ABSTRACT
In order to simplify things when commencing the hydraulic study of canals, it is usually assumed that the speed is constant and that the water surface is always parallel to the bottom, however, this very rarely happens in practice in a permanent fashion. The inverted syphons, which have already been examined, together with
any constructions installed in the canal, will introduce specific head losses that vary with the circulating flow rate. This means they will not coincide with slopes and drops already included in the canal design, which has been calculated for a flow rate that generally coincides with the design maximum. The water surface is then no longer parallel to the bottom, usually for long periods of time and this leads to a regime thatwe called “varied” inChapter 1. Moreover, it is also quite usual to have to vary flow rates or levels
artificially over time, or even in the same day, by means of gates that allow more or less flow rate or raising or lowering the levels arriving at the hydraulic regime which we called “variable over time” in Chapter 1. This is sometimes necessary in order to adjust the flow rates to those
required and on other occasions it is necessary to resolve problems in secondary and tertiary canals and turnouts due to a lack of adequate water levels. All canals suffer from the problem of having a very long response time
between introducing a flow rate variation at its beginning until this is available at its destination. This makes canal regulation difficult, where “regulation” is understood as being all the control operations that have to be carried out in order to have the necessary flow rates available at all times along the entire canal. This becomes quite obvious by thinking of the advance warning that is required to modify the feed flow rate at the intake of a canal that is twenty or even fifty kilometres long, with water speeds of between 1 and 1.5m/s in order to change the consumption at a secondary or tertiary canal located at the other end. This regulation is significantly more complicated in irrigation canals,
and even more so if the irrigation network is
taken into consideration as a whole, consisting of secondary canals leading from the main one which, in turn, feed smaller ones until the actual plot turnouts are reached. The variation of desired flow rates at a certain point requires the modifica-
tion of the associated gate for derivation or canal level regulation or both, an operation that will propagate the level variation both upstream and downstream from this point and, obviously, that of the circulating flow rates. This modification affects the turnouts located downstream and, in general,
will also affect the upstream turnouts. An exception occurs in those cases where there is a drop or chute, the supercritical speed of which will guarantee non-propagation of any perturbation in a direction against the current. This fact leads to a modification of the flow rates in front of the gates,
which means that in order to maintain a good level of service, these gates will have to be adjusted, leading to flow rate and level modifications at other gates, so that the requirement for gate adjustment eventually becomes necessary throughout the entire network. It therefore turns out that the manual operation of a single gate in an
irrigation network that includes conventional regulation elements not only requires the theoretical reiterated adjustment of a large number of gates, but also requires this in practice. The associated human costs would be enormous. However, the worst part of this is the fact that, in spite of everything,
success is not guaranteed. It is all too easy to act in an overzealous fashion when opening or closing a gate with the resulting danger of oscillation and increasing instability, together with the very great risk of over-spilling, with the corresponding loss of water or insufficient water flow being supplied. Such a situation, from an operational point of view, is quite unacceptable.
