Classification of spillways

There are several types of spillway design. A general classification of spillways used in small hydraulic works is provided below. This classification focuses on the spillway's position in relation to the earthfill and to the valley's principal stream (see figure 3.1):

A - at the earthfill's centre, on the axis of the main stream,
B - lateral to the earthfill, out of the axis of the main stream,
C - external, out of the axis of the main streambed, discharging into a secondary side valley.

The first spillway typology (A) is characteristic of all kinds of gabion weirs (diversion weir, debris/check dams and water spreading dams). The spillway is a simple gabion weir with a stilling basin on its downstream side. It is generally inserted in the earthfill embankment.

The other two spillway typologies are characteristic of dams. In the first case (B) the spillway is positioned sideways the earthfill embankment, in the second (C), the spillway discharges the excess flow into a secondary lateral valley. The last solution is the most appropriate for small dams, because its cost is generally inferior to that of the other types. Another fundamental advantage of this spillway type (C) is the complete independence that it realises between earthfill and spillway, the main structures composing the hydraulic work, which can therefore be built at different moments in time. Otherwise, it would be necessary to build these two structures simultaneously, having to cope with all the problems of works co-ordination. Finally, this solution keeps water from flowing nearby the earthfill embankment, eventually causing problems in the area of contact between earthfill and gabion structures, especially if these have not been executed thoroughly.

A

B

C

The spillway is generally composed (B and C) by a channel, that carries excess water from the impoundment to the exit, and by a drop system, for the water's restitution to the natural streambed. In the first spillway typology (A), the channel is absent and the excess flow is directly evacuated in the drop system.

The channel is generally excavated in the natural soil, and its characteristics (i.e. shape, slope and dimensions) have to be adequately predisposed to evacuate the design flow. The drop system can be composed of one or more weirs, according to the entity of energy dissipation and to the characteristics of the materials used in weirs building.

Sometimes, a stilling basin is realised downstream the weirs in order to concentrate the water's energy dissipation in this zone.