RIVER TRAINING STRUCTURES: - GROYNES

By

Arun Lila Mahendra Chaudhari Minimol Korulla

Design Engineer Design Engineer GM Design

Maccaferri Environmental Solutions Pvt. Ltd.

ABSTRACT

River training is the stabilization of the channel in order to maintain the desired cross section and alignment.

Training structures are then necessary in order to protect the channel against the changes that occur due to this

disturbance. River training has assumed considerable significance in India due to huge annual recurring damage

caused by the floods. The paper attempts to highlight the requirement of Groynes, followed by detail design

consideration such as planview shape and cross section of Groynes, length and spacing of Groynes, orientation and

permeability of the Groynes, as per International and Indian standards. This paper also emphasize on the scour

protection work near the Groynes due to localized scour around the Groynes as per Indian standards. A working

example of installation of Groynes using Gabion as a construction material is presented to understand the design

procedure. The design instructions given in this paper must be used as guide line and not interpreted as a strict code

of practice.

Key Words: River training work, Groynes, Gabions

1. INTRODUCTION

River training is the stabilization of the channel in order to maintain the desired cross section and alignment. The

practice of training a river dates back to the sixteenth century where the Yellow River in China was trained by

building embankments along its banks so that the flow would be confined to a single deep channel, which would

transport the sediment load to the sea. Modern river training practice, however, started in Europe in the nineteenth

century, driven by the demands of the industrial revolution for the purpose of maintaining sufficient channel depth

and a better course for navigation.

In general, the objectives of river training may be summarized as:

� To increase the safety against flooding by accommodating the flood flow

� To improve the efficiency of the sediment transport

� To minimize bank erosion by stabilizing the course of flow

� To direct the flow to a desired river stretch

� To reduce the probability of meandering

� And in most of the cases the primary objective of river training is to improve navigation by maintaining

channel depth

Natural processes and human interference may disturb the equilibrium between the sediment load contributed to the

channel and the transport capacity of the flow. Seasonal variations in the flow, dredging of the river, construction of

a reservoir, and deforestation in the catchment area are all examples of causes of disturbance. Training structures are

then necessary in order to protect the channel against the changes that occur due to this disturbance. They could be

classified into:

I. Longitudinal structures

II. Transverse structures

In this paper, the focus will be only on the Groynes as a transverse structure.

2. TRANSVERSE STRUCTURES (GROYNES)

Groynes are structures constructed at an angle to the flow in order to deflect the flowing water away from critical

zones. They are made of stone, gravel, rock, earth, or piles, beginning at the riverbank with a root and ending at the

regulation line with a head. They serve to maintain a desirable channel for the purpose of flood control, improved

navigation and erosion control.

Functions of Groyne

(1) Prevention of river bank erosion:- In particular, a strong flow in a bight of a river causes sediment in

environs to move, which brings out problems related to erosion. The main object of installation of Groyne in

rivers is to prevent the breaking of a bank caused by sediment erosion as a flood.

(2) Flow Control:- There are two main effects controlling flow, expected by installation of a bank. The first

effect is that an existing thalweg can be led to other direction, so the direction of flow would be controlled.

Then another is as the strong velocity of a river bank in a bight can be reduced, a flow would be

delayed. These effects controlling flow cause a flow to concentrate affect maintenance of the depth of water

for navigation and reduce the velocity of a river bank. As a result, a river bank would be protected.

(3) Improvement of ecological environment and scenery:- A Groyne is a protection technique which can

protect a bank and improve scenery in environs at the same. In particular, as the velocity in a Groynes field

would be reduced enough compare to a main stream. According to this phenomenon, not only various

habitats but also a refuge as a flood for fishes and microorganisms are provided. In short Groynes contribute

to create ecological environments.

2.1 TYPES OF GROYNES

Various types of Groynes can be distinguished according to their construction, action on stream flow and

appearance. Beckstead (1975), (as reported by Przedwojski et al. 1995) considers the following, necessary for a full

description of Groynes:

I. Classification according to the method and materials of construction: Groynes may be permeable

allowing the water to flow through at reduced velocities or impermeable blocking and deflecting the

current. Permeable Groynes are fabricated from piles; bamboo or timbers whereas impermeable

Groynes also called solid Groynes, are constructed using rock, gravel, or gabions.

II. Classification according to submergence: Groynes may be designed either as submerged or as non-

submerged. Which of the two types will be used is dictated by the design conditions. Usually

impermeable Groynes are designed to be non-submerged since flow over the top of solid Groynes may

cause severe erosion along the shanks. For submerged conditions, on the other hand, permeable

Groynes may be designed owing to the fact that they disturb the flow much less than solid Groynes.

III. Classification according to the action on the stream flow: Groynes may be classified as attracting,

deflecting or repelling Groynes. Attracting Groynes point downstream, they serve to attract the stream

flow towards themselves and do not repel the flow towards the opposite bank. Deflecting Groynes are

generally short ones and used for local protection. They serve to change the direction of flow without

repelling it. Repelling Groynes point upstream. They serve to repel the flow away from themselves.

IV. Classification according to their appearance in plan: Groynes may be built with different plan view

shapes. Examples are straight Groynes, T-head, L-head, hockey shaped, inverted hockey shaped

Groynes, straight Groynes with pier head, wing, or tail Groynes.

2.2 DESIGN OF GROYNES

The most important considerations involved in Groynes design are plan view shape, length of the Groynes, spacing

between Groynes, orientation to the flow, crest elevation and slope, cross-section, construction materials and scour;

Alvarez (1989), Richardson et al. (1975), and Przedwojski et al.(1995).

The purpose of Groynes design to ensure a set of structures which

� Are of a length and height to maximize their effectiveness

� Are located and oriented to maximize their effectiveness

� Are able to withstand bed scour adjacent to the structures and

� Have sufficient structural strength to withstand hydraulic and debris forces without failure

Firm design rules for Groynes not exist. The range of conditions, functions and construction material means that

design becomes a judgmental process which must rely heavily on the experience and common sense of the designer.

The design instructions given in this paper must be used as guide line and not interpreted as a strict code of practice.

Flexibility in the design process can be lead to innovative implementations and major cost saving without sacrificing

effectiveness. Providing the major principle are recognized and incorporated.

2.2.1 PLANVIEW SHAPE

Of the above mentioned types of Groynes according to their appearance in plan view, the straight Groynes is set at

an angle from the bank and has a rounded head to provide extra volume and area for scour protection at the outer

end. The T-head Groynes is normally set at a right angle from the bank and it has a straight shank with a rectangular

guide vane at the outer end. L-head, wing or tail Groynes have larger sediment deposits between Groynes, less scour

at their head, provide greater protection to the banks and are more effective in channelization for navigation when

the length closes 45 to 65 percent of the gap between Groynes. Hockey-shaped Groynes have scour holes that are

more extensive in area than the T-head Groynes.

2.2.2 LONGITUDINAL EXTENT & LENGTH OF THE GROYNES

The longitudinal extent of channel bank requiring protection is discussed in Brown (1985)(1,3) .

Figure 1 was

developed from USACE(1981)(11)

studies of the extent of protection required at meander bends. The minimum

extent of bank protection determined from Figure 1 should be adjusted according to field inspections to determine

the limits of active scour, channel surveys at low flow, and aerial photography and field investigations at high flow.

Figure:- 1. Extent of protection required at a channel bend (after USACE (1981)(11)

)

Groynes length depends on the location, purpose, spacing, and economics of construction. The total length of the

Groynes includes the anchoring length, which remains embedded in the bank, and the working length, which stays

in the flow. The length can be established by determining the channel width and depth desired. The working length

is usually kept between the lower and upper limits of the mean depth and a quarter of the mean width of the free

surface respectively. The anchoring length on the other hand is recommended to be less than a quarter of the

working length.

The length of both permeable and impermeable Groynes relative to channel width affects local scour depth at the

Groynes tip and the length of bank protected. Laboratory tests indicate that diminishing returns are realized from

Groynes lengths greater than 20 percent of channel width. The length of bank protected measured in terms of

projected Groynes length is essentially constant up to Groynes lengths of 20 percent of channel width for permeable

and impermeable Groynes. Field installations of Groynes have been successful with lengths from 3 to 30 percent of

channel width. Impermeable Groynes are usually installed with lengths of less than 20 percent while permeable

Groynes have been successful with lengths up to 25 percent of channel width. However, only the most permeable

Groynes were effective at greater lengths.

As per IS 8408 : 1994 the Groynes length should not be less than required to keep the scour hole formed at the nose

away from the bank. Thus assuming angle of repose of sand to be 2.5H : 1V & anticipated maximum depth of scour

below bed be ds., the length should be more than 2.5ds. Normally the effective length of Groynes should not exceed

1/5th

of width of the flow in case of single channel. In case of wide, shallow & braided rivers, the protrusion of the

Groynes in the deep channel should not exceed 1/5 th

of the width of the channel on which the Groynes is proposed

excluding the length over the bank.

2.2.3 SPACING BETWEEN GROYNESS

The spacing between Groynes is measured at the riverbank between their starting points. It is related to river width,

Groynes length, velocity of flow, and angle to the bank, orientation to the flow, bank curvature, and purpose.

However, it is often expressed as a multiple of the Groynes length. Richardson et.al. (1975) recommends a spacing

of 1.5 to 6 times the upstream projected Groynes length into the flow. In order to obtain a well defined deep channel

navigation, to keep a spacing of 1.5 to 2 times the Groynes length is recommended, whereas for bank protection the

ratio of spacing to Groynes length is less and distances from 2 to 6 times the Groynes length are generally used,

although there exists successful examples of bank protection with short Groynes spaced apart 10 to 100 times their

length where the banks are protected with riprap or vegetation. If the spacing between Groynes is too long, a

meander loop may form between Groynes. Long and far apart spaced Groynes may contract the flow resulting in

channel degradation and bank erosion, and cause a hindrance to navigation. If the Groynes are spaced too close

together on the other hand, construction costs will be higher and the system would work less efficiently without

making best use of each individual Groynes.

Figure:- 2. Relationship between Groynes length and expansion angle for Groynes permeability (Brown (1985)(2)

)

Groynes spacing is a function of Groynes length, Groynes angle, permeability, and the degree of curvature of the

bend. The flow expansion angle, or the angle at which flow expands toward the bank downstream of a Groynes, is a

function of Groynes permeability and the ratio of Groynes length to channel width. This ratio is susceptible to

alteration by excavation on the inside of the bend or by scour caused by the Groynes installation. Figure 2 indicates

that the expansion angle for impermeable Groynes is an almost constant 17°. Groynes with 35 percent permeability

have almost the same expansion angle except where the Groynes length is greater than about 18 percent of the

channel width.

Groynes spacing in a bend can be established by first drawing an arc representing the desired flow alignment (Figure

3). This arc will represent the desired extreme location of the thalweg nearest the outside bank in the bend. The

desired flow alignment may differ from existing conditions or represent no change in conditions, depending on

whether there is a need to arrest erosion of the concave bank or reverse erosion that has already occurred. If the need

is to arrest erosion, permeable retarder Groynes or retarder structures may be appropriate. If the flow alignment must

be altered in order to reverse erosion of the bank or to alter the flow alignment significantly, deflector Groynes or

retarder/deflector Groynes are appropriate. The arc representing the desired flow alignment may be a compound

circular curve or any curve which forms a smooth transition in flow directions.

Next, draw an arc representing the desired bankline. This may approximately describe the existing concave bank or

a new theoretical bankline which protects the existing bank from further erosion. Also, draw an arc connecting the

nose (tip) of Groynes in the installation. The distance from this arc to the arc describing the desired bank line, along

with the expansion angle, fixes the spacing between Groynes. The arc describing the ends of Groynes projecting into

the channel will be essentially concentric with the arc describing the desired flow alignment.

Figure:- 3. Groynes spacing in a meander bend (Brown (1985)(2)

)

As per IS 8408 1994 the spacing of the Groynes is normally 2 to 2.5 times its effective length. For site specific cases

model studies may be conducted.

2.2.4 ORIENTATION OF THE GROYNESS

Groynes orientation refers to Groynes alignment with respect to the direction of the main flow current in a channel.

Figure 4 defines the Groynes angle such that an acute Groynes angle means that the Groynes is angled in an

downstream direction and an angle greater than 90° indicates that the Groynes is oriented in a upstream direction.

Groynes may be oriented perpendicular to the flow or be inclined either upstream or downstream. Each orientation

affects the stream in a different way and results in different deposition of sediment in the vicinity of the Groynes. A

Groynes pointing downstream is an attracting Groynes, which attracts the stream flow towards itself. Repelling

Groyne, which repel the flow away, and deflecting Groyne, which deflect the flow away from the bank, point

upstream. A Groyne that is oriented upstream causes more deposition than a perpendicular one at the downstream

bank and also at the area upstream where a reverse eddy is formed and causes suspended load to settle. The amount

of deposition between Groynes is maximized in case of upstream inclination due to their ability to protect bank areas

upstream and downstream of themselves. Therefore, Groynes of this kind are best suited for bank protection and

sedimentation purposes.

Figure:- 4. Definition sketch for Groynes angle (Karaki (1959)).

Groynes that are perpendicular to the flow have protection over a smaller area. Downstream facing Groynes are not

suitable for bank protection purposes due to their attracting effect on the flow. The flow towards the root of the

downstream Groynes threatens the surrounding bank area as well as the Groynes itself. For the purpose of

maintaining a deep channel to improve navigation on the other hand, best performance is obtained by perpendicular

or downstream pointed Groynes. Permeable retarders Groynes are usually designed to provide flow retardance near

the streambank, and they perform this function equally as well without respect to the Groynes angle. Since Groynes

oriented normal to the bank and projecting a given length into the channel are shorter than those at any other

orientation, all retarder Groynes should be constructed at 90° with the bank for reasons of economy.

Figure:- 5. Scour adjustment for Groynes orientation (Richardson et.al. (2001))

No consensus exists regarding the orientation of permeable retarder/deflector Groynes and impermeable deflector

Groynes. There is some agreement that Groynes oriented in an upstream direction do not protect as great a length of

channel bank downstream of the Groynes tip, result in greater scour depth at the tip, and have a greater tendency to

accumulate debris. Groynes orientation at approximately 90° has the effect of forcing the main flow current

(thalweg) farther from the concave bank than Groynes oriented in an upstream or downstream direction. Therefore,

more positive flow control is achieved with Groynes oriented approximately normal to the channel bank. Groynes

oriented in an upstream direction cause greater scour than if oriented normal to the bank, and Groynes oriented in a

downstream direction cause less scour.

It is recommended that the Groynes furthest upstream be angled downstream to provide a smoother transition of the

flow lines near the bank and to minimize scour at the nose of the leading Groynes. Subsequent Groynes downstream

should all be set normal to the bank line to minimize construction costs. Figure 5 can be used to adjust scour depth

for orientation. It should be noted that permeability also affects scour depth. A method to adjust scour depth for

permeability is presented in the following section.

The lateral extent of scour can be determined from the depth of scour and the natural angle of repose of the bed

material the expansion angle downstream of Groynes, i.e., the angle of flow expansion downstream of the

contraction at the Groynes is about 17° for impermeable Groynes for all Groynes angles. The implication is that

Groynes orientation affects the length of bank protected only because of the projected length of the Groynes along

the channel bank.

2.2.5 GROYNES PERMEABILITY

The permeability of the Groynes depends on stream characteristics, the degree of flow retardance and velocity

reduction required, and the severity of the channel bend. Impermeable Groynes can be used on sharp bends to divert

flow away from the outer bank. Where bends are mild and only small reductions in velocity are necessary, highly

permeable retarders Groynes can be used successfully. However, highly permeable Groynes can also provide

required bank protection under more severe conditions where vegetation and debris will reduce the permeability of

the Groynes without destroying the Groynes. This is acceptable provided the bed load transport is high.

Scour along the stream bank and at the Groynes tip are also influenced by the permeability of the Groynes.

Impermeable Groynes, in particular, can create erosion of the stream bank at the Groynes root. This can occur if the

crests of impermeable Groynes are lower than the height of the bank. Under submerged conditions, flow passes over

the crest of the Groynes generally perpendicular to the Groynes as illustrated in Figure 6. Laboratory studies of

Groynes with permeability greater than about 70 percent were observed to cause very little bank erosion, while

Groynes with permeability of 35 percent or less caused bank erosion similar to the effect of impermeable Groynes.

Richardson et. al (2001)

Figure:- 6. Flow components in the vicinity of spurs when the crest is submerged (Brown (1985)(2)

)

Permeability up to about 35 percent does not affect the length of channel bank protected by the Groynes. Above a

permeability of 35 percent, the length of bank protected decreases with increasing permeability. Figure 7 shows the

results of laboratory tests of the effects of permeability and orientation on the expansion angle of flow downstream

of Groyness. For this figure, Groynes lengths were 20 percent of the channel width projected normal to the bank.

(Brown (1985)(2)

)

From the above discussion, it is apparent that Groynes of varying permeability will provide protection against

meander migration. Impermeable Groynes provide more positive flow control but cause more scour at the toe of the

Groynes and, when submerged, cause erosion of the stream bank. High permeability Groynes are suitable for use

where only small reductions in flow velocities are necessary as on mild bends but can be used for more positive flow

control where it can be assumed that clogging with small debris will occur and bed load transport is large. Groynes

with permeability up to about 35 percent can be used in severe conditions but permeable Groynes may be

susceptible to damage from large debris and ice.

Figure:- 7 Groynes permeability and spur orientation vs. expansion angle (Brown (1985)(2)

)

2.2.6 CREST ELEVATION, SLOPE & CROSS SECTION OF GROYNES

The crest elevation of Groynes depends on the purpose and possible problems due to overbank flow. For bank

protection, the crest should be at least as high as the bank. To avoid overtopping the crest elevation should be higher

than the expected levels of water. Crests may be either level or sloping downwards from the bank towards the end of

the Groynes (Figure 8).

Figure:- 8 Groynes Crest Sloping and Level (Courtesy of Maccaferri )

For bank protection, sloping-crested Groynes are recommended by Alvarez (1989) with a slope of 0.1 to 0.25 due to

their advantages of reducing scour at the Groynes end, less material needed for construction, faster deposits of

sediment between Groynes. For navigation channel control, level crested Groynes work best normal to the flow or

angled downstream, whereas, sloping crested Groynes work best normal or angled upstream, Richardson et al

(1975). The crest width ranges from 1to 6m and side slopes from 1:1.25 to 1:5. The minimum crest width of 1m is

controlled by the equipment placing the Groynes and wider crests make placing easier.

Impermeable Groynes are generally designed not to exceed the bank height because erosion at the end of the

Groynes in the overbank area could increase the probability of outflanking at high stream stages. Where stream

stages are greater than or equal to the bank height, impermeable Groynes should be equal to the bank height. If flood

stages are lower than the bank height, impermeable Groynes should be designed so that overtopping will not occur

at the bank. Bank erosion is more severe if the Groynes are oriented in the downstream direction. The crest of

impermeable Groynes should slope downward away from the bank line, because it is difficult to construct and

maintain a level Groynes. Use of a sloping crest will avoid the possibility of overtopping at a low point in the

Groynes profile, which could cause damage by particle erosion or damage to the stream bank. Groynes permeability

and Groynes orientation vs. expansion angle (Brown (1985)(2)

). Permeable Groynes, and in particular those

constructed of light wire fence, should be designed to a height that will allow heavy debris to pass over the top.

However, highly permeable Groynes consisting of jacks or tetrahedrons are dependent on light debris collecting on

the Groynes to make them less permeable. The crest profile of permeable Groynes is generally level except where

bank height requires the use of a sloping profile.

In general, straight Groynes should be used for most bank protection. Straight Groynes are more easily installed and

maintained and require less material. For permeable Groynes, the width depends on the type of permeable Groynes

being used. Less permeable retarder/deflector Groynes which consist of a soil or sand embankment should be

straight with a round nose. The top width of embankment Groynes should be a minimum of 1 m (3 ft.). However, in

many cases the top width will be dictated by the width of any earth moving equipment used to construct the

Groynes. In general a top width equal to the width of a dump truck can be used. The side slopes of the Groynes

should be 1V:2H or flatter.

As per IS 8408 1994: the top width of Groynes should be 3 to 6m as per requirement the slopes of the side & nose

the Groynes would be 2:1 to 3:1 depending upon the material used.

2.2.7 SCOUR PROTECTION FOR THE GROYNES

River training works should be designed to resist scour, in particular erosion of the bed adjacent to the river training

structure. For Groynes scour can be localized, general or a combination of both. The expected scour near the

structure during the construction and during the service is one of the most important aspects to consider during

design. Most failures of the river training structures result from an underestimation of depth of scour. Therefore the

expected scour depth should be taken into consideration in the determination of the base depth of the Groynes.

Scour Apron should be placed on the upstream and downstream faces as well as on the nose of the Groynes to

inhibit erosion of the Groynes.

The depth of scour for different portions of Groynes can be adopted as per IS 8408: 1994

o Nose 2.0 D to 2.5 D

o Transition from nose to shank and first 30 to 60m in upstream 1.5D

o Next 30 to 60M in upstream 1.0 D

o Transition from nose to shank and first 15 to 30m in downstream 1.0 D

Where, D = The depth of scour below HFL estimated using Lacey’s formula,

In which

D = 0.473 (Q/f)1/3

------------------------------Eqn 1

Where, Q = discharge in cum/sec, and

f = silt factor = 1.76 d1/2

, Where d is the mean diameter of river bed material in mm.

When the discharge intensity is known, following formula may preferably used

D = 1.33 (q2/f)

1/3 ------------------------------Eq

n 2

Where, q=intensity of discharge in cum/sec/m.

Shape and Size of Launching Apron as per IS 8408: 1994

o Nose at upstream 1.5 Dmax

o Transition from nose to shank and first 30 to 60m in upstream 1.5 Dmax

o Next up to upstream bank line Nominal or No Apron

o Nose at downstream 1.5 Dmax

o Transition from nose to shank and first 15 to 30m in downstream 1.0 Dmax

o Next up to downstream bank line Nominal or No Apron

Detailed plan & Sections drawings of straight Groynes as per IS 8408:1994 is shown in Appendix:-1

Thickness of Loose stone Pitching as per IS 8408: 1994

Thickness of pitching should be equal to two layers of stones determined for velocity from equation given below in

the case of free dumping stone

( )6

31

02323.0V

SK

SW

s

s

−= ------------------------------Eq

n 3

Where

21

2

2

sin

sin1

−=

φ

θK ------------------------------Eq

n 4

,

W – Weight of stone in Kg, Ss – Specific gravity of stones, θ - Angle of sloping bank, φ - Angle of repose of

protection material, V – Velocity on m/s.

Thickness of protection layer should be checked for negative head created due to velocity from the following

formula

( )12

2

−=

sSg

VT ------------------------------Eq

n 5

Where, V – Velocity in m/sec, T – Thickness in m, Ss – Specific gravity of stones

Thickness of Wire Crates or Gabion as per IS 8408: 1994

In case of crates, the mass specific gravity of the crate is required to be worked out to account for the porosity. For

estimating it an empirical relation between void ratio and mean diameter of stone in mm is as follows:

( ) 21.0

50

864.0245.0

De += ------------------------------Eq

n 6

D50 – Mean diameter of stone used in crates in mm.

The stone size should be larger than opening of the crates. The mass specific gravity of the crates can be worked out

from the formula given below

Sm = (1-e)Ss ------------------------------Eqn 7

For working out the volume of crates Sm should be used instead of Ss in Eqn 3. the shape of the crates as far as

possible cubical. Crates should be of made of G.I wire of adequate strength and should be with double knots, should

be PVC coated as per International standards.

The thickness of crates or Gabions is decided on the basis of the Eqn 5 the condition that the mass of each crate

should not be less than that determined on the basis of velocity consideration in Eqn 3 for Gabions or crates.

2.2.8 DESIGN EXAMPLE OF GROYNE INSTALLATION

We have assumed a location at which a migrating bend threatens an existing bridge as shown in Figure. 9. Existing

bank line conditions are shown with a solid line. Ultimately, based upon the following design example, seven

Groynes will be required. Although the number of Groynes is not known in advance, the Groynes (and other design

steps) are shown as dashed lines on Figure 9 as they will be specified after completing the following design

example. We have assumed that the width of the river from the desired (North) bank line to the existing bank line is

50m and the other river flow parameters are assumed and given in appendix 2.

Objectives of Groynes installation:-

• To establish a different flow alignment & to reverse erosion of concave (outer side) bank.

• To stop migration of the meander before it damages the highway stream crossing.

• To reduce scour at the bridge abutment & piers by aligning flow in the channel with the bridge opening.

Step 1. Type of Groynes

Impermeable deflector Groynes are suitable to accomplish these objectives and the stream regime is favorable for

the use of this type of countermeasure. It has been assumed that for this particular site the conditions are favorable

for the use of Gabions as a construction material. It has been desired to achieve the 30% porosity of the Gabions,

indeed will allow the water to pass through it and will help in protecting downstream bed. For the present scenario

as per plan view appearance the straight Groynes is set an angle from the bank and has a rounded head to provide

extra volume and area for scour protection at the other end.

Step 2. Length and Expansion angle of Groynes

As per IS 8408:1994 the effective length of Groyne should not exceed the 1/5th

(20 percent) of the desired river

width. Therefore for the present example the effective length of Groyne is = 1/5 x 50 = 10 m. The expansion angle

for this Groyne type is approximately 17° for a Groyne length of about 20 percent of the desired channel width, as

indicated in Figure 2.

Figure 9. Example of Groynes design.

Step 3. Sketch Desired Thalweg

The third step is to sketch the desired thalweg location (flow alignment) with a smooth transition from the upstream

flow direction through the curve to an approach straight through the bridge waterway (Figure 9). Visualize both the

high-flow and low-flow thalwegs. For an actual location, it would be necessary to examine a greater length of

stream to establish the most desirable flow alignment. Then draw an arc representing the desired bank line in

relation to thalweg locations. The theoretical or desired left bank line is established as a continuation of the bridge

abutment and left bank downstream through the curve, smoothly joining the left bank at the upstream extremity of

eroded bank .

Step 4. Sketch Alignment of Groyne Tips

The forth step is to sketch a smooth curve through the nose (tip) locations of the Groynes, concentric with the

desired bankline alignment. Using a guideline of 1/5th

(20%) of the desired channel width for impermeable Groynes

(see Section 2.2.2) the distance, L, from the desired bankline to the Groyne tips (Figure 9) would be:

The effective length of Groynes = 1/5 x 50 = 10 m

Step 5. Locate First Groyne

Step number five is to locate Groyne number 1 so that flow expansion from the nose of the Groyne will intersect the

stream bank downstream of the abutment. This is accomplished by projecting an angle of 17° from the abutment

alignment to an intersection with the arc describing the nose of Groynes in the installation or by Eqn 8. Groynes are

set at 90° to a tangent with the arc for economy of construction. Alternatively, the first Groyne could be considered

to be either the upstream end of the abutment or guide bank if the Groyne field is being installed upstream of a

bridge. Thus, the Groyne spacing, S, would be:

S=L x cot θ=10 x cot17° = 33m ---- Eqn 8

As per IS 8408: 1994 the minimum spacing of Groyne should be 2 to 2.5 times of its effective length

, therefore Minimum S = 10 x 2.5 = 25 m, Hence 33m spacing is ok.

It may be desirable to place gabion mattress protection on the stream bank at the abutment. Furthermore, the size of

the scour hole at the Groyne directly upstream of the bridge should be estimated. If the extent of scour at this

Groyne overlaps local scour at the pier, total scour depth at the pier may be increased. This can be determined by

extending the maximum scour depth at the Groyne tip, up to the existing bed elevation at the pier at the angle of

repose.

Step 6. Locate Remaining Groynes

Groynes upstream of Groyne number 1 are then located by use of Eqn 8, using dimensions as illustrated in Figure 3

(i.e., the spacing, S, determined in Step 5). Using this Groyne spacing, deposition will be encouraged between the

desired bank line and the existing eroded bank.

The seventh and last Groyne upstream is shown oriented in a downstream direction to provide a smooth transition of

the flow approaching the Groyne field. This Groyne could have been oriented normal to the existing bank, and been

shorter and more economical, but might have caused excessive local scour. Orienting the furthest upstream Groyne

at an angle in the downstream direction provides a smoother transition into the Groyne field, and decreases scour at

the nose of the Groyne. Last Groynes should be anchored well into the bank to prevent outflanking.

Step 6. Design of Scour protection work using Gabions as per IS 8408:1994

Scour depth & scour apron length for different parts of Groynes is calculated as per section 2.2.7. The thickness of

the Gabion mattress is calculated on the Groynes slope on river bed. The calculation & supported assumed data for

the presented working example is shown in appendix 2. Detailed plan of the Groyne as per design is shown in

appendix 3, also three alternatives of Groynes cross section using Gabions has been shown in the appendix 3. The

selection of suitable alternative depends upon important of structure, cost, construction viability, availability of stone

& other geotechnical, environmental & hydraulics stability.

2.2.9 SUMMARY AND CONCLUSION

1. Among the different types of structures for river Groynes are most nature friendly, which make them

popular worldwide.

2. Firm design rules for Groynes not exist. The design instructions given in this paper must be used as guide

line and not interpreted as a strict code of practice.

3. To install Groynes properly it is required to take four-steps: the fundamental design, model experiments for

a relevant river, construction, and then the river itself should follow the nature system to create topography

formation.

4. The ecological connectivity of Gabion Groynes increases through the opening in the Gabion Groynes. The

opening allows the better migration of species, Vegetation growth and prevents scouring of downstream

bank.

REFERENCE

1. Alvarez, J. A. M. (1989). “Design of groins and Groynes dikes.” Proceedings 1989 National Conference On

Hydraulic Engineering, New Orleans, 296-301.

2. Brown, S.A., 1985, "Design of Groynes-Type Streambank Stabilization Structures, Final Report," FHWA/RD-

84-101, Federal Highway Administration, Washington, D.C.

3. Brown, S.A., 1985, "Design of Spur-Type Streambank Stabilization Structures, Final Report," FHWA/RD-84-

101, Federal Highway Administration, Washington, D.C.

4. Brown, S.A., 1985, "Streambank Stabilization Measures for Highway Engineers," FHWA/RD-84. 100 Federal

Highway Administration, McLean, VA.

5. Brown, S.A. and E.S. Clyde, 1989, "Design of Riprap Revetment," Hydraulic Engineering Circular No.11,

FHWA-IP-89-016. Prepared for the Federal Highway Administration, Washington, D.C.

6. IS 8408:1994 Planning and Design of Groynes in alluvial river Guidelines.

7. Karaki, S.S., 1959, "Hydraulic Model Study of Groynes Dikes for Highway Bridge Openings," Colorado State

University, Civil Engineering Section, Report CER59SSK36, September, 47 pp.

8. Przedwojski, B. (1995). “Bed topography and local scour in rivers with banks protected by groynes.” Journal of

Hydraulic Research, 33(2), pp. 257-273.

9. Przedwojski, B., Blazejewski, R., and Pilarczyk, K. W. (1995). River training techniques fundamentals, design

and application, A.A. Balkema, Rotterdam.

10. Rajaratnam, N., and Nwachukwu, B. (1983). “Flow near groyne-dike structures.” Journal of Hydraulic Div.,

ASCE, Vol. 109, No. HY3, pp. 463-480.

11. Richardson, E.V., D.B. Simons, and P.F. Lagasse, 2001, "River Engineering for Highway Encroachments -

Highways in the River Environment," Report No. FHWA NHI 01-004, Hydraulic Design Series No. 6, Federal

Highway Administration, Washington, D.C.

12. Richardson, E. V., Stevens, M. A., and Simons, D. B. (1975). “The design of Groyness for river training.”

XVIth , IAHR congress, Sao Paulo, Brazil, 382-388.

13. U.S. Army Corps of Engineers, 1981, "The Streambank Erosion Control Evaluation and Demonstration Act of

1974," Final Report to Congress, Executive Summary and Conclusions.

APPENDIX:-1

PLAN AND SECTION OF GROYNES (As per IS 8408:1994)

APPENDIX:-2

PROTECTION WORK CALCULATION FOR WORKING EXAMPLE AS PER SECTION 2.2.7

ASSUMED DATA

Velocity of Water (V) 3.8 m/s

Discharge (Q) 2000 cum/sec

Desired Width of River (W) 50 m

Discharge Intensity (q) 40 cum/s/m

Angle of Repose of Fill Material ( Ф ) 30 Degree

HFL 4.5 m

Free Board 1 m

Specific gravity of stones (Ss) 2.6

Thickness of Gabion Protection work on Bed for Scour Protection

Assumed Opening of Gabions 100mm x 120 mm

Assumed D50 of stones in Gabions 150 mm

Void ratio (e) From Eqn. 6 0.27517

The mass specific gravity of Gabions (Sm) From Eqn. 7 1.88457

From Eqn. 3 Weight of Stone W= 190.446 Kg

Thickness of Gabion From Eqn. 5 0.46 m

Assume Gabion Mattress of 0.5 m thickness with diaphragms at every 1 m

Volume of Gabion (1 x 1 x 0.5 )= 0.5 cum

Mass of Stone 942.282 Kg

Weight of Stone in each gabion is higher than that computed by Eqn. 3

Hence 0.5 m thick Gabion Mattress can be adopted for the Launching apron on Bed

Thickness of Gabion Protection work on Side Slope of Groynes

Assumed Side slope of Groynes is 2H:1V 26.56 Degree

Slope Factor K From Eqn. 4 0.44753

From Eqn. 3 Weight of Stone W= 425.551 Kg

Thickness of Gabion From Eqn. 5 0.46 m

Assume Gabion Mattress of 0.5 m thickness with diaphragms at every 1 m

Volume of Gabion (1 x 1 x 0.5 )= 0.5 cum

Mass of Stone 942.282 Kg

Weight of Stone in each gabion is higher than that computed by Eqn. 3

Hence 0.5 m thick Gabion Mattress can be adopted for the Launching apron on Bed

Calculation For Depth of Scour

Assumed Silt Factor f = 1

Lacey's Scour Depth (D) From Eqn. 1 15.5558 m

Maximum Scour Depth 11.0558 m

For Nose Portion consider Dmax = 2 D 22.1115 m

Transition from nose to shank and first 30 to 60m (Assumed up to Scour

Depth) in upstream 16.5837 m

Next up to upstream River Bank 11.0558 m

Transition from nose to shank and first 15 to 30m (Assumed up to 50 % of

scour Depth ) in downstream 11.0558 m

Next up to downstream River Bank 11.0558 m

Calculation For Length of Scour Apron

Nose at upstream 33.1673 m

Transition from nose to shank and first 30 to 60m(Assumed up to Scour

Depth) in upstream 24.8755 m

Next up to upstream (Assumed up to Scour Depth) River Bank line 11.0558 m

Nose at downstream 33.1673 m

Transition from nose to shank and first 15 to 30m (Assumed up to 50 % of

scour Depth ) in downstream 11.0558 m

Next up to downstream (Assumed up to Scour Depth) bank line (50% of

Maximum Scour Depth ) 5.5279 m

Check For Length of Groynes

As per Desired Width of River Effective Length 10 m

Total Length of Groynes should be Greater than 2.5 times Scour depth 27.6394 m

Calculation For Top Width of Groynes

Top Width of Groyne 3 to 6 m

Assumed Width of Groynes 3 m

Calculation For Height of Groynes

Top level of Groyne 5.5 m

Spacing of Groynes

Minimum Spacing of Groyne should be 2 to 2.5 times of its effective length

Assumed 2.5 times of Effective length than Spacing S = 25 m

APPENDIX:-3

PLAN VIEW AND SECTION OF WORKING EXAMPLE