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])

mailto:[email protected]

mailto:[email protected]



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



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!"



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.



:$en command starts to be lost inta&es can easily be moved furt$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.



%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.



%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



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



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 .



%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.



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.



$e sediment trap efficiency for different sediment si=es varies widely as a basin fills wit$

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.



%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



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/.



'%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.



.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.


division.

6a&e provision for risin command levels.


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

$ead reac$.


division.

6a&e provision for risin command levels.


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$



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

Managing Sedimentation in Spate Irrigation Schemes

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