Contents preface (VII) introduction 1—37

Fig. 11.8 Different canal alignments between offtake and watershed
When the crossing site is such that the canal FSL is well above the stream HFL the choice between aqueduct and siphon aqueduct is made depending on the stream discharge. For larger stream discharges (i.e., when the stream bed is much wider) an aqueduct is more suitable than a 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

CROSS-DRAINAGE STRUCTURES

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crossing (Fig. 11.9). This is due to the wide flood cross-section of streams in plains and the requirements that the canal must be well above the HFL, and the aqueduct has to be constructed in a smaller part of the cross-section of the stream.

Canal bed

Stream H.F.L.

Canal	Waterway	Canal
embankment	at	embankment
		crossing