Guide Bund Railway

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Technical Instruction No. 66th October, 2005

RIVER TRAINING AND PROTECTION

1. INTRODUCTIONRailway Engineers are constantly engaged in tackling with flowing

water. The flow may be small or huge coming from small streams or avery large river. At times, these cross Railway alignments requiringbridging. At other times, such flow occur close to a running line, whichdid not cause any problem at the initial stage but on subsequentdevelopments resulting in changes in the pattern of flow, the flow eitheractually damages the embankment or poses a grave threat to do so.

During various interaction with Railway engineers at differentlevels, it has come to light that there is general ignorance about the

behaviour or pattern of river flow. This is mainly because, many of themmay not have enough exposure in dealing with river training works orthey may have dealt with the problem in rather ad-hoc manner. In eithercase, the result has not been satisfactory.

The Indian Railways have a long history of over 150 years ofdealing with rivers of various description in the sub-continent. In fact,the Indian Railway is uniquely placed as far as all important rivers of thecountry are concerned in as much as they have bridged all of them.Illustrious bridge engineers of the past acquired in-depth knowledge indesign and construction of such structures. Somehow, many of simplerules and knowledge have been lost in course of time. Today even oldFigure No.s are also not readily available. Some Railways have their ownstandard specifications dealing briefly about some of them. Butsurprisingly, they have been largely gone out of use.

Through this note, an attempt has been made to present the basicsconcerning the river behaviour and approach to tackle them. Although alarge number of literatures are available but this attempt is to bring in aconcise form what is considered necessary to appreciate the dynamics ofriver flow. Those, who wish to examine higher level of the science, arewelcome to study further material available in the References given at theend of the note. I must, however, state that these aspects, if they are

followed and practiced, will take care of most of general problemsassociated with the flow mechanism. They are time tested and are costeffective.

2. RIVER BEHAVIOUR : MEANDERING - CUT-OFF

The flow in river is derived from two sources - tidal and freshwater. Some basic knowledge of river types and their behaviour isessential, whenever any construction work is involved, be it riverprotection / erosion etc. River reaches can be divided according totopography of river basins - upper, middle and end reaches. The upper


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structures, e.g. bridges, barrages, dams, guide banks, marginal or floodembankments etc.

SHAPE OF A MEANDER

Figure No. - I

Dimension of meander:

LR = length along max. depth LR = Tortuosity ratioLV

LV = Valley lengthW = Width of the channel

R = Radius of bend MB = Meander Belt

ML = Meander length Q = Angle of bend

Meander shape can be circular / sinusoidal or parabolic. In case ofcircular curve, arc of a circle is used as the elementary form to describe ameander. A tortuosity ratio of 1 describes a straight channel. A ratio of5.5 is a limiting value, when consecutive bends cut into one another.Tortuosity ratio uniquely determines the shape of a meander.

Meander size:

The meander size is defined by the ratio of the radius of center lineof bend or a meander and surface width of the channel.

Size = R or MB

W W

Implication of Meander:

In a meandering river, position of deep channels and shoalschange continuously in the wide khadir of the river. The shift of the mainchannel cannot be predicted. This will call for necessary measures toprotect a structure.

ii) Cut-off

Meander is a hydraulic phenomenon dependant upon the


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discharge, bed slope, sediment load and capability of river flow to erodethe bed and banks. This will have the function of velocity also. Meandershape is, thus, the most efficient hydraulic flow under these given flowconditions.

Any change in the above pattern is only possible, if the present

channel flow condition becomes hydraulically less efficient and the riveris able to find a more efficient path. One such condition is an alternativechannel known as cut-off. This can be a natural development by the riveritself. Alternatively, it can be artificially developed by cutting a pilotchannel.

Depending upon the location of cut-off, it can be either a neck orloop cut off and chute cut off.

Loop cut off occurs due to progressive bank erosion at the neck ofacute bends, as shown in the Figure No.. This is more commonlyoccurring natural case of cut off. A chute cut off occurs at the flat of a

meander and is less common as compared to neck cut off. This is shownin the Figure No. 2:

Cut - off

Figure No. - 2

This generally develops along a remanant side channel, which developsfurther due to high floods and because of growing resistance on thenormal meander course due to presence of sand bars etc.

Following factors contribute towards formation of a cut off:

(a) High ratio of length of the bend to that of the chord. Veryroughly this is tortuosity ratio. A value of over 5.5 to 7 arelikely to suit development of a cut off.

(b) Flood duration

Short duration flood may not succeed. With a long enoughflood, the main channel say about 10 / 16 km along the curvewith velocity due to bed slope of 0.2 m / km and a side chordhaving a length of 3 km with the same fall, will have much


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better hydraulic flow property. More flow and consequent highvelocity would develop cut off faster.

(c) Sharp curvature for discharge:

Too sharp a curvature may induce a cut off. Ratio r/Q, where r

is radius of curvature and Q is discharge can also be animportant consideration for development of a cut-off as shownin some cases.

iii) Effects of cut-off

Immediately after a cut off takes place, there are lot of changes inthe flow pattern both up and downstream of the cut off. There are heavyerosions of banks and the new channel readjusts itself to the newalignment. There can be local deposits on the downstream side, wherethe cut off channel meets the main channel. This, however, is removed ina season or two during succeeding floods.

iv) Artificial cut off

The artificial cut off can be designed, if proper hydraulicconsideration are taken into account. Important considerations are:-

a) The alignment of pilot channel should be tangenital to the mainflow at both ends.

b) Its curvature should be a lot flatter than that of the mainchannel.

c) The entrance to the cut off channel should be bell mouth. Onthe outlet side, it is not necessary.

d) If the flow is unlikely to cause scour in the artificial channel, itshould be cut as deep as the main flow, if not more.

e) Laceys Regime formula, RS (R hydraulic mean radius of theparticle, S slope) is applicable. Assuming the particle size asthe same, RS of the cut channel should be at least as high asthe dominant channel.

Since slope of cut and river is inversely proportional to thelength, in order that the cut channel is self-scourable R/L ofthe cut channel should be greater than the dominant channel,where L is the length of the reach.

f) The pilot cut should be as deep as possible, as the tractive effortis directly proportional to the depth. Thus, a deeper cut wouldhelp development of cut off rapidly.

For cut off, a dozer, suction dradger, drag line or excavator can be usedwith imagination and to suit the particular situation and design to divertflow into cut off. Use of permeable screen has also been used with gooddesign and imagination. Most importantly, we must use hydraulic flowbasis to select cut off, location as well as its alignment.


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4. FLOOD DATA COLLECTION

For design of any hydraulic structure, it is essential to know thedesign discharge and other related parameters. This is in a broad sense

knowing the hydraulic geometry, which is used to establish relationshipsbetween width, depth, velocity of flow and the characteric discharge.River slope is also included in the key parameters.

Different approaches have been made to determine such arelationship by treating these aspects of hydraulic geometry asindependent variables. In emperical approach, regime type of equationhas been derived by using observed data. In statistical approach, arelationship is derived by statistical means between each of channeldimensions and other relevant parameters.

Discharge can be expressed in terms of product of velocity and

cross sectional area of the channel. For rigid bed channels, Mannings orChezys formula has been used. In case of mobile bed channels, wherebed form changes, coefficient of roughness n in Mannings formula andC, Chezys coefficient are used.

Without going into nuances of a host of various approaches, inRailways, the method of estimation of discharge is described in the Codeof practice for Bridges & substructures. This has been used withsuccess and confidence so far and, therefore, this may be adopted for allgeneral use. The discharge estimation is derived from, wherever possibleusing procedures evolved from actual hydro meteorological observationsof the same or similar catchment. If the discharge values are actually

available for sufficiently long time, design discharge can be computed byeither using actual data or by statistical method for the desiredrecurrence interval.

It is very important to do gauging of the stream to establish stage-discharge relationship and the discharge can be found at the knownHFL.

Following are important hydraulic parameters to be used:

(i) Discharge: By known meteorological detail or by flood hydrographas described in the Code.

(ii) Frequency: Design discharge is normally for 50 years returnperiod. For important bridges, return period of 100years have been used. More than this is notrecommended. The risk factor can be calculated by R =1 (1 1/T)n where T is return period and n design lifeof structure both in years.

(iii) Design Discharge for foundation/ protection works:For catchmentarea less than 500 sq.km., 30% more. For catchment area more than500 sq.km. to 5000 sq.km. varying linearly from 30% more to 20% more.For catchment of more than 5000 sq.km. to 25000 sq.km., 20% more to


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10% more. For catchment area greater than 25000 sq.k.m. less than10%. This will not be used for fixing free board.

(iv) Water way: For alluvial beds, Laceys formula has been found to be

quite suitable Pw=1.811 CQ

Pw= wetted perimeters, which is nearly equal to width or waterwayQ= discharge in cusecs

C=2.67 but may vary between 2.5 to 3.5 according to local conditiondepending upon bed slope and bed material Sharper the slope andsmaller the bed material, smaller is value of C

In case, the bed materials are not alluvial or not erodible, Laceysformula for waterway is not applicable. Width, in this case, should bedetermined by actual flow pattern and presence of firm banks throughwhich the flow is largely negotiated.

In case of river of wide and shallow sections, the bed beingalluvium, the discharge is generally confined to smaller width. The valueof width as obtained from Lacey is also small. In such cases, the flowshould be constricted by constructing well designed guide bund/s forboth hydraulic efficiency and economy.

The value of scour depth can be determined using guidelinesavailable in Bridge and substructure codes.

5. MOST IMPORTANT CONCERN OF ENGINEERS AS FAR ASHYDRAULIC STRUCTURES ARE CONCERNED

Unlike design/construction of concrete and steel structures for otherthan hydraulic purposes, there is a large variation in the way hydraulicstructures are designed or constructed. Where as for other structures,basic loading and behaviour patterns can be determined with great dealof accuracy, it is not so in case of hydraulic structures. Even parameterslike discharge and bed slopes can vary and estimation of them are largelyemperical. Even for construction, the parameters can be different whichmust be appreciated properly in order to succeed in the effort. Mostimportantly, there cannot be any surety of the measures succeeding andthere are chances of failures, cost of which at times can be very high

indeed. However, based on vast amount of experience already availableand proper application, it is possible to tackle such problems moreeffectively. Some of important considerations are given below forguidance:

(i) The river flow pattern is broadly different for alluvium andinerodible beds. Most of rivers in Northern India including those inGangetic/Brahmaputra plains follow similar characteristics except riversin bouldry stage.

Southern rivers are generally having inerodible bed.


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Difference between the two is mainly due to erosion of the bed.Thus, during high floods, alluvium rivers may initially register high HFLbut this may come down as soon as full scour takes place. Rise,therefore, may not be high. In other cases, since practically no scourtakes place, there is very high spill and water may overflow the banks

causing large scale inundation.(ii) For hydraulic design, most important parameters are discharge,gauge level/HFL, scour and physical layout of the channel.

The details are required for flood discharge. For not very importantrivers, flood discharge may be calculated based on guidelines given in theBridge and Substructure Code. For important rivers, data are generallyavailable. It requires to be properly interpolated or extrapolated.Sometimes the discharge is worked out on the basis of Gauge-Dischargecurve. This is also a fairly reliable estimate. Apart from designdischarge, it is also necessary to define the return period. This is already

given in the Bridge and Substructure Codes. Only in case of veryimportant structures, return period is taken as 200 years. For othercases, 50-60 years is generally adequate.

The discharge should be known for design of pier as well asprotection works. Most importantly, discharge should also be known forthe lean period, when construction work will be undertaken. This willdecide how temporary works should be designed to enable constructionactivities.

More often, it is for the lower discharge, when pitching, bankslopes fail. This is because of high surface velocity at lower discharge,

when scour has not fully developed.Gauge level at the site of construction is another very important

field data. If there is no gauge fixed, this should be done as quickly aspossible. For past records, nearest available gauge level should be usedafter making correction for bed slopes, which is also roughly the waterlevel. With gauge level, discharge can be co-related. This will also giveuseful data on the maximum time available for construction activities.The pitching, temporary protection works, bottom of well cap levels etc.can be fixed with relation to low water level given by the gauge.

Scour data can also be had from the evidence left behind by the

river for past discharges. This requires careful survey and inquiry madeby local people. Known waterholes or sounding of some structuresupstream can give very useful information. This needs to be verified bycalculations either made by some emperical methods or by hydraulicmodel studies. Again scour depth likely to be there for lower dischargeduring construction is also very important data. This is necessary fordesigning temporary works required during construction.

Physical layout of channel upstream and downstream is veryimportant. In fact its importance cannot be overemphasized.

The meander, curvilinear flow and approach to the bridge are very


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important considerations. In general, the flow can have more than onestreams/channels carrying varying discharge during different parts ofthe year. The flow may be braided. The channels may be spaced atconsiderable distance in the Khadir*. Particularly, while designingwaterway, it is a very important consideration in such a situation. The

water way may have to be adjusted to suit the construction dependingupon physical layout of the channel. For example, if the mole head ofthe guide bund is in a flowing channel, it will be difficultt to constructthe guidebund, unless the discharge is small during construction leanperiod and it is possible to divert the flow. Otherwise span length mayhave to be increased only on this consideration.

Irrigation, flood control and Bridge Engineers are all concernedwith river flow. Where as Flood Control Engineers most importantconcern is the flood discharge, velocity and scour, Irrigation Engineersare concerned about available discharge for irrigation during non-floodconditions. For maintenance/construction engineers of Bridges,although flood discharge is an important consideration but it is so mostlyfor design. For construction, it is non flood flow data which are ofparamount importance. These data are sometimes not readily availableor are not correctly appreciated.

6. USEFUL MEASURES FOR PROTECTION WORKS INCLUDINGTEMPORARY WORKS/MAINTENANCE AND CONSTRUCTION

For Railways, approach embankments, guide bunds, bridgepiers/abutments etc are required to be adequately designed for all kinds

of river discharge. If these are properly designed and correctlyconstructed, there are normally no occasion to have any threat or aproblem. Occasions, however, occur when a river changes its coursebecause of meanders and accordingly causes an attack on the portion ofunarmoured/unprotected embankment or even on the bridge itself. Insuch cases, some protection works or measures are required to be takenin order to protect the works. It may be necessary also to repair theworks and thereafter provide suitable protection measures according tothe hydraulic design.

There is no doubt that before such a thing happens, the river gives

Note:Khadir is not an English word. But in terms of river hydrology, Khadir is used to define extentor width of the river bed within which the river is likely to flow either during lean or flood season.Khadirs are generally bounded by high and firm banks. It can be very wide (6 to 8 Kms) and is

particular to rivers in alluvium terrain.


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enough of warning. Only when they are not heeded or ignored, a gravesituation may emerge causing for emergency measures.

Some useful action in this regard are:

(i) History of flow/channel to be kept.

(ii) Any important addition of hydraulic structure on the regime of theflow should be taken note of, both on upstream as well as ondownstream. On downstream side, effects are rarely beyond 5 kms.Such changes may be in the form of additional spans/worksprovided by highways on upstream side. This may be also due toconstruction of barrage/dam/waterstorage structures. These areconsidered as Railway affecting works and any change may causeserious implication on the Railway bridge.

(iii) Shift in flow channel because of meander: This should be studiedparticularly with reference to acuteness of the meander given by

tortuosity ratio described earlier(iv) Any large scale deforestation on upstream will cause heavy

sediment load leading to change in flow pattern.

It is important to have a plane table survey of the floor so that anyserious change can be taken note of. Nowadays, satellite pictures takenby Remote Sensing Agencies also give very accurate picture for over allappreciation of flow pattern.

7. SOME IMPORTANT DESCRIPTION OF PROTECTIONMEASURES

Cardinal principle about river training or protection is to use theriver to aid protection and training. Brute force has seldom succeeded.Apart from being very expensive, they are difficult to construct. Some ofthe measures described as under are time-tested, proven and are veryinexpensive. They do not cause heavy changes suddenly in the flowpattern. But by gradual change caused in the flow direction, sedimentload of the flow bring in the desired effect.

(i) Porcupines

It is not known how this name has been derived. These are madeof 50/65 mm dia-bamboos nailed in a square shape of 600 mm plaindimension as per the typical Figure No. 3. They are suitable up to 1200mm depth (4 feet) of flow. They can form a typical permeable spur or ifused in combination can work as screen. For good effect, they are filledwith bushes of branches of tree/grass. In case of screen, in between aseries of porcupines, a nylone rope is tied to which a number of twigs,branches with leaves/bushes are tied. They are very effective indampening the flow and they cause heavy siltation of the bed and if usedintelligently, they can be very effective in causing diversion of the existingflow into a different channel. A typical arrangement is shown in FigureNo.-4:


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Fig - 3


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Figure No. - IV

Porcupines

Figure No.- 4

These porcupines are weighed down either with boulders or sand bags.As scour takes place, they get fixed in the bed. Even if they fall theyremain as effective.

(ii) Permeable Screen

For depth of flow more than 4 feet, permeable screen made ofbamboos are more effective. A typical arrangement is shown in FigureNo. 5. They are also filled with bushes/branches/grass. They also servethe same purpose as in case of porcupines. They can be convenientlyput to function like a permeable spur.


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Permeable Screen

Figure No.- 5

(iii) Boulder Crates

They are formed by filling boulders in a crate made of galvanizedwire generally of 4 SWG in important works. For ordinary works, 6SWG wire may be used. Size of the crate is dependent on the discharge.Ordinary principle is that the crated boulder should not be lifted bywater flow. According to IS 8408-1979, the size of stone required for avelocity of 4m/sec. is 300 kg, if isolated boulders are used. Relationshipis V=4.893 d1/2 where V is velocity and d is diameter of stone. For 300


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kg stone, crated boulder is an option. Size of crate should be determinedon these considerations.

(iv) Sausage Crates

These are crated boulders of circular cross section of diameter

varying from 2-0 to 3-0 (600 mm to 900 mm). They can be long andare usually very good for protecting slopes of an embankment. A typicalarrangement is shown in Figure No. 6.

Figure No.- 6


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(v) Spurs

They are either made of permeable or solid type constructed acrossthe flow. They can be at right angle to the flow and may be at angle awayor towards the flow. Spurs must be properly anchored at the bank.Otherwise they are likely to be outflanked. A byepassed spur is useless

and will serve no purpose. Permeable spurs are less expensive and moreuseful than solid ones. Permeable spurs can be made of porcupines,permeable screens or sal ballies driven in the bed. Solid ones are similarto guide bund and having strong protection of slopes as well asnose/shanks. They are also known as groynes.

SpursFigure No.- 7

More common and safe are normal spurs. Other two typesshould not be done without proper model study. They can do more harmthan relief if not properly selected designed and constructed. Length ofspur should not be less than 21/2 times local scour depth. Local scourcan be taken as 2 to 2.5 times the normal scour depth calculated byLaceys formula D=0.47 (Q/f)1/3, where Q is discharge in cumecs and f issilt factor. If the depth is taken from model study, it need not bemultiplied by 2 or 2.5 factor to find local scour. This is to keep scour

hole away from the bank. Location of spur is most important. If thepurpose is to deflect the flow away from the bank, it would beadvantageous to provide on convex portion/curvilinear flow as shownbelow.


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Figure No.- IX

Figure No. - 8

8. DESIGN FOR FLOOD PROTECTION

(A) Minor Bridges

Most of minor bridges are on open foundation. They have to beproperly protected by a well designed flooring system. This will includefloor, curtain and drop wall. Length of floor and depth of drop wall willbe on the basis of scour depth. This can be determined either by localobservation or by using empirical value of Lacey, based on designdischarge. Depth of drop wall will be 1.25 times the normal scour depth.Floor should cover the entire width and length of abutment includingwing wall as shown in the typical Figure No.- 9 and 10.

Figure No. - 9


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Figure No.- 10

By and large, this takes care of protection in case of minor bridges. Theslope of floor should match the bed slope and also the wall should matchthe slope. If necessary, local dressing may be done. It is essential to doproper protection of the box culvert which relies on uniform ground

support for its designed structural behaviour. If the underside isscoured, the box culvert gets unevenly supported. For this purpose,properly designed floor system as described above should be provided.Sometimes, instead of splayed wing wall, straight return wall is providedparticularly on high bank or in case of a box, another box is provided tofunction like a wing wall. Similar protection work is called for in suchcases.

(B) Major/Important Bridges

As far as bridges on open foundations are concerned, it is generallyon rocky/in-erodible bed and not requiring any particular protection. Inother cases, flooring with drop walls as in minor bridges may have to beprovided.

For well/pile foundations, they are designed for the scour andhence no protection is necessary even in case of a local scour.


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The bridge itself, however, may need a well designed guide bundwith proper protection on the approach embankments. Design of guidebunds is not dealt with in Bridge Substructure Codes. In case ofimportant bridges, the guide bund design is finalized in course of modelstudies. But in case where no model studies have been done and where

it is not considered necessary to do so, the design can be done on thebasis of guidelines given in this technical note. River Training &Protection Works for Railway Bridges, published by IRICEN has also anyuseful details. River Behaviour Management and Training PublicationNo.204 of Central Board of Irrigation and Power has also many usefulguidelines. They may be referred to for more details. But for all practicalpurposes, this technical note should serve the purpose of a PractisingEngineer. Most important consideration for guide bundh design are :-

(i) Shape in Plan

The guide bundhs can be either divergent, convergent or parallel.They are shown in the Figure No. 11 For important bridges, ellipticalguide bunds have found favour particularly for wide and shallow rivers toinduce the flow to hug the guide bund better. This is also advocated inIS-8408-1976. Straight guide bund with composite curve on upstreamhas also proved very well. Ganga Bridge at Mokamah is such anexample. A good example of elliptical guide bundh is on river Gandak ofChhitauni - Bagah Railway Line. As a rule, unless model studies havebeen conducted and for smaller works, straight parallel guides should beprovided.


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Figure No.- 11

(ii) Length of Upstream/Downstream Guide Bund

There are a large number of opinions on this. But the mostimportant consideration for determining the length are maximumobliquity of the current which must be limited to a reasonable value and


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that it should provide protection to the approach embankments againstmaximum possible embayment behind the nose of the training works.

An obliquity of 300 to 340 flow through bridge axis isconsidered the upper limit.

Upstream length of guide bund should vary between 1.25 Lto 1.5 L depending upon the discharge linearly between 21000 m3/sec to42500 m3/sec. Here L is length of bridge between abutments.

Most of the rivers have their own channel pattern and guide bundshould be designed to take care of these characteristics.

From the river survey, most acute bend has to be found. Theradius of curvature (R) may be taken as average curvature. A largenumber of studies have revealed that the radius of worst embaymentloop can be taken as 2 to 2.5 of the average radius R. For rivers ofmaximum discharge above 5660 m3/sec, this may be taken as 2 and for

those up to 5660 m3

/sec it may be taken as 2.5.Downstream size of the guide bund 0.25 L for all sizes of rivers.

Typical effects of obliquity and embayment is shown in the FigureNo.- 12 and 13 below :-

Figure No.- 12


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Figure No.- 13

Worst embayment behind head of guide bundh shown single or doubleloop.

(iii) Radius of Curved Head

This is radius of curved upstream mole head. This may be takenas 0.45 L (L is waterway width determined from Laceys formula subjectto minimum of 150 m and maximum of 600 m). The downstream curvedtail may be kept as 0.3 to 0.5 times the radius of upstream curved head.The angle of sweep of curved head should range from 1200 to 1450according to river curvature that of the tail head may be kept as 450 to600.

For smaller rivers, one single radius is good enough. For smootherflows, for important rivers multi radii are selected generally after modelstudies.

(iv) Top Width and Side Slopes

Top width of the shank of the guide bundh should be wide enough topermit plying of trucks and for keeping reserve boulders formaintenance. From these considerations, top width should be between 6m to 9 m (20 feet to 30 feet).

Side slope should be 2:1. It is customary to allow construction ofthe guide bundh with local river bed material. This is the case also withapproach embankment. For borrowing earth (which can be generallynon-cohesive, sandy material after top vegetation layer is cleared), itshould be taken from upstream side, not from the downstream side.


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(v) Slope Protection of Approach Embankment and Guide Bund

Side slopes need protection on following counts :-

a) Wave action on the upstream side.

b) Water current along the slopes

c) Wind action.

d) Rain cuts/Rain Water.

On upstream of approach embankment and rear of guide bundh,there can be heavy ponding of water. This water body is particularlylarge in case of long guide bunds. Due to wind, there is heavy build upof water wave causing severe splash, which causes washing away of coreof the guide bund or approach embankment. On upstream/river side,there is heavy water current. There will be large water current on the

rear side of guide bund and upstream side of approach bank, when pondlevel goes down. This has also the same effect and particularly toe of thebanks are severely affected.

The slopes of guide bund and approach embankment, therefore, needsprotection. Most common method is to provide stone pitching. It isnecessary to provide graded filter 20 cm to 30 cm thick, satisfying thestandard criteria conforming to IS 8237-1976, below the pitching.

Stone used for pitching is generally man size boulder of 35 to 55 kgso that they cannot be easily displaced by the current. For small works,one stone thick pitching (25 to 30 cm) will suffice. Gaps in between

could be filled up by smaller pieces.Thickness of stone pitching for larger works can be worked by

formula given by Inglis.

T=0.06 Q1/3 T=Thickness stone in feet and

Where Q=discharges in cusecs

The thickness is not less than 25/30 cm but not more than 1.0 m.Size of stone has been recommended by IS 8408-1976, which is relatedto velocity. V=4.893 D1/2 where D is diameter of the stone. The weight Velocity Diameter is given by the curve shown in (Figure No. 14). It

may be seen that up to velocity of 2.5 m/sec, 35-55 kg stone is generallygood enough. Beyond that either they have to be in a grid, with properpointing. From these considerations, concrete blocks have also beenrecommended. It is, however, very difficult to maintain. After long andgeneral experience, concrete blocks should not be provided unless thebed is inerodible and in general there is no chance of scour and thecurrent is high. Such cases are on foot hills where river is in boulderystage. Typical arrangement for the laying of pitching has been shown inthe Figure No.-15 for Chitauni-Bagah bridge and the bridge overBramhaputra at Tezpur.


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In case of guide bundh, the pitching should continue right up tothe top of the formation for the riverside, including the curved head onboth sides, tail head. For important rivers or having large ponding etc.,the pitching should be done on the rear side of the guide bund also. Forapproach embankment, on the upstream side, the pitching should

continue up to the free board level which should be determined not onlyon HFL but also to take care of velocity head (V2/2g), wave action etc.

For the downstream side, pitching may be done up to the waterlevel, observed either in case of hydraulic model study or the generalwater level observed. Otherwise, toe protection with about a 1 m height

Figure No.- 14


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Figure No.- 15

should be good enough only in Khadir portion of the river. In addition to

this, a toe protection with a short apron width of 1 m wide, one stonethick 25 cm to 30 cm should be considered alright. For rest of the slope,a well designed turfing is generally considered adequate. No filter isgenerally provided on downstream of the approach embankment.

A good drainage is a key for protection of slopes from rain cuts,particularly on high banks of over 6 m. Longitudinal and cross drainswill have to be provided. Arrangements made for this purpose, in case ofTezpur bridge and Chhitauni bridge are shown in the Figure No. 16 forguidance.


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Figure No.- 16


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(vi) Apron

The guide bund particularly on the river side has to be providedwith suitably designed apron to take care of scour of the bed due to highcurrent. The slope pitching at the toe is particularly to be protected. For

currents higher than 2m/sec, normally crated boulders of man sizeweighing 30 to 55 kg stone are provided. The idea is to allow the apronto launch because of scour, so that the entire scoured slope is protectedwith launched boulders. Adequate quantity of stone is to be provided toensure this and to take care of inevitable loss in course of launching ofapron. For alluvium beds, the apron should be like a carpet and shouldbe capable of adjusting itself due to uneven scour. In such cases,concrete blocks are not useful and are not recommended. However, forin-erodible bed/rocky/bouldery beds having very high current (over4m/sec) concrete blocks are provided which are properly chained andanchored. For ordinary situations, boulder aprons with or without crates

are considered proven and established solutions.Following elements require designing of apron :-

a) Thickness of apron.

b) Level at which the apron is to be laid

c) Width of apron.

a) Thickness

Thickness of apron is governed by thickness of slope of the guidebund. In general, thickness of launched apron is to be kept at 1.25 T(where T is thickness of slope pitching). In more severe case, this may bekept as 1.50 T.

b) Level at which Apron is to be Laid

This is the most important consideration. Normally apron shouldbe laid on dry bed, as low as possible. Since this is done manually, theapron is not possible to be laid beyond water depth of knee, i.e., 2 to 3feet of water (0.6 to 0.9 m). If this is not available, the bed should bemade up by temporary filling. In case of current, the stream has to bediverted by taking useful measures. Generally, the bed may beexcavated up to LWL to lay the Apron. The excavated material is used forconstruction of guide bundh, approach bank.

c) Width of Apron

This has to be determined by depth of scour. For ordinaryalluvium conditions, where Laceys formula is applicable, normal scourdepth is worked out by D=0.47 (Q/f)1/3, where D is depth of scour belowHFL, Q is discharges in m3/sec and f is silt factor. Depending upon thelocation, scour depth varies. This may be adopted as 2.5D for theupstream curved head of guide bund and as 1.5D for the straight reachof the guide bund including tail on the downstream side of the bank. Forvery large radius of guide bund, having severe attack, depth up to 2.75D


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has also been adopted. But these are special situations, normallyindicated by the model studies.

Side slope of apron is taken as 2:1, which is the same as the slopeof guide bund.

The width of apron is generally kept as 1.5 Dmax, where Dmax isthe normal scour depth multiplied by the factor 1.5/2.5 as mentionedabove due to the locations of part of guide bund.

This thickness of guide bund is generally kept on wedge shapehaving thickness equal to the slope pitching at the inner and linearlyvarying to 2.25 at the outer end.

Total volume of boulders required is worked out by followingrelationship :-

Figure No.- 17

R=Rise of flood above low water or the level at which the apron is laid

D=Depth of scour below the level, the apron is laid

F=Free Board


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r1=Radius of the nose at the top of the slope

r2=Radius at the toe of the slope

r3=Radius at the toe of the laid apron

r4=Radius at the toe of the launched apron

r=r2+r42

T=Thickness of stone pitching on slope

Thickness to be adopted = 1.25 t

Surface area to be covered after full launching = r5D

Volume= r5DX1.25TSince r= r2+r4 = r1+2(F+R)+D

2

Volume of stone = 2.81DT[(r1+2(F+R)+D]

This quantity is to be laid in an area = (r32 r22)

2

All cardinal elements are shown in the figures 18 and 19 for Tejpurbridge over Brahmaputra and Gandak bridge respectively

Figure No.- 18 (Brahmputra Bridge)


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Figure No.- 19 (Gandak Bridge)

The apron arrangement in typical important bridge over River Gandak isshown in Figure No.19. On the rear side of the guide bund 15m wideapron has been provided.


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Figure No.- 20

The approach embankment is generally provided a smaller width ofapron on the upstream side. In most of the ordinary cases, a width of1m is considered adequate. For other important works, 3 to 5 meterwideapron, 30 cm thick, is considered adequate.


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9. REPAIR, MAINTENANCE AND PROTECTION OFEMBANKMENTS

If embankments are properly designed and executed, there should

not be occasions to have any serious problems in their properfunctioning or any difficulty in their maintenance. Some reasons, whichmay lead to problems, are as follows :-

(i) Faulty Construction

This will include construction of guide bund and approachembankments particularly in khadir of the river when done in twostages or in two seasons without proper construction joints. As a matterof rule, these must be constructed in one go in one season. It is verydifficult to provide continuity in two stage construction, particularly, ifsubjected to floods. Classical example of failure is the guide bund of old

Chitauni bridge over River Gandak in 1924. The guide bund wasconstructed in two seasons without proper joint. This was breached atthe joint. In case this becomes unavoidable, at least, a wedge size equalto angle of internal friction of the old construction should be removedand the next construction should be done with proper benching. An overlap may be provided of slope protection and also an apron.

A high bank, with inadequate cross and longitudinal drain caninduce very high run off velocity causing frequent failures. Properdrainage is a must. Good turfing is also very essential. In constructioninvolving river bed material, geogrid (of jute) etc. is very effective both forgrowth of vegetation as well as for dampening the effect of run off along

the slopes and preventing rain cuts. This can be laid on the slopes andfixed as per procedures prescribed by the manufacturers.

Good construction also includes proper quality control e.g.compaction, moisture control and proper layer by layer constructions.

Toe protection is very essential in Khadir. This has been alreadydescribed earlier.

(ii) Change of River Course

This happens mainly due to meandering effect of the river. In case ofmeander, it is possible to know the maximum radius of curvature from

which it should be possible to estimate the likely area of effect. Onething is quite clear that this does not happen suddenly. The river givesadequate prior warning. It is, therefore, necessary to keep the behaviourof the river under close watch and one should take remedial action intime. If adequate study and data are kept, design of protection measuresmay be easier, cost effective and simpler.

As explained earlier, meander is basically a matter of hydraulic energywhich is dissipitated in a particular manner. So in case the tortutosity


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ratio is high, the help can be got by artificially increasing the resistanceto flow in the concave part and by doing necessary deflection in theconvex part of the curvilinear flow. For this purpose, permeable spurmade of permeable screens, porcupines etc. are very effective. With theirhelp, smaller spills can be closed also. This was successfully

demonstrated in diversion/closure of multiple stream of river Gandak forconstruction of guide bunds and approach embankments. This waspresented as a technical paper in National Seminar on BridgeEngineering in North-East organized by Indian Institution of BridgeEngineers in October, 1998.

For protecting or repairing a bank affected by such flow, the bestway is to provide a series of spurs with proper toe protection. Afterproviding suitable pitching, a layer or two or sausage crates followed byaprons are very effective. This was done for Bagmati River approachbank. A typical Figure No. will illustrate the arrangement and principlebehind the same (Figure 21). It is very essential to provide properanchorage to the spur against scour. This should be protected by properpitching, apron or using sand bags.

(R.R. JARUHAR)

MEMBER ENGG.


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LIST OF FIGURE NO./FIGURE NO.S

1. Figure No. 1 Dimension of meander

2. Figure No. 2 Neck and chute cut off3. Figure No. 3 Typical details of porcupines4. Figure No. 4 Typical arrangement of dampening of flow to

induce siltation5. Figure No. 5 Details of permeable screen7. Figure No. 6 Details of sausage crate8. Figure No. 7 Types and cardinal dimension of spur9. Figure No. 8 Location of Spur to aid diversion10. Figure No. 9 Plan view of bridge floor with curtain and drop

wall11. Figure No. 10 Typical section of bridge floor

12. Figure No. 11 Different forms of guide bundh13. Figure No. 12 Typical dimension of guide bundh showing

extend of obliquity14. Figure No. 13 Worst embayment behind head of guide bundh

showing single or double loop15. Figure No. 14 Size of apron stone V/s.Velocity16. Figure No. 15 Typical arrangement of laying of stone pitching

on slope for bridge over river Brahmaputra nearTezpur and river Gandak for Chitauni-BagahProject

17. Figure No. 16 Typical arrangement of longitudinal and cross

drains for approach embankment provided forbridge over river Brahmaputra near Tezpur

18. Figure No. 18 Details of guide bund (Brahmputra Bridge)19. Figure No. 19 Details of guide bund (Gandak Bridge)20. Figure No. 20 Apron arrangement/guide bundhs for Chitauni-

Bagah over river Gandak and RiverBrahmaputra at Tezpur

21. Figure No. 21 Protection arrangement for embankment inBagmati River

===


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References

1. River Behaviour Management and TrainingPublication No.204, Central Board of Irrigation & Power.

2. River Training and Protection Works, Director/IRICEN/Pune.

3. Key Design Parameters.4. Bridging River Brahmaputra, Published by N.F.Railway

(Construction)5. Chitauni Bagha Bridge over River Gandak.6. North Eastern Railway Engineering Department Standard

Specifications.7. IRS Bridge and Sub-structure Code.


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Documents

Guide Bund Railway