Site Loader

LOCATION

510 5th Ave, New York, NY 10036, USA

1.1 General
Rivers in alluvial plains are highly variable in their behaviour and often unpredictable to an average man. A stream, which is quite trouble free during low flow, may attain a threatening condition during high stages. It may develop unforeseen meander, break through embankment, attack town and important structures, outflank bridges and in general may create havoc. Therefore, whenever any hydraulic structure is built across an alluvial stream, adequate measures in the form of river-training works must be taken to establish the river course along a certain alignment with a predetermined cross-section.

River training refers to the structural measures which are taken to improve a river and its bank. River training is an important approach in the prevention and mitigation of floods and general flood control, as well as in other activities such as ensuring safe passage of a flood under hydraulic structure. For flash flood mitigation, the main aim is to control the water discharge regime in the watercourse (Colombo et al. 2002). River training measures also manages sediment transportation and thus minimization of bed and bank erosion is observed. Many river training structures are implemented in combination with bioengineering techniques to lessen the negative effects on environment and landscape. There are a number of types of river training structure. The selection and design of the most appropriate structure depends largely on the site conditions.

We Will Write a Custom Essay Specifically
For You For Only $13.90/page!


order now

River training is necessary in those reaches of the river where the river encounters excessive erosion of bed. Braiding, meandering, breaching of embankments, damages of hydraulic structures, roads and railways, etc. are some of the consequences of an untrained river. Various river training measures which are commonly deployed include spurs/groynes (permeable and impermeable), submerged vanes, bank pitching, guide vanes, bandalling etc. Details about planning, layout, design, and maintenance of permeable and impermeable type spurs are covered in IRC: 89 (1997) & IS: 8408(1994).
Porcupine systems have also been installed in big river in India like Brahmaputra and Ganga and have yielded fairly good results. After repeated failures of earthen spurs upstream and downstream of the Farakka barrage on the Ganga River (India), Central Water Commission (CWC) used RCC to protect the erosion of left bank. It is reported that the porcupines were very effective and helped in siltation of the left bank. Aamir and Sharma (2015) have developed a rational design methodology for riverbank protection using RCC porcupines.

1.4 Aim:
To carry qualitative analyses sediment factor with introduction of porcupine system, study the flow condition with the different setup of porcupine system with orientation and assign variation in manning sand “n” value (Roughness coefficient) with the end of porcupine system.

1.5 Objectives:
The objectives of the study are:
To reduce the momentum of flow to encourage sediment riverbank.

To assize the effectiveness of RCC porcupines with the different orientation and density of Porcupine.

1.4 THEORY OF RIVER TRAINING
1.4.1 BACKGROUND
River training is an ageold practice resulting in incessant development and application of human ingenuity to correct vagaries of the rivers. It requires deep and precise study of river mechanism and behavior discussed heretofore. River training has assumed considerable significance in India due to huge annual recurring damage caused by the floods; 80 per cent of which accounts for loss of crops. River training, in its broad aspects, covers all engineering works constructed on a river to guide and confine the flow to the river channel, and to control and regulate the river bed configuration for effective and safe movement of floods and river sediments. In essence, river training envisages training and stabilizing a river within a suitable waterway and along a certain alignment for a variety of purposes. River training works involve large outlays and it is essential to select the type of the training work and materials of construction so as to make optimum utilization of funds, and effective and economical utilization of the available construction.

The various objectives of river training are, (i) To guide the axis of flow at ordinary and low stages and safe passage of floods without overtopping the banks. (ii) To protect the banks from erosion and generally improve their alignment by stabilizing the river channel, (iii) To train the river flow along a safe course, thereby avoiding damage by flooding or erosion of valuable lands, habitation, crops, factories, etc., (iv) To prevent outflanking of a bridge, barrage or weir by directing the flow in a defined stretch of the river, (v) To prevent river from changing its course, (vi) To confine a river channel which has become too wide by swinging from side to side and to reclaim the land from river bed, (vii) To check certain devastations like that of flash torrents, (viii) To trap bed load in areas of superfluous width, (ix) To transport efficiently bed load and suspended sediment load, (x) To provide sufficient depth of flow for safe navigation, (xi) To establish channel boundaries where braiding has created too wide a section divided into small channels separated by island, and (xiii) To correct disorderly banks or flow conditions
3.2. Classification of river training works
3.2.1. Classification Based on Purpose
1.High Water Training: River training aimed at flood protection is called high water training or training for discharge. It envisages provision of adequate waterway for safe passage of maximum flood by proper location, alignment and height of embankments for a given flow discharge without tending to change river bed conditions.

2.Low Water Training: Also termed as training for depth. It envisages to provide adequate water depths during low water periods in the river channel for navigation by concentrating flow in a desired channel and closing other channels by the method of bandalling i.e., contracting the width of the river cannel with the help of groynes, etc.

Mean Water Training: Also termed as training for sediment. It is by far the most important type of river training. It envisages rectification of the river bed configuration and efficient movement of suspended and bed load for maintaining the channel in good condition. The maximum aggrading capacity of a stream occurs in the vicinity of mean water or dominant flood discharge, and as such tends to change the river bed in accordance with that stage of bed flow. Mean water training includes river training for efficient sand exclusion from canals by correcting adverse river curvature to locate the canal off take.

3.2.2 Classification Based on Structure Alignment
1.Longitudinal structure: These aim at guiding the axis of flow at ordinary and low stages, protecting the banks from erosion, generally to improve their alignment, trapping bed load in the areas of superfluous width, and establishing channel boundaries where braiding has created too wide a section divided into small channels separated by islands. Longitudinal training works are preferred to projecting training works in rivers carrying small bed loads, and with narrow channels having steep sloped and swift currents. In rivers with unstable bed conditions, longitudinal works are susceptible to damage by undermining.

2.Projecting training works. Projecting training works aim to protect the bank from which they project into the river by deflecting the current away from the bank. They are more suitable in rivers with unstable bed conditions. Cross connecting dikes to the bank at intervals add strength to withstand flood action and promote sedimentation in the closed off channel space.

3.3. Principles of river training
Two approximate theories, the theory of tractive force for bed load, and regime theory for suspended load are available for application to problems of river training.

3.3.1. Tractive Force Theory
Tractive force approach is essentially lan tractive force for the incipient conditions of bed movement. The alluvial rivers carry sediments in suspension as well as bed load. The capacity of river channel is designed to cater for the discharge and sediment load by the following formula:
Qs =0.17/m3/4 r2 BR2 S2
Also Q=1/n BR5/ 3 S1/ 2 (Manning’s formula)
Qs = silt bed charge (Cum/s), m = dia. Of sand (mm), r = specific weight,
3.3.2. Regime Flow Theory
In incoherent alluvium, Lacey’s regime the dimensions, although there is a marked divergence when applied to large rivers due to omission of an important parameter, viz., quantity of silt in suspension are important where silt charge in the flow predominates.

3.4. Methods of river training
Planning and design of river training works is done by empirical methods and on the intuitive judgments of engineers. The method of river training applicable depends mainly on the type of river and sediment load, as under. Under certain cases, where river is very unstable, training is almost impossible and the situation has to be met as it arises.

3.4.1. Stable rivers:
Stable rivers are characterized by stability of alignment, slopes and regime as they mould their beds to carry into sea almost all the silt brought down by them. They are amenable to river training to attain ultimate stability with the aid of training works such as spurs and guide banks.

3.4.2. Aggrading rives:
Aggrading rivers have inherent instability and are not equally amenable to river training. Stability cannot be imposed, e.g., bank protection works may either be destroyed by severe erosion or get bur.

3.4.3. Degrading rivers:
Degrading rivers too have inherent instability and not easily amenable to river training. Stability cannot be imposed, e.g. training works may be destroyed by undermining due to bed scour. Gradient control by constructing dams and weirs is a prerequisite to attempting any other modifications in the river. Local training works may be possible if their repercussions on river regime upstream and downstream are not drastic.

3.5. Planning for river training measures
The training measures are planned with due regard to the limitations imposed by the type of river. The various steps involved are as follows:
3.5.1. Alignment: Alignment of training work is generally based on the layout and number of training structures involved. A better way to arrive at radius of curve is to study to photographs of alignment of a stable stretch of river on a similar grade and soil conditions in the vicinity of the erosion site and from this stable section measure the radius of curves. An attempt is the made to develop an alignment with curves of similar radius to those measured. Once the proposed alignment is decided, the methods of control to stabiles the channel to its alignment are considered.

3.5.2. Cross section: The cross section is that built by the river itself, i.e., when the channel is uniform and does not show signs of accretion of retrogression. The proposed section is either by river contraction or by river diversion and is compared with the normal cross section with regard to gauge and discharge relation, velocity of flow, and total sediment runoff. The section is modified by successive analysis to bring it in line with the normal corss section as far as possible.

3.6. Types of River Training Works
There are 7 types of river training works are as follows.

(1) Embankments
(2)Guide Banks or Bell Bunds
(3)Spurs or Groynes
(4)Impermeable Groynes
(5)Permeable Groynes
(6)Bed Pitching and Bank Revetment and
(7)Dredging of River.

3.6.1.Embankments:
The floods may be prevented from submerging the country by constructing earth embankments. They are generally constructed up to a height of 12 m. They are designed and constructed in the same way as an earth dam. The embankments are generally constructed parallel to the river channel.

Depending upon the position of the embankments subdivisions made are:
i. Marginal embankments or dykes or levees,
ii.Retired embankments.

The marginal embankments are constructed as close to the banks as possible to restrict the flood water from submerging the area behind them. Position of marginal embankments.

Figure: 3.1. Marginal embankment or levees

They are designed to hold up the water up to a maximum anticipated HFL without the possibility of overtopping and with a view to withstand all external pressures. This condition is met with by providing sufficient freeboard, bed width, top width and stone protection on adequate slopes.

As the height of the embankment increases it becomes necessary to provide key treach, zoned section etc., to make the embankment stable. Like earth dams embankments are also likely to fail due to overtopping, piping, rat holes, seepage and caving in of river side sloping face. It is therefore necessary to adopt adequate sections for various heights.

The following sections are generally adopted for various heights.

Figure: 3.2 Sections of marginal embankment

3.6.1.1. Advantages of Embankments:
(i)They are very widely used river training work.

(ii)It is cheaper and quick as well as simple in construction. They can be constructed with locally available material.

(iii)Maintenance of embankments is similar to canal bank maintenance and does not involve intricate methods.

(iv)Embankment can be constructed reach by reach to extend extent of protection.

(v)They protect large areas by comparatively small investment.

3.6.1.2. Disadvantages of Embankments:
(i) By restricting the waterway it raises the flood levels.

(ii)Unpredictable flood flows attack the embankment and hence chances of its failure are quite high.

(iii)During flood constant vigil is required on the embankments. It increases cost of maintenance.

(iv)They interfere in laying irrigation canal system and also reduce cultivable area.

Retired embankments are constructed at a distance from the river banks. Thus retired embankments are the intermediate type between the case of marginal embankments and river with no embankments. Retired embankments are generally constructed on a lower ground away from the banks.

Though they are costly due to increased height and risky, they have some mentionable advantages:
i. They do not interfere in the process of raising of the ground by deposition of silt.

ii.They make it possible to store more water for longer period.

iii.They provide wider waterway in times of high floods.

3.6.2. Guide Banks or Bell’s Bunds:
Rivers in flood plains submerge very large areas during flood periods. Naturally when some structure is to be constructed across such a river (for example, bridge, weir, etc.), it is very expensive to construct the work spanning whole width of the river. To economies some training work may be constructed to confine the flow of water within a reasonable waterway.

Guide banks are meant for guiding and confining the flow in a reasonable waterway at the site of the structure. The design of the guide banks is based on the theory developed by Mr. Bells. Hence, guide banks are also known as Bell’s bunds. This river training work has been devised from a study of the natural river channel in alluvial reach.

The river has a tendency to meander over large width of low lying land thereby flooding it occasionally. But it was observed that the same stream passes through narrow and deep sections where high and stiff permanent banks are available on either side without appreciable afflux or abnormal velocity.

The guide banks guide the river flow past a bridge or any other hydraulic structure without causing damage to the work and its approaches. The guide banks are constructed parallel or approximately parallel to the direction of flow. They extend both upstream and downstream of the abutments of the hydraulic structure. The guide banks may be provided on either side of the hydraulic structure or on one side as required. The guide banks consist of four parts mainly:
i. Upstream curved head or impregnable head,
ii.Downstream curved head,
iii.Shank or a straight portion which joins the two curved heads, and
iv.Slope and bed protection, it includes apron.

Generally the core of the bund is built with sand. The sloping faces are protected with stones. An apron is also provided for protecting the bed against scouring. Sufficient freeboard and top width are also provided. The curved heads are laid with adequate curvature. The Guide banks mainly serve two purposes:
i. They protect the approach embankment for the bridge from attack of the water. Approach embankments extend from the bank of the river to the guide banks generally in perpendicular direction to both.

ii. They control the river and induce it to flow through the bridge more or less axially.

Rivers in alluvial plains are highly variable in their behaviour and often unpredictable to an average man. A stream, which is quite trouble free during low flow, may attain a threatening condition during high stages. It may develop unforeseen meander, break through embankment, attack town and important structures, outflank bridges and in general may create havoc. Therefore, whenever any hydraulic structure is built across an alluvial stream, adequate measures in the form of river-training works must be taken to establish the river course along a certain alignment with a predetermined cross-section.

River training refers to the structural measures which are taken to improve a river and its bank. River training is an important approach in the prevention and mitigation of floods and general flood control, as well as in other activities such as ensuring safe passage of a flood under hydraulic structure. For flash flood mitigation, the main aim is to control the water discharge regime in the watercourse (Colombo et al. 2002). River training measures also manages sediment transportation and thus minimization of bed and bank erosion is observed. Many river training structures are implemented in combination with bioengineering techniques to lessen the negative effects on environment and landscape. There are a number of types of river training structure. The selection and design of the most appropriate structure depends largely on the site conditions.

River training is necessary in those reaches of the river where the river encounters excessive erosion of bed. Braiding, meandering, breaching of embankments, damages of hydraulic structures, roads and railways, etc. are some of the consequences of an untrained river. Various river training measures which are commonly deployed include spurs/groynes (permeable and impermeable), submerged vanes, bank pitching, guide vanes, bandalling etc. Details about planning, layout, design, and maintenance of permeable and impermeable type spurs are covered in IRC: 89 (1997) & IS: 8408(1994).
Porcupine systems have also been installed in big river in India like Brahmaputra and Ganga and have yielded fairly good results. After repeated failures of earthen spurs upstream and downstream of the Farakka barrage on the Ganga River (India), Central Water Commission (CWC) used RCC to protect the erosion of left bank. It is reported that the porcupines were very effective and helped in siltation of the left bank. Aamir and Sharma (2015) have developed a rational design methodology for riverbank protection using RCC porcupines.

1.4 Aim:
To carry qualitative analyses sediment factor with introduction of porcupine system, study the flow condition with the different setup of porcupine system with orientation and assign variation in manning sand “n” value (Roughness coefficient) with the end of porcupine system.

1.5 Objectives:
The objectives of the study are:
To reduce the momentum of flow to encourage sediment riverbank.

To assize the effectiveness of RCC porcupines with the different orientation and density of Porcupine.

1.4 THEORY OF RIVER TRAINING
1.4.1 BACKGROUND
River training is an ageold practice resulting in incessant development and application of human ingenuity to correct vagaries of the rivers. It requires deep and precise study of river mechanism and behavior discussed heretofore. River training has assumed considerable significance in India due to huge annual recurring damage caused by the floods; 80 per cent of which accounts for loss of crops. River training, in its broad aspects, covers all engineering works constructed on a river to guide and confine the flow to the river channel, and to control and regulate the river bed configuration for effective and safe movement of floods and river sediments. In essence, river training envisages training and stabilizing a river within a suitable waterway and along a certain alignment for a variety of purposes. River training works involve large outlays and it is essential to select the type of the training work and materials of construction so as to make optimum utilization of funds, and effective and economical utilization of the available construction.

The various objectives of river training are, (i) To guide the axis of flow at ordinary and low stages and safe passage of floods without overtopping the banks. (ii) To protect the banks from erosion and generally improve their alignment by stabilizing the river channel, (iii) To train the river flow along a safe course, thereby avoiding damage by flooding or erosion of valuable lands, habitation, crops, factories, etc., (iv) To prevent outflanking of a bridge, barrage or weir by directing the flow in a defined stretch of the river, (v) To prevent river from changing its course, (vi) To confine a river channel which has become too wide by swinging from side to side and to reclaim the land from river bed, (vii) To check certain devastations like that of flash torrents, (viii) To trap bed load in areas of superfluous width, (ix) To transport efficiently bed load and suspended sediment load, (x) To provide sufficient depth of flow for safe navigation, (xi) To establish channel boundaries where braiding has created too wide a section divided into small channels separated by island, and (xiii) To correct disorderly banks or flow conditions
3.2. Classification of river training works
3.2.1. Classification Based on Purpose
1.High Water Training: River training aimed at flood protection is called high water training or training for discharge. It envisages provision of adequate waterway for safe passage of maximum flood by proper location, alignment and height of embankments for a given flow discharge without tending to change river bed conditions.

2.Low Water Training: Also termed as training for depth. It envisages to provide adequate water depths during low water periods in the river channel for navigation by concentrating flow in a desired channel and closing other channels by the method of bandalling i.e., contracting the width of the river cannel with the help of groynes, etc.

Mean Water Training: Also termed as training for sediment. It is by far the most important type of river training. It envisages rectification of the river bed configuration and efficient movement of suspended and bed load for maintaining the channel in good condition. The maximum aggrading capacity of a stream occurs in the vicinity of mean water or dominant flood discharge, and as such tends to change the river bed in accordance with that stage of bed flow. Mean water training includes river training for efficient sand exclusion from canals by correcting adverse river curvature to locate the canal off take.

3.2.2 Classification Based on Structure Alignment
Longitudinal structure: These aim at guiding the axis of flow at ordinary and low stages, protecting the banks from erosion, generally to improve their alignment, trapping bed load in the areas of superfluous width, and establishing channel boundaries where braiding has created too wide a section divided into small channels separated by islands. Longitudinal training works are preferred to projecting training works in rivers carrying small bed loads, and with narrow channels having steep sloped and swift currents. In rivers with unstable bed conditions, longitudinal works are susceptible to damage by undermining.

2.Projecting training works. Projecting training works aim to protect the bank from which they project into the river by deflecting the current away from the bank. They are more suitable in rivers with unstable bed conditions. Cross connecting dikes to the bank at intervals add strength to withstand flood action and promote sedimentation in the closed off channel space.

3.3. Principles of river training
Two approximate theories, the theory of tractive force for bed load, and regime theory for suspended load are available for application to problems of river training.

3.3.1. Tractive Force Theory
Tractive force approach is essentially lan tractive force for the incipient conditions of bed movement. The alluvial rivers carry sediments in suspension as well as bed load. The capacity of river channel is designed to cater for the discharge and sediment load by the following formula:
Qs =0.17/m3/4 r2 BR2 S2
Also Q=1/n BR5/ 3 S1/ 2 (Manning’s formula)
Qs = silt bed charge (Cum/s), m = dia. Of sand (mm), r = specific weight,
3.3.2. Regime Flow Theory
In incoherent alluvium, Lacey’s regime the dimensions, although there is a marked divergence when applied to large rivers due to omission of an important parameter, viz., quantity of silt in suspension are important where silt charge in the flow predominates.

3.4. Methods of river training
Planning and design of river training works is done by empirical methods and on the intuitive judgments of engineers. The method of river training applicable depends mainly on the type of river and sediment load, as under. Under certain cases, where river is very unstable, training is almost impossible and the situation has to be met as it arises.

3.4.1. Stable rivers:
Stable rivers are characterized by stability of alignment, slopes and regime as they mould their beds to carry into sea almost all the silt brought down by them. They are amenable to river training to attain ultimate stability with the aid of training works such as spurs and guide banks.

3.4.2. Aggrading rives:
Aggrading rivers have inherent instability and are not equally amenable to river training. Stability cannot be imposed, e.g., bank protection works may either be destroyed by severe erosion or get bur.

3.4.3. Degrading rivers:
Degrading rivers too have inherent instability and not easily amenable to river training. Stability cannot be imposed, e.g. training works may be destroyed by undermining due to bed scour. Gradient control by constructing dams and weirs is a prerequisite to attempting any other modifications in the river. Local training works may be possible if their repercussions on river regime upstream and downstream are not drastic.

3.5. Planning for river training measures
The training measures are planned with due regard to the limitations imposed by the type of river. The various steps involved are as follows:
3.5.1. Alignment: Alignment of training work is generally based on the layout and number of training structures involved. A better way to arrive at radius of curve is to study to photographs of alignment of a stable stretch of river on a similar grade and soil conditions in the vicinity of the erosion site and from this stable section measure the radius of curves. An attempt is the made to develop an alignment with curves of similar radius to those measured. Once the proposed alignment is decided, the methods of control to stabiles the channel to its alignment are considered.

3.5.2. Cross section: The cross section is that built by the river itself, i.e., when the channel is uniform and does not show signs of accretion of retrogression. The proposed section is either by river contraction or by river diversion and is compared with the normal cross section with regard to gauge and discharge relation, velocity of flow, and total sediment runoff. The section is modified by successive analysis to bring it in line with the normal corss section as far as possible.

3.6. Types of River Training Works
There are 7 types of river training works are as follows.

(1) Embankments
(2)Guide Banks or Bell Bunds
(3)Spurs or Groynes
(4)Impermeable Groynes
(5)Permeable Groynes
(6)Bed Pitching and Bank Revetment and
(7)Dredging of River.

3.6.1.Embankments:
The floods may be prevented from submerging the country by constructing earth embankments. They are generally constructed up to a height of 12 m. They are designed and constructed in the same way as an earth dam. The embankments are generally constructed parallel to the river channel.

Depending upon the position of the embankments subdivisions made are:
i. Marginal embankments or dykes or levees,
ii.Retired embankments.

The marginal embankments are constructed as close to the banks as possible to restrict the flood water from submerging the area behind them. Position of marginal embankments.

Figure: 3.1. Marginal embankment or levees

They are designed to hold up the water up to a maximum anticipated HFL without the possibility of overtopping and with a view to withstand all external pressures. This condition is met with by providing sufficient freeboard, bed width, top width and stone protection on adequate slopes.

As the height of the embankment increases it becomes necessary to provide key treach, zoned section etc., to make the embankment stable. Like earth dams embankments are also likely to fail due to overtopping, piping, rat holes, seepage and caving in of river side sloping face. It is therefore necessary to adopt adequate sections for various heights.

The following sections are generally adopted for various heights.

Figure: 3.2 Sections of marginal embankment

3.6.1.1. Advantages of Embankments:
(i)They are very widely used river training work.

(ii)It is cheaper and quick as well as simple in construction. They can be constructed with locally available material.

(iii)Maintenance of embankments is similar to canal bank maintenance and does not involve intricate methods.

(iv)Embankment can be constructed reach by reach to extend extent of protection.

(v)They protect large areas by comparatively small investment.

3.6.1.2. Disadvantages of Embankments:
(i) By restricting the waterway it raises the flood levels.

(ii)Unpredictable flood flows attack the embankment and hence chances of its failure are quite high.

(iii)During flood constant vigil is required on the embankments. It increases cost of maintenance.

(iv)They interfere in laying irrigation canal system and also reduce cultivable area.

Retired embankments are constructed at a distance from the river banks. Thus retired embankments are the intermediate type between the case of marginal embankments and river with no embankments. Retired embankments are generally constructed on a lower ground away from the banks.

Though they are costly due to increased height and risky, they have some mentionable advantages:
i. They do not interfere in the process of raising of the ground by deposition of silt.

ii.They make it possible to store more water for longer period.

iii.They provide wider waterway in times of high floods.

3.6.2. Guide Banks or Bell’s Bunds:
Rivers in flood plains submerge very large areas during flood periods. Naturally when some structure is to be constructed across such a river (for example, bridge, weir, etc.), it is very expensive to construct the work spanning whole width of the river. To economies some training work may be constructed to confine the flow of water within a reasonable waterway.

Guide banks are meant for guiding and confining the flow in a reasonable waterway at the site of the structure. The design of the guide banks is based on the theory developed by Mr. Bells. Hence, guide banks are also known as Bell’s bunds. This river training work has been devised from a study of the natural river channel in alluvial reach.

The river has a tendency to meander over large width of low lying land thereby flooding it occasionally. But it was observed that the same stream passes through narrow and deep sections where high and stiff permanent banks are available on either side without appreciable afflux or abnormal velocity.

The guide banks guide the river flow past a bridge or any other hydraulic structure without causing damage to the work and its approaches. The guide banks are constructed parallel or approximately parallel to the direction of flow. They extend both upstream and downstream of the abutments of the hydraulic structure. The guide banks may be provided on either side of the hydraulic structure or on one side as required. The guide banks consist of four parts mainly:
3.6.2.1. Selection of Site and Section of Guide Banks
The site for guide banks should be selected in such a way that there is no side channel flowing parallel to the guide banks. The side channel if present may breach the approach embankment. The guide banks should be so designed that no swirls are produced.

The top width of bank should not be less than 3 m. Side slopes should be 2:1 and free board 1.25 to 1.50 meters. While providing the free board due weightage should be given for heading up of the water and also for settlement of banks (generally 10 per cent of height). The inside slope should be protected with stone pitching and outside slope with good earth.

The waterway is given by Lacey’s regime perimeter formula:
Pw = 4.825 Q1/2
Where- Pw is waterway in metres and
Q is discharge in cubic metres/sec. The length of upstream part of the guide bank should be 10 per cent more than the length of a bridge or any other structure between the abutments. The length of downstream part of the guide bank should be 1/5 of the structure.

3.6.2.2. Dimensions of Guide Banks
The radius of curvature of the upstream curved head should be such as not to cause intense eddies. The radius of downstream curved head may be kept half that of upstream curved head. The heads should be curved well round to the back of the guide bank. Upstream curved head generally subtends an angle from 120° to 145° to the centre and downstream head from 45° to 90°. The upstream curved head is also called “Impregnable head”.

Figure: 3.3 Dimensions of guide banks

To protect the face of the guide bank at the river bed level a thick stone cover is laid on the bed. It is called an apron. When the scour undermines the river bed the apron comes down or launches to cover the face of the scour. Hence it is called Launching apron also. The quantity of stone in the apron should be adequate to insure complete protection of the scoured face. Figure shows the details of a guide bank. After launching, the apron does not remain uniform in thickness.

3.6.2.3. Details of Guide Bank:
Generally apron thickness is kept 1.25 times thickness of the pitching. For rivers in which deep scour is likely to take place thickness of the apron may be increased to 1.5 times.

Figure: 3.3 Details of Guide bank

3.6.3. Spurs or Groynes:
Spurs are the structures constructed transverse to the river flow. They extend from the bank into the river. These structures are known by several names, viz, groynes, spurs, spur dykes, transverse dykes and constitute probably the most widely used training works. Spurs can be classified as in table-3.1. figure shows the various types of spurs in practice.
Spurs or Groynes serve following purposes:
They protect the river bank by keeping the flow away from it.

They create still pond along a particular bank with the aim of silting up the area in the vicinity.

They train the river to flow along a desired course by attracting, deflecting or repelling the flow.

They contract the wide river channel for improving the navigation depth.

Table: 3.1 Classifications of Spurs
Sr. no. Types of spur Basis of classification
1. Permeable or non-permeable Mode of construction
2. Submerged or non-submerged Height of spur below high water levels
3. Attracting, repelling, deflection or sedimenting Spur action and its effect on flow condition
4. Denehy’s T-headed, Hockey, Inverted Hockey, Straight, L-type, Kinked etc. Special types based on shape, geometry etc.

5. Slotted, sloping 4. Special type
For example, Denehy’s “T” headed groynes. Hockey spurs, etc. When a river is to be confined to a definite channel impermeable type of groyne is most suitable. For excessively silt-laden rivers permeable groynes are suitable. The groynes may be used singly or in series or in combination with other training work depending upon the problem in hand.

When training or protection is to be given over a long and straight river reach groynes are used in series. Spacing of 2 to 2.5 times the length of groynes is a general practice. In a curved reach river can be trained by limited number of spurs. They can also be used in combination with other training measures.

3.6.4. Impermeable Groynes:
The groynes may be aligned either perpendicular to the bank or inclined, pointing upstream or downstream. When a groyne points upstream then it is called a repelling groyne. The reason being, this type has a property of repelling the river flow away from the bank .This is accomplished by creation of a still pond on the upstream. Obviously the river starts following beyond the still pond and in the process the river flow goes away from the bank.

Figure: 3.5. Repelling Groynes

On the contrary, when a groyne points downstream it is called an attracting groyne as. It attracts the river flow towards the bank from which it takes off.

Figure: 3.6 Defecting groynes

After successfully conducting model experiments various designs for groyne heads have been evolved. A groyne with head normal to the groyne direction of called ‘T’ headed groyne.

Figure: 3.7 Special type of spurs or groyne

From this it is clear that deflecting, repelling, attracting, T headed, hockey type, etc., all come under the impermeable type of groynes. The section of groyne is like a guide bund or an embankment.It is protected on both sides by stone pitching or concrete blocks etc. At the river bed launching apron is also provided. Top of spur is generally kept 3 m wide. Side slopes of 2:1 is general practice. The spurs are built by sand, gravel and boulders.

3.6.5. Permeable Groynes
Common type of permeable groynes are tree groynes and pile groynes. They are temporary in nature and get washed away during floods. Therefore they are constructed every time before floods. A tree groyne consists of a thick wire rope (2.5 cm dia) firmly anchored at one end to the bank and tied at the other to a heavy buoy. Sometimes this wire may be stretched across the river and anchored at its ends. It may be supported at intermediate points on tripods.

Entire leafy trees are taken and about 30 cm up the stem a hole is drilled through each tree. Then an iron ring is drawn through the whole and attached to the wire rope. Dimensions of trees may vary from 6 to 12 m in height and 0.50 to 1.2 m in girth.

A pile groyne consists of a series of piles driven 6 to 9 m into the bed 2.5 m to 3 m apart. There may be two or three rows. The rows are spaced 1 to 2 m apart. Each row is closely intertwined by brushwood branches. For stability upstream row is braced to the downstream row by transverse laterals and diagonals.

The permeable groynes lower the velocity of flow. As a result sedimentation occurs. Hence permeable groynes may be said to be of segmenting type according to the function served. The cost of construction of this type is about 40 per cent that of impermeable type of same length. This type of groynes may be constructed even if there is flow in the river. Thus construction is easy and rapid.

Figure: 3.8 Details of spur or groyne

To summarise, the factors which influence the choice and design of groynes are:
i. Fall and velocity of flow in the river.

ii.Character of bed load carried by the river.

iii.Depth of waterway, maximum HFL and nature of flood hydrograph.

iv.Width of waterway, at high water, low water, and mean water.

v.Availability of funds and construction materials.

3.6.6. Bed Pitching and Bank Revetment
Sometimes to protect the bed and bank against action of water, protection is provided by laying a closely packed stone blocks or boulders or even concrete blocks. This permanent revetment and pitching counteracts the general tendency of the water to notch away the material from bed and banks.

3.6.7. Dredging of River
To improve navigability of the river channel the river section may need to be excavated.

Post Author: admin

x

Hi!
I'm Chloe

Would you like to get a custom essay? How about receiving a customized one?

Check it out