Physical Causes of Flooding

 Flooding occurs when a river's dicharge exceeds the capacity of its channel to carry that discharge. the river overflows its banks. Flooding may be caused by a number of natural causes or physical factors:

Excessive levels of precipitation occurring over a prolonged period of time. This eventually leads to saturation of the soil. When the water table reaches the ground surface, there is increased overland flow or runoff

Intensive precipitation over a short period of time particularly when the ground surface is baked hard after a long period without rainfall. In such circumstances the infiltration capacity is such that the ground cannot soak up the rainfall quickly enough, so more water reaches the river than would normally be the case

The melting of snow particularly when the subsoil is still frozen, so that infiltration capacity is reduced

Climatic hazards such as cyclones in Bangladesh, hurricanes in the Gulf of Mexico or deep low-pressure weather systems in mid-latitudes bring abnormally large amounts of precipitation

The nature of the drainage basin has an influence on the  likelihood of flooding. Some drainage basins are more likely to flood than others. Relief, vegetation, soil type and geology all have a part to play. In areas of the world vegetated by dense forest, interception and uptake by plants reduce the risk of flooding during time of heavy rainfall.

Reference:
 http://www.acegeography.com/causes-of-flooding.html