Types of River Training Works: 6 Types | Geography

River training works may be directive or protective or both. Types of river training works depend on the purpose to be achieved.

The following are the usual types of river training works:

Type # 1. Guide Banks:

They are used to guide the river to pass through the constrained width of the river at the structure. Guide bank system of river training at bridges and diversion weirs, was first introduced by Mr. J.R. Bell. Hence guide banks are also known as Bell’s Bunds in his honour.

The guide banks are provided in pairs, symmetrical in plan. Both the guide banks may be kept parallel, slightly converging or slightly diverging. Mostly parallel system of guide banks is used. The length of the guide bank may be equal to the lineal waterway of the weir or barrage. In plan guide banks have curved ends. The angle subtended at the centre, by the curved end at U/S side, ranges from 120° to 145° and that at D/S end ranges from 45° to 50°.

If one bank of the river is quite high and stable it can be used as one of the guide banks. In that case only one guide bank will be required to be constructed.

Purpose of Guide Banks:

(i) In order to reduce the cost of construction of diversion works their length has to be reduced. The length of the diversion works is constrained or restricted between the guide banks.

(ii) They confine the river within a reasonable water way.

(iii) They direct the flow in such a manner that ensures safe and expeditious passage through the work.

(iv) They protect the works from being outflanked.

(v) They protect adjacent land from being flooded due to afflux caused by the construction of the work in the river.

The effects of confining the river between the guide banks are the following:

(i) Water level on the U/S side is increased.

(ii) Slope of water surface U/S of the work is reduced.

(iii) Velocity of flow and scouring action between marginal bunds is increased.

(iv) Increase the rate at which the flood wave travels down the stream.

(v) Increase the maximum discharge at all point D/S.

Design Features of Guide Banks:

i. Wetted perimeter in the case of rivers is almost equal to width of the river. The confined width of the river between guide banks may be found out by 4.75 √Q formula.

L = 4.75 √Q

This equation is for the wetted perimeter. Since the width is slightly more than the perimeter, the constrained width of river (L) can be found from following formula-

L = 5 √Q

ii. Total length of guide bank is taken 1.5 to 1.75 times the confined width L.

Total Length of the guide bank = 1.5 L to 1.75 L.

Out of this length 1.25 L to 1.50 L is provided on the U/S side of the weir and 0.25 L on the D/S side.

iii. The shank or straight portion of the bond should have 6 m wide top. Side slopes vary from 2:1 to 3:1 depending upon the type of material.

iv. 1 m to 1.50 free board should be provided.

v. The inside slope should be protected by stone pitching; the usual thickness of pitching varies from 40 cm to 60 cm. The thickness of pitching as recommended by Inglis may be determined from following formula t = 0.60 Q^1/3, where t is thickness in cm and Q is discharge in cumecs.

vi. The radius of the curve at the U/S end of the bank is 150 m to 300 m and central angle varies from 120° to 140°.

Mr. Spring suggested a value of R equal to 180 m to 250 m for rivers having velocities 2.4 m/sec to 3.1 m/sec gales on the other hand suggested a value of 250 m for rivers having flood discharge between 7000 to 20000 cumecs. Sharper curves are permissible for discharge less than 7000 cumecs.

A value of 580 m is recommended for discharge varying from 40000 to 70000 cumecs. The radius may be obtained by interpolation for discharges between 20000 and 40000 cumecs. The value of R can be obtained by relation R = 0.45 L.

Radius of curvature of D/S end varies from 90 m to 240 m and central angle from 45° to 60°.

vii. Maximum scour depth (D) can be calculated from formula-

Scour depth at the bond may be assumed 1.5D to 1.75D.

viii. The toe of the slope should be protected by the Launching apron, the thickness of which is generally kept 1.25 times the thickness of pitching. At places where deep scour is expected, it may be kept 1.5 times the thickness of pitching. The thickness of apron is kept same as pitching at toe. Apron thickness on the U/S curved head is generally kept 1.25 times the normal apron thickness.

Type # 2. Marginal Bunds:

They are also called Levees. They are earthen bonds whose face towards the river is generally pitched. They are constructed parallel to river banks to protect the marginal land from inundation caused by floods. They should be located at the limiting line of meandering of river i.e. they should be beyond the meander belt of river. Effects of levels or marginal bunds are same as those enumerated in case of guide banks.

Advantages of Marginal Bunds:

i. The restricted area of the river causes increased depth of water. This causes improvement in Navigation facilities.

ii. It prevents submergence of the adjoining land during floods.

iii. Since water is made to flow in between the dykes, the velocity of flow gets increased, causing the peak discharge to flow D/S.

iv. They prevent course changing tendencies of the river.

Disadvantages of Marginal Bunds:

i. There is rise in H.F.L.

ii. They require heavy maintenance.

iii. If fail, they inundate large low lying areas thus causing loss of property and life.

iv. They can be easily eroded.

v. The rise in water level increases sub-soil water level also. This makes surrounding areas, water logged.

vi. Spread of silt on the country is prevented.

Design Features of Marginal Bunds:

a. Spacing and Height:

Both of these elements are interdependent. If levees are located closer their height has to be more. If reverse is the case i.e., levees are retired their height will be less. 1 m to 1.25 m free board should invariably be provided over the maximum rise expected during floods.

b. Top Width:

Minimum Top width should be 3 m. Top width may be kept more than this if required.

c. Side Slopes:

Internal side slope may be 3:1 to 5:1.

Outer side slope may be 4:1 to 7:1 Banquette or counter-berms may be added on outer side, to prevent sloughing of high banks: Muck trenches should be constructed to act as cut-off to check the seepage along the foundation plan. In order to collect and dispose off any seepage that may be occurring, ditch is provided at the outer toe of the levee.

Type # 3. Revetment of Banks:

It is a protection of boulders or concrete blocks to the river bank. A flexible apron from the toe of bank, towards river side is also provided. Bank revetment also exerts an attracting influence by drawing the river water towards it on account of deep scours formed at the Toe of the bank. The river channel-is, in this way held permanently at the pitched banks.

Bank Protection:

Any protective work whose purpose is to maintain the stability of the bank against erosive effect of water is known as bank protection.

The purposes of bank protection are the following:

(i) To resist erosion of banks by water currents.

(ii) To prevent sliding of soil due to drawdown of flood.

(iii) To prevent piping of water through the banks.

(iv) To afford facilities for water transportation.

(v) To protect hydraulic structures.

(vi) To protect flood embankments.

If bank slopes are subjected to strong currents, the bank protection may be provided in form of vegetal cover, either by turfing or growing low shrubs. If the water currents are very strong, protection has to be provided by stone pitching or various types of mattresses such as willow, asphalt, or articulated concrete.

Stone is the most commonly used material for protection of banks where available locally. The thickness of the pitching is governed by the velocity of the current near the bank. Mr. Spring gave following thicknesses of pitching for different types of soils and different bed slopes of the river.

15 cm thick filter layer should be provided behind the pitching to prevent failure by sucking action of high velocity flow. Launching apron is a must at the Toe, to protect the bank against the effects of the flood scour.

Type # 4. Artificial and Natural Cut-Offs:

When meandering river develops very sharp horse-shoe bends, the land between successive peaks near the same bank of river is very much reduced. If small cut is given to connect the peaks, the water of the river will rush through the cut­off and develop into a full-fledged channel section.

Rush of water takes place because steeper slope is available along the cut-off rather than the curved path. The cut-off may be artificial or natural, usually it is artificial. Cut-off can be achieved without giving any cut, by concentrating more of river water at the bottleneck by putting a spur or groyne in the natural flow at the curve.

Cut-off induces following effects:

(i) Steep slope being available along the cut-off, river water flow is more through the cut-off rather than curved path.

(ii) Discharge in curved path, being considerably reduced, is silted up in due course of time and river flow becomes straight.

(iii) Water level in the river on U/S side is reduced.

(iv) Due to scouring action, the regime of D/S stream is disturbed.

(v) Channel taking off from just D/S of cut-off may get silted up.

Development of Cut-Off:

Cut-off develops due to following reasons:

i. Development of Bars at Infliction:

The flow through the main channel is reduced because of the growth of bars at inflections. This induces flow through already existing shallow side channels. Discharge is main channel being reduced continues to silt and more and more discharge gets diverted to the side channel which ultimately leads to the formation of cut-off.

ii. Formation of Drops at the Junction of Main Channel with Side Channel.

The side channel usually has a smaller velocity and smaller length with the same head difference as the channel. It, therefore, joins the main channel at a local drop. This local drop cuts back into the side channel and tends the development of a cut-off.

iii. Steep Slope and Unfavourable Cross-Section:

The steep slope of the side channel along with unfavourable cross-section generate such velocities which cause erosion of the bed and bank of the side channel.

iv. Bend Erosion:

Erosion of the concave banks increase the curvature of the river. The erosion might occur to such an extent that the arms of a loop cut into one another and cut-off occurs.

v. Duration of Flood:

The duration of flood should be sufficient so that stream may erode its bed and banks.

Type # 5. Spurs or Groynes:

Groynes are structures, built transverse to the river flow, extending from the bank towards the river. Groynes, or spurs are also sometimes known as transverse dykes.

They perform the following functions:

(i) They contract the width of the river. Thus depth of water in the river increases, and navigation during slack periods becomes possible.

(ii) They protect the river banks by keeping the flow away from it.

(iii) They create slack flow in the vicinity and thus cause silting of the area.

(iv) They train the river to flow along a specified coarse.

Classification of Groynes:

Following are the possible classifications of the groynes:

I. According to permeability.

II. According to height.

III. According to their actions.

IV. Special groynes.

I. According to Permeability:

According to this classification the groynes may be:

(i) Permeable groynes.

(ii) Impermeable groynes.

Permeable groynes may be made of bushes, bamboos, piles, mattresses. They are cheap and popularly used but they are temporary. The water can pass through them gradually the silt getting deposited in the permeable spurs make them almost impermeable.

Impermeable spurs may be made of earth embankments with necessary pitching and flexible apron. They do not allow water to pass through them. They are used at important places to protect the river banks.

II. According to Height:

The spurs may be submerged type or non-submerged type. If spins remain projecting above water level during, floods they are known as non-submersible spurs. Those that get sub-merged are known as sub-merged spurs.

III. According to their Actions:

According to this classification the spurs may be:

(i) Attracting Groyne:

It is a groyne which remains pointing towards the D/S of the flow. Its main purpose is to attract river flow towards that bank from which the spur is originating. This spur has to be very strong as it has to bear the full frontal attack of the river on it U/S face.

(ii) Repelling Groyne:

This spur is constructed pointing towards U/S. The angle of inclination with the normal to the bank varies from 10° to 30°. The head of the spur should be strong to resist the swirling action of the currents. Its best location is just at the beginning of the damaging river loop. One spur is effective for a length of about 1 to 2 times its length. The distance between spur is kept more in case of convex banks and small for concave banks.

(iii) Deflecting Spur or Groyne:

It is a spur that is short in length than repelling groyne. It is constructed perpendicular to the bank.

IV. Special Groynes:

(i) Hockey Type Groyne:

It is curved spur. It has attracting characteristic of flow. It is not very helpful for bank protection.

(ii) Denehy’s T-Headed Groyne:

It is a T-shaped spur. The longer arm of the T-head remains in the U/S direction of the flow. The T-head is protected by stone pitching. Such groynes may be spaced at a distance of say 800 m or so.

Type # 6. Pitched Islands:

It is an artificially created island in the river. It may be made of masonry or earth embankment, but pitched around. On account of the disturbances created by the islands placed in series; a deep river channel is formed and flow is diverted away from the bank. The measure is not successful if river is shallow and wide, as scouring velocities cannot be developed by pitched islands.