design criteria for canals
TRANSCRIPT
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Feasibility Study for the Construction of 95km Irrigation DESIGN REPORTCanal from Jari Outlet to Iftikharabad District Bhimber- J!" #ol-$%Design Criteria
Design Criteria for Canal
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3 Canals & Structures
3.1 Scope
&his section describes the criteria and 'rocedures used for the detailed design of 95
"m canal( its irrigation net)ork and a''urtenant hydraulic structures* &he design of all
)orks )ill follo) the same engineering 'rinci'les as e+'lained in the design criteriauntil and unless some s'ecial conditions encountered during the e+ecution* &he
design criteria( including formulae( coefficients and constants )ill be used in all
hydraulic designs as a''licable*
3.2 Design Discarge of !ain Canal
Design discharge of the main canal is ,$ )hich )as )orked out as command area
irrigation re.uirements in a study by /0S1" in year 2334 cusecs under the study
ssessment of Irrigation 6ater 7e.uirement for 8oernment of J!" from :angla
7eseroir and ;''er Jhelum Canal< =&able-5*24>*
3.3 "ongitu#inal $ater Surface Slope of !ain Canal
&o bring ma+imum area under command by graity irrigation( main canal as )ell as
distribution canals hae to be as flat as far as 'ossible consistent )ith the need for
minimum elocities to trans'ort the sediment and to minimi?e its settlement in the
canal*
6ith these considerations( the longitudinal )ater surface slo'e in the main canal has
been fi+ed as 3*3$ @ =$ in $3(333>*
3.% Canal lign'ent
3.%.1 !ain Canal
&he to'ogra'hy along the antici'ated route of 95 "m main canal 'rinci'ally com'rises
on 'arts of 'otohar 'lateu( residual hill( foothill( 'lain and starches land* ;nder these
conditions alignment of the main canal )ould logically follo) the contours of terrain*
&he selected alignment of main canal )ill be fine tuned considering the follo)ing
maAor re.uirements%
• oid shar' bends
•
Crossing the drainage nullahs( channels and roads en-route 'ossibly'er'endicular and drains 'er'endicular to the centerline of main canal(
• oid settlements or sites of s'ecial significance =e*g* archaeologically im'ortant
sites>
• oid heay cut and fill areas to o'timi?e e+caation and borro) material fill
.uantities so as to minimi?e cost*
3.%.2 Distri(utaries) !inors & $atercourses
ike eery other irrigation channel( a distributary or minor should be aligned on the
local )atershed and its command area confined bet)een drainage lines* Follo)ingbasic considerations hae goerned 'lanning of distribution system% -
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Feasibility Study for the Construction of 95km Irrigation DESIGN REPORTCanal from Jari Outlet to Iftikharabad District Bhimber- J!" #ol-$%Design Criteria
• lignment of distributaries ado'ted )ould%
a* Eae shortest length as )ell as ensure effectie distribution of )ater to the land*
b* oid illages( roads( cart tracks( mos.ues( graeyards and other aluable
'ro'erties( as far as 'ossible*
• Distributary off-take )ould be located at the highest 'oint so as to commandma+imum area*
• Off-take regulators of distributariesminors )ould be located at a 'oint a)ay from
cures in the 'arent channel*
• lignment )ould attem't to 'roide minimum de'th of cutting or filling* In order to
stick to this criterion( fall structures of arying heights )ould be 'roided( )here
re.uired*
• lignment )ould hae minimum number of cures and acute cures )ould be
aoided 'articularly* It is a kno)n fact that such channels e+'erience non-uniform
elocity as )ell as silting on inner side of cure*
• lignment )ould cater for the commanded area in such a )ay that length of the
)atercourses does not e+ceed 2 miles* 6here re.uired( minors )ould be
introduced*
• lignment )ould be such that it does not gie rise to Chaks )ith long )atercourse
as )ell as hel' to create contour Chaks reducing thereby leeling and grading
re.uirements*
• lignment )ould aoid any costly cross drainage )orks*
• 0sca'es )ould be 'roided at 'ro'er 'laces to act as safety ales*
3.%.2.1 "a*out Proce#ures for Distri(utar* Co''an#
Outlets =:oghas> of a''ro'riate ty'e )ould be 'roided from a distributary or minor*
=/o direct outlets )ould be allo)ed from the :ain Canal>* Ca'acity of each outlet(
)hich is de'endent on the si?e of commanded area )ould be limited to *3 cusecs
considering the )ater allo)ance and ease of handling by the farmers* &ail clusters
shall be 'roided on the tail of minordistributary( )here re.uired*
For layout of )ater courses( one s'ot leel 'er acre of the command area )ould be
obtained )ith contour interal of $ft on the surey ma'* Considering the )ater
allo)able and discharge limitation of cusecs( Chak si?e )ould ary bet)een 253-
,33 acres of CC*
In order to determine FS in the distributary at an outlet location( highest 'oint in the
related chak )ould be considered*
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3.%.3 +en# Ra#ii
s suggested by ;SB7( for lined channels( radius to the canal centerline should be
three to seen times the )ater surface )idth =the larger ratio for larger ca'acity>
de'ending on the si?e or ca'acity of canal*
7adius of curature is also linked )ith free board by certain authors* Due to rise in
)ater leels on outer side of the cure the free board can be affected* It can be
calculated as belo)% -
gR
bvh
2
=
6here(
h G change in )ater surface eleation across channel =ft>
G sub-critical aerage elocity =fts>b G )idth of channel =ft>
g G acceleration due to graity =ft2s>
7 G distance from cure center to the channel center line =radius of the cure
in ft*>
0ffect of rise in )ater leel on outside of cure is negligible )hen the ratio of the
radius of curature to the distance to the center of the canal is greater than three
times the bed )idth of the channel*
Ta(le 3.1, #alues of minimum radii of channel cures for different channel ca'acities
Cannel Capacit* -cusec !ini'u' ra#ius of cur/ature -ft
ess than $3 25
$3 to $33 533
$33 to $333 $333
3.0 Canal Cross Sections
3.0.1 Pris' Geo'etr*
3.0.1.1 !ain Canal & Distri(utaries
&he main canal and distributaries shall be constructed as in-situ concrete lined
tra'e?oidal section* &he )ater 'rism of these canals shall be )orked out using
:anningHs formula* &his formula( )hich )as em'irically deried to)ards the end of the
$9th century( relates discharge to area of flo)( the slo'e of the energy line( the sha'e
of the channel and the roughness of the boundaries of the channel* :anningHs formula
is%
2/13/2486.1
S RnV ⋅=
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6here%
# G Flo) elocity =fts>
n G :anningHs roughness coefficient
S G longitudinal )ater surface slo'e =ft*ft>
7 G hydraulic radius G 1 =ft*>
1 G )etted 'erimeter of the flo) =ft*>
:anningHs roughness coefficient n< is an im'ortant 'arameter and should be realistic*
If n< alue is lo)er than realistic both for side slo'e and bed( the canal( could
e+'erience heading u'( encroachment of free board( decrease in discharge ca'acity
and siltation in bed* Such conditions must be aoided for effectie o'eration and
maintenance*
Ta(le 3.2% #alues of :anningHs 7oughness Coefficient( n( for #arious :aterials =7ef
#*&* Cho)>
T*pe of Cannel an# Description !ini'u' Nor'al !ai'u'
Float Finished Concrete 3*3$ 3*3$5 3*3$4
Concrete Finish )ith 8rael on Bottom 3*3$5 3*3$ 3*323
Straight and ;niform 0+caated 0arthChannel in Clean but )eathered Condition
3*3$ 3*322 3*325
Straight and ;niform 0+caated Channel)ith Short 8rass and Fe) 6eeds
3*322 3*32 3*3
&he consultants hae 'ro'osed manningHs n< alue of 3*3$4 for design of concrete
lined main canal and distributaries*
Cange of Distri(utar* Canal Section, For 'ur'ose of distributary design( most
im'ortant consideration in selection of reaches )ould be that ariation in the sections
of t)o adAoining reaches )ould not be e+cessie* &o)ards tail of the channel( )here
discharge is small( these reaches must be short* /ear the head( the reaches )ould be
longer * s a rough rule( the reaches )ould be so set that the section of the channel
)ould not change by more than $3 @ in )idth as )ell as de'th from one reach to the
ne+t*
3.0.1.2 !inors & $atercourses
6ith regard to lined canals( in-situ canal lining is 'articularly )ell suited to main canal
and distributaries but as flo)s get smaller to)ards the tail ends( it is much more
'ractical to use a 'refabricated 'recast system for )ater distribution* 1recast
'arabolic canal segments shall be for minors and )atercourses* &heir hydraulic
design shall be carried out using :anningHs e.uation*
3.0.2 Si#e Slopes
&he maAor limitations to the stee'ness of hard lined channel side slo'es are sli''age
of the lining and soil stability* Sli''age may be caused by insufficient friction bet)een
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the lining and the sub-grade in combination )ith effects of e+ternal hydrostatic
'ressure* &he follo)ing side slo'es may be ado'ted*
Ta(le 3.3, Side Slo'es for ined Channels
Flo) De'th( D =ft> Side slo'e =$#%+E>
*3 $*3
*3 to *3 $*3 to $*5
K *3 $*5 to 2*3
&he consultants hae ado'ted concrete lined side slo'es of $%$*5 for the main canal*
3.0.3 ree(oar#
3.0.3.1 !ain Canal
Freeboard for the canal lining and banks shall be in accordance )ith the ;SB7
guidelines and based on the ma+imum flo) 'reailing in a reach*
In main canal the 'ro'osed freeboard shall be 2*3 ft* )hich includes $*3 ft* lining
freeboard and $*3 ft* bank freeboard* Similarly minimum clearance under bridge decks
or girders shall be ke't as 2*3 ft*
In reaches )here the main canal is crossing nullah hill torrent through a cross
drainage structure( there )ould be 'ossibility of raised )ater leels afflu+ on the
u'stream side of nullah* t such locations the bank of main canal facing nullah
u'stream shall be conerted into flood embankment* &he crest leel of flood
embankment )ill be 'roided )ith the minimum freeboard of *3 ft* from highest flood
leel in the nullah to 'rotect against oer-to''ing*
3.0.3.2 Distri(utaries) !inors & $atercourses
For distributaries( minors and )atercourses( the recommended freeboards are as
follo)s%
Ta(le 3.%, 7ecommended Bank &o' Freeboard
Canal ree(oar# -ft.
Distributaries $*53
:inors $*25
6atercourse 3*5
3.0.% +er' $i#t
&he berm is the narro) stri' of land on either side of the canal( bet)een the canal
'rism and toe of the embankment* Berms are generally 'roided in unlined canals
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)here de'th( D of flo)( is greater than *3 ft* In lined channels berms are clearly not
re.uired to accommodate change in channel 'rism or additional discharge*
In main canal( $*3 ft* )ide concrete berm shall be 'roided on the lining to' to
stabili?e the toe of earthen bank*
Stabili?ing berms of different si?es =based on slo'e stability re.uirements> shall be
'roided in high earth-fill embankments and dee' cut slo'es along the main canal as
'er geotechnical design criteria*
3.0.0 Canal E'(an'ents
igure 3.1, &y'ical Cross section
0mbankments )ould be constructed out of the s'oil material from e+caation of the
main canal and from borro) areas along the alignment* fter establishing the cross
section of canal embankment( before 'lotting the actual 'hreatic( line of hydraulic
gradient =assumed line of saturation in the embankment body to )hich 'orous soils
offer less resistance to 'ercolation> )ill be dra)n on the cross-section starting fromflood )ater leel at rierside u' to the e+it face at land side* It is customary that E8
must not only fall )ithin the embankment but should hae a minimum coer of 2 ft* &o
adAust this( ne) embankments )ill be resi?ed )hereas for e+isting embankments a
back berm =or 'ushta> )ill 'roided to kee' the hydraulic gradient )ithin the cross-
section*
Eydraulic gradient lines for arious ty'es of soils are considered as follo)s%
Ta(le 3.0, Eydraulic gradient lines for arious ty'es of soils
T*pe of soil 4G" slope -5,48ood soil =Clay> $ % ,
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erage soil =sandy loam> $ % 5
Bad soil =Clayey loam> $ % 4
Fine silt $ % 4
Fine sand $ %
Coarse sand $ % $3
&he 'ro'osed hydraulic gradient for canal and the distributaries is 'ro'osed as$ in 5
for both lined and unlined sections* Bank back slo'es are chosen to maintain the
see'age 'hreatic surface at least 3*m =$ft> )ithin the toe of the embankment for
canals in fill*
For the :ain canal( and any branch distributary canals )here 'atrol roads are
re.uired( one 'atrol road 23 ft* )ide on the to' of a bank shall be 'roided* /on-'atrol
bank )idth is ado'ted as $2ft*
Ta(le 3.6, 7ecommended /on 1atrol Bank &o' 6idths
Canal =ft>
:ain$3*3 to $2*3
Branch
Distributary *3 to $3
:inor 5*3
*3
6atercourse 2*3
3.6 Canal "ining
3.6.1 "ining Ticness an# Concrete Strengt
:ain canal lining shall be un-reinforced , inches thick concrete 'laced on the
moistened and com'acted sub-grade )hereas thickness of un-reinforced lining for
Distributaries and :inors shall be inches*
&he concrete for lining shall hae minimum cylinder com'ressie strength of the (333
'si at 2-days*
3.6.2 7oints
ongitudinal contraction dummy Aoints formed as #- grooes and filled )ith
elastomeric sealant )ould be 'roided on canal slo'es at one-third and t)o-third
distances from the toe*
&ranserse construction Aoints and 'rofile )alls on slo'es and in bed s'aced $5 feet
a'art and longitudinal construction Aoints and 'rofile )alls )ould be 'roided at the
centre of bed and along toes*
3.6.3 Canal Structures for lo8 Regulation & Control&hese structures may be described as follo)s%
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• Cross regulators to head u' )ater in the 'arent channel to diert some of it
through an off take channel( like a distributary
• Distributary head regulator to control the amount of )ater flo)ing in to off take
channel
•
Dro's and falls to lo)er the )ater leel of the canal
• 0sca'es( to allo) release of e+cess )ater from the canal system
igure 3.2, Canal Structures for Flo) 7egulation and Control
3.6.3.1 Cross Regulators -Cecs
&he main functions of cross regulator are to% -
i* 0nable effectie regulation of the canal system as a )hole*
ii* Eel' raise )ater leel and feed the off-taking distributaries o'erating in rotation
to their ma+imum authori?ed discharge( )hen the )ater leel in :ain Canal is
lo)er than 4@ of the design discharge*
iii* Eel' in closing the breaches in the lo)er sections of the canal*
i* 1reent 'ossibilities of breaches in the tail reaches by absorbing fluctuations in
arious u''er reaches of the canal system*
* 1roide road crossing to facilitate communications*
&he number and )idth of s'ans should allo) the design discharge to 'ass )ithout
causing backu' or dra)do)n of flo)* Crest leel of head regulator should e.ual( or be
higher than( the u'stream bed leel* ;'stream and do)nstream transitions can
reduce head losses and non-se'aration of flo) do)nstream*
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For the distribution system( flo) elocity though check structures using sto' logs
should be limited to about *4 fts* 6ith gates the elocity may be increased to about 5
fts* &he use of sto' logs is not recommended for flo)s greater than 53 cusecs*
3.6.3.2 Distri(utar* 4ea# Regulators -Turnouts
:ain functions of a head regulator are% -
i* &o control su''lies entering into a distributary from :ain Canal*
ii* &o control entry of sediment into the distributary*
iii* &o )ork as a meter for the measurement of discharge*
i* &o act as a check and hel' in shutting off the su''lies )hen the distributary is
re.uired to be closed*
* Eead regulator dierts )ater from the su''ly channel to a smaller channel*
&he structure usually com'rises an inlet( a conduit to coney )ater through the
bank of the su''ly channel and an outlet transition* 8ates are usually used inthe inlet to control the flo)* Crest leel of the distributary head regulator is
generally ke't $ft to 2 ft higher than crest leel of cross regulator*
igure 3.3, Cross 7egulator and Eead 7egulator
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Head Regulator
Cross Regulator
Feasibility Study for the Construction of 95km Irrigation DESIGN REPORTCanal from Jari Outlet to Iftikharabad District Bhimber- J!" #ol-$%Design Criteria
igure 9 3a, Cross Regulator an# 4ea# Regulator
Off:taing lign'ent, &he angle at )hich a distributary canal off-takes from the 'arent
canal has to be decided carefully* &he best angle is )hen the distributary takes off smoothly(
as sho)n in Figure ,=a>* nother alternatie is to 'roide both channels =off-taking and 'arent>
at an angle to the original direction of the 'arent canal =Figure ,b>* 6hen it becomes
necessary for the 'arent canal to follo) a straight alignment( the edge of the canal rather
than the centre line should be considered in deciding the angle of off-take =Figure ,c>*
igure 3.%, lignment &y'es of off taking canal from 'arent canal
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&o 'reent e+cessie entry of silt de'osition at the mouth of the off-take( the entry
angle should be ke't to bet)een 43 and 93 degree* &he )ater entering in to the off-
taking distributary canal from the 'arent canal may also dra) sus'ended sediment
load* &he distributary should 'referably be designed to dra) sediment 'ro'ortional to
its flo)( for maintaining non-siltation of either the 'arent canal or itself*
3.6.3.3 Canal alls -Drops
:ain functions of a canal fall are% -
i* &o lo)er the )ater leel of canal by dissi'ating hydraulic energy*
ii* &o negotiate the change in bed eleation of the canal necessitated by difference of
ground slo'e and canal slo'e*
iii* &o )ork as a meter for the measurement of discharge*
igure 3.0, &y'ical location for 'roiding canal dro' or fall
"ocation of all ; Drop, &he location of fall is 'rimarily influenced by the to'ogra'hy
of the area and the desirability of combining a fall )ith other masonry structures such
as bridges( regulators etc* 0conomy in the cost of e+caation is to be considered*
Ideally( there should be balance bet)een e+caation and filling* In case of channels
)hich irrigate the command area directly( a fall should be 'roided before the bed of
the channel comes into filling* &he full su''ly leel of a channel can be ke't belo)
ground leel for a distance of about $533 ft* do)nstream of fall as command in this
reach can be irrigated by the channels off-taking u'stream of the fall*
T*pes of Canal all, &he follo)ing t)o ty'es of canal falls shall be considered for
this 'roAect%
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1 Glacis all
glacis fall usually consists of an inlet transition( a control section( a glacis dro'( a
stilling basin and an outlet transition* &he inlet 'ortion of the structure transitions the
flo) from the channel u'stream to the control section* &he control section is re.uired
to 'reent dra)-do)n of the u'stream flo) and to 'reent racing and scouring of the
u'stream channel* &he inlet andor control section should hae cutoffs andor sufficient length to 'roide the re.uired 'ercolation 'ath as com'uted by aneHs
)eighted cree' method* &he glacis section connects to the stilling basin( and the
outlet transition ensures against se'aration of flo) and formation of )aes* &o 'roide
tail)ater for the hydraulic Aum'( the )ater surface at the outlet must be controlled* &he
do)nstream transition may be used to 'roide back)ater by raising the floor of the
transition*
;nless the falls are gated( or hae 'roision for sto' logs only 'ercolation( scour and
u'lift are likely to be critical* For small lined channels( the channel lining 'rotects
against both scour and 'ercolation failure* For larger lined channels( it )ould be'rudent to assume that the lining fails and 'roide cut offs andor u'stream see'age
a'rons*
igure 3.6, ongitudinal Section of 8lacis Fall
a.Transitions
&ransitions 'reent formation of )aes )hich can be troublesome as they trael
through the glacis dro' section and energy dissi'ater( and 'reent flo) se'aration
and eddies do)nstream* s a general consideration the ma+imum deflection angle in
the )ater surface should be about $%$ =inlet section> and $% to $%, =outlet section>*
&he transitions should be symmetrical about the centreline of the canal*
(.Control Section
&o 'reent sediment de'osition u'stream of the fall a high crest is aoided( and
=shallo)> ertical transitions to and from the glacis 'roided* 7acing of flo) u'stream
of the fall is 'reented by constriction of the flo)* &he a''ro'riate discharge formula
should be used*
c.Glacis Section
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&he usual section for the glacis section is rectangular* s friction in the glacis is
negligible( the continuity e.uation and BernoulliHs =energy balance> e.uation may be
used to com'ute the flo) ariables =2 and y2> at the bottom of the glacis by
balancing )ith the to' of the glacis* &he energy balance and continuity e.uations(
gien belo)( are soled by trial and error for 2 and y2*
y$ L M L $22g G y2 L 222g =energy balance>
N G $ $ G2 2 =continuity>
)here%
y$ G de'th of flo) u'stream at location $ ftP
M G difference in height bet)een locations $ and 2*
$ G flo) elocity u'stream of glacis at location $ ftsP
N G discharge ftsP
g G graitational constant( 2*2 fts2P
#.Stilling +asin
&he stilling basin ensures that energy of flo) is dissi'ated in a hydraulic Aum'*
Eori?ontal ;SB7 ty'e stilling basins shall be usually 'roided*
2 5ertical:Drop all
&he ertical-dro' fall uses a raised crest to head u' )ater on the u'stream of the
canal section and allo)s it fall )ith an im'act in a 'ool of )ater on de'ressed floor
)hich acts like a cushion to dissi'ate the e+cess energy of the fall*
igure 3.
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&o 'reent sediment de'osition u'stream of the dro' a high crest is aoided* Dra)
do)n and racing of flo) u'stream of the dro' may be 'reented by a hori?ontal
constriction of the flo)* &he free flo) broad crested )eir formula is a''licable and
used to determine the )idth of the control section( bc%
N G *39 b h$*5
6here%
N G design discharge =fts>
h G u'stream de'th of flo) =ft>
b G crest )idth =ft>
f. Stilling +asin
&he stilling basin do)nstream of the ertical - dro' fall shall be a cistern ty'e* &he
cistern< is defined as that 'ortion of the fall structure in )hich sur'lus flo) energy of
the )ater leaing the crest is dissi'ated and subse.uent turmoil stilled( before the)ater 'asses into lo)er leel channel* Cistern is ery efficient )hen dro' is sufficient
so that falling stream becomes almost ertical*
&he length c< and de'th +< of the cistern can be determined only by em'irical
e+'ressions gien by different inestigators*
&o 'reent the falling na''e from adhering to the masonry face of the fall( aeration of
the na''e is necessary and is 'roided by 'i'es embedded in )ing )alls Aust
do)nstream of crest*
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&he symbols used in these
relations and their alues are
in meters
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Selection of Canal all, &he follo)ing criterion shall be a''lied for the selection of
a''ro'riate ty'e of canal fall%
Drop 4eigt -ft T*pe
Distributaries $*53
:inors $*256atercourse 3*5
3.6.3.% Canal Escapes
Canal 0sca'es are side channels 'roided for the follo)ing 'ur'oses% -
• &o release the e+cess )ater from the canal
• &o o'erate as a tem'orary control during construction of the canal* s each section
bet)een the esca'es is commissioned( flo)s )ould 'ass do)n the canal and out
of these esca'es into a nearby nullah until the ne+t section is commissioned*
• &o 'ass the canal full su''ly discharge to the nullah in the eent of sudden closure
of the cross regulator do)nstream*
• &o facilitate canal eacuation for 'eriodic ins'ection during O!:*
• &o flush sediment )hich accumulates in the canal reach u'stream due to heading
u'*
• &o )ork as a safety ale in case of canal breach do)nstream*
• &o 'ass accidental inflo)s either from oerto''ing of any su'er- 'assage flume or
from breach of the right flood 'rotection embankment of :ain Canal*
0sca'es are designed for full su''ly designed discharge at head of the canal reach*
ocations are selected according to the 'hysical to'ogra'hy of the terrain( )here
there is enough difference in eleation to drain the canal discharge into a nullah*
igure 3.=% ongitudinal section of different 0sca'es
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1 Canal Outlets
3.0.6 "ocation
n outlet )ould( in general( be located u'stream of a dro' control structure* It )ould
be at the u'stream boundary of the Chakmain )atercourse so that ma+imum area is
commanded*
3.0.< Co''an# rea
s already mentioned( command area of an outlet )ould be ke't bet)een 253 acres
and ,33 acres of CC )ith limiting ma+imum discharge of Q:oghaH as *3 cusecs*
3.0.= T*pes of Outlets
Follo)ing fie ty'es of outlet( de'ending u'on situation and location( )ould be used in
the distribution system of the 1roAect%-
i* dAustable Orifice Semi :odule =OS:>*
ii* O'en Flume )ith 7oof Block =OF7B>*
iii* O'en Flume =OF> ;'stream of FallDro' Structure
i* O'en Flume at &ail Cluster
* 1i'e or Barrel Outlet
In the first four ty'es i*e* OS:( OF7B or OF( the discharge( though de'endent on the
)ater leel in the 'arent channel( is inde'endent of the )ater leel in the )atercourse(
so long as the :inimum :odular Eead =mmh> re.uired for its )orking is aailable*
In the fifth ty'e of outlet i*e* 'i'e or barrel outlet( the discharge is a function of difference bet)een )ater surface in the channel and the )atercourse( if the
do)nstream end is submerged* In case of free fall( the difference in the canal )ater
surface and centre of 'i'e at e+it )ould control the discharge*
ll outlets )ithin 233 ft u'stream of fall structures )ould be designed as O'en Flumes
=OF>*
3.0.=.1 OS! Outlet
8enerally( all outlets shall be of OS: ty'e e+ce't the outlets at tail u'stream of fall
and those )hich hae small )orking heads* O'en flume =OF> ty'e )ould be ado'ted)here OS: ty'e cannot be used* OS: ty'e of outlet is a long throated flume )ith
a roof block( ca'able of ertical adAustment( introduced into the u''er end of the
'arallel throat* So long as the standing )ae is steady and remains clear of the e+it of
the orifice( the discharge co-efficient does not alter* &he roof block is so sha'ed that
the Aet is made to fill the e+it of the orifice and Aet contraction is su''ressed* Cast iron
or 're-cast 7CC roof block )ould be used* 7oof block )ould hae its u'stream end at
distance of three => inches do)n the throat from the crest and its bottom at a height
of y =aries> aboe the crest*
Discharge of this ty'e of outlet is com'uted by the formula%
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21)(***3.7 Hs y Bt q =
6here
. G Discharge of outlet =fts>
Bt G 6idth of the orifice =gullet> in ft =minimum )idth is 3*23 ft>
y G Eeight of bottom of roof block aboe crest in ft*
QyH is usually bet)een $*5 Bt and 2 Bt as far as 'ossible and in no case y should
e+ceed 3*5 E )here E is the height of full su''ly leel in channel aboe the crest
of the outlet*
Es G de'ression of roof block belo) )ater surface leel in the 'arent channel
in feet G =E-y>*
Setting of the outlet )ould be e.ual to 3*3 of the full su''ly de'th* t this setting( its
sediment dra)l ca'acity )ould be is $39* 'ercent*:inimum modular head =::E> re.uired for this ty'e of outlet G 3*2R Es Bt2
RHC | AGES | 2015 -$
igure 3.>, &y'ical rrangement for OS: Outlet
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Feasibility Study for the Construction of 95km Irrigation DESIGN REPORTCanal from Jari Outlet to Iftikharabad District Bhimber- J!" #ol-$%Design Criteria
3.0.=.2 Open lu'e Outlet 8it Roof +loc -OR+
&his ty'e of outlet is 'roided )here OS: cannot be used on account of small
)orking head aailable* &his outlet is sim'ly a smooth )eir )ith throat constructed
sufficiently long to ensure that the controlling section remains )ithin the 'arallel throat
at all discharges u' to ma+imum*
&he discharge is gien by the formula%
23** H Bt C q =
6here(
. G Discharge of the outlet in fts*
Bt G 6idth of outlet in ft =minimum 3*23 ft>*
E G Eeight of the )ater leel in the channel aboe crest leel*
C G Co-efficient( )hich remains constant as long as the ::E re.uiredfor modularity is aailable* #alues of C< are as under%
For Bt< 3*23 ft to 3*3 ft G 2*93
For Bt< 3*3 ft to 3*,3 ft G 2*95
For Bt< 3*,3 ft G *33
Setting of the outlet )ould be ke't e.ual to 3*9 times the full su''ly de'th* In order to
check the oer-)ithdra)al by the outlet( a roof block )ould al)ays be fi+ed in the
gullet* &he roof block )ould hae its u'stream end at a distance of E do)n the throat
from the crest and its bottom at a height of 3* E aboe the crest* ;'stream bottomcorner of the block )ould be rounded to a radius of $-$2 inches* &he :inimum
:odular Eead =::E> re.uired for this ty'e of outlet is 3*2 E*
s )idth of outlet is limited to minimum of 3*23 feet( it is not 'ossible to design the
outlets )ith 3*93 full su''ly de'th setting for discharges lo)er than those sho)n belo)
for different full su''ly de'ths% -
D =ft> $*3 $*5 2*3 2*5 *3 *5 ,*3
N =fts> 3*53 3*9$ $*3 2*25 2*3 *$5 *43
3.0.=.3 Open lu'e -O Outlet ?pstrea' of all;Drop Structures
ll outlets )ithin 233 ft u'stream of a fall structure for lined sections and 533 ft for
unlined section )ould be designed as o'en flume =OF> )ithout roof block* In such
cases( the crest leel of o'en flume outlet )ould be the same as that of bed in the
'arent channel*
3.0.=.% Open lu'e -O Outlet at Tail Cluster
t tail( o'en flume )ithout roof block )ould be used* E( the height of full su''ly leel
in the channel aboe crest leel of the OF outlet )ould ke't as $*33 ft or 3*5 ft in an
unaoidable case* Mero of the gauge )ould be fi+ed at crest leel* 8auge reading at
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Feasibility Study for the Construction of 95km Irrigation DESIGN REPORTCanal from Jari Outlet to Iftikharabad District Bhimber- J!" #ol-$%Design Criteria
tail )ould(
therefore(
directly
indicate
)hether or
not the tailis getting
its
authori?ed
discharge*
Crests of
all outlets
at tail
)ould be
ke't at the
same
leel*
RHC | AGES | 2015 -$9
igure 3.1@, &y'ical rrangement for O'en Flume Outlet )ithout 7oof Block
igure 3.11, &y'ical rrangement of &ail Cluster Outlets
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Feasibility Study for the Construction of 95km Irrigation DESIGN REPORTCanal from Jari Outlet to Iftikharabad District Bhimber- J!" #ol-$%Design Criteria
3.0.=.0 Pipe or +arrel Outlet
&his ty'e of outlet )orks as semi module( if it has a free fall* In dro)ned condition( it
)orks non-modular *
;se of this ty'e of outlet is aoided( as far as 'ossible* Its use )ould be Austified in
case of lift outlets )hen the )orking head aailable or the discharge of the outlet is so
small that a semi-modular outlet cannot be designed*
Discharge of 'i'e or barrel outlet is gien by the formula%
. G
21
)( H CA
6here(
. G discharge of outlet in =fts>
G area of the 'i'e or barrel in ft2*
E G distance in feet from the )ater leel in the channel to the centre of
the 'i'e or barrel( in case the outlet has free fall and if it is dro)ned i*e* it
discharges into a )atercourse in )hich the )ater leel is aboe to' of the
barrel( then TET is the difference in leels in the channel and the )atercourse*
C G co-efficient )hich is taken as 5*3 for free fall outlet and 4*, for
dro)ned outlet*
igure 3.12, T*pical Plan & cross:section of an Outlet
3.6 Cross:Drainage Structures
Cross drainage structures are constructed to negotiate an aligned channel oer( belo)
or at the same leel of stream crossing that channel* &hese structures can beclassified under the follo)ing three broad categories%
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$> Structures for a carrier channel oer a natural stream =.ueducts and
Culerts>
2> Structures for a carrier channel belo) a natural stream =Su'er-'assages and
Si'hons>
2> Structures for a carrier channel crossing a natural stream at same leel =eel
Crossings>
*4*$ Aue#ucts
n a.ueduct is a cross-drainage structure constructed )here the drainage flood leel
is belo) the bed of canal* Small drains may be taken under the canal through culerts
but for streams crossings it is economical to flume the canal oer the stream*
&he structure )ould consist of% flume trough to carry the canal )ater( su''orted on
'iers or 'iles foundation sometimes a bridge to maintain continuity of the canal road
su''orted foot bridge on trough )alls u'stream and do)nstream flared out
transitions in canal u'stream guide banks in the stream to guide the flo) through the
crossing )ithout inducting e+cessie scour u'stream and do)nstream stone a'rons
stone 'itching for slo'e 'rotection*
3.6.2 Superpassages
6here the canal section is 'rimarily in cut as it crosses a nullah( the storm runoff
)ater )ould be coneyed oer the canal( clear of the canal )ater surface* Such
structure called su'er-'assage is a''ro'riate )here the nullah inert leel is bet)een
canal bed leel and full su''ly leel =FS> or aboe FS* &he canal section geometry
)ould be ke't unchanged to minimi?e head losses*
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igure 3.13, &y'ical 1lan ! cross-section of an .ueduct
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Feasibility Study for the Construction of 95km Irrigation DESIGN REPORTCanal from Jari Outlet to Iftikharabad District Bhimber- J!" #ol-$%Design Criteria
&he structure )ould consist of% trough to carry the nullah )ater( su''orted on 'iers or
'iles foundation do)nstream bridge to maintain continuity of the canal road
su''orted foot bridge on trough )alls stilling basin for energy dissi'ation u'stream
and do)nstream flared out transitions u'stream guide banks to guide the flo)
through the crossing )ithout inducting e+cessie scour u'stream and do)nstream
stone a'rons stone 'itching for slo'e 'rotection u'stream and do)nstream cutoffsdo)nstream 1CC blocks oer inerted filter( and reinforced concrete canal lining
underneath the trough*
Flume )idth to 'ass the nullah design discharge )ould be goerned by heading u' of
flood )ater on the u'stream side* &he crest of flumetrough shall be fi+ed )ith sub-
critical flo) and generating ma+imum elocity of ftsec* 8uide banks )ould 'roide a
free board of *3 feet aboe the raised )ater leel*
3.6.3 Cul/erts
6here the canal section is com'letely in fill( the nullah )ater be freely flo)ing under
the canal through concrete barrels* &he structure )ould com'rise% multi'le barrels
carrying nullah )ater under canal section u'stream head )all do)nstream
transitions do)nstream cistern floor u'stream and do)nstream stone a'rons stone
'itching for slo'e 'rotection and cutoffs*
:any irrigation structures incor'orate a culert or a bridge in order to facilitate
ehicular access )ithin the scheme* 6here a road crosses a channel at some
distance from any other structure( it )ill be necessary to 'roide an inde'endent
culert or bridge* 7oad crossings usually com'rise either 'i'e or bo+ culerts to
coney channel )ater under the road*
&he culert acts as a constriction and creates a back)ater effect to the a''roach flo)(
causing a 'ondage of )ater aboe the culert entrance* &he flo) )ithin the barrel
itself may hae a free surface )ith subcritical or su'ercritical conditions de'ending on
the length( roughness( gradient( and u'stream and do)nstream )ater leels of the
culert* If the u'stream head =E> is sufficiently large the flo) )ithin the culert may or
RHC | AGES | 2015 -22
igure 3.1%, &y'ical Cross-section of a Su'er-'assage
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may not fill the barrel( and its hydraulic 'erformance de'ends u'on the combination of
entrance and friction losses( length of barrel( and the do)nstream back)ater effects*
3.6.3.1 Design Consi#erations for Pipe Cul/erts
1i'e culerts are often designed to flo) full( )ith the 'i'e soffit submerged by at least inches both u'stream and do)nstream* &he 'i'e ty'ically has a fall of 3*35 m*
Ta(le 3.
For culerts under 'atrol roads )ithin an irrigation scheme( the minimum de'th of
coer to the 'i'e should be about 3*93 m* If this is im'racticable( then consideration
should be gien to%
• surrounding the 'i'e in concrete
• using smaller 'i'es =for e+am'le by using 2 + 3*43 m diameter 'i'es instead of a
single 3*93 m diameter 'i'e>
RHC | AGES | 2015 -2
igure 3.10, Flo) through a Culert
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Feasibility Study for the Construction of 95km Irrigation DESIGN REPORTCanal from Jari Outlet to Iftikharabad District Bhimber- J!" #ol-$%Design Criteria
&ransitions are re.uired at both the inlet and outlet of the culert barrel* n
accelerating )ater elocity usually occurs at the inlet( and a decelerating elocity at
the outlet* &ransitions reduce head losses and 'reent channel erosion by making the
elocity changes less abru't*
1i'e collars may be re.uired to reduce see'age flo)s along the outside of the 'i'e*
&his should be checked using aneHs see'age 'ath length method( 'articularly for anyculert )here the channel )ater surface is significantly higher than the 'otential 'oint
of relief of the 'ercolating )ater* 1i'e collars may also be necessary to discourage
rodents from burro)ing along the 'i'e*
Channel erosion 'rotection is re.uired in earthen channels( and usually takes the form
of dry stone 'itching 'laced u'stream and do)nstream of the transitions* In addition(
for large flo)s( cutoffs may be 'roided to the transition structures e+tending belo)
scour de'th*
3.6.3.2 Design Consi#erations for +o Cul/erts
Bo+ culerts can be single or multi'le( )ith s.uare or rectangular section* &hey can be
constructed from reinforced concrete( or com'rise concrete floor and roof slabs )ith
masonry )alls* &he minimum height of the culert should be about ft* since anything
smaller than this is 'robably more easily constructed using concrete 'i'es* Bo+
culerts are normally designed for free flo)( )ith the )ater surface belo) soffit leel*
Eead loss across the culert can be calculated by summing entry and e+it losses( 'lus
friction loss in the culert )hich may be determined using :anningHs e.uation*
Bo+ culerts can be designed to flo) full( in )hich case both the u'stream and
do)nstream soffits should be dro)ned by at least inches to 'reent gul'ing