Spillways

Earlier, in Section 10.3, we mentioned the need for the cross section of the canal to be designed with the theoretically necessary depth and a margin of safety that we called freeboard. Several reasons cited there may cause the freeboard to be partially or totally full under abnormal conditions. In order to cope with the excess circulating water, which occasionally

causes overflows, provision should be made in the canal for structures able to empty them automatically into streams or wasteways foreseen for that purpose. This will prevent uncontrolled overflows elsewhere, which could cause damage to crops or to the canal itself. Such structures are the so-called “lateral spillways” and allow the canal

downstream to operate normally, or almost normally. These may be of two different types. One is a fixed-crest weir, coinciding

with the theoretical water surface level, parallel with, or little divergent from, the axis of the canal. When the water rises above the set level, the water spills laterally into a chamber that rids it of its excess energy and is then conducted through an auxiliary course to the place where it is emptied back into the drainage canal (Figure 16.1). The other type is the syphon spillways. These are made up of a tube

with two bends running along the side of the canal at a height at which, when the water level passes a predetermined figure, the syphon fills up and spills all its water, taking advantage of the difference in level between the entrance and the lowest exit, as a result of which both the speed and the flow of water are great, despite the small amount of space (Figure 16.2). A factor that must be kept in mind in selecting the location of the spillways

is the proximity of, or possibility of having available, an evacuation channel, although many times this may require the construction of a canal to carry spillage. The canal’s crossings with creeks and streams should always be considered as possible spillway locations. Irrigation canals and some for the supply of human settlements have the

due to the fact that when they feed

a smaller canal the theoretical flow diminishes and the following section of the canal is sized with a smaller capacity. Obviously, in such places there may be danger of flooding when, e.g. the total flow reaches them and due to abnormal circumstances (breakdowns, unforeseen change in operational orders, etc.) the derivative inlet is closed. The section of canal downstream to carry the total flow of water.