A dam is an engineering structure constructed across a valley or natural depression to create a water storage reservoir. Such reservoirs are required because of three main purposes – provision of a dependable water supply for domestic and/or irrigation use; flood mitigation and generation of electric power.
But, they can fail because of one or a combination of some of these reasons:  overtopping caused by floods that exceed the capacity of the dam; Deliberate acts of sabotage;  Structural failure of materials used in dam construction; Movement and/or failure of the foundation supporting the dam; Settlement and cracking of concrete or embankment dams;  Piping and internal erosion of soil in embankment dams; Inadequate maintenance and upkeep.
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