They mostly happen in south America or in Mexico where the land is dry is where the floods occur. Floods most commonly occur near a river or a coastal area, they commonly occur near low lying areas and areas below sea level. Floods can also be contributed to by mountain runoff
In tropical regions, this can occur seasonally - such as during the monsoon season of India, Pakistan, Bangladesh and other countries. Otherwise, floods mostly happen after it rains a lot - as in much more than usual for a given geographic area.