Cross Drainage Structures

7/28/2019 Cross Drainage Structures

http://slidepdf.com/reader/full/cross-drainage-structures 1/18

UNIT 11 CROSS DRAINAGE WORKS

Structure

11.1 IntroductionObjectives

11.2 Necessity of Cross Drainage Works

11.3 Aqueduct

11.4 Syphon Aqueduct

11.5 Syphon

11.6 Superpassage

11.7 Inlet andcutlet

11.8 Level Crossing

11.9 Selection of a Suitable I)lpe of Cross Drainage Work to Conform to Site

Conditions

11.10Design Parameters

11.11 Summary

11.12Key Words

11.13 Answers toSAQs

1 1 . INTRODUCTION

When a canal takes off from a river, it has to cross some streams or rivers before it canreach the top of the intended watershed for the purpose of irrigating an area. Such

crossings constitute various types of cross drainage works that are required to be

provided on the canal alignment. Cross drainage works are required to carry the canal bynegotiating it above, below or at the same level as the stream across which the canal is

meant to be aligned.

Objectives

By studying this unit you should be able to conceptualize the basics of the following

cross drainage works :

an aqueduct,

a syphon aqueduct,

an nlet and outlet, and

a level crossing.

Further you should be able to :

select a suitable type of cross drainage work that is appropriate to given siteconditions, and

get an understanding of the various design parameters regarding cross

drainage works.11.2 NECESSITY OF CROSS DRAINAGE WO RKS

Figure 11.1 shows a canal aligved between the headworks and the main watershed. The

canal taking off from point A on the river has to cross some streams or drainages,a, b, cand d, before it can be taken on the watershed at B. At the intersections of the canal and

the tributaries or streams, major cross drainage works will be required. These cross

drainage works are provided on the alignment of the canal such that the water in the canal

is taken across the stream, above, below or at the same level as the stream it is intended to



cross, as per practical conditions. Usually no cross drainage works are needed once thecanal mounts the w atershed except in places where the canal is required to leave thewatershed as at DEFG or a short distance and the c ross streams at E and F.

OBDG- ATERSHED

ABDEFG- U U L AlI&NMEWT

Figure 11.1 r Cmd A l i p e n t - Offbake toMainWatershed

SAQ 1

Why are cross drainage works needed? Why do they cross the nalural drainage atdifferent levels?

11.3 AQUEDUCT

An aqu educt is a hydraulic structure which carries a canal (through a trough or a duct)across and above the drainage, just as a bridge carries the road o r rail traffic over a river.111 this case the highe st flood level of the drainage should remain low er than the level ofthe underside of the canal trough.

Types of an Aqueduct

There are three types of aq ueducts as described below :

TOP PLAN O F A C WE WC T

WERTED LRC-c.8-L LEN HI\ -MCCYS-~Rf LTER C m m . , MERE0 nUER

Figure 11.2 r 'QpiulP&I Pad SectionofanAqueduct (lyp )



Type I . : In this type of structure the entire section of the canal (w ithout any change in it)along with its earthen banks is carried across the drainage. Hence the length of the barrelsthrough which the drainage water is passed under the canal has to be long so as to supportthe section of flow of water and the canal banks (Figure 11.2). In this type of wcrk thereis no tluming of the canal section (i.e., there is no reduction in the width of the section),and so the width of the structure across the can al is maximum possible. T he advantage ofthis type of aqueduct is that this structure saves on the canal w ings and bank connectionsand is recommended only for sma ll streams where, obviously, the length of the sttucturealong the canal is small. An ex treme exam ple of this type wo uld be where a small 5lreilnIis passed through a large pipe o a series of pipes laid under the bed of the canal with tliepipes flowing freely.

Type I1 : n this type of structure, which is similar to 5 p e , the outer slopes of theearthen canal banks are replaced t y retaining walls which reduces the length of theculvert wh ile the irlner slope s of t:ti, canal are retaine d as earthen sections . Streanls ofintermediate size are negotiated by ti:, ype of structure (Figure 11.3).

r NSPECT'ON RO* r BERM

BRICK MnSOWYIN CEMENT MORTAR

STONE PITCH1

CEMENT CONCRETEBLOCKS OVERINVERTED FILTER

Figure 11.3 : Typical SectionofanAqaedoct (Type Ir )

Type I11 :Here the nor ma l earthen banks of the canal are replaced by a &med trough ofconcrete or masonry (Figure 11.4) over and across the drain for carrying the canaldischarge. Th e side walls of the trough are extended on the upstream and downstream ofthe canal and coilnected to the earthen canal banks by means of suitable wing wallsdesigned to provide a smooth transition for the flow. Th is provides the shortest length ofthe barrels for the dra inage and is thus suited to large streams requiring considerable

length of aqueduct between the abutments. T hus, the cost of the barrels for the drainwould be least while the cost of the flumed canal trough w ould be the highest.Figure 11.5 shows a typical sectio n of an aqueduct of Type 111.

0

TRANSITIONQOS

SECTION AT Z Z

Figure 11.4 : ypical Plansod Section of an Aqueduct (Type 10

CrosDrainageWorks



rWEaRiNG COAT

HFL =5 E +-i=s-s~X-==s

CONCRETE OR

l2 1STREAMBEDI

;-- - - - - - - -- - -- - - - - - ----------LEAN CONCRETE

Figure 11.5 : nlarged wealSectionof ao Aqueduct (Type I I I )

S A Q 2

Discuss the factors that irlflucncc Lhe chtiice o f ;UI open aqueduct ou r or tlie r l l ~ e c

types discussed above. What is t h ~mportance o f cost factor ~ 1 1 1 1 ~hoosillg ;111

alternative out of these types?

11.4 SYPHON AQUEDUCT

A sypho n aqueduct is a cross drainage structure similar to an ordinary aqueduc t exceptthat the bed of &stream is depressed locally where it passes under the trough of thecanal, and the b arrels discharge the stream flow under pressure. In order to control theshape of the barrel, the stream bed is usually provided with a concrete or masonry floor.

Types of SyphonAqueduct

Syphon aqueducts are of three types almost exactly like the ordinary aquedu cts except

that the HFL of the stream is higher than the underside of the canal trough. Figures11.6,

11.7 and 11.8 show the three types of syphon aqueducts -Tvpe I, Type I1 and Type 111,respectively.

TOP R A N OF SYPHON AQUEDUCT

Rgum 11.6:%leal PlnnPndl

Seetiamuf

SyphonAguednct (Type I )





























Documents

Cross Drainage Structures