The following points highlight the three main types of cross drainage work. The types are: 1. Aqueduct 2. Super-Passage 3. Level Crossing.
Type # 1. Aqueduct:
It is a structure which carries an irrigation canal over a drain. It is an open conduit which spans the drain depression. Basically it is similar to a rail or road bridge except that there is a trough at the top in place of a road or rails. When the length of an aqueduct is short (that is when the drain is not quite wide at the site) the canal may be taken over the drain unaltered. (Fig. 16.1).
Hence section of the canal above the drain is exactly same as the normal section.
When the length of an aqueduct is more (that is when the river or the drain is very wide) it becomes prohibitively costly to carry the unaltered canal over the drain. Then the width of the canal or that of the aqueduct over the drain is reduced by fluming. (Fig. 16.2)
In such cases a rectangular masonry or concrete trough is constructed over the depression. This restricted or flumed length of the canal is connected by smooth transitions or wing walls to the normal canal section.
When the width of the drain is moderate the canal section is carried over the drain unaltered but the outer slopes of the canal banks are replaced by retaining walls.
Depending upon the constructional features the following sub-types of aqueducts may be recognized:
a. Pipe Aqueduct:
When an irrigation canal is very small the drain depression may be crossed by carrying the irrigation water through a pipe line (Fig. 16.3).
The pipe line spans over the depression. The pipe is generally supported in between.
b. Irrigation Box Culvert:
It is also called slab drain type aqueduct. When the drain is narrow (say maximum upto 3 m width) the drain may be passed through rectangular tunnel or a culvert. There may be more than one tunnel. To span the depression and to provide a flat roof to the culvert r.c.c. slab is laid on the top of the piers. The canal passes over the culvert in unaltered and unobstructed condition.
c. Irrigation Culvert:
When the width of a drain is from 2.5 to 16 m this type is adopted. It consists of small culverts over which irrigation canal passes. (Refer Fig. 16.1) Culverts may consist of one or more spans to provide ample water-way for the drain. Here arched culverts are generally adopted.
Maximum number of spans or culverts may be from 6 to 8. The span length of the culvert may be from 2.5 to 2 m. In this type the canal section over the drain is not contracted or altered. To retain the earth and to protect the sides of the drain short wing walls may be provided on both sides of the culvert.
d. Aqueduct:
When the drain is very wide it is made to pass through number of spans. The spans may be rectangular or arched in shape. This type of cross drainage work is much similar to a rail or road bridge in construction features except that it carries a trough shaped irrigation canal on the top. (Refer Fig. 16.2) As the length of the aqueduct is more, to reduce the cost of construction the irrigation canal is generally made smaller (trough shaped) over the drain.
It is called fluming of the irrigation canal. In the normal course the canal is trapezoidal in shape with banks on both sides. The shape of the canal over the drain is like a rectangular trough. To achieve the change of section smoothly wing-walls are provided on both sides of the canal trough.
An aqueduct requires four sets of wing walls, two for the canal and two for the drain (Fig. 16.4).
The canal wing walls protect and retain the earth in the canal banks. The foundation of the canal wing walls is not left in the embanked earth but taken deep upto the sound foundation in the natural ground.
The drainage wing walls are provided on the upstream and the downstream of the barrel to protect and to retain the natural sides of the drain. As the bed of the drain gets scoured during floods the drainage wing walls are taken deep into the foundation below maximum scour depth.
All the wing walls are taken back sufficiently into the top of the guide banks. Fig. 16.4 shows the arrangement of wing walls. The wing walls help in keeping the flow in the canal and the drain precisely within the provided waterway.
Generally a roadway is also provided by the side of the aqueduct. It may be mentioned here that in all the types mentioned above the drain water does not touch the top of the waterway. The water in the drain below the canal thus flows under atmospheric pressure through the barrel or the culvert.
e. Siphon Aqueduct:
This type is adopted when the drain water touches the roof of the drain waterway during high floods. Then the bed of the drain below the crossing, that is the floor in a barrel, is depressed below its normal level.
When the drain comes out of the barrel the bed is again taken back to normal bed level. If the bed level is not carried back to the normal level then the work cannot be said to be a siphon aqueduct but then it will be called an aqueduct with a fall in drain under the crossing.
The barrel is just like U. It is like an inverted siphon under the canal trough. This type is called siphon aqueduct. Here the drain water does not flow under atmospheric pressure but there exists a positive hydrostatic pressure in the barrel. Fig. 16.5 shows the cross sectional end view of a siphon aqueduct. Sometimes it is also called drainage siphon.
The shape of the barrel may be arched or rectangular. There may be various barrels through which drainage water passes under pressure. It helps in keeping the barrels silt free.
Type # 2. Super-Passage:
In this type of cross drainage work a drain is carried over the irrigation canal. Thus in this type of work the relative positions of the canal and the drains are reversed with respect to an aqueduct. The canal flows through tunnels or barrels. The barrels do not run full and hence the canal water flows under atmospheric pressure through the barrels.
To reduce the length of the super-passage the canal waterway may be restricted at the crossing. As the drain is carried through the trough the drain waterway may also be restricted at the site. Fig. 16.6 shows a cross sectional end view of a super-passage.
If a small irrigation canal for example a distributary or a minor is crossing the drain then such a canal may be passed through the pipe line laid below the drain. Such structure is then called pipe super-passage. The design criteria and the constructional features for this type are similar to that of aqueduct.
Siphon Super-Passage:
This type of structure is many times recognized by name siphon only. It is another type of super passage. This type is adopted when the canal water touches the roof of the waterway. Then the barrel runs full. In this type of structure the bed of the canal under the drain is depressed below normal canal bed level (Fig. 16.7).
Thus the siphon is much similar in principle to a siphon aqueduct except that the relative positions of the canal and the drain are reversed. Canal water flows under pressure through the barrel. The principles of design of the siphon super-passages are more or less similar to the siphon aqueduct.
Type # 3. Level Crossing:
Like canal headworks it is also an assemblage of various hydraulic structures. When the canal bed and the drainage bed are at the same level this type of cross drainage work may be adopted. Fig. 16.8 gives a line sketch of the level crossing.
In this type of work the drain water and the canal water is allowed to mix with each other. To maintain the suitable discharge conditions regulatory structures are provided.
They are as follows:
An escape weir is constructed across the drain at the upstream junction of the drain and the canal. Top of the weir is fixed at the canal F.S.L. When the water level in the drain rises above the F.S.L. of the irrigation canal the drain water starts spilling over the weir.
To relieve the canal of this excess water a regulator with quick falling shutters is constructed across the drain at the downstream junction of the drain with the canal. Also a cross regulator is constructed across the canal at the downstream junction of the canal with the drain.
When the water level in the drain on the upstream side is below the top of the escape weir the drain regulator is kept closed as there is no addition of water to the canal flow. The cross regulator is kept full open to allow required discharge in the canal below the crossing.
When the water level in the drain on the upstream side is above the top of the escape weir drain water enters the canal. This extra addition is then discharged through the drain regulator below the crossing in the drain. Required supply on the downstream side of the canal may be obtained through the cross regulator.
Sometimes level crossing is constructed just like headworks with two regulators across the canal one drops water in the pond and other takes forward the flow in the canal.
Inlet and Outlet:
When a small drain intercepts an irrigation canal the drain water may be allowed to enter into the canal.
It is of course true that such entry is permissible only when:
(i) The discharge of the drain is low;
(ii) The drain water is not bringing great silt load; or
(iii) It is possible to release this additional contribution to the canal through some suitable outlet.
Obviously this type of work should consist of an inlet and a suitably located outlet. The drain water flows into the canal through the inlet and it escapes through the outlet (Fig. 16.9).
The inlet is nothing but an opening in the bank of the canal. This opening is in the form of a tunnel with its sill preferably at the canal F.S.L. Drain water is taken into the canal through the tunnel. The inlet may also be designed as an escape weir. The inlet is constructed with masonry. Sometimes a pipe may be used for the purpose.
An outlet is designed as a regulating work. The discharge through the outlet is disposed-off safely in some other drain. To reduce the cost of maintenance the inlet and outlet are made self-regulating.