with small woring heads
•
$t should be simple so that construction is easy
•
The total cost of installation and maintenance should be minimum
T!pes of Outlets • "on#$odular Outlets % $t is one in which the discharge is dependent upon the di&erence of head in water course and parent channel. % 'ence( a variation in either a&ects the discharge.
• %emi#$odular &'le(i)le* % $t is one in which the discharge depends upon the water level in distributary only and is independent of water level in water course. % This is achieved by producing hydraulic )ump within the *ume length.
• $odular &Rigid* Outlets % $t is one in which the discharge is independent of the water level in water course and parent channel. % $t can be +xed for any discharge value. This is achieved by creating a free vortex and destroying any extra head more than allowed for in the designed discharge.
T!pes of Outlets /emi-odular
odular
!0lexible"
!1igid"
,on-odular Pipe or 2arrel Type( with drowned *ow at d3s /cratchley outlet !a pipe type( with a cistern d3s"
4eir-Type
Ori+ce-Type !O/"
'arvey /toddard outlet
6ennedy 7auge Outlet
Crumps open *ume outlet
Crumps ad)ustable proportion al module !Crumps AP"
Characteristic of Outlets • 'le(i)ilit!+ $t is de+ned as the ratio of rate of change of discharge in outlet to the rate of change of discharge in parent channel. ' &d-*-&d.-.* &m-n*&D-/* !'39" is the setting of an outlet n is exponent of discharge eq !EF6 :9n" for canal G m is exponent of discharge eq !qF6 >'m" for outlet. 0or Trap. channel with H8: side slope( n is I3( and for open *ume outlet m is >3. Therfore 0 FJ.; 93'. $f we set the crest of outlet at J.;9 depth below water level( the 0 will be : G the outlet will be proportional outlet.
• 0lexibility is the capacity of an outlet to vary its discharge with the change in the discharge of the distributary. • $f 0F: • $f 0K: • $f 0L:
Proportional 'yper-proportional /ub-proportional
Characteristic of Outlets • %ensitivit!+ $t is the ratio of rate of change of discharge of an outlet to the rate of change in the level of distributary water surface( i.e. normal depth of channel / F !dq3q"3!d739"
• 'ere( / is the sensitivity and
G is the gauge reading of a gauge which is so set that G F J corresponds to the condition of no discharge through the outlet ! i.e.( QJ F J".
• /ensitivity can also be de+ned as the ratio of the rate of
change of discharge of an outlet to the rate of change of depth of *ow in the distributary channel.
% n' n F I3 for wide trape#oidal channel with side slope H8:
Characteristic of Outlets • E0cienc!+ $t is equal to the ratio of the head recovered
!or the residual head after the losses in the outlet" to the input head of the water *owing through the outlet.
• $inimum $odular /ead+ it is the minimum head required for the proper functioning of the outlet as per its design.
• $odular Limits+ The extreme values of any parameter at which a module or a semi module ceases to be capable of acting as such.
• $odular Range+
The range of conditions between the modular limits within which a module or semi module wors as designed.
Characteristic of Outlets • Coe0cient of Discharge+
$n order to use the outlet as a measuring device the coeMcient of discharge should remain constant in the full modular range.
• %ilt Dra1ing Capacit!+ $t is vital that the outlets should draw their fair share of silt. This avoids silting or scouring and consequently remodeling of distributary.
• $n a distributary system the absorportion losses are generally taen as :J-:IN and therefore the silt conducting power of outlets should be around ::J-::IN as compared to :JJN of distributary to enable them to draw their proportional share.
Characteristic of Outlets • 2d3usta)ilit!+ The ad)ustment of module may range from complete reconstruction to the provision of some mechanical arrangement by which read)ustment can be made at little cost. 1ead)ustments are required in view of the revision of areas under command and because of change conditions in the distributary.
• Immunit! from Tempering+ There
is tendency on the parts of cultivators to draw more than their lawful share of water by tampering with the outlets. Therefore the outlets must be tamper proof.
%election of T!pe T!pe of Outlet • A rigid module !odular" or a 0lexible module !/emimodular" with a constant coeMcient of discharge is the best selection if the discharge and the water levels are constant in the distributary and necessary woring head is available.
• 2ut the problem of choice becomes quite complex when both the discharge and levels are liely to change.
• The following points may be noted % 0or a temporary discharge variation a proportional semi module is desirable to distribute both excess or de+ciency in the parent channel. % /easonal variation in the slope require the use of outlets of low *exibility( i.e.( sub-proportional.
%election of T!pe T!pe of Outlet % 0or channels running with full supply for a certain period and remaining closed for certain other periods( i.e. rotational running( it is desirable to have hyper-proportional or high *exibility outlets in the head reaches. % The silt drawing capacity of outlet must be ::J-::IN assuming a :J-:IN loss in parent channel. % $n general rigid modules are desirable in the following circumstances • 9irect outlets on a branch canal sub)ect to variation in supply • $n channels which sometimes carry extra discharge for speci+c reasons lie leaching.
Open 'lume Outlets • This is a smooth weir with a throat constricted suMciently long to ensure that the controlling section remains with in the parallel throat for all discharges up to the maximum
• /ince a hydraulic )ump forms at the control section( the water level of the watercourse does not a&ect the discharge through this type of outlet. 'ence this is a semi-modular outlet.
• This type of structure is built in masonry( but the controlling section is generally provided with cast iron or steel bed and chec plates.
Open 'lume Outlets
Open 'lume Outlets • The discharge formula for the open *ume outlet is given as8 E F C 2t '3>
• 4here8 E is related to the coeMcient of discharge( C( as given in the table below 2 t is the width of the throat and ' is the height of the full supply level of the supply channel above the crest level of the outlet in ft. 2t
C
J.> ft % J.>; ft( J. ft % J.; ft Over J.= ft
>.;J >.;I .JJ
Crump4s Open 'lume Outlet Design • 9ata8 • Outlet 9ischarge F q F = cfs • 0ull /upply 9epth F 9 F .I ft • 4oring 'ead F 'w F :.J ft • 9ischarge of 9istributory F E F
/ide /lope :8J.I
9FID 2FBD
Crump4s Open 'lume Outlet Design • %etting+ 7F /etting of outlet
F J.; 9 F .:I ft
'ead above crest of outlet F .:I ft
• Throat 5idth+ • qF Cd2t73> • qF >.;2t73>
Assuming Cd F >.;
• =F>.;2t!.:I"3> • 2tF J.>=<> F J.>I ft • ,ote8 The value of 2 t lies in between J.> to J.>; ft. !min. is J.>ft"
Crump4s Open 'lume Outlet Design • Length of Crest+ @ength of Crest F >.I 7 F >.Ix.:IF .BIF.; ft
• Radius of Transition+ 1 F > 7F<.J ft
• %etting 6ack+ This
distance by which the wall parallel to the distributary axis is to be set bac bears the same ratio to the width of distributary as the discharge of the outlet to that of the distributary. /etbac3width of distributary F q3E /etting 2ac F >.I ft
Crump4s Open 'lume Outlet Design • Transition in )ed+ The transition in bed is given by curve of radiusF > 7 F >x.:IF<. ft
• D-% Transition+ The slope of d3s glacis !:8:J to :8:I" is not de+ned as it depend upon the bed level of the water course.
• $inimum $odular /ead+ ' F J.>7 FJ.>x.:I F J.< ft L woring headF:.J ft !O6"
Crump4s Open 'lume Outlet Design • 'le(i)ilit!+
0F!m3n" !937" F!3>3I3"!:3J.;"
•
&m7-8, n9-7*
F: %ensitivit!+ / F n0 F I3 0 F I3
• E0cienc!+ F:JJx'ead recovered3'ead put in F:JJx!'ead put in-4oring head"3'ead put in F:JJ x !.:I-:"3.:IF IN
2d3usta)le Proportional $odule &2P$* • $n this type of outlet( a cast iron base( a cast iron
roof bloc and chec plates on either side are used to ad)ust the *ow and is set in a masonry structure
• This outlet wors as a semi-module since it does not depend upon the level of water in the watercourse. • The roof bloc is +xed to the chec plates by bolts which can be removed and depth of the outlet ad)usted after the masonry is dismantled. • This type of outlet cannot be easily tampered with and at the same time be conveniently ad)usted at a small cost. • The AP is the best type of outlet if the required woring head !'" is available and is the most economical in ad)ustment either by raising or lowering the roof bloc or crest. 'owever( it is generally costlier than the other types of outlets and also requires more woring head.
%chematic Diagram of Outlet Outlet discharge F q F Cd R 2t !'s":3>
0/@
R o o f 6 l o c k
0ull supply depth in parent channel F 9 4oring head F 'w 9ischarge of canal F E 4idth of throat F 2 t 9epth of water above crest u3s F 7 0lexibility F 0 in. odular 'ead F 'm 9istance from tip of roof bloc to 0/@ F 's 'sSR F 7( 'sFR(
9
7
's Q
'm 5 R
Tail Cluster • 4hen the discharge of a secondary( tertiary or quaternary canal diminishes below :IJ l3s !about I cusecs"( it is desirable to construct structures to end the canal and distribute the water through two or more outlets( which is called a tail cluster. ?ach of these outlets is generally constructed as an open *ume outlet
