In this article we will discuss about:- 1. Introduction to Cross Drainage Works 2. Types of Cross Drainage Works 3. Selection of Site 4. Selection of Suitable Type.
Introduction to Cross Drainage Works:
Whenever an irrigation channel intercepts a natural drainage, which may be small stream or river, a masonry work has to be constructed to pass one above the other or to pass at the same level. Such masonry works are known as cross drainage works. Canals are mostly run at ridges, and hence there should not be any necessity of such works.
Following are some of the situations where necessity of cross drainage works occurs:
(i) In the head reaches where a main canal is led to the water shed, it has to cross a number of natural streams.
(ii) If, due to certain reason, the canal alignment leaves the watershed and catches it again after some distance, the area enclosed by the watershed and the alignment of canal, has to be drained. This requires construction of a cross drainage work.
(iii) When canal is aligned as contour canal it requires cross drainage works.
(iv) When various canal systems have to be linked with one another, necessity of cross drainage work may arise.
Cross drainage works are generally very costly works and should, as far as, possible be avoided.
If they cannot be avoided at all, their number should at least be reduced by adopting following measures:
(i) The canal may be aligned in such a way that it does not cross large number of small drains but crosses smaller number of large size drains. Several tributaries after meeting each other form a large drain. The canal should be aligned to cross the large drain rather than smaller tributaries.
(ii) Several large independent drains can be connected together by link channels and canal should cross them at one point only.
Types of Cross Drainage Works:
Cross drainage works can be grouped under following three heads:
I. Cross drainage works when canal crosses over the drainage.
II. Cross drainage works when drainage crosses over the canal.
III. Cross drainage works when drainage and canal waters are allowed to intermix.
I. Cross Drainage Works when Canal Passes over the Drainage:
In this case, irrigation canal passes over the drain.
The cross drainage works connected with this particular condition are the following:
(i) Aqueduct:
This is constructed when bed level of the canal is sufficiently higher than the H.F.L. of the drainage. This is the most commonly used cross- drainage work. The main advantages being that canal will not be led into cutting and canal is open for inspection. Moreover floods in drains do not cause any damage to the canal as its bed is kept will above the H.F.L. of the drainage.
Aqueduct structure may be arch culvert slab culvert, or even pipe culvert. If drainage and canal both are of substantial size a large masonry or concrete bridge like structure has to be constructed. Aqueduct is just a highway bridge, with the only difference that aqueduct carries canal above whereas highway bridge carries highway traffic.
For channels of very small capacity, the pipe aqueducts may be constructed. The channel water is carried through pipes instead of water troughs. The pipes are supported on piers along the width of the drainage. The head wall and toe wall should be provided at both the ends. The pipes should be capable of discharging the full supply discharge of the channel.
(ii) Siphon Aqueduct:
If difference between bed level of canal and bed level of drain is not substantial, the drainage water passes under the canal by touching the underside of the canal. If H.F.L. of the drain is much higher than the bed level of the canal, the water passes under the canal, under syphonic action. In case of syphon aqueduct the bed of the canal is subjected to very large uplift pressure and thus have to be quite strong.
Sections of aqueduct of syphon aqueduct may be of three types as follows:
(i) Full canal section along with full side earthen banks are maintained at the aqueduct. Fig. 25.2 (i).
(ii) Full canal section is maintained as usual but outer slopes of both the banks are replaced by masonry walls. Fig. 25.2 (ii).
(iii) Canal section is flamed and confined into a trough and sides are made of concrete or masonry. Fig. 25.2 (iii).
In case I, Fig. 25.2, width of the aqueduct will be quite large and hence cost per unit width will be maximum. In this case, bank connections are not required. Hence choice of this type will depend upon the relative cost of aqueduct proper and the cost of bank connections. The cost of bank connection is independent to the length of the aqueduct. Hence this type of construction is suitable when length of aqueduct is small.
For case III Fig. 25.3, the width of the aqueduct is minimum and thus cost of aqueduct per unit width is minimum. However the cost of bank connections will be additional. Hence this type is suitable when length of the culvert is very large (big drains). Type II Fig. 25.2 section is suitable for intermediate conditions. In short type III is suitable for very large drains while type I is suitable for very small drains. The correct way of choosing the type is to work out cost of all the three types and adopt the cheapest alternatively. Design of aqueduct is given ahead.
II. Cross Drainage Works when Drainage Passes over the Canal:
In this case, drainage is carried over the canal. This is found suitable when bed level of the canal lies below the bed level of the channel.
Cross drainage works of this category are:
(i) Super passage and,
(ii) Canal Syphon.
(i) Super Passage:
Where the bed level of the drainage is well above the F.S.L. of the canal passing below, the cross drainage work is known as super-passage. In this case canal water does not touch the bottom of the drainage. Super passage is possible when F.S.L. of canal is sufficiently below drain bed.
(ii) Canal Syphon:
When the canal discharge is small in comparison to the drain and the canal bed level is lower than the F.S.L. of canal, syphon is the preferred structure to be adopted. In this case canal is syphoned below the drain.
The siphon is disadvantageous in the respect that the canal bed is likely to silt up, and that the siphon barrel has to resist the undue uplift pressure on the top.
The siphon essentially consists of a tunnel or a barrel, laid under the drainage of for carrying canal water. The siphon may be designed with vertical drops at the ends and connected by a horizontal barrel in the centre. The vertical drop type is not suitable for canal water with heavy charge of silt.
The siphons may be constructed with a curved passage for water, so as to cause less disturbance to the canal flow and reduce the silting trouble. But constructing barrels of curved type is more difficult.
The siphons with gradually sloped inlet and exit are more suitable for canals having muddy flow.
Design of super passage and siphons are done similar to the design of aqueducts and siphon aqueducts.
III. Cross Drainage Works when Drainage and Canal Waters are Allowed to Intermix:
The works connected with this situation are level crossing, inlets and outlets.
When bed level of canal and that of the drainage are at or about the same level, the canal water is carried across the drainage by mixing with it. The drainage and canal water mix with each other at the site of the cross drainage work. The discharge of the drainage decides whether level crossing or inlet alone should be adopted.
When the drainage flow is small and drain is not perennial, it is generally admitted into the channel by the inlets. If die discharge in the channel is too much and admitting the drainage may raise the water level in the channel considerably, an outlet also becomes a necessity to drive out the excess excepted discharge.
If the drainage is small and runs throughout the year the water may be led to the channel by an inlet. This water helps to supplement the discharge of the channel and can be safely used for irrigation.
When the discharge of drainage is large and intermittent, the cross drainage work is provided by means of level crossing which consists of a cross-regulator across the channel and also cross the drainage. The work is similar to a diversion head work. After mixing of the two waters, the canal draws its designed discharge through a head regulator.
The advantage of the regulator across the stream may be taken to raise the level of water so that the command of the channel D/S of crossing may be further increased. The canal regulator is generally kept closed during floods and flood discharge is passed through the regulator across the drainage. When floods recede, and the water is clear in the stream, the canal regulator is opened for drawing water for the channel.
When drainage is a big river in which the flow is perennial, permanent cross drainage works are necessary. Such a work consists of a dam or pick up weir across the river. The canal water is discharged into the river through the regulator gates at one side and at the other side water is drawn into the canal through another head regulator. Escapes may be provided to deal with the surplus water and also to remove silt from canals.
Inlet:
The inlet is constructed to admit the flow of small perennial streams into the channel. The bank of channel is cut open and the sides and bed protected by pitching. If bed of the stream is slightly at higher level than the bed of canal, the stream may be admitted to the channel through inlets provided with drop. For very small streams inlet may be mere pipe laid in the bank of the canal.
Outlet:
Its job is reverse to that of inlet. It is an arrangement in the bank of a channel to discharge out the surplus water from the channel. The outlet becomes necessary when discharge admitted into the channel by an inlet, raises the water level above the maximum water level of the channel. The outlet is similar in action to a surplus escape, and is provided at suitable site where surplus water can be easily discharged into a side valley.
Pipe Aqueduct:
The inlets and outlets can be replaced by pipes in case of small drainages. The whole discharge of the drainage is carried across the canal by providing pipes extending from one bank to the other bank. The pipes are laid above the F.S.L. of the channel with a free board of 60 cm to 1 m. If the drainage bed width is more, the pipes are supported on piers. The bed width, in such cases, should be suitably widened so that the natural water way is not decreased by piers.
Selection of Site for Cross Drainage Works:
While selecting site for a cross drainage work, the following points should be borne in mind:
1. Soil of good quality should be available so that safe foundations may be laid on it.
2. The natural banks should be high and stable, otherwise large cost is required for making good bank connections.
3. The canal and drainage, should cross each other at right angles. There should be straight reaches in both on the U/S and D/S sides.
4. The section of the drainage should he uniform. The bed of the drainage should be even and the flow should be free from turbulences.
5. The type of the cross drainage work should be carefully decided with reference to:
(i) Bed levels.
(ii) Discharge of the canal and drainage.
(iii) Overall cheapness of construction.
Selection of Suitable Type of Cross Drainage Work:
While making selection of suitable type of cross drainage work following points should he carefully considered:
(i) If Bed level of the canal is sufficiently above the H.F.L. of the drainage, file construction of aqueduct is the obvious choice.
(ii) If bed level of drain is sufficiently above the F.S.L. of the canal super passage is the obvious choice.
(iii) If head way below the canal bed level and above H.F.L. of drainage is not adequate, it can be increased by shifting point of crossing D/S of the drainage. If however, shifting of canal alignment, D/S is not possible, syphon aqueduct should be provided. In this case H.F.L. of the drainage would be above the bed level of the canal.
(iv) If canal bed is lower than the H.F.L. of the drainage, canal syphon should be provided.
(v) If canal and drainage cross each other more or less at the same level, level crossing may be preferred. Level crossing should as far as possible be avoided.
If we want to change the type of cross drainage work it can be done by shifting the crossing point either D/S or U/S side. By doing so, relative heights of each work can be changed and hence type of work can be changed.