River Training: Objectives and Methods

After reading this article you will learn about the objectives and methods of river training.

Objectives of River Training:

River-training measures aim at achieving one or more of the following objectives:

(i) Flood Protection:

Floods cause enormous damage to life and property almost every year.

River training for flood protection (also known as ‘high water training’ or ‘training for discharge’) can be achieved by one or more of the following methods:

(a) By constructing levees or embankments to confine water in a narrower channel.

(b) By increasing the discharge capacity of natural channels by some suitable methods such as straightening, widening or deepening.

(c) By constructing reservoirs.

(d) By providing escapes or diversion from rivers.

(ii) Navigation:

The measures to achieve adequate depth of flow in a river for navigation (also known as Tow water training’ or ‘training for depth’) include dredging the shallow reaches of rivers and using spurs to contract the river channel and, thus, increase its depth. Canalisation makes a non-navigable river navigable and is ac­complished by building a serk of dams (or weirs) and locks.

(iii) Sediment Control:

River training for sediment control (also known as ‘mean water training’ or ‘training for sediment’) aims at attaining efficient movement of sediment load for keeping the river channel in a state of equilibrium. Spurs and pitched islands are normally used for training the river for sediment.

(iv) Guiding the Flow in River:

Construction of structures (such as head-works or bridges) requires that river in the vicinity of these structures flows between the abutments of these structures. For this purpose a system of guide banks (also known as Bell’s guide bunds) on one or both banks of the river is built.

Sometimes, the flow in a river needs to be deflected away from a bank in order to protect some portions of the river bank. This is achieved by constructing one or more spurs projecting into the river from the bank.

(v) Stabilisation of River Channel:

Stability of river banks is achieved by training methods, such as stone pitching or lining of banks.

River Training Methods:

The planning and design of river training structures are based on:

(i) Empirical methods,

(ii) Intution and judgement of experienced river engineers, and

(iii) Model investigations.

One or more of the following methods are generally used for river training.

Levees:

A levee (also known as an embankment, dike or bund) is constructed along a river bank to protect the adjacent area from getting flooded. This is the oldest method used for flood protection.

The method of flood control by levees is fairly simple and also economical as it uses locally available material and labour for their construction. Levees have been constructed recently on many major rivers of the world, such as the Ganga, the Kosi, the Mahanadi, the Gandak and others.

The levees should follow the general alignment of the river keeping in view:

(i) The desirability of having high discharge capacity of the river for a given stage, and

(ii) The requirement that the entire meander belt (Fig. 9.1) be within the levees so that they are not attacked by river flow.

The top width of levees is generally kept between 3 to 8 m and its height is decided so that it is able to contain a 500-year flood with a free board of about 1 to 2 m. The side slopes of levees may vary from 2H: IV to 6H: IV.

Spurs:

Spurs (also known as spur dikes, groynes or transverse dikes) are generally made of locally available earth material in the form of an embankment constructed transverse to the flow extending from one of the river banks into the river.

Spurs are the most widely used river training structures, and serve the following functions in river regulation:

(i) Training the river along a desired course by attracting, deflecting or repelling the flow in a channel.

(ii) Creating a slack flow with the object of silting up the area in the vicinity.

(iii) Protecting the river bank by keeping the flow away from it.

(iv) Contracting a wide river channel, usually for the improvement of depth for navigation.

Spurs can be used either singly or in series or in combination with other river training measures.

Their design depends on:

(i) River discharge,

(ii) Angle of attack,

(iii) Sediment load,

(iv) Meander length,

(v) Curvature of river, and

(vi) Upstream and downstream river training measures.

Spur length is kept longer than 1.5 to 2 times the depth of flow, and is usually restricted to less than 20% of the river width. Spacing between adjacent spurs is generally kept 2 to 2.5 times the spur length.

The top width of a spur should be between 3 and 6 m, and freeboard of 1 to 1.5 m above HFL (high flood level) should always be provided in case of non-submerged spurs. It is always advisable to finalize the spur designs only after conducting model studies.

Spurs can be classified based on:

(i) The method and material of construction – permeable and impermeable,

(ii) The height of spur with respect to HFL – submerged or non-submerged,

(iii) The functions – attracting, deflecting and repelling, and

(iv) The plan shape of spur – T-headed type, hockey type etc.

Usually, permeable spurs are either tree spurs or pile spurs. Tree spurs (Fig. 9.2) are constructed by tying leafy trees with strong stem and branches to a rope which is firmly anchored to the bank at one of its ends and to a concrete block at the other end.

Pile spurs are constructed by driving two to three rows of inter-twined piles of timber or RCC or sheet-piles up to about 6 to 9 m inside the river bed. The space between the rows of piles is filled by alternate layers of 1.8 m thick brushwood weighted down by boulders or sand bags. Stone pitching, graded filter and a launching apron (to protect stone pitching) are always provided in case of impermeable spurs constructed as an earthen embankment (Fig. 9.3).

The side slopes vary between 1H: IV to 5H: IV with nose of the spur sloping at a flatter slope of about 5H: IV. When spurs are oriented to point upstream, they direct the flow away from the bank and are called repelling spurs.

On the other hand, spurs pointing in the downstream direction attract the flow towards the bank and are known as attracting spurs. Spurs oriented with their axes normal to the bank are known as deflecting spurs which help in contracting the river channel.

Guide Banks:

Guide banks are artificial embankments constructed along the flow direction both upstream and downstream of the abutment on one or both sides of the river for guiding the river flow past a bridge (or other hydraulic structures, such as weirs or barrages) without causing damage to the bridge and its approaches. Clear waterway between guide banks can be equated to Lacey’s wetted perimeter [Eq. 5.6].

The overall waterway between guide banks is obtained by adding thickness of piers to the clear waterway. Figure 9.4 shows the plan and section of a typical guide bank. The elevation of the top of the guide banks is kept 1.5 to 2.5 m above the HFL of 100-year flood.

The top width of guide banks is generally kept between 6 and 9 m. Locally available earth materials are used for constructing guide banks. To prevent erosion of the material from the river-side surface of guide bank, the surface is stone-pitched. This pitching is continued up to the rear side of the curved head. The size of stones used for pitching is given by

d = 0.046U³

where d is equivalent diameter in meters and U is the average flow velocity in m/s. Angular and graded stones weighing between 450 and 1800 N (i.e., 45 and 180 kg) are generally used for this purpose.

The thickness of stone pitching T in meters is related to river discharge Q (in m³/s) by the following empirical relation:

T = 0.06Q^1/3

The thickness of stone pitching is increased for the curved head region. Scour of the river bed may cause undermining of the stone pitching thereby resulting in failure of the guide banks. This can be prevented by providing launching apron of stones on the river bed beyond the toe of the guide banks, as shown in Fig. 9.4.

Bank Protection:

Bank failures due to either wave action or erosive action of river flow can lead to river breach causing huge losses in terms of human lives, property, agricul­ture and other utilities. Measures of bank protection such as revetment, riprap, etc. provide a shield against erosion of bank material. Bank revetments are mattresses of suitable material, such as reinforced asphalt and articulated concrete, laid over the bank surface to be protected.

Riprap of hard angular rock fragments (or concrete blocks) laid on a thick layer of rubble or quarry chips is cheap and durable method of bank protection. Another way of providing bank protection is by means of flexible brick pitching (Fig. 9.5).

For this purpose bricks having a pair of holes across their full width are laid on the bank slope and for some distance on the river bed at the toe of the bank. A G.I. wire passes through the brick holes. The wires are knotted at their ends and suitably anchored so that the bricks are held together and also small movement of bricks is possible.

Pitched Islands:

An island created artificially on river bed with sand in its core and protected by stone pitching on all its sides is called pitched island. It can be used to correct an oblique approach upstream of weirs, barrages and bridges.

Marginal Bunds:

Marginal bunds are embankments which are constructed along the river banks upstream of weir-like structures to protect low-lying areas from being flooded. These bunds extend to regions where the ground level is the same as that of top of the bund.

Cutoffs:

Sometimes alluvial river flowing along curves or bends abandons a particular bend and establishes its main flow along a comparatively straight and short channel which is called cutoff.

At times, it is advantageous to make an artificial cutoff by excavating a pilot channel of small cross-section and letting it develop by itself in due course of time so that the river in that region abandons the curved path and adopts the new course along the artificial cutoff.

Artificial cutoff avoids the chaotic or non-equilibrium conditions which prevail during occurrence of natural cutoff. Artificial cutoffs shorten the travel distance, and increase ease of maneuvering of a navigating vessel. Artificial cutoff also diverts the river from a curved path which might be endangering important land area.