A detention dam is a dam built to catch surface runoff and stream water flow in order to regulate the water flow in areas below the dam. Detention dams are used to reduce the damage caused by flooding and to manage the flow rate through the downstream channels. The reservoir behind the dam is normally dry, and will only fill during severe rainfall.Failure of a detention dam usually occurs from overtopping or piping:Overtopping of a detention dam occurs when the water level behind the dam exceeds the dam crest height. The dam crest is the top edge of the dam. Overtopping is caused by extreme flooding or severe waves. The severe waves can be a result of high winds, landslides, and earthquakes. If overtopping occurs, the dam may fail due to the erosion caused by the overtopping waters. Piping occurs when seepage through the body of the dam becomes so great that the material that makes up the dam itself is washed away, and the dam can no longer hold the water behind it. In both failure modes, water, silt and debris would flow down the channel, causing widespread destruction to anything in the flow path.Most failures of dams occur quite rapidly, and as such there is unlikely to be any substantial warning time for those in the immediate downstream vicinity. However, since the detention dams only fill during times of heavy rain, (and thus these times are the only times the dam is likely to fail) most people are already aware of flooding hazards.
