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RIVER TRAINING AND PROTECTION WORKS

Editor's Note: The river training and protection works are very important part of management of the bridges. The following is the extract of the chapter VIII of the IRBM. Members are requested to improve the content

Training / Protection of various types of rivers

The object of river training/protection works is to prevent the river from damaging railway formation, bridges and other structures. The training/protection works will have to be decided depending on the reach in which the river is situated namely:

a) Upper reaches (Mountainous)

b) Submontane reaches (Foot hills)

c) Quasi-alluvial reaches (Trough)

d) Alluvial reaches, and

e) Tidal reaches

Upper Reaches (Mountainous Rivers)

1. Characteristics:

These streams have narrow and deep cross section with very steep bed slopes. The gorge is often deep and narrow with formation of rapids . The discharge is extremely variable and their beds are interspersed with large size bed material consisting of rock, boulders, shingle and gravel. The rise of flood in them is very sudden and flashy. The water is heavily sediment laden, with high concentration of suspended load.

2. Suggested protective measures:

The following protective measures are suggested/ recommended for adoption.

a) Suitable protective fenders of concrete, rolled steel or rails may be provided upstream of the bridge to reduce the impact on piers and abutments due to rolling boulders down steep slopes.

b) Measures for controlling soil erosion and landslips, improving stability of side slopes and arresting bed load in boulder bedded hilly terrains should be undertaken.

c) The formation of gullies by the water coming down the hills can be prevented by afforestation, construction of gully/ check dams, contour bunding, debris basins, chambers or wells. These should be cleaned as frequently as necessary.

d) Stability of side slopes can be improved by provision of adequate drains, breast and toe walls etc.

e) Properly designed chutes with paved apron at the entrance with adequate free board may be used in Railway cutting for leading the water of mountainous streams down the hill slopes. Alternatively, suitably designed catch water drains on water-shed side may be constructed.

Submontane Reaches (Foot Hills)

1. Characteristics:

The rivers in these regions have a flatter bed slopes generally form 1 in 50 to 1 in 500. The velocity and its sediment transporting capacity gets reduced encouraging deposition of excess sediment load.

Medium size boulders, gravel and coarse sand are generally found in the beds. The floods are of flashy nature. These streams are highly erosive and the erosion proceeds through grinding of bed load material during transport, formation of deep holes through whirlpools and plunging action along with cliffs forming the banks, in the higher reaches. These actions go on widening the bed and deepening the channel. The flow in the channel, except during highest stages, is insufficient to transport the detritus which gets deposited blocking the original channel; another channel may then be formed and in course of time the river bed may become a net work of such channels with islands in between. Such streams are called “Braided” stream (Annexure 8/1). All these channels normally overflow during high floods and the river acquires very wide and shallow cross section. The rivers in this reach are prone to progressively raise their beds by sediment deposition. Such rivers are known as “Aggrading” type.

In these case, the over bank spills increase year after year, until occurrence of abnormal floods in a year, when sudden change of course may take place.

2. Suggested protective measures:

It is not desirable to locate bridge in such reaches. However, if a bridge is to be provided, training measures in the form of marginal bunds, extending right upto the high ground in the hills are required to shift the point of aggradation downstream. To reduce the erosive action on the marginal bund

a) Suitable slope protection with boulders or concrete slabs,

b) adequate toe protection in the form of two rows of in-situ concrete blocks or boulders in wire crates and

c) boulders in wire crates forming flexible type apron may be provided.

Quasi-Alluvial Reaches (Trough)

1. Characteristics:

In this reach, the bed slope varies from 1 in 500 to 1 in 2,500. The bed consists of small size gravel and medium sand. The channel has generally a well defined course.

2. Suggested protective Measures:

Bridging such rivers normally involves constriction in width and provision of guide bunds. Assistance of specialised agency, undertaking hydraulic model studies may be availed of, as considered necessary.

Alluvial Reaches

1. Characteristics:

In this reach, the river bed slope varies from 1 in 2,500 to 1 in 25,000. The river flows on an almost flat bed built by its alluvium. The alluvial river meanders as a whole within its “Khadir” (a strip of low land within which a river meanders and its flood rises). (Annexure 8/2).

The main difference between the alluvial and quasi-alluvial rivers is that the former meanders as a whole within its “Khadir” while the latter has well defined banks and it is only during periods of low water that the channels meander. Rivers in alluvial reaches are normally stable with no perceptible lowering or raising of the river bed in the course of the years.

2. Suggested protective Measures:

The training of alluvial rivers is generally on the same lines with guide bund system as described for quasi-alluvial rivers. The meanders do not remain fixed but usually travel down stream. Every effort should be made to keep the river to its original course near the bridge . In some cases it is observed that the main current of the river starts flowing along the railway bank on the upstream side due to the meander travel downstream. The building of spurs along the railway embankment is not a good remedy, as it perpetuates the main channel along the Railway alignment. In such cases every effort should be made to divert the river to its original course. In some cases the construction of a second control point consisting of a spur about 0.4 times the meander length has been found to be satisfactory. The exact length and location of such works however, should be determined through model studies in a hydraulic laboratory.

A technical paper on the issues related to the river training and protection works required for the railway embankment protection against river SHARADA in alluvial soil may be seen here...

Tidal reaches

At the confluence of the river with the sea, the tidal effects predominate. Constriction of the water way is to be avoided in these reaches and tidal regime is to be kept in view, while designing bridges.

South Indian Rivers

The river system in south India is geologically older and stable. Tendency for shifting of the river bed course and aggradation/degradation is insignificant. Problems of river training and protection normally do not arise except in the deltaic region/tidal reaches.

River Training Works

The following types of river training works are generally adopted on the Indian Railways:

1. Guide Bunds;

2. Spurs (Groynes);

3. Marginal Bunds;

4. Closure Bunds; and

5. Assisted cut offs.

Guide Bunds

1. Necessity:

Guide bunds are meant to confine and guide the river flow through the structure without causing damage to it and its approaches. They also prevent the out flanking of the structure.

2. Shape and Design Features:

a) The guide bund can either be divergent upstream or parallel. In the case of divergent guide bund, there is possibility of formation of a shoal at the center. Parallel guide bunds minimise obliquity and separation of flow along the flanks. According to geometrical shape, the guide bunds may be straight or elliptical. In the case of certain type of alluvial rivers with sandy bed and meandering pattern, elliptical shape appears preferable to minimise obliquity and separation of flow.

Various types of guide bunds are shown in Annexure 8/3.

b) Normally the upstream shank of the guide bund is between 1.0 to 1.5. times the length of the bridge, while the downstream shank is between 0.2 to 0.4 times the length of the bridge.

c) The tail bund on the downstream side is provided to afford an easy exit to the water and to prevent formation of vertical whirlpools or rollers which give rise to scour. These tail bunds are also curved at their ends and should be properly armoured.

d) The guide bund is provided with a mole head on its upstream side. The mole head bears the brunt of the attack and should be provided with adequate protection in the form of slope pitching and properly designed launching apron. The shank i.e. the portion behind the curved mole head of the guide bund should also be similarly protected on the river side. The slope in the rear of the guide bund need not necessarily be provided with pitching and may be protected by planting grass or shrubs as found suitable.

e) No spurs projecting from the guide bunds should, in any case, be provided.

f) For design and construction of guide bunds/launching aprons reference may be made to IS: 10751-1983 and IRC: 89-1985.

3. Apron Protection :

Apron is provided beyond the toe of the slope of the guide bund, so that when bed is scoured, the scoured face will be protected by launching of the apron stone or wire crate containing stone. A typical lay out of a guide bund is shown in Annexure 8/4.

4. Maintenance :

a) Substantial reserve of pitching stone should be maintained on the guide bund for use during emergency. This should be stacked at the top of the guide bund.

b) The track on the guide bund, where provided, should be maintained in a satisfactory condition and should be capable of taking boulder trains at any time. The Permanent Way Inspector and the Assistant Engineer should inspect the track soon after the monsoon every year and carry out necessary repairs well before the next monsoon.

c) Every effort should be made to ascertain whether the apron is launching to the intended position and this should be done by probing after the flood season is over. Plotting of the levels will indicate the efficacy of the launching.

d) Disturbance of pitching stone on the slope indicates dangerous condition and additional stones should be placed in position immediately as necessary.

5. Failures and remedial measures:

The conditions under which an apron of the guide bund can fail and remedial measures to be adopted are stated below:

a) If the launching takes place beyond the capacity of the stone in the apron and results in leaving the bank material exposed to the current and wave action, more stone will have to be added to the apron.

b) If stones are carried away by high velocity current from the launching apron and the toe of the bund, the apron should be strengthened against severe attack by laying large sized stones at the outer edge of the apron.

c) If slips and blow-outs in the bund occur due to a steep sub soil water gradient resulting from a rapidly falling flood in the river, the bank should be widened to reduce the hydraulic gradient. This equally applies to marginal bunds.

d) Wherever disturbance is noticed in rear of guide bund due to wave lash or other causes, the slope pitching should be adopted as a remedial measure.

e) An apron can launch satisfactorily only if the material scours easily and evenly and the angle of repose of the underlying material is not steeper than that of the stone.

In all these cases action should be taken to dump the boulders on the toe of the bank and make up irregular surface.

Spurs (Groynes)

1. A spur/groyne is a structure constructed transverse to the river flow and is projected form the bank into the river.

2. Type of spurs /groynes:

i) They may be either “Permeable ” or “Impermeable”. Permeable spurs are constructed by driving wooden bullies or bamboos, filled in with brush wood, with sarkanda mattresses or other suitable material. These are helpful in causing quick siltation due to damping of velocity. They are useful when concentration of suspended sediment load is heavy; they allow water to pass through. Impermeable spurs are made of solid core, constructed of stones or earth and stones with exposed faces protected by pitching. These spurs can with stand severe attack better than permeable spurs.

ii) Spurs may be classified as (a) repelling (deflecting) (b) attracting and © neutral (sedimenting ). Repelling (deflecting) spurs are those which incline upstream at an angle of 60 degree to 70 degree to the river course and deflect the current towards the opposite bank. They cause silting in still water on the upstream pocket. Attracting spurs incline downstream and make the deep channel flow continuously along their noses. They cause scour just on the downstream side of the head due to turbulence. The river flow is attracted towards the spur. Normal (holding or sedimenting) spurs are those which are built at right angles to the bank to keep the stream in a particular position and promote silting between the spurs. They have practically no effect on the diversion of the current and are mostly used for training of rivers for navigational purposes.

iii) Spurs are also classified as full height spurs and part height spurs. Where top level is higher than HFL, it is called a full height spur.

iv) Spurs are also constructed extending into the stream with a “T” head or hockey stick shaped head, properly armoured to hold the river at a distance. A series of such spurs/groynes correctly positioned can hold the river at a position away from the point intended to be protected. The edge of the “T” head should be curved somewhat in the manner of a guide bund to avoid swirls. Sketches of the various types of spurs may be seen in Annexure 8/5.

3. Location and salient features of a spur/groyne:

i) The space between spurs or groynes generally bears a definite ratio to their length. The common practice is to keep the spacing at about 2 to 2.5 times the length so as to effectively protect the bank.

ii) If designed as a full height spur, care should be taken to see that spurs are built sufficiently high so that they are not overtopped and out flanked by the current during high floods. Free board of 1 metre is provided.

iii) The side slope of spurs are generally 2:1.

iv) The spurs should be anchored on to high ground.

v) The head of the spur is most vulnerable point for scour and should be well protected on slopes by pitching and at toe by an apron designed for scour depth of 2.5 to 2. 75 times D lacey at the mole head.

vi) Spurs should never be constructed at a point where severe attack is taking place but at some distance upstream.

vii) Spurs/groynes should be used only in-situation where they are absolutely necessary.

viii) The design of spurs may be finalised preferably through hydraulic model studies.

4. Maintenance of spurs/ Groynes:

In all cases, satisfactory arrangement should be made for the maintenance of spurs/groynes by providing access to them during all seasons of the year and keeping boulders as reserve.

The maintenance procedures specified for guide bunds apply equally to spurs/groynes also.

Marginal Bunds

Marginal bunds are provided to contain the spread of the river when the river in flood spills over its banks upstream of the bridge site over wide area and likely to spill in the neighbouring water courses or cause other damages. The marginal bund should normally be built well away from the active area of the river. The slope should be well protected by turfing. Where a marginal bund has to be built in the active area of the river, it should be protected with pitching and apron. The earth for the construction of marginal bund should preferably be obtained form the river side. The upper end of the marginal bund should be anchored into high ground well above HFL. Marginal bunds should be inspected every year along with the annual bridge inspection and necessary repairs should be carried out before the onset of monsoon. Cattle crossing and rodent holes across the marginal bund should be specially watched and deficiencies made good.

Closure Bunds

Sometimes it may be necessary to entirely block one or more channels of the river in order to prevent the discharge of such channels developing into a main river channel after the construction of the bridge. This is done by providing a closure bund. The bund is designed as an earthen dam. The same is generally constructed at some distance from the railway line. special care should be exercised to guard it against its failure. It should be inspected every year after the monsoon and necessary repairs carried out.

Assisted Cut- Offs

Sometimes when very heavy meandering develops near bridges and there is a danger of its encroaching too heavily into the still water area or otherwise dangerously approaching the railway embankment, it becomes necessary to dig a cut-off channel which will ultimately develop and help in the diversion of water through it. To effect economy, a pilot channel cut is usually made when there is low flow in the river and full development of the channel takes place during the flood. This cut off channel should preferably have (i) atleast three times the river’s straight regime slope and (ii) the upstream end should take off from where the bed load of main channel has less than the average amount of coarse material i. e. from the active part of the channel where the velocity is more. The entrance to the pilot cut should be bell shaped to facilitate entry of water. The chord loop ratio should normally be greater than 1 to 5 if a successful channel is to develop. Cut off should be planned with care taking all relevant factors into account (Annexure 8/2).

Protection of Approach Banks

1. Approach banks of bridges may be subjected to severe attack under the following conditions:

i) When the HFL at the bridge is very high and there is spill beyond the normal flow channel.

ii) When the stream meets a main river just downstream of the bridge.

iii) In the case of bridges with insufficient water way.

iv) The wave action on the approach bank of bridges situated in a lake/large tank bed may have a detrimental effect.

In all the above cases the pitching of the approach bank upto HFL with sufficient free board is an effective solution. Provision of toe wall and narrow apron in some cases will also be useful.

2. If deep borrow pits are dug near the toe of approach banks, the water flows through these pits and forms a gradually deepening water course which may eventually threaten the safety of the approach bank. In this case it will be useful to put rubble “T” spurs across the flow to reduce the velocity and expedite silting of the course.

3. Whenever the water level on either side of an approach bank is different, there may be seepage of water and to ease the hydraulic gradient , widening of banks, provision of sub banks and toe filters etc may be resorted to.

4. At locations with standing water against the embankment, special watch should be kept when the water level recedes rapidly and when slips are likely to occur.

Design of river training works through model studies

In case of large alluvial river where training/ protection works involve a heavy financial outlay, model studies should be resorted to, to arrive at the most economical and effective solution.

 
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