The spillway is a very important component of any hydraulic work. The spillway crest's main function consists in fixing the maximum water level upstream the structure, and preventing water overflows. The spillway must be accurately dimensioned, so that it can evacuate the design flow calculated for a hydraulic work.

The main procedures used for designing the structures that generally compose a spillway (e.g. spillway channel, weirs) are briefly illustrated in this chapter, with reference to both hydraulic theory and stability computation procedures. These procedures are taken from elaborate theories, which, for the sake of simplicity, will be only briefly mentioned here. For an in-depth treatment of these theories, readers should refer to specific publications.

The problem of runoff's restitution to the original river bed

In natural streams, the total hydraulic energy is uniformly dissipated along the streambed. However, if a small dam or weir is built, the energy dissipation on the dam's upstream side results substantially lower than it would be in natural conditions and the potential energy level is therefore high. When this high hydraulic energy is dissipated downstream the structure, it could cause serious scour problems in the streambed, unless the rise in energy created by the structure's installation is dissipated immediately beyond the structure. This can happen naturally, if the characteristics of the streambed in question allow it, or artificially, with the creation of a 'stilling basing'. Here, the water flowing throughout the spillway must lose a portion of its total energy so as to reach a lower energy level downstream, i.e. a level equal to the one it would have had in the absence of the dam or weir.

The energy dissipation that takes place as a result of the construction of a hydraulic structure can give rise to important erosion phenomena in the streambed. Locally, this will threaten the structure's stability.

Downstream, it will scour the river's bed for a long reach. Therefore, avoiding the negative consequences of energy dissipation is one of the principal problems to be dealt with when designing a hydraulic work.

A common way to solve this problem consists in concentrating the energy in a circumscribed area, called 'stilling basin'. For the importance of its function, this area should be carefully designed and realised.