Contents preface (VII) introduction 1—37

Fig. 11.9 Canal embankments near crossing site
Siphon aqueducts are more suitable when the stream size is small compared to the canal size. In case of siphon aqueducts, the relative differences of water and bed levels of the canal with those of the crossing stream is small and, hence, embankments of only small height are required.
If the stream HFL is well above the canal FSL, superpassage is generally preferred in comparison to siphon as the latter involves considerable head loss in the canal. In addition, the construction of a siphon under a stream with an erodible bed requires heavy protection works. The foundations of the superpassage and siphon have 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, then sufficient headway should be provided for the passage of boats.
If the bed and water levels of the canal and stream at the crossing site are approximately the same, a level crossing is provided. Sometimes, due to prohibitive costs of siphons and siphon aqueducts, the canal alignment between the offtake and watershed is suitably altered so that the level crossing can be provided at the crossing site. The initial cost of a level crossing is generally much lower 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 crossing requires 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 FSL is higher than the general ground level, the HFL of the stream would increase on the upstream side of the crossing site causing submergence of the land. To prevent such submergence of the land, marginal banks are provided.
In addition to 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 chocking 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

386 IRRIGATION AND WATER RESOURCES ENGINEERING
structures. An accurate estimate of the cost and method of dewatering must be worked out when design involves laying of foundations below the ground water table.