DMPQ- What are the primary causes for Floods ?

Excessive rainfall in river catchments or concentration of runoff from the tributaries and river carrying flows in excess of their capacities.
Backing water in tributaries at their confluence with the main river.
Synchronization of flood peaks in the main rivers or their tributaries.
Intense rainfall when river is flowing full.
Poor natural drainage system.
Landslides leading to obstruction of flow and change in the river course.
Cyclone and very intense rainfall when the EL Nino effect is on a decline.