How can a flood be stopped in dams?is there an alarm for it? ?

Question

How can a flood be stopped in dams?is there an alarm for it? ?

Answer 1

I assume that you mean that when a flood occurs upstream of a dam. What are the effects? First it depends on how full the reservoir is at that particular dam. If it is a flood control such as we have in California, the operators of the dam try to always have enough room to contain any flood waters. If there is not enough room, then part of the flood is allowed to pass through the dam.  It will be controlled in such a way that does not exceed the carrying capacity of the downstream river.

Floods are detected by watching rain patterns in drainage basin. This allows them to predict how much water will be coming down and the reservoir level is adjusted to take this inflow into account. By the time the water level at the dam starts to rise, it is too late.  

Answer 2

There is no alarm for floods but dams use up the river's water and distribute it so extra water would not cause a flood