Managing Sedimentation in Spate Irrigation Schemes
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Transcript of Managing Sedimentation in Spate Irrigation Schemes
8/13/2019 Managing Sedimentation in Spate Irrigation Schemes
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Managing sedimentation in spate irrigation schemes
Philip Lawrence1
1 Independent consultant, ( [email protected])
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Contents
1.0 Introduction
2.0 Overview of sedimentation problems encountered in spate systems
2.1 Sediment transport in wadis
2.2 Sediment manaement in traditional systems
2.! Sediment manaement in modernised systems
2." # sediment manaement stratey for spate irriation systems
!.0 #pplication of models to aid t$e desin of sediment manaement structures in spate
sc$emes
!.1 Sediment e%cludin inta&es
!.1.1 '%amples of sediment e%cludin inta&es
!.1.2 $ysical models!.1.! $ree dimensional numerical modellin
!.1." *asic inta&e model
!.2 Secondary sediment control + e%tractors and settlin basins
!.2.1 esin of ravel traps
!.2.2 -ybrid e%tractorflus$ed basin for lare sc$emes
".0 Spate canal desin met$ods
/.0 redictin future command levels
.0 Sediment manaement options
.1 atc$ment conservation
.2 Sediment manaement options for a rane of sc$eme types
.0 Sediment manaement desin software
3.0 4eferences
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1.0 Introduction
'ffective manaement of sedimentation is a &ey factor in improved spate irriation systems.
$is paper describes t$e sediment problems encountered in spate sc$emes and describes
met$ods t$at now can be used to estimate5ate t$e performance of sediment control structures.
6ost of t$e sediment control strateies described in various papers in t$e first 7#O sponsored
spate irriation e%pert consultation, (7#O 183), remain valid 9 t$e laws of p$ysics $ave not
c$aned over t$e last two decades. :$at $as c$aned is t$e development of numerical models
t$at now, wit$in limits, allow t$e performance of sediment manaement structures to be
investiated wit$out t$e use of p$ysical models, w$ic$ enerally cannot provide ;uantitative
predictions in t$is application. <umerical models provide ;uantitative predictions of t$e si=es
and ;uantities of sediment diverted at spate irriation off ta&es, t$e performance of sediment
e%tractors and settlin basins, and t$e sediment transportin capacity and patterns of
deposition wit$in canal networ&s. #doptin a ;uantitative approac$ to sediment manaement
based on numerical simulations $as many advantaes, t$e most important bein t$e capability
to rapidly compare a rane of sediment manaement options, optimisin t$e performance of
sediment control structures, and enablin costs of constructin sediment control structures to be compared wit$ a realistic estimate of t$e benefits.
$e ;ualification above arises from t$e ;uality of predictions provided by even t$e best of
currently available sediment transport functions, w$ic$ underpin all sediment transport
simulations, and t$e eneral lac& of reliable field data, particularly from spate systems, w$ic$
is needed to set up and verify numerical sediment transport models. 6odels can only be
improved and developed, and sedimentation issues properly ;uantified, t$rou$ comparisons
wit$ reliable field information. -owever w$en applied by enineers wit$ an understandin of
t$e models and t$e processes bein simulated, numerical sediment transport models can
provide an invaluable aid to t$e desin of spate irriation diversions and water distribution
systems.
$e first section of t$e paper summarises sedimentation problems encountered in spate
sc$emes, and sets out a sediment manaement stratey. 6odels t$at can be applied to aid t$e
desin of sediment control structures and canals are t$en described, wit$ some e%amples. $e
last section of t$e paper lists sediment manaement options for sc$emes wit$ a rane of
c$aracteristics.
2.0 Overview of sedimentation problems encountered in spate systems
2.1 Sediment transport in wadis
6orp$oloy and sediment transport processes in spate wadis are described in uidelines
prepared in 200/2. In summary t$e main features of t$e sediment loads arrivin at typicalspate diversion sites are>
:adi bed materials can rane from boulders and cobbles to silts, wit$ t$e lower
reac$es of wadis usually $avin sand beds. Sediments of all t$e si=es represented in a
wadi bed are transported in t$e larest floods.
otal load sediment concentrations risin to and e%ceedin 100,000 ppm, or 10
percent by wei$t, occur in floods in some wadis. Sediment concentrations up to /
percent by wei$t in floods are common.
2 Improvin community spate irriation, ?awrence and r. 7 van Steenberen. 4eport O 1/" -4:allinford, A. (an be downloaded free of c$are from> $ttp>eprints.$rwallinford.co.u&1!"
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Sediment transport is dominated by t$e finer sediment fractions transported in
suspension. ypically t$e proportion of fine silts and clay in annual sediment load
ranes between /0B and 80B. -i$ concentrations of fine sand are also transported
in suspension. $e suspended sand load is ;uite fine, enerally between 0.1 and 1
mm, w$en compared wit$ t$e parent bed material, as s$own in 7iure 1!.
$e proportion of t$e sediment load represented by larest sediments, transported mostly by rollin and slidin alon a :adi bed, (bed load), is typically only / percent or less of
t$e annual sediment load. Sediments in t$is si=e rane, (coarse sand, ravel, cobbles, and
in some cases small boulders), will, if diverted, settle and bloc& inta&es and canals.
0
10
20
30
40
50
60
70
80
90
100
0.01 0.1 1 10 100 1000
Size mm
% F
i n e r
W ad i Z abid W ad i Laba
Bed Material
Suspended Sediment
larer t!an 0.06 mm
Figure 1 Bed material and suspended bed material (sand) load
size grading for two Wadis
#s only t$e very larest floods are usually allowed to pass beyond spate irriated areas most
of t$e sediments transported to t$e first diversion point in a spate sc$eme are deposited wit$in
t$e irriated area. oarser sediments settle in t$e :adi c$annels and canals, and finer
sediments are deposited on t$e fields, w$ere farmers welcome sedimentation as a source of
fertility. Spate systems build up t$eir own soils, and older systems are c$aracterised by fine
sediment deposits t$at are many metres deep. Sediment deposition rates on spate irriated
fields reported in t$e uidelines rane between 1 to more t$an /0 mm year.
2.2 Sediment management in traditional spare irrigation systems
raditional spate systems $ave several features t$at t$at $elp control sedimentation>
Inta&es are was$ed away by lare floods, reatly reducin t$e volume of water
diverted to a canal durin periods of very $i$ flow in t$e wadi, w$en $i$
concentrations of coarse sediments are bein transported.
raditional canals are steep, and flow at muc$ $i$er velocities t$an would be
allowed in conventionally desined irriation c$annels.
sually all t$e flow in a canal is diverted fields at a sinle point, maintainin t$e full
canal disc$are and sediment transportin capacity from t$e wadi inta&e to t$e field.
! $e si=e rane of suspended bed material can be appro%imately estimated from t$e bed material si=edistribution. $is facility is available in -4 :allinfordCs S-#4 software.
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:$en command starts to be lost inta&es can easily be moved furt$er upstream to
reain command.
2.3 Sediment management in modernised systems
:$en systems are modernised wit$ new permanent diversion structures muc$ larer
disc$ares can be diverted from flood flows. onse;uently water carryin $i$
concentrations of coarse sediments is diverted to canals. In some systems modernised in t$e
1830Cs t$e canals were constructed wit$ ;uite low slopes, and a limited sediment transportin
capacity. $e combination of a lare input of sand and larer sediments and a limited canal
sediment transportin capacity inevitably resulted in severe canal sedimentation problems.
:$ile later systems were provided wit$ canals wit$ steeper slopes, canal de+siltin remains as
a sinificant maintenance burden in many modernised sc$emes.
Dated scour sluices are usually provided at inta&es. $ey are intended to be operated to
e%clude t$e coarse sediments t$at are transported alon and close to t$e wadi bed floods.
6anual operation of sluice ates in rapidly varyin spate flows, to follow idealised ateoperation rules $as enerally proved difficult or impossible. #part from t$ese practical
difficulties, t$e first priority of farmers is to divert as muc$ water as possible. $ey may be
e%tremely reluctant to open, (or allow aency employed ate operators to open) sluice ates,
e%cept durin t$e very larest floods.
In some larer sc$emes flus$ed sedimentation basins are provided, w$ere t$e incomin water
is slowed down, and larer sediments are deposited in a lined basin. $ese are t$en flus$ed
bac& to t$e wadi w$en t$e basin is full. If water for flus$in is not available, or flus$in is
rearded as bein too difficult to oranise, basins are e%cavated mec$anically. :$en properly
desined t$ese structures function well at inta&es in perennial rivers, but t$ey $ave $ad a
mi%ed trac& record w$en used in spate sc$emes.
If twin sedimentation basins or a bypass c$annel are provided it is possible, at least in t$eory,
to maintain canal supplies and flus$ basins durin t$e periods w$en t$e wadi flows e%ceed t$e
disc$are needed for t$e canal. Eery slic& operational procedures, almost certainly reliant on
electrically powered ates, are re;uired for t$is to be feasible. :$ile a flow duration curve
may s$ow t$at substantial volumes of water are available w$en t$e wadi disc$are e%ceeds
t$e canal demand, t$ese Fe%cessF flows occur durin numerous flood pea&s, at unpredictable
times over a flood season. In many floods e%cess flow is only available for very s$ort periods,
sometimes only a few minutes. $us unless a very lare flood $appens to occur w$en a basin
needs flus$in it is necessary to utilise canal flows for sediment flus$in. $is is always t$e
case w$en a sinle sedimentation basin is provided.
7armers $ave stron obGections to w$at t$ey perceive as wastae of water t$at could bediverted for irriation, and t$ere are e%amples of inta&es provided wit$ sop$isticated sediment
manaement facilities t$at are rarely operated due to pressure from farmers, or ot$er powerful
local interests, on ate operations staff. $e result of t$ese operational difficulties is t$at
substantial ;uantities of sand and larer sediment are diverted to canals w$ere t$ey settle,
reducin diversion and conveyance capacities. $e need for fre;uent, often annual, removal
of sediment deposits from t$e upstream reac$es of canals places a $eavy burden on farmers or
sc$eme operators.
4isin command levels due to sediment deposition on t$e fields eventually results in some
parts of upstream irriated areas oin out of command. In sc$emes wit$ permanent inta&es
weir crests and t$e sill levels of water control structures eventually $ave to be raised in
e%pensive re$abilitation proGects.
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%igure 2 &iversion structures wit! sediment e"cluders for perennial and spate irrigation
inta#es
$e structure on t$e left diverts only a small proportion of t$e river flow durin t$e wet
season. urin t$ese periods t$e sluice ates are left partially open, providin a stron
sluicin flow sweepin t$e bed load past t$e canal ates. In t$e dry season, w$en most if not
all of t$e river flows are diverted, t$e sluice ates are opened intermittently (still pond
operation) to flus$ t$e sediment deposits t$at build up in t$e sluicin poc&et formed by t$e
divide wall. $e canal ates are closed durin sluicin. $is type of perennial river inta&e is
not suitable for spate irriation, w$ere all t$e wadi flows are diverted to canals for most of t$e
time t$at t$e wadi is flowin, t$e sediment loads are muc$ larer, and still pond operation is
not feasible.
In t$e spate diversion structure s$own on t$e ri$t t$e inta&e and sluice ates are placed
downstream from t$e weir. $ere is no divide wall, w$ic$ would obstruct flows approac$in
t$e inta&e from across t$e wadi. ($e anle at w$ic$ flows a spate inta&e can vary durin, and
between floods, and in flood recessions flows may approac$ t$e inta&e parallel to t$e weir.)
Sluice ates are provided and supposed to be opened durin t$e s$ort periods of $i$ flow,
w$en e%cess water is available for sluicin, and t$e $i$est concentrations of coarse sediment
are transported. $e curved c$annel provides some additional e%clusion of coarse sedimentsdue to t$e bed load sweep effect t$at moves lare bed load sediments towards t$e inside of t$e
bed, and t$rou$ t$e sluiceway.
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%igure 3 'ed load sweep at a c!annel bend ()s * flow at surface )b * flow at bed +
$e canal inta&e is alined at a s$allow anle to t$e flow direction in $i$ flows, so as to
minimise t$e diversion of bed load."
3.1.2 ,!ysical models
$ysical models, usually wit$ a mobile bed, $ave often been used to aid t$e desin of
sediment e%cludin inta&es. $e desin of many of t$e larer spate inta&es constructed since
t$e 1830Cs were developed wit$ t$e aid of p$ysical model tests. 7or e%ample model tests were
carried out on several curved c$annel e%cluders similar to t$at s$own in fiure 2, (Smit$
183, and osswell, 1838), and s$owed t$at in a p$ysical model curved c$annel e%cluders can
e%clude / B or more of (bed) sediments from a canal, w$en operated wit$ a flus$in
disc$are of around !0 B of t$e canal disc$are.
-owever usin a p$ysical model to ma&e ;uantitative predictions of sediment e%clusion is
frau$t wit$ difficulty. $e problem is principally one of scalin. If sediment si=e is scaled in
proportion to t$e main model scale, t$en t$e material re;uired becomes so fine t$at it e%$ibitsvery different properties from t$e prototype. #rtifices suc$ as t$e use of oversi=e li$twei$t
sediments or tiltin models $ave been used wit$ some success but alin (181) s$owed t$at it
is impossible to satisfy t$e p$ysical laws for scalin sediments w$en water is used as t$e
model fluid. In models of a reasonable scale only t$e larer sediments, movin as bed load
can be represented.
$ese difficulties are demonstrated in fiure ", w$ic$ is based on studies carried out by
-ydraulic 4esearc$, :allinford, A, and includes comparisons of sediment e%cludin
performance predicted in p$ysical model studies wit$ t$at later measured in t$e field.
In t$e fiure sediment e%cludin performance is e%pressed as a performance ratio (4)
defined as>
4 J 1 + sediment concentration enterin canal
oncentration bein transported by t$e river
# performance ratio of 1.0 indicates complete sediment e%clusion, =ero indicate neutral
performance, wit$ t$e inta&e neit$er reducin nor en$ancin sediment concentrations, w$ile a
" $is contradicts t$e advice iven in 7#OCs 2002 irriation desin manual, (fiure !8 volume 2 module
), w$ic$ recommends ri$t anled inta&es for silt laden rivers. $ysical and numerical models and fielde%perience all demonstrate t$at frontal inta&es divert t$e minimum of bed load to canals. $e proportion
of bed load in t$e diverted flow increases as t$e diversion anle is increased. $e reason is t$at t$e lowermomentum of flows near t$e bed of a c$annel ma&es t$em more easily divertible if t$e flow is turned. In
spate inta&es t$e anle of diversion is of course only relevant durin $i$ flows, w$en water is passint$rou$ t$e sluiceway or over t$e weir
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neative ratio would indicate t$at t$e inta&e is wit$drawin a $i$er sediment concentration
t$an t$e mean concentration in t$e river.
7iure " clearly demonstrates t$e overestimation of sediment e%cludin performance in
p$ysical models, w$ic$ is not always made clear in reports from modellin oranisations.
It is stressed t$at t$is conclusion relates only to sediment e%clusion at inta&es, p$ysical
models are a valuable tool t$at are still widely used for a lare rane of applications.
%igure 4 Observed at predicted performance of some sediment e"cluding inta#es.
3.1.3 -!ree dimensional numerical modelling
<umerical modellin of t$e flow and sediment movement in t$e vicinity of an inta&e $as t$e
potential to ma&e ;uantitative predictions of its sediment e%cludin performance, wit$out
problems in representin a wide rane of rain si=es. # sediment e%cluder $as a lare effect
on t$e sediment rain si=e distribution carried in flows diverted to a canal, and t$is can be
more important t$an its effect in reducin t$e overall sediment concentrations. #n inta&e wit$
a moderate overall e%clusion performance may e%clude all t$e coarser material, so t$at only
fine material enters t$e canal. If t$e canal is able to transport t$is fine material, t$e inta&e will
prevent w$at would ot$erwise be a severe sedimentation problem. Suc$ conditions can only be assessed wit$ a si=e+by+si=e prediction of sediment e%cludin performance. # second
advantae of numerical modellin is t$at it can be very muc$ c$eaper t$an commissionin a
p$ysical model.
-4 :allinford developed a t$ree dimensional computational model specifically for studies
of t$e performance of sediment e%cludin inta&es in 188/. (-4 :allinford, 188/). $e
model represents t$ree dimensional effects, for e%ample t$e $elicoidal flow w$ic$ develops at
a river bend or curved c$annel sediment e%cluder, and also ta&es account of> momentum,
turbulence, bed eometry, bed friction, sediment settlin velocities, and of course t$e
eometry of an inta&e and associated any sediment e%clusion features.
4esults from t$is model are compared wit$ field performance data in fiure ". #reement between t$e model predictions and t$e sediment e%cludin performance measured in t$e field
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is, (for sediment transport predictions), e%cellent. $e model enables predictions of t$e
variation of sediment e%cludin performance wit$ rain si=e, w$ic$ is essential if t$e impact
of an e%cluder in on limitin canal sedimentation is to be assessed, see fiure /.
%igure Sediment e"clusion as a function of sediment si/e
$e settin up, calibration and operation of t$is type of model re;uires specialist numerical
modellin and sediment transport e%pertise, and application of t$e model would only be
relevant in lare spate modernisation proGects, w$ere lare sums are to be spent in improved
infrastructure. 7or t$ese applications it provides desiners wit$ a capability to rapidly assess
and optimise t$e sediment e%cludin performance of a comple% inta&e.
3.1.4 'asic inta#e model
6ore basic inta&es are provided on smaller wadis. #n e%ample of a newly constructed inta&e
in 'ritrea is s$own below as fiure .
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%igure 'asic inta#e in a sand bed river
'stimates of sediment e%cludin performance of t$is type of inta&e, wit$ a simple sluicin
arranement, can be made usin a simpler model, as described in -4 :allinford, 2001.
$is model $as a flow component, and a sediment component. $e flow component
determines t$e oriin of t$e flow diverted to t$e canal from a cross section located upstream.
#n envelope containin t$e diverted flow wit$in t$is cross section is predicted, based on an
assumption t$at t$e flow re;uirin t$e least momentum c$ane to be diverted will enter t$e
canal inta&e. $e sediment component of t$e model computes t$e sediment si=es and
concentrations transported in t$e diverted flow envelope, t$ereby predictin t$e sediment
si=es and concentrations enterin t$e inta&e.
#n e%ample of t$e output provided by t$is model for a simple inta&e wit$ a sluice is s$own in
fiure below>
Impact of Sluicing
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 2 4 6 8 10 12 14 16 18
Eexcluder discharge m3/s
P R
%igure $ffect of sluicing disc!arge on predicted sediment e"cluding performance
$is model can also be used to estimate e%clusion for t$e rane of sediment si=es present in
t$e c$annel bed upstream from t$e inta&e, t$us provin uidance on t$e sluicin disc$ares
needed to ma%imise e%clusion of selected (lare) sediment fractions.
// In t$e fiure 4 is a performance ratio defined earlier in t$is section.
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3.2 Secondary sediment control e"tractors and settling basins
'ffective sediment e%clusion at an inta&e may be not be possible, or may not be snsufficient
to e%clude all t$e larer sediments t$at will settle in canals. 'ven at well desined inta&es
t$ere is always t$e possibility t$at sluicin will not be carried out. Some form of secondary
sediment control is often considered, particularly, in larer sc$emes. $e c$eapest and most
tec$nically attractive option is a vorte% tube e%tractor. $is e%tracts flow and sediments from
near t$e canal bed in t$e canal $ead reac$, and pass it bac& to a river. $e advantae of t$is
approac$ is t$at t$e si=e rane of t$e sediments t$at are e%tracted can be controlled to some
e%tent by adGustin t$e proportion of t$e canal flow, often around 10B, w$ic$ is e%tracted.
-owever as t$ese devices re;uire water to be continuously KwastedF t$ey are not usually
appropriate for spate sc$emes. (6odels used to desin e%tractors are described in (-4
:allinford, 2001).
Settlin basins are a second met$od of secondary sediment control, t$at in perennial sc$emes
enable sediment to be removed from t$e flows enterin a canal at t$e e%panse of only t$e few
B of t$e incomin flow volume, w$ic$ is used for intermittent flus$in.
3.2.1 &esign of gravel traps
In spate sc$emes t$e obGective is to trap lare sediments at t$e $ead of a main canal. $e
relatively small basins used to ac$ieve t$is are often called ravel traps. Several e%amples of
sedimentation basins used in spate sc$emes are s$own in -4 :allinford 200/. ($e
problems of flus$in ravel traps in spate sc$emes were discussed earlier.)
$e advantae of a ravel trap over t$e alterative of removin sediment deposits from a canal
$ead reac$ is t$at some sedimentation can occur wit$out limitin t$e flows t$at can be
diverted to or conveyed by a canal, w$ic$ may be important w$ere t$e available $ead is
limited. If sediments are removed and e%cavator rat$er t$an by flus$in t$en de+siltin is
mostly focussed at t$e basin, rat$er t$an alon t$e canal system.
Settlin basin desin can be carried out usin -4 :allinfordCs ossbas software pac&ae.
$is includes two numerical models t$at simulate t$e performance of basins operatin in t$e
deposition and sluicin modes. $ese models provide several advantaes over earlier basin
desin met$ods, w$ic$ can only be used to predict t$e trappin efficiency of an idealised
basin, as t$ey assume =ero sediment transportin capacity wit$in t$e settlin =one, and t$us
do not predict t$e variations in trappin efficiency as t$e basin fills wit$ sediment.
sin t$e models a desiner can ma&e several &ey predictions t$at assist in refinin and
optimisin a settlin basin desin.
$ese are>
$e variation in sediment concentrations and rain si=es passin t$rou$ a basin and
enterin t$e downstream canal networ& as t$e basin fills wit$ sediment.
'stimates of t$e fre;uency of sediment sluicin or de+siltin operations.
:$en a basin is flus$ed, t$e time period re;uired to flus$ t$e basin and t$e volume of
water needed for flus$in.
$e slope and cross section re;uired for t$e escape c$annel to convey sediment
flus$ed from a basin to t$e river or disposal point.
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$e sediment trap efficiency for different sediment si=es varies widely as a basin fills wit$
sediment. :$en empty, and supplied wit$ a very low disc$are, a conventionally desined
basin will trap considerable volumes of fine silts. $ese ta&e up storae in t$e basin, and are
anyway usually valued by farmers. -owever basins for spate sc$emes can be desined to
minimise trappin fine sediments, t$e results of numerical simulations s$owin t$at>
*asins s$ould be relatively narrow, so as to &eep t$e operatin velocities fairly $i$,
wit$ sediment storae obtained by increasin t$e lent$, rat$er t$an t$e widt$ or
dept$ of t$e basin. (See for e%ample t$e basin desined for :adi 4ima in emen
s$own in (7#O, 183)
eposition of some fine sediment is inevitable if a series of low flows carryin $i$
concentrations of fine sediments enter t$e basin, particularly w$en t$e basin is
initially empty. $is may occur ;uite often in spate sc$emes, w$en a series of small
floods arrive at an inta&e. If it is considered necessary substantial reductions in t$e
trap efficiency for fine sediments can be made if t$e tail water level in t$e basin is
lowered for very low basin disc$ares. $is can obviously be ac$ieved by providin a
ate to control downstream water levels, but ate operation can be problematic inspate flows. #not$er possibility is to provide a notc$ed weir at t$e basin e%it, so t$at
tail water levels are substantially lowered w$en t$e basin disc$are is very low.
3.2.2 ybrid e"tractorsettling basin for large sc!emes
It is possible to combine t$e benefits of vorte% tube sediment e%tractor, wit$ t$e muc$ lower
wastae of water needed for flus$in provided by a settlin basin, w$ic$ can be =ero if a
basin is mec$anically e%cavated. # $ybrid system of t$is type was proposed by consultants
for t$e wadi 6awr system in emen, but was reGected by t$e client and :orld *an& due to
concerns of t$e sustainability of a Get pump t$at was to be used to evacuate t$e basin.
-owever a $ybrid vorte% tube settlin basin system was later constructed in a sc$emesupplied from a seasonal ravel bed river in t$e $ilippines in 188/. $e river transports very
$i$ sediment concentrations, some derived from mine tailins.
In t$is system, desined usin t$e models described in (-4 :allinford, 2001), a vorte% tube
sediment e%tractor was located upstream from t$e first drop structure alon t$e main canal. It
disc$ares to a small flus$ed settlin basin, wit$ t$e settlin basin disc$are bein passed
bac& to t$e canal system downstream from t$e drop. In t$is case a flus$ed basin was used, but
in spate sc$emes a mec$anically e%cavated basin could be considered, allowin e%traction of
coarse sediments from t$e canal wit$out any loss of water.
$e system proved to be e%tremely successful, t$e substantial reduction in de+siltinre;uirements provided by t$e e%tractor installation $altin, and t$en reversin, a lon term
decline in t$e irriation service area, and providin very lare economic returns, (-4
:allinford, 188, 6erritt, 2002). Systems li&e t$is could be considered for lare spate
sc$emes, w$ere investment in infrastructure can be Gustified by a reatly reduced O and 6
costs.
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%igure 5 )orte" tube used in combination wit! a settling basin
4.0 Spate canal design met!ods
anals for perennial sc$emes are often desined usin ma%imum and minimum water
velocities set by Kno scourin 9 no siltinF criteria, as described in, (7#O 2002). #s
concentrations of sand diverted to canals is far larer in spate sc$emes t$an is t$e case in
perennial sc$emes, and canals are operated at a fraction of t$eir desin capacity for most of
t$e time, spate canals desined in t$is way rapidly silt up.
K4eimeF desin met$ods $ave been applied in spate sc$emes. $ese are sets of empirical
e;uations derived from observations of canal systems t$at are relatively stable, or Lin reimeL.
$e met$ods are simple to apply as usually only t$e disc$are $as to be specified. Some
met$ods, suc$ as t$e (?acey, 18!0), e;uations are still widely used in t$e Indian subcontinent
but as most are based of observations of canals t$at carried low sand loads, t$ey are not
appropriate for use in spate sc$emes.
#n e%ception is t$e Simons and #lbertson reime e;uations, based on a lare data set
collected from canal systems in India and <ort$ #merica t$at includes e;uations for canals
wit$ sand beds and co$esive ban&s carryin K$eavyF sediment loads (2000 9 3000 ppm).
amac$o in (7#O, 183) reports t$at t$ese e;uations were used to desin canals in some of
t$e modernised emen systems.
Sediment transport t$eory s$ows t$at t$ree e;uations are re;uired to determine t$e slope,
dept$, and widt$ of a strai$t alluvial c$annel w$en t$e incomin water and sediment
disc$ares, and bed material si=e is specified. $e first two are provided by alluvial friction
and sediment transport e;uations. $e t$ird, a widt$ relations$ip, can be obtained from
minimum stream power or ma%imum sediment transport efficiency concepts. # roup of
canal desin met$ods based on t$ese relations$ips (rational met$ods) is available, and provide
t$e most loical met$od of desinin canals to ac$ieve a specified sediment transportin
capacity. Of t$ese t$e met$od due to $an (183/) provides predictions of slopes and bed
widt$s t$at are similar to wide s$allow canals observed in many spate systems.
#lbeit wit$ a number of adGustments to t$e coefficients
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onventionally t$e pea& or desin disc$are is used to determine canal bed slopes and cross
sections. If t$is is followed for spate canals t$ere could be serious siltation problems, as spate
canals flow at t$eir full desin disc$are for very s$ort time periods. Indicative fiures can be
derived from spate flow duration curves, s$ow t$at a typical canal supplied from a wadi mi$t
flow at disc$ares larer t$an /0 B of its desin disc$are for only a few B of t$e time.
amac$o, in (7#O, 183), suests t$at t$e slopes of spate canals s$ould be computed for a
disc$are of /B of t$e ma%imum, but notes t$at t$is is in fact ;uite conservative. $e
desin disc$are capacity is provided by specifyin ade;uately si=ed cross sections.
$ese computations can be conveniently carried out usin -4 :allinfordCs KorcF software
pac&ae, w$ic$ includes a wide rane of canal desin met$ods. Slopes obtained wit$ t$e
$an desin met$od are compared wit$ t$e averae slope of traditional canals in t$e wadi
Mabid system in emen in t$e table below.
$e computations are for a median bed sediment diameter of 1mm, and an input sediment
concentration of 000 ppm, a typical sand load sediment concentration carried by a small
flood.
&isc!arge
(m3s+
Sediment
concentration (sand
load6 ppm+
Slope (m#m+
/0 (from survey) na ".0 (from survey)
/0 000 !.
!./ (/B) 000 !.3
2/ (/0B) 000 !.8
:it$ t$e selected bed sediment si=e and sediment concentration t$e $an met$od predicts a
slope similar to t$at observed if it is based on /0 B of t$e full supply disc$are. -owever t$is
would c$ane if a different sediment concentration or median sediment diameter $ad beenselected. It is recommended t$at canal desins in modernised sc$emes are based on t$e slopes
and cross sections of stable e%istin traditional canals. $e desin of enlared, e%tended or
new canals can t$en be derived usin t$e $an e;uation, wit$ a Gudicious c$oice of input
parameters to provide a ood matc$ wit$ t$e slopes and cross sections observed in e%istin
canals.
In new sc$emes parameters suc$ as canal bed material si=es and t$e incomin sediment
concentration $as to be estimated if rational desin met$ods are to be used. $is is beyond t$e
scope of t$is paperN suitable procedures are described in (-4 :allinford, 2001).
.0 ,redicting future command levels
In e%istin sc$emes t$e future rise in command levels is estimated from $istorical rates of rise
of field and command levels, derived from surveys, corin or trial pits, and t$e e%tent of
upstream movement of traditional diversion structures. In new sc$emes, w$ere t$ere are no
spate systems in t$e vicinity, t$e limited sediment yield information t$at is available for
catc$ments in reions w$ere spate irriation is practiced can be used.
$e 7#O data base of sediment yields of t$e worlds rivers (part of #;uastat) is a ood
startin point, alt$ou$ a searc$ s$ows t$at t$ere is now far more information available in t$e
public domain t$an w$en t$is information was compiled in 188/.
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'%amples of sediment yield data from catc$ments in 't$iopia and 'ritrea are iven below.
$e information is derived from t$e 7#O data base, supplemented wit$ data collated durin
proGect studies.
"ield # 3209 $rea%0.21
&2 # 0.36
10
100
1000
10000
1 10 100 1000 10000 100000 1000000
Catchment Area m!
" i e l d t / m ! / #
't!i(pia
'ritrea
%igure 5 sediment yields from catc!ments in $ritrea and $t!iopia
$is sort of information can be used derive preliminary, albeit appro%imate, estimates of
future increases in command level in spate irriated areas. :$ere sediment yield data is
available for nearby catc$ments of similar si=e, rainfall, slopes and veetation etc., it can be
used directly. Ot$erwise it mi$t be assumed t$at spate irriated areas will be located in
reions wit$ $i$ sediment yields, and estimates based on yield fiures from t$e upper
;uartile of t$e available data, for catc$ments of similar si=e to t$e catc$ment of t$e wadi
supplyin t$e sc$eme t$at bein considered. 7ield rise rates can be derived usin assumptions
as to t$e proportion of t$e annual sediment load t$at will be diverted to a sc$eme, t$e sc$emecommand area, a bul& density for settled silts, and an indication of t$e li&ely variation in
sedimentation rates between upstream and downstream fields.
.0 Sediment management options
.1 Catc!ment conservation
It is sometimes assumed t$at t$e introduction of soil conservation prorammes in river
catc$ments will rapidly reduce river sediment loads, and t$us sedimentation problems in
downstream irriation canals. $e information t$at is available suests t$at t$is is not t$e
case over t$e lifetime of typical irriation structures, e%cept for sc$emes supplied from very
small catc$ments. $e principle reason is t$at in semi arid reions t$e vast store of sediments
available for remobili=ation in catc$ments t$at $ave suffered from $i$ rates of soil erosion in
t$e past, continue to contribute to downstream sediment yields for lon periods, decades or even
centuries, even if catmint wide conservation was possible. . Soil and water conservation
prorammes will not usually provide sinificant s$ort term reductions in canal sedimentation
in spate irriation systems. -owever construction of series of low c$ec& dams can provide
some benefit. #lt$ou$ t$e storae provided by c$ec& dams soon fills wit$ coarse sediments,
t$e reduction in and control of river bed radients provided by c$ec& dams can reduce bed
and ban& scour durin floods. #s t$ese measures reduce t$e (unwanted) sand si=es
transported by wadis t$ey mi$t be considered as part of wider soil and moisture conservation
prorammes w$en spate irriation is bein developed in new areas.
In wadi ?aba in 'ritrea mean sedimentation rates in upstream fields were about twice t$e mean rate.
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.2 Sediment management options for a range of sc!eme types
$ese are summarised in t$e table below>
ntake!scheme type "ediment management options
*asic inta&e wit$out a weir ?ocate inta&e at t$e outside of a c$annel bend
?imit flows enterin canal wit$ flow t$rottlin
structure and reGection spillway.
If provided close ates durin periods of very $i$
wadi flows.
rovide steep canals, minimise pondin and flow
division.
onsider arranements for and sustainability of
canal de+siltin
*asic (probably small) inta&e
wit$ a low weir
?ocate inta&e at t$e outside of a c$annel bend
rovide a simple sediment sluice.
#lin canal inta&e to minimise diversion anle. ?imit flows enterin canal, close ates durin
periods of very $i$ wadi flows Pflow t$rottlin
structurereGection spillway.
onsider if mec$anically e%cavated ravel trap is
appropriate.
rovide steep canals, minimise pondin and flow
division.
6a&e provision for risin command levels.
onsider arranements for and sustainability of
canal de+siltin
-i$er cost inta&es ?ocate inta&e at t$e outside of a c$annel bend
Incorporate sediment sluice, consider curvedc$annel sediment e%cluder if bed sediments are
coarse.
#lin canal inta&e to minimise diversion anle.
?imit flows enterin canal, close ates durin
periods of very $i$ wadi flows Pflow t$rottlin
structurereGection spillway.
onsider if mec$anically e%cavated ravel trap is
appropriate, or w$et$er flus$ed settlin basin
mi$t be feasible.
:$ere $i$ investments costs mi$t be Gustified
by reduced de+siltin costs, consider $ybrid
e%tractor settlin basin system located in t$e canal
$ead reac$.
rovide steep canals, minimise pondin and flow
division.
6a&e provision for risin command levels.
onsider arranements for and sustainability of
canal de+siltin.
n all cases it is essential to ensure that any planned sediment management measures
are both understood and accepted by farmers# and that they are also sustainable over
the long term$
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5 :eferences
$an - -, 183/. Design of stable alluvial canals in a system. #S' Rourn -ydr 'n, Eol
11.1 <o -I.
7#O , 2002, Planning, Development Monitoring and Evaluation of Irrigated Agriculture
with Farmer Participation, Eolume I, 6odule
7#O<. 183. Spate Irrigation Proceedings of the Subregional E!pert "onsultation on
#adi Development for Agriculture in the $atural %emen . +10 ecember, 183. #den, 4
emen. nited <ations evelopment roramme. 7ood and #riculture Oranisation of t$e
nited <ations. 130pp.
-4 :allinford , 188/ A $umerical Model for Predicting Sediment E!clusion at Inta&es,
'dmund #t&inson, 4eport O 1!0, 7ebruary 188/.
-4 :allinford, 188, A method for evaluating the economic benefit of of sediment control
in irrigation systems, $ancellor 7, ?awrence #t&inson '. -4 :allinford report O <
31
-4 :allinford ,2001, Procedure for the selection and outline design of canal sediment
control , ec$nical and software manuals ?awrence ' #t&inson Spar& ounsell.8
-4 :allinford, 200/, Improving community spate irrigation, ?awrence and r 7 van
Steenberen, -4 :allinford report O 1/"10
?acey D, 18!0. Stable channels in alluvium 6inutes of t$e roc., Institute of ivil
'nineers, ?ondon, A. <o 228, pp2/8+282.
6erritt S 2002, #ater for Agricultures, Irrigation economics in international perspective ,
Spon press, ?ondon and <ew or&, IS*< 0+"1/+2/2!3+/
Smit$ AE-, -ewlett 46D ?awson R. # -eadwor&s for Spate Irriation Systems. ivil
'nineerin 6aa=ine, Sept 183,
osswell , 1838. 'apunga (ice Pro)ect, report on hydraulic model investigation for
irrigation headwor&s. ivil 'nineerin epartment, $e niversity, Sout$ampton, A.
alin 6S 181 *heory of hydraulic models, 6acmilan ?ondon
8 $ttp>www.dfid+&ar+water.netw/outputssoftware.$tml
10 $ttp>boo&s.$rwallinford.co.u&acataloInternationalevelopmentpae1.$tml