Sketch a neat diagram of layout of a diversion head work ensure the different component on it

Sketch a neat diagram of layout of a diversion head work ensure the different component on it

1. Introduction to Diversion Head Works
2. Types of Diversion Head Works
3. Layout of a Diversion Head Works
4. Location of Diversion Head Works
5. Effects of Construction of Diversion Head Works on the Behaviour of Rivers

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1. Introduction to Diversion Head Works:

The purpose of this head work is to divert river water into the off taking canal. It is located across the river near the point of takeoff of the canal.

A diversion head work performs following functions:

(i) It regulates the flow in the off taking canal.

(ii) Silt entrance into the canal is controlled.

(iii) It raises the level of water in the river. This causes diversion of the river water into the off taking canal, under gravity flow.

(iv) By raising the level of water in the river with the help of diversion work, and consequently raising the off take level of the canal, more area maybe brought into the command of the proposed canal system.

(v) Level of water in the rivers remains fluctuating due to changing flow. Diversion works reduce such fluctuations in river water level.

(vi) Although the purpose of diversion head work is not to store water but still some water definitely gets stored. This stored water can be used to augment the short supplies of the river. This type of augmentation is possible for few days only, because stored water is not much in amount.

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2. Types of Diversion Head Works:

Diversion works can be classified under following two heads:

1. Temporary bunds or spurs.

2. Permanent weirs and barrages.

Temporary bunds or spurs are temporary works, which have to be constructed every year after floods. These bunds cannot sustain the assault of floods and get washed away. Immediately after floods they are again constructed. Temporary bunds can be constructed on small streams only. Such works are generally carried out by the nearby villagers jointly, so as to make arrangements for irrigation for their fields for lean months of flow.

Such temporary spurs or bunds can be economically constructed in boulder reaches of the river. However, for important diversion works permanent weirs and barrages have to be designed and constructed.

Weir:

Weir is a solid obstruction, constructed across the river. It is used to raise the water level in the river and then divert it into the canal. The weirs can also be used to store surplus flood water, to tide over the shortages likely to occur during lean months. In such a case, the weir is known as storage weir.

The main difference between storage weir and dam, is only is regard to the height and duration of storage. Dams store large amounts of water and for longer durations than storage weirs. One more difference is that water in the reservoir never overtops the dam, but in case of weirs the water can flow over the weir crest. In case of dams the surplus flood water is disposed off D/ S through spillways.

Barrage:

Functions of barrage are the same as those of weirs; the only difference being in mode of heading up of water. The weirs cause heading up of water by obstructing the flow by themselves, but in case of barrages water is headed up with the help of gates. In case of weirs the high flood water passes over the crest but in case of barrages, gates are lifted up and flood water passed D/S.

In case of barrage, crest level is maintained at low water level which is almost in level with bed level of the river. Because gates can be lifted for passing the flood water and shut down for storing water, control on the water level in the river is better exercised with the help of barrage. However barrages are much more costlier than the weirs. A road bridge is generally constructed over the barrage, with a little additional cost. Hence a barrage may be used to act as bridge also.

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3. Layout of a Diversion Head Works:

A typical layout of a diversion head work is shown in Fig. 15.1.

Following is the list of component parts of a diversion head works:

1. Weir or barrage.

2. Scouring sluices or under sluices.

3. Divide wall.

4. Fish ladder.

5. Log chutes.

6. Canal head regulator.

7. River training works.

8. Silt control devices

Out of all these component parts river training works include lot of other elements also. Before we take up description and design of all these component parts of a diversion head works, let us first of all discuss location of head works.

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4. Location of Diversion Head Works:

The total length of the river may be divided into following four stages:

1. Mountainous Stage:

The first length of the river originating from hills is known as mountainous stage of the river. The longitudinal slope in this stage is very steep and velocity of flow of water, very fast. The width of the river in this region is generally very small.

This region is found most suitable for the construction of storage works like high dams. Large storages are possible as very deep valleys are easily available here. Moreover the length of the dam is also relatively small. Materials of construction are locally available. This stage is however not suitable for diversion works.

2. Sub-Mountainous Stage:

The bed and banks of the river in this stage are made of gravel. The velocity of flow is considerable though smaller than first stage. The river water contains lot of silt load. The river generally does not flow in a single channel, but is form of small several channels. Longitudinal slope is less than first stage but still considerable. Diversion works can be located in this region of the river.

3. Alluvial or Trough Stage.

This length of the river is in plains. The section of the river is made of alluvial soils or silt. The longitudinal slope of the river is small and velocity of flow moderate.

This length of the river is the largest of all other lengths. Diversion works are mostly located in this length. The area to be irrigated lies very near to this length and thus reduces the overall length of the canal network.

4. Delta Stage:

This is the last length of the river before it falls into the sea. This length of river is only a few kilometres long. Here velocity of flow is very small and silting rate very high. Because of high rate of silting, the rivers frequently change their course. No diversion work is constructed in this region. Secondly available command area near this region is also small.

Canals cannot be taken from mountainous stage, as lot of cuttings and fillings are involved and thus canal construction becomes very costly. Lot of falls will have to be constructed adding still more to the cost.

After having decided the region in which diversion head work is to be located, following additional points should also be taken into account:

(i) All the construction materials should be available in the vicinity.

(ii) In order to keep the length of the canal minimum, diversion head works should be located as near the area to be irrigated as possible.

(iii) The elevation of the water in the river should be such that water may flow to the area under gravity. If level of water is low, the position of the head works should be shifted a few kilometres upstream so that increased elevation of water is available.

(iv) The canal should run in such a way that canal is partly in cutting and partly in filling.

(v) The river at site of diversion, should be neither too wide nor too narrow. The river should neither be scouring nor silting.

(vi) The river should be straight and flowing in one channel. Its bed and banks should be stable.

(vii) The canal alignment should be such that minimum streams cross it. This will reduce the number of cross-drainage works to be constructed.

(viii) The canal should take off, either at 90° or greater than 90° with the river. Canal should not run parallel to the river. In such circumstances the canal may be eroded during floods.

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5. Effects of Construction of Diversion Head Works on the Behaviour of Rivers:

Following are the changes that take place in the behaviour of the river after the construction of a diversion head works:

1. Silt carrying capacity of river is decreased because heading up of water causes flattening of the surface slope on the U/S side of the weir. Because of reduction in silt carrying capacity, the pond formed on U/S side starts silting.

2. Because of silting on U/S side, the water passing D/S of the weir contains less amount of silt. To make up for the deficiency of the silt, the D/S flowing water starts scouring the bed and banks. The scouring may lead to undermining the stability of the weir.

3. Because of silting of the pond, the afflux goes on increasing and thus more and more areas on U/S are submerged. At last a stage is reached when no more silt is deposited and the normal silt charge is passed on to the D/S side. Devices like silt excluder also discharge more silt D/S.

4. The silt discharge D/S is more than the silt carrying capacity at low discharges. This will cause silting D/S side. In other words where scouring was taking place in the initial stages, silting will start. This process is known as recovery process of D/S bed levels. The recovery of levels on D/S, may lead to loss of control on silt excluding devices. Because of this, sufficient margin should be left between F.S.L. of canal and the pond level so that if need be, crest level of the head regulator could be raised later