Contents preface (VII) introduction 1—37

Table 11.2 Maximum permissible velocities (1)

Type of floor	Maximum permissible velocity (m/s)
Steel-and cast iron-lined face	10
Concrete-lined face	6
Stone masonry with cement pointing	4
Stone (or brick) masonry with cement plastering	4
Hard rock	4
Brick masonry with cement pointing	2.5

In aqueducts, the height of barrels is fixed such that the canal trough is about 0.6 m above the HFL of the stream. The requirement of the distance between the full supply level of canal and the bottom of the stream trough is, however, less in case of superpassage as FSL is relatively a certain quantity. But, in siphon aqueducts, the required waterway area is calculated on the basis of permissible scouring velocity (generally 2-3 m/s) through the barrels. Velocities higher than the permissible velocity will result in higher afflux upstream of the structure resulting in higher and longer marginal banks.

In aqueducts as well as siphon aqueducts, it is necessary to have sufficient headway between the bed level of the stream (downstream of the crossing) and underside of the culvert roof. This headway should be at least 1 m or half the height of the barrel, whichever is less. In the absence of a clear headway, there will exist the risk of the barrels being blocked because of silting. To fulfil the requirements of the headway, the stream bed may have to be lowered by providing a fall upstream of the crossing.