River Training Works

River Training Works Points : river training works, objects of river training, classification of river training works Objects of River Training: The expression river training implies various measures adopted on a river to direct and guide the river flow, to train and regulate the river bed or to increase the low water depth. The purpose of river training is to establish the channel along a certain alignment. There may be various objects for training a river these are described below:
1. High flood discharge may pass safely and quickly through the reach.
2. Sediment load including bed and suspended load may be transported efficiently.
3. To make the river course stable and reduce bank erosion to minimum.
4. To provide a sufficient draft for navigation as well as good course for it.
5. To fix direction of flow through certain defined reach, Classification of river training works; 1) Keeping in view the objects of river training as stated above, river training can be classified into the following:
2) High water training. This is also called training for discharge. The river is (rained to provide sufficient and efficient cross-sertional area for the expeditious passage of maximum flood. It concerns mainly with alignment and height of embankment for a given flood discharge.
3) Low water training. In this case the river is trained to provide sufficient depth for navigation during low stage of river. This is also called training for depth and is usually achieved by contraction of the width of the channel.
4) Mean water training. In this case the river is trained to correct the configuration of river bed for the efficient transport of sediment load in order to keep the channel in good shape. It can be called training for sediment.
5) Out of the various kinds of training, mean water training is the most important training. Any attempt to change the configuration of river bed in alignment or in cross-section must obviously by designed in accordance with that stage of the river at which maximum movement of sediment takes place. The river is most active during high stage which persists for a very short duration and is practically inert at low stage which persists for a very long duration. Some where between these two there is a stage at which the combined effect of forces causing sediment and the time for which such forces are maintained is maximum. The maximum bed building stage is some where in the neighborhood of mean water Mean water training, therefore, forms the basis on which both high and low water training are planned. Types of river training works: The following are different types of river training works, provided for training of rivers: 1. Marginal embankment (Dyke or Levee): The marginal embankment or dyke is an earthen embankment of trapezoidal section constructed approximately parallel to .the bank of the river to confine the flood water to the minimum possible cross-section of the river in between the embankments. A margin of about 2 metres is provided between the toe of the embankment and top edge of the river bank. These are provided on those rivers in which the flood level is so high that the adjoining areas get flooded.
2. Guide bank: The guide bank is an earthen. embankment with curved heads on both the ends. When a barrage is constructed across a river which flows through the alluvial soil, the guide banks must be constructed on both the approaches to protect the structure from erosion. The guide banks may be divergent, convergent or parallel guide banks.
3. Groynes: A groyne is a permanent structure constructed transverse to the river low and extend from the bank into the river up to a certain limit. They are constructed in order to
(a ) protect the river bank, by keeping away the river flow,
(b) train the flow along a certain course,
(a ) contract a river channel to Improve Its depth and
(d) silt up the area in the vicinity by creating, a slack flow.

The length of the groyne depends upon the width and mature of the river. The top width of varies from 3 m to 4 m. The side slope may be 4 : 1 or 2 : 1. The spacing between the adjacent groyne is generally kept as twice the length of the groyne.

The groynes, depending upon the materials of construction, may be permeable and impermeable groynes. The groynes which permit the flow of water through them are called permeable groynes. These groynes obstruct the flow, reduces its velocity and reduce the erosive action of the stream. The groynes which do not permit any flow through them are known as impermeable groynes or solid groynes. These groynes may be constructed by a core of locally available material like sand, clay and gravel. The impermeable groynes may be classified as follows:

(a) Deflecting groynes; (b) Attracting groynes; (c) Repelling groynes; (d) Hockey groynes; and (e) T-shaped or Denehy’s groynes.

The deflecting groynes are constructed perpendicular to the river bank to deflect the flow away from the bank. The attracting groynes are constructed obliquely to the bank by making an angle of 600 to 750 towards the downstream. It has the property to attract the flow of water towards the river bank. The repelling groynes are constructed in such a way that it is pointing towards upstream at an angle of 30°. It has the property to repel the flow of water away from it.

The hockey groynes are constructed in such a way that the head of the groyne is curved towards downstream in the shape of a hockey stick. It behaves like an attracting groyne. The T-shaped or Denehy’s groynes are the special types of groynes with the head in T-shape.