What is a flood??? 

A flood is an overflow of an expanse of water that submerges land, a deluge. In the sense of "flowing water", the word is applied to the inflow of the tie, as opposed to the outflow or "ebb".It is usually due to the volume of water within a body of water, such as a river or lake, exceeding the total capacity of the body, and as a result some of the water flows or sits outside of the normal perimeter of the body. It can also occur in rivers, when the strength of the river is so high it flows right out of the river channel , usually at corners or meanders. These of course, are not applicable in such instances as sea flooding.

Causes

Floods from the sea can cause overflow or overtopping of flood defences like dikes as well as flattening of dunes or bluffs. Land behind the coastal defence may be inundated or experience damage. A flood from sea may be caused by a heavy storm (storm surge), a high tide, a tsunami, or a combination thereof. As many urban communities are located near the coast this is a major threat around the world. Many rivers flow over relatively flat land border on broad flood plains. When heavy the deposition of silt on the rich farmlands and can result in their eventual depletion. A flood occurs when an area of land, usually low-lying, is covered with water. The worst floods usually occur when a river overflows its banks.

Primary effects

Physical damage-Structures such as buildings get damaged due to flood water. Landslides can also take place.
Casualties-People and livestock die due to drowning. It can also lead to epidemics and diseases.

Secondary effects

Water supplies-Contamination of water. Clean drinking water becomes scarce.
Diseases-Unhygeienic conditions. Spread of water-bourne diseases
Crops and food supplies-Shortage of food crops can be caused due to loss of entire harvest. Tertiary/long-term effects
Economic- Economic hardship, due to e.g. temporary decline in tourism, rebuilding costs, food shortage leading to price increase etc, especially to the poor.