Managing the drain-off of floodwater can have consequences for the choice of dam type, depending on the hydrological and topographic conditions of the site in question.
In actual fact, it is more difficult to incorporate a floodwater spillway into an earth-fill dam, especially in the case of larger structures. In such a case, the designers may well prefer a concrete dam.
In addition, an earth-fill dam turns out to be more likely to becoming inundated during work than a concrete one.
Floodwater drain-off in actual fact is made up of two aspects:
during the dam construction phase,
once this is finished.
During on-site construction, the site will be protected against a given recurrent frequency of seasonal floodwater, for a minimum of ten years - often more, depending on how long construction lasts.
The risk is calculated in such a way that the economic consequences can be borne and that no human life may be lost.
Some diversionary structures constructed at that time, particularly tunnels, will then be reused later as part of dam operation proper, for example as a final low-level outlet or floodwater spillway.
Once the dam is in service, there are various solutions for drain-off of floodwater. But the increasing height of the structures, and the energy involved in the floodwater which must be dissipated generally persuade designers to opt for the following solution. This takes the form of a weir, controlled or otherwise by gates, followed by a spillway ski-jump chute to dissipate the energy, with, possibly, a stilling basin at its end.