The requirements divide flooding into three major causes: floods due to precipitation and other causes; water waves induced by earthquakes or other geological phenomena; and floods and waves caused by failure of water control structures. Flooding can be caused by one or more concurrent natural phenomena, such as heavy precipitation and rapid snow melt, high tides and storm surges, seiche and wind waves, among many other combina­tions. Flooding hazards from tsunamis associated with marine earthquakes or close to lakes and large rivers, and seiches originating from geological causes should be quantified to design protective measures. On river sites, floods and waves caused by the failure of upstream dams or other water retention structures have to be analysed to determine the level above the river surface where safety structures should be built to avoid floods or to protect key structures, systems and components.

An IAEA safety guide considers flood hazards for nuclear power plants sited on coastal and river sites (IAEA, 2003c). The guide considers each one of the flooding causes and identifies deterministic and probabilistic parameters which should be included in the design basis of the plant. Due consideration is given to the concurrence of different causes of flooding and the potential changes in the initial parameters due to climate changes or geographical modifications. The stability of shorelines and river beds is also a matter for consideration.