Cross drainage works is defined as when an irrigation canal is intercepted by a drain which is required to be crossed over the canal, some suitable structure is required to be constructed. Thus the engineering works constructed to cross the canal by the drain is called cross drainage works. These also include the crossing of rivers, streams, canals and natural water course.
Types of Cross Drainage Works
Following are the various types of cross drainage works which are constructed under different condition:
These are just like bridges except that in place of carrying a road or railway over these, they carry canal on the top and drainage is passed under the canal such that its high flood level (H.F.L) is always lower than the under side of the supporting structure for the canal. The primary consideration in deciding whether should be an aqueduct or a syphon aqueduct is the relative sizes of canal and the drainage to be crossed. In case of syphon aqueduct, as the canal bed is not clear above the high flood level. The relative difference of levels between the two canals is much smaller and heavy banks are practically eliminated. Thus it can be said here that when the drainage is small as compared to the canal, syphoning of the drainage will mostly be preferred for taking the canal in high banks. The difference of bed levels between canal and drainage can be suitably altered by changing the canal alignment.
Depending upon the constructional features, the aqueduct is of following types:
a) Pipe Aqueduct
When the canal is very small, it passes over the drain through pipe line which is called pipe aqueduct.
b) Irrigation Culvert
Where the width of drain is from 2.5m to 16m small culverts are provided for drain over which canal is passed. Canal passes unobstructed over the culvert.
c) Irrigation Box Culvert
It is also called irrigation slab drain type aqueduct. When the width of drain is small i.e., up to 3m, the drain may be passed through rectangular tunnel or a culvert.
d) Syphon Aqueduct
Syphon is the structure in which case the canal water passed through inverted syphon under a drainage. The canal water is allowed to flow under pressure when tail water level below a syphon is higher than the under surface of the culvert and uplift pressure is exerted on the covering of the roof. To ensure the safety the work under worst conditions, it should be assumed that there is no water in the canal at that time. The uplift pressure at the downstream end of the barrel roof is equal to the difference between downstream water level and level of the underside of the roof. At any other point along the barrel, the uplift pressure is given by the ordinate between the hydraulic gradient line and the under side of the roof covering.
2. Supper passage
When the drain is carried over the irrigational canal, the cross drainage is called a super passage. The canal flows in barrels or tunnels under atmospheric pressure and do not run fall and the water touches the roof of waterway, the bed of the canal under the drain.
Super passage works exactly similar to aqueducts and with the same difference as in syphon that in this case the drainage is on top and the canal is carried below. As the canal is passed under the drainage, bridge should be separately constructed on the drainage trough to carry the service road across the drainage. Syphon and super passages are more expensive and difficult to construct and require extensive training works and a large drainage trough to take the high flood discharge.
3. Level Crossing
When the bed of canal and drainage is at same level a level crossing is provided. An escape weir is constructed along the drain at upstream junction. Top of weir is kept at the full reservoir level (F.S.L.) of the canal. When the water level in drains rises above full reservoir level (F.S.L.) of canal it spills over the escape weir and mixes with the canal water and is taken out from the canal at down stream junction by providing regulators.
4. Spill Way
It is structure which is provided to dispose off surplus water from the reservoir. Spill way works are safety works for drain and provides it structural stability. It also passes the flood water level to rise above high flood level. There are many types of spill way, an ogee shape spill way. the other part of spill ways are straight drop or free over fall spill way, syphon spill way, gate spill way, chute spill way, and tunnel spill way.
To supply the water from the reservoir to the area below the drain, outlets are provided which are called sluices. Sluices are also provided to remove the sediments from the reservoir. They are provided with gates which are operated from a platform at the top. Top of platform is kept above full reservoir level (F.S.L,). Another type of sluices are Belgrain type outlet head wall outlet, Dharwar type outlet and Pipe outlet.