After reading this article you will learn about:- 1. Meaning of Diversion Headworks 2. Location of Division Headworks 3. Lay-Out 4. River Training.
Contents
Meaning of Diversion Headworks:
An irrigation channel takes its supplies either from its source (a river or storage reservoir) in case of main canal, or from a channel in case of branch canal or a distributary. The structures constructed across a river source at the head (i.e., the upstream end) of an off taking main canal are termed canal head works or simply headworks. The headworks can be either diversion headworks or storage headworks.
Diversion headworks divert the required supply from the source to the off taking channel and serve the following functions:
(i) Diversion headworks regulate the supply of water into the canal.
(ii) Diversion headworks raise water level in the river so that,
(a) Excavation in the head reach of the off taking canal is reduced,
(b) Command area is increased, and
(c) The command area can be easily served by flow irrigation.
(iii) Diversion headworks control the entry of sediment into the off taking canal.
Diversion headworks can be either:
(i) Temporary, or
(ii) Permanent.
For temporary diversion headworks, spurs or bunds are constructed across the source river. Since floods in the source river can damage such bunds, it may be necessary to repair such bunds rather frequently, and construct them after every flood that occurs in the source river.
Permanent headworks have a permanent structure in the form of a weir (or a barrage) constructed across the river. Most of the headworks of important canal systems are of permanent type. Sometimes temporary headworks are constructed in the beginning and they are replaced by permanent headworks when the demand for irrigation has developed sufficiently.
Location of Division Headworks:
Most of the important rivers have four stages, viz., the rocky, boulder, trough (or alluvial) and delta stages. Of these, the rocky and delta stages are generally unsuitable for siting headworks.
Usually, the command area is away from the hilly stage, and it would, therefore, involve avoidable expenditure to construct a channel from headworks located in the hilly stage to its command area. In the delta stage, the irrigation requirements are generally less, and the nature of the river at this stage poses other problems.
The boulder and alluvial stages of a river are relatively more suitable sites for locating headworks. The choice between the boulder stage and the alluvial stage is mainly governed by the command area. If both stages are equally suitable for siting the headworks from command area consideration, the selection of the site should be made such that it results in the most economical alternative.
The following features of the two stages should be considered while selecting the site for headworks:
(i) The initial cost of headworks in the boulder stage is generally smaller than that in the alluvial stage because of:
(a) Local availability of stone,
(b) Smaller width of river (requiring smaller length of weir),
(c) Smaller scour depths which reduce the requirements of cutoffs and other protection works, and
(d) Close proximity of higher banks which means need for less extensive training works.
(ii) The irrigation canal off taking from a river in the boulder region will have a number of falls which may be utilised for generation of electricity. There is almost no scope for the generation of electricity in this manner in the alluvial reach of a river.
(iii) If the existing irrigation demand is less but is likely to develop with the provision of irrigation facilities, it is possible to divert the river water into the irrigation channel by constructing a temporary boulder bund across the river. This bund will be washed away every year during the floods and will be reconstructed every year.
This will, no doubt, delay the Rabi crop irrigation, but it is worthwhile to use temporary bunds for a certain period, and when the irrigation demand develops, permanent headworks may be constructed. In this manner, it would be possible to get returns proportional to expenditures incurred on the headworks. Construction of temporary bunds is generally not possible in the alluvial stage of the river.
(iv) An irrigation channel off taking in the boulder stage of the river will normally require a large number of cross-drainage structures.
(v) Because of the nature of the boulder region, there is always a strong sub-soil flow in the river bed. This causes considerable loss of water and is of concern during the periods of short supply. Similarly, there will be considerable loss of water from the head reach of the off taking channel. In alluvial reach of the river, this loss of water is much less.
(vi) The regions close to the hills usually have a wet climate and grow good crops. The irrigation den ad in the head reach of the channel off taking in the boulder stage is, therefore, generally small. However, this demand would increase with the provision of irrigation facilities. In alluvial regions, the demand for irrigation is high right from the beginning.
Once the stage of a river has been chosen for locating the headworks, the site of the headworks is selected based on considerations of its suitability for the barrage (or weir), the under-sluices and the canal head regulator. For irrigation purposes, the site for headworks should result in a suitable canal alignment capable of serving its command area without much excavation.
For siting the headworks, the river reach should, as far as possible, be straight and narrow and have well-defined and non-erodible high banks. In the case of a meandering river, the headworks should be located at the nodal point.
From sediment considerations, the off taking channel should be located at the downstream end of the outside of a river bend so that it has the advantage of drawing less sediment. However, a curved reach would need costly protection works against the adverse effects of cross currents. Moreover, if canals take off from both the banks, the canal taking off from the inner bank draws relatively more sediment.
In order to ensure adequate supply to the off taking canal at all times, the under sluices should be sited in the deep channel. A river reach with deep channels on both banks and shallow channel at the centre is more suitable when canals off take from both sides. Besides, the site must be accessible and suitable for making the river diversion and other related arrangements at a reasonable cost.
Lay-Out of Diversion Headworks:
Diversion headworks (Fig. 10.1) mainly consist of a weir (or barrage) and a canal head regulator. A weir has a deep pocket of under sluice portion upstream of itself and in front of the canal head regulator on one or both sides. The under sluice bays are separated from other weir bays by means of a divide wall.
In addition, river training structures on the upstream and the downstream of weir and sediment excluding devices near the canal head regulator are provided. Detailed model investigations are desirable to decide the location and layout of headworks and its component units. A typical layout of diversion headworks is shown in Fig. 10.1.
(i) Weir (or Barrage):
A weir is an un-gated barrier across a river to raise the water level in the river, and diverts the water into the off taking canal situated on one (or both) of the river banks just upstream of the weir. Weirs are usually aligned at right angles to the direction of flow in the river. Such weirs will have the minimum length and normal uniform flow through all the weir bays thereby minimising the channels of shoal formation and oblique flow.
To increase the water level, the weir crest is raised above the river bed. Part of the raising of the water level is obtained by shutters provided at the top of the weir crest. These shutters are dropped down during floods so that the afflux is minimum.
The afflux is defined as the difference in water level between the upstream and the downstream of a structure under free flow conditions as a result of construction of the structure across a river. Controlling pond levels by means of shutters becomes difficult when the difference between the pond level and the crest level is higher than 2.0 m. In such cases, a gate-controlled weir, better known as barrage, is preferred.
Barrage is a gate-controlled weir with its crest at a lower level. The ponding up of river for diversion of water into the off taking canal is achieved by means of gates (instead of shutters). Barrage and weir are similar structures, and differ only in a qualitative sense.
Barrages are considered better than weirs due to the following reasons:
(i) Because of the lower crest level of a barrage, the afflux during floods is small.
(ii) Barrages offer better control on the river outflow and the canal discharge.
(iii) A roadway across the river can be easily provided at a small additional cost.
Weirs (or barrages) can be either masonry weirs (with vertical upstream face) or rock-fill weirs (with sloping apron) or concrete weirs with glacis. On pervious foundations only concrete weirs (Fig. 10.2) are constructed.
(ii) Scouring Sluices (or Under Sluices):
The construction of a weir (or barrage) across a river results in ponding of water, and causes considerable sediment deposition just upstream of the canal head regulator. This sediment must be flushed downstream of the weir.
This is done by means of scouring sluices (also known as under sluices) which are gate-controlled openings in continuation of the weir with their crest at a level lower than the level of the weir crest and are located on the same side as the off taking canal.
In case of two off taking canals, one on each of the two banks of the river, scouring sluices are provided at both ends of the weir (Fig. 10.1). The scouring sluices are also useful for passing low floods, after meeting the requirements of the off taking canal, without having to drop the weir shutters raising of which is a cumbersome task.
(iii) Divide Wall:
The divide wall is constructed parallel (or nearly parallel) to the canal head regulator for separating the main weir base from the base of scouring sluice. As shown in Fig. 10.1, the wall extends on both sides of the weir. The divide wall separates the weir floor from the floor of the scouring sluices which is usually at a lower level than the weir floor.
The divide wall also isolates the canal head regulator from the main river flow, and creates a still pond of water in front of the canal head regulator. This results in relatively sediment- free water entering into the off taking canal. The divide wall also improves scouring of the deposited sediment in the under-sluices by ensuring straight approach.
(iv) Fish Ladder:
Different kinds of fish in a river migrate from upstream to downstream in the beginning of the winter in search of warmth, and return upstream before the monsoon for sediment-free clear water.
While constructing a weir across a river, a narrow opening between the divide wall and the scouring sluices (where water is always present) is provided to- allow for free movement of fish. This opening is called fish ladder or fish way or fish pass (Fig. 10.3) in which baffles or staggering devices are provided so as to keep flow velocity in fish ladder less than 3.0 m/s so that fish can easily travel upstream.
(v) Canal Head Regulator:
A canal head regulator (Fig. 10.4) regulates the discharge into the off taking canal and also controls the entry of sediment into the canal. The head regulator is usually aligned at an angle of 90° to 110° to the barrage axis to minimise entry of sediment into the off taking canal, and prevents backflow and stagnation zones in the under-sluice pocket upstream of the regulator. The discharge through the regulator is controlled by steel gates which are generally of 6 to 8 m width.
The pond level in the under sluice pocket, i.e., just upstream of the canal head regulator is obtained by adding the working head of about 1 to 1.2 m to the designed full supply level of the off taking canal. The crest of the head regulator should be higher than the sill of the scouring sluices to prevent entry of sediment into the canal.
The crest level is obtained by subtracting from the pond level, the head over the crest required to pass the full supply discharge in the canal at the specified pond level. Provision of sediment excluder in the under sluice pocket will also affect the crest level of the head regulator.
The width of waterway in the canal head regulator should be such that the canal can be fed its full supply with about 50% of the working head provided. If this width of waterway is more than bed width of the canal, a converging transition is provided downstream of the regulator to attain the required canal width.
The required head over the crest, H (in m) for passing a discharge Q (in m3/s) with an overall waterway L (in m) is computed from the following relation:
Q= C(L – KnH)H3/2 (10.1)
in which n is the number of end contractions and K is a coefficient which ranges from 0.01 to 0.10 depending upon the shape of the abutment and the pier nose. The value of C can be taken as 1.71 (1.84 if the width of the crest is less than 2.5 times the head over the crest) in S.I. units.
The height of the gates is the difference of the pond level and the crest level of the regulator. But, during high floods, the water level in the river would be much higher than the pond level and the flood water may spill over the gates.
To prevent such spilling of flood water into the canal, an RCC breast wall (Fig. 10.4) between the pond level and the HFL, and spanning between adjacent piers is always provided. A bridge and a working platform (for the operation of gates) are also constructed across the head regulator.
(vi) Sediment Excluder (or Silt Excluder):
Sediment entering into an off taking canal, if excessive, causes silting and thus reduces canal capacity. As such, it is necessary to control the amount of sediment entering into the off taking canal. This is done by constructing a sediment excluder in the river bed immediately upstream of the canal head regulator.
Tunnel-type sediment excluder (Fig. 10.5) prevents the bottom layers of water, which have maximum sediment concentration, from entering the off taking canal and allows only the top layers of the stream, containing relatively less sediment, to enter the off taking canal.
(vii) Sediment Ejector (or Silt Ejector):
Sediment ejector, Fig. 10.6, is constructed in the off taking canal (downstream of the head regulator) to eject the sediment that has entered the canal. Sediment ejectors eject the near-bed water layers having the largest sediment concentration from the canal at a suitable location downstream of the head regulator.
The main components of an ejector include a diaphragm, tunnels, control structure and an outfall channel. The lower side of the upstream end of the diaphragm is bell-mouthed.
The ejector spans the entire width of canal, and is divided into a number of main tunnels which, in turn, are subdivided with turning vanes which gradually converge so as to accelerate the escaping flow. The outflow from the ejector is led to a natural drainage through an outfall channel which is designed to have a self-cleansing velocity.
River Training for Canal Headworks:
River training structures for canal headworks are required for the following purposes:
(i) To prevent outflanking of the structure.
(ii) To minimise possible cross-flows through the weir (or barrage).
(iii) To provide favourable curvature of flow at the head regulator from considerations of entry of sediment into the off taking canal.
Following types of river training structures are generally provided at canal headworks:
(i) Guide banks,
(ii) Approach embankments,
(iii) Afflux embankments, and
(iv) Spurs.
A typical lay-out of river training structures for canal headworks is shown in Fig. 10.7.
