31-03-2012, 03:26 PM
CROSS-DRAINAGE STRUCTURES
NEED 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.
11.1) 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 a 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. 11.1) for a short distance between D and F, and
may cross tributaries (as at e and f). Cross-drainage structures are constructed to negotiate an
aligned channel over, below, or at the same level of a stream (1).
Structures for a Carrier Channel Over a Natural Stream
The structures 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 (FSL) of a canal is much higher than the high flood level (HFL) of a
stream which, in turn, is lower than the bottom of the canal trough, the canal is carried over
the stream by means of a bridge-like structure, which is called an aqueduct.
DESIGN OF CROSS-DRAINAGE STRUCTURES
Before undertaking detailed designs of any important cross-drainage structure, collection of
relevant field data is required. Besides, a note regarding the several alternative alignments
surveyed and reasons for final selection of a particular crossing site is also necessary. A location
map of the site along with the results of subsurface explorations of the site, and stream crosssections
at different locations around the site should also be prepared. The following specific
hydraulic data regarding the canal and stream should be made available.
WATERWAY AND HEADWAY OF THE STREAM
Economic and safety considerations limit the waterway of the stream. Lacey’s regime perimeter
equation [Eq. (8.29)] can be used for determining the permissible waterway for structure without
rigid floor. In structures with rigid floors, however, the waterway can be further reduced, but
keeping the flow velocities within permissible limits (Table 11.2).
NEED 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.
11.1) 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 a 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. 11.1) for a short distance between D and F, and
may cross tributaries (as at e and f). Cross-drainage structures are constructed to negotiate an
aligned channel over, below, or at the same level of a stream (1).
Structures for a Carrier Channel Over a Natural Stream
The structures 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 (FSL) of a canal is much higher than the high flood level (HFL) of a
stream which, in turn, is lower than the bottom of the canal trough, the canal is carried over
the stream by means of a bridge-like structure, which is called an aqueduct.
DESIGN OF CROSS-DRAINAGE STRUCTURES
Before undertaking detailed designs of any important cross-drainage structure, collection of
relevant field data is required. Besides, a note regarding the several alternative alignments
surveyed and reasons for final selection of a particular crossing site is also necessary. A location
map of the site along with the results of subsurface explorations of the site, and stream crosssections
at different locations around the site should also be prepared. The following specific
hydraulic data regarding the canal and stream should be made available.
WATERWAY AND HEADWAY OF THE STREAM
Economic and safety considerations limit the waterway of the stream. Lacey’s regime perimeter
equation [Eq. (8.29)] can be used for determining the permissible waterway for structure without
rigid floor. In structures with rigid floors, however, the waterway can be further reduced, but
keeping the flow velocities within permissible limits (Table 11.2).