After reading this article you will learn about:- 1. Meaning of Cross-Drainage Structures 2. Types of Cross-Drainage Structures 3. Selection 4. Requirements.
Meaning of Cross-Drainage Structures:
Aligning a canal on the watershed of an area is necessary so that water from the canal can flow by gravity to fields on both sides of the canal. However, a canal taking off from a river at A (Fig. 8.1) has to necessarily cross some streams or drainages (such as at a, b, c and d in the figure) before it can mount the watershed of the area at B. In order to carry the canal across the streams, major cross-drainage structures have to be constructed.
Once the canal is on the watershed at B, usually no cross-drainage structure is required except in situations when the canal has to leave a looping watershed (such as, DEF in Fig. 8.1) for a short distance between D and F and may cross tributaries (as at e and f). The cross-drainage structures are constructed to negotiate a channel over, below or at the same level of a stream.
Types of Cross-Drainage Structures:
Such structures can be classified under three broad categories depending on whether the structure is built to negotiate a carrier channel over, below or at the same level as the stream channel.
1. Structures for a Carrier Channel over a Natural Stream:
The structure’s falling under this category are aqueducts and siphon aqueducts. Maintenance of such structures is relatively easy as these are above ground and can be easily inspected. When the full supply level (F.S.L.) of a canal is much higher than the high flood level (H.F.L.) of stream, the canal is carried over the stream by means of a bridge-like structure which is called aqueduct. The stream water passes through the space below the canal such that the H.F.L. is lower titan the underside of the canal trough (Fig. 8.2).
Siphon aqueducts (Fig. 8.3) are the aqueducts in which the bed of the stream is depressed when it passes under the canal trough, and the stream water flows under pressure below the canal. In siphon aqueducts, the stream bed is usually provided with a concrete or masonry floor.
Aqueducts and siphon aqueducts are further classified into the following three types:
In this type of structure, the earthen canal banks are carried as such and, hence, the culvert length (i.e., the length of barrels through which the stream water is passed under the canal) has to be long enough to support the water section as-well as the earthen banks of the canal [Fig. 8.4 (a)].
In this type of structure, the canal section is not flumed and remains unaltered. Hence, the width (across the canal) of the structure is maximum. This type of structure, obviously, saves on canal wings and bank connections, and is justified only for small streams so that the length (along the canal) of the structure is small. An extreme example of such a structure would be to carry the stream by means of a pipe laid under the bed of the canal.
This type of structure is similar to the Type I with a provision of retaining walls to retain the outer slopes of the earthen canal banks [Fig. 8.4 (b)]. This reduces the length of the culvert. This type of construction can be considered suitable for streams of intermediate size.
In this type of structure, the earthen canal banks are discontinued through the aqueduct, and the canal water is carried in a trough which may be of masonry or concrete, Fig. 8.4 (c). The earthen canal banks are connected to the respective through walls on their sides by means of wing walls.
The width of the canal is also reduced over the crossing. In this type of structure, the width of the structure is minimum and, hence, the structure is suitable for large streams requiring considerable length of aqueduct between the abutments.
2. Structures for a Carrier Channel Underneath a Natural Stream:
The structures falling under this category are super-passages and siphons. The maintenance of such structures is relatively difficult as these are not easily accessible. Super-passage (Fig. 8.5) is like an aqueduct, but carries the stream over the canal. The canal F.S.L. is lower than the underside of the stream trough and, hence, canal flows with a free surface.
Siphon (Fig. 8.6) carries the canal water under pressure through barrels below the stream trough. For siphoning small discharges, precast RCC pipes will be economical.
For siphoning higher discharges, horse-shoe shaped, rectangular or circular barrels, single or multiple, are adopted. Roof of rectangular barrels are, at times, arch-shaped for economy. For discharges under high pressures, circular or horse-shoe shaped barrels are more suitable.
3. Structures for Carrier Channel Crossing a Natural Stream at the same Level:
Structures falling under this category are level crossings and inlets. Inlets are, at times, combined with escapes. When the canal and the stream meet each other at practically the same level, a level crossing (Fig. 8.7) is provided. The level crossings involve intermixing of the canal and the stream waters.
These are usually provided when a large-sized canal crosses a large stream which carries a large discharge during high floods and when siphoning of either of the two is prohibitive on consideration of economy or non-permissibility of head loss through siphon barrels. Across the stream and at the upstream end of the junction with the canal is constructed a barrier with its top at the canal F.S.L.
The regulators are provided across the stream and the canal at the downstream junctions of the level crossing. These regulators control the flow into the canal and the stream downstream of the crossing. This type of arrangement is also useful in augmenting the canal supplies with the stream discharge.
When the stream is dry, the stream regulator is kept closed and the canal regulator is opened so that the canal water flows in the canal itself without interruption. When the stream is bringing water, it mixes with the canal water, and the stream regulator is used to dispose of that part of stream water which is not used to augment the canal supply.
Selection of Cross-Drainage Structures:
Relative difference in bed and water levels of the canal and stream as well as their discharges are the main factors for deciding the type of cross-drainage structure at a site. By suitably changing the alignment of the canal between off taking point A and the watershed (Fig. 8.8), the relative difference between the bed levels of the tributaries and the canal at the crossing site can be altered.
Consider three possible alignments ABC, ADE and AFG of a canal taking off from a river at A and intersecting a tributary HBDFI at B, D and F before mounting the watershed at C, E and G, respectively (Fig. 8.8). The distances AB, AD and AF are almost the same and, hence, the canal reaches the crossing site with its bed more or less at the same level.
But, the bed levels of the tributary at B, D and F are significantly different due to higher slope of the tributary. Obviously, the bed level of the tributary is the highest at B and the lowest at F in the reach BDF.
Thus, if F is a suitable crossing site for aqueduct, site D may necessitate construction of siphon aqueduct or level crossing, and site B may require construction of a siphon or a super-passage. Thus, the type of cross-drainage structure can be changed by suitably altering the crossing site.
When the crossing site is such that the canal F.S.L. is well above the stream H.F.L. the choice between aqueduct and siphon aqueduct is made depending on the stream discharge. For larger stream discharges (i.e., the streambed is much wider), an aqueduct is more suitable than the siphon aqueduct which requires lowering of the stream bed by a drop.
Besides being costly, lowering of the bed may result in silting on the lowered stream bed which increases the risk of failure. However, an aqueduct necessitates heavy canal embankments towards the crossing (Fig. 8.9). This is due to the wide flood cross-section of streams in plains and the requirements that the canal must be well above the H.F.L. and the aqueduct has to be constructed in a smaller part of the cross-section of the stream.
Siphon aqueducts are more suitable when the stream size is small compared to the canal size. In case of siphon aqueducts, the relative difference of water and bed levels of the canal and stream is small and, hence, embankments of only small height are required.
If the stream H.F.L. is well above the canal F.S.L., the super-passage is generally preferred in comparison to the siphon as the latter involves considerable head loss in the canal. In addition, construction of siphon under a stream with erodible bed requires heavy protection works.
Foundation of both super-passage and siphon has to be carried up to much below the erodible bed of the stream. A separate bridge across the stream trough has to be provided to carry the canal road across the stream.
The construction of these structures is relatively difficult and costly due to the requirements of extensive training works and large stream trough to carry the high flood discharge. If the canal serves navigation needs also, sufficient headway should be provided for the passage of boats.
If the bed and water levels of the canal and the stream at the crossing site are approximately the same, level crossing is provided. Sometimes due to prohibitive costs of siphons and siphon aqueducts, the canal alignment between the off-take and the watershed is suitably altered so that level crossing can be provided at the crossing site.
The initial cost of a level crossing is generally much smaller than the cost of other cross-drainage structures. Also, the perennial discharge of the stream can be diverted to the canal to provide additional irrigation. However, the level crossings require permanent staff for continuous watch, maintenance and operation of gates.
Also, when the stream is passing the high flood discharge, the canal may have to be closed down to prevent the sediment load of the stream from entering the canal and silting it. Further, if the canal F.S.L. is higher than the general ground level, the H.F.L. of the stream would increase on the upstream side of the crossing site, and submerge the land. To prevent such submergence of the land, marginal banks are provided.
Besides the above factors, the topography of the terrain, foundation conditions, regime of the stream, and dewatering requirements would also affect the choice of the type of cross-drainage structures. Detailed examination of the terrain topography and the foundation is necessary to locate a stable reach of the stream with good foundations and permitting preferably a right-angled crossing.
For streams carrying high sediment discharge, the possibility of choking up of the siphon and the effect of fluming of the stream should be kept in mind. Dewatering of foundations is necessary in the construction of foundations for cross-drainage structures. An accurate estimate of the cost and method of dewatering must be worked out when designs involve laying of foundations below the ground water table.
Requirements of Cross-Drainage Structures:
Any cross-drainage structure should preferably be located in a straight reach of the stream crossing the canal at right angles as far as possible. Alignment of the canal should also be such that it results in minimum lengths of embankments (for aqueduct and siphon aqueduct structures).
If required, the site of the structure may even be shifted away from the existing stream channel, when it is possible to divert the channel and also keep it there by reasonable training works. One obvious advantage of such an alternative would be that the construction will be carried out in dry conditions.
The type of foundation for cross-drainage structures will depend primarily on the depth of scour, calculated from Lacey’s equation (Eqs. 5.9 or 5.10), and the bearing capacity of the soil. The depth of scour around piers is taken as twice the depth of scour calculated from Lacey’s equation.
In alluvial streams, well foundation is usually provided where deep foundation is required. With the provision of an impervious floor (necessary for siphon and siphon aqueduct) along with cutoff walls, the depth of foundation may be reduced. The floor itself may be designed as either a gravity floor or a raft.
The floor is designed to resist the total uplift pressure caused by subsoil water and the water seeping from the canal. The uplift pressure is counterbalanced by the dead weight of the gravity floor.
The worst condition occurs when there is no water in the barrel and, hence, the weight of water in the barrel is not included. At times, it may be economical to design the floor as a raft so that the uplift is counterbalanced by the entire weight of the superstructure.
The spacing of the piers (i.e., the span) depends on structural and economic considerations. Fewer piers (i.e., longer span) are preferable at sites which require costly foundation. In case of siphon aqueducts and siphons, the drop at the upstream end of the culvert may be vertical (generally economical) or sloping.
But, at the downstream end of the culvert, the rise should always be at a slope flatter than 1 in 4 so that the bed load can be moved out of the siphon barrel. The culvert floor should extend upstream of the barrel inlet by a distance equal to the difference between the H.F.L. and the culvert floor level. Barrel inlet should be bell-mouthed to reduce the head losses.
Suitable arrangement has to be provided to pass the service road across the stream. This requirement does not pose much problem in structures of type 1 and II in which earthen embankments are continued. In structures of type III, the simplest arrangement is to carry the road on either side (or only on one side for economic reasons) by providing slabs and arches on either side (or on one side) of the canal trough.
The piers are suitably raised to keep the road and bank slabs at a level higher than the canal bed so that the flood water may find clear entry and exit (if roads on both sides of canal trough are provided) conditions at the siphon barrels. Also, the quantity of masonry is reduced.
The sides of the canal trough are generally designed as beams in reinforced concrete structures. The bottom slab is suspended from these beams. Additional beams, if required, are projected into the canal to divide the canal trough into number of parallel channels.
For wider troughs having intermediate beams, the service road may be provided on one of the compartments. Canal troughs of the smaller width can be constructed as a hollow box girder and the service road can be provided on the top slab.
Wing walls of stream are suitably connected to high ground. The stream should be guided towards the structure by means of suitable river training works. Similarly, the canal banks, adjacent to the crossing, should be protected by measures, such as pitching, launching apron, etc., wherever necessary.