ABSTRACT
These are structures that are similar to bridges and transport the canal water over rivers, other watercourses and obstacles.
14.1.1 Purpose
Canals normally have only very small gradients and this means that their most economical route is usually almost coincident with a contour line, in relation to which it descends a few centimetres every kilometre at the same time as slightly separating from it. However, there are times when the canal route meets a watercourse that
presents the design engineer with two possibilities. One is to follow the same contour line, bordering the watercourse. The other is to cross it by means of a structure, which is really a bridge, known as an elevated flume, supporting the canal. This second option opens up a whole range of variants since the crossing point can be located over the watercourse either further upstream or further downstream. It is, in any case, essential for this new construction to allow the passage
of rainwater that comes along the watercourse and has to cross the canal, either above or below it. If the flume solution is adopted, very careful attention should be paid to
the fact that most roadway bridges that have collapsed throughout history have done so due to insufficient capacity to drain away the river or watercourse waters. If the available section under the flume is insufficient for the natural passage of the waters, then the flume will be loaded with a water level upstream from the flume that is higher than the downstream level. Pressurized water will then circulate under the flume at high speed, which will lead to undermining of the pile foundation with subsequent general collapse. If it is decided to adopt the solution of crossing the watercourse with a
upstream or downstream, will have to be
chosen so that it has sufficient capacity to allow safe passage of the water carried by the watercourse. Other devices are required if it is decided to completely route the canal
alongside the watercourse. At times, an overpass is constructed over the canal, which will transport the rainwater arriving via the watercourse to the lower section. This can be seen in Figure 14.1, which shows the existing construction for the Carrizal Canal in Mendoza, Argentina. On other occasions, preference is given to one or more pipes passing
underneath the canal, provided that the only moderate flow rates are to be carried. A syphon inlet is employed, but it is very important for the outlet to be as free as possible, without the water having to overspill via a syphon outlet because the watercourse waters will be carrying stones and sediment that could block the pipes. At times, when the watercourse flow rates are very small, water is allowed
to enter the actual canal itself, using the canal freeboard for this purpose.
